Complexes of Ni(I): a “rare” oxidation state of growing importance

Chun-Yi Lin * and Philip P. Power *
Department of Chemistry, University of California, Davis, California 95616, USA. E-mail:

Received 24th March 2017

First published on 4th July 2017

Nickel plays an important role in areas as diverse as metallurgy, magnetism and biology as well as in chemical applications such as the catalytic transformation of organic substrates. Despite nickel's importance, the investigation of its compounds in various oxidation states remains uneven and those in the +1 oxidation are less common than those in the neighboring 0 and +2 oxidation states. Nonetheless, in recent years, the volume of work on Ni(I) complexes has increased to the extent that they can be no longer regarded as rare. This review focuses on the syntheses and structures of Ni(I) complexes and shows that they display a range of structures, reactivity and magnetic behavior that places them in the forefront of current nickel chemistry research.

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Chun-Yi Lin

Chun-Yi Lin was born in Taichung, Taiwan and grew up in nearby Fengyuan City (means flourishing plain) and received a BS in chemistry with a Minor in physics from National Tsing Hua University in Taiwan in 2009. His undergraduate research with Prof. Yi-Chou Tsai focused on the synthesis and reactivity of the quintuply-bonded dichromium species. After a one-year military service in the Taiwanese Air Force. He joined Prof. Philip Power's lab to pursue a PhD, where his research interest lies in inorganic synthesis, reactivity and magnetic properties of low-coordinate transition metal complexes, in particular, using attractive dispersion forces to stabilize unusual species.

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Philip P. Power

Philip Power received a BA from the University of Dublin in 1974 and a DPhil under M. F. Lappert from the University of Sussex in 1977. After postdoctoral studies with R. H. Holm at Stanford University, he was appointed to the Department of Chemistry at the University of California, Davis in 1981, where he is a Distinguished Professor. His main interests lie in the exploratory synthesis of new main group and transition metal compounds. A major theme of his work has been the use of large ligands to stabilize, via steric blocking and/or dispersion force attraction, species with new types of bonding, coordination numbers, and/or reactivity toward important small molecules such as hydrogen, ammonia, or carbon monoxide.

1. Introduction

Compounds of Ni(I) are less numerous than those of the neighboring oxidation states Ni(0) and Ni(II).1 Although for many years, Ni(I) complexes were considered to be unstable,1 they have been known for over a century (1914) and the number of structurally characterized Ni(I) species continues to grow rapidly (Fig. 1).2–4 The recent growth is mainly due to their use in catalysis and their biological relevance.3–5 In the latter case, the Ni(I) ions have been proposed to exist in several enzymes, such as in the active site of cofactor F430 (Fig. 2) of the enzyme methylcoenzyme M reductase (MCR), in NiFe-hydrogenase (Fig. 3) or acetyl-CoA-synthase/carbon monoxide dehydrogenase (ACS/CODH) (Fig. 4).6–20 In addition, the development of nickel species to replace existing expensive palladium and platinum catalysts employed in organic transformations has also generated interest in the investigation of the properties of Ni(I) complexes in general.21–24 Although a majority of catalytic reactions are proposed to involve Ni(0)/Ni(II) cycles, some have been thought to operate through a Ni(I)/Ni(III) cycle or a radical mechanism that involves Ni(0)/Ni(I)/Ni(II).21–24 Understanding the role of Ni(I) in catalytic reactions could have a huge impact on the design of better catalysts. The relevance of nickel for catalysis was first demonstrated by Sabatier, who later summarized his findings in his 1912 Nobel lecture:25 “…This catalytic hydrogenating power of nickel appeared to me so perfect that I then thought of using it for a major reaction, in which the various known hydrogenating agents had shown themselves ineffective, i.e. the hydrogenation of benzene. … During the period 1901 to 1905, together with Senderens, I showed that nickel is very suitable for the direct hydrogenation of nitriles into amines and, no less important, of aldehydes and acetones into corresponding alcohols. Carbon monoxide and carbon dioxide are both changed immediately into methane, which can therefore be synthesized with the greatest ease.
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Fig. 1 Timeline of some major developments in Ni(I) chemistry.

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Fig. 2 A schematic diagram (left) and crystal structure (right) of the cofactor F430 at the MCR active site. Hydrogen atoms and the bound coenzyme M (CoM) are not shown. The crystal structure is derived from Protein Data Bank code 1HBN.26 The figure was produced with UCSF Chimera package.27

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Fig. 3 A schematic diagram (left) and crystal structure (right) of the NiFe-hydrogenase active site. The crystal structure is derived from Protein Data Bank code 3UQY.18 The figure was produced with UCSF Chimera package.27

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Fig. 4 A schematic diagram (left) and crystal structure (right) of the ACS active site. The crystal structure is derived from Protein Data Bank code 1OAO.20 The figure was produced with UCSF Chimera package.27

Despite still being regarded as a rare or unstable oxidation state,1–4 there are now more than 280 structurally characterized Ni(I) complexes having a range of coordination numbers (2–6) that are stabilized by a variety of ligands. Although several Ni(I) species have been electrochemically and spectroscopically characterized, this review focuses mainly on complexes that have been isolated and whose structures have been determined. Detailed structural characterization usually furnishes the most straightforward and generally accurate method for the assignment of oxidation state. However, other characterization methods such as EPR, magnetic (Evans’ method or SQUID magnetometry), reactivity and computation studies are also key for the derermination of the oxidation state. Nonetheless, it should be borne in mind that none of these methods is infallible, for example, interatomic distances are affected by several factors besides oxidation state, in addition, ligands may be redox active, and in metalloid clusters (not covered in this review) oxidation state of individual atoms has limited relevance.

In general, there are four routes used for the synthesis of Ni(I) complexes: (1) chemical reduction of a Ni(II) complex, (2) chemical oxidation of a Ni(0) complex, (3) comproportionation reactions of Ni(0) and Ni(II) complexes and (4) spontaneous reduction or oxidation to form Ni(I) species due the instability of an original complex with a different oxidation state (Scheme 1). Due to the diverse nature of the ligands used to form Ni(I) complexes, there is no straightforward or convenient way to classify their structures other than by the simple expedient of coordination number. Some early Ni(I) complexes have been discussed as part of a 19802 review (6 structurally characterized complexes) and in the first3 (14 structurally characterized complexes) and second4 editions (36 structurally characterized complexes) of Comprehensive Coordination Chemistry.

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Scheme 1 General synthetic route to Ni(I) complexes.

2. The first Ni(I) complex

In 1914 Bellucci and Corelli published an article titled “Verbindungen des einwertigen Nickels” (Compounds of Monovalent Nickel) in Zeitschrift für Anorganische Chemie.28 They reported the synthesis and characterization of the dark red cyano complex K4[Ni2(CN)6], which was prepared by the reduction of K2[Ni(CN)4] with sodium amalgam. Although this so-called “Bellucci's salt” had been analyzed by some preliminary X-ray crystallographic data and a series of IR and Raman spectroscopic analyses,29–36 its detailed structure was not determined until 1970 when single crystal X-ray data were reported by Jarchow, Schulz and Nast.37,38 The dinuclear Ni2(CN)64− tetraanion that it was shown to contain has two planar Ni(CN)3 units connected by a short Ni–Ni bond 2.32 Å long (Fig. 5), which is one of the shortest among all complexes with NiI–NiI bonding (cf. Table 5) and is consistent with the pairing of an electron from each of the d9 Ni(I) ions. The two Ni(CN)3 planes are almost perpendicular to each other with a dihedral angle of 82°. A similar structure was observed for the rubidium salt, Rb4Ni2(CN)6, which has an average Ni–Ni bond distance of 2.31 Å.39 The reactivity of K4[Ni2(CN)6] was also explored, and the reaction of K4[Ni2(CN)6] with water generates H2 and regenerates the initial Ni(II) complex K2[Ni(CN)4].
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Fig. 5 Drawing of the structure of the anion Ni2(CN)64−.37–39

3. Six-coordinate Ni(I)

The only currently known, structurally characterized six-coordinate Ni(I) complex was reported by Doyle and coworkers in 2015. It arose during the investigation of cross coupling reactions of acetals and aryl iodides catalyzed by nickel species. The octahedral, dimeric Ni(I) complex, [(bpp)Ni(μ-Cl)(EtOH)]2 (bpp = 2,6-di(1H-pyrazol-1-yl)pyridine), was synthesized as dark green crystals in 58% yield via the comproportionation reaction of Ni(COD)2 (COD = 1,5-cyclooctadiene), NiCl2(DME) (DME = 1,2-dimethoxyethane) and two equivalents of the bpp ligand in THF and subsequent crystallization from ethanol (Scheme 2).40 The X-ray crystal structure of [(bpp)Ni(μ-Cl)(EtOH)]2 revealed two Ni(I) ions bridged by two chloride ligands with each nickel center also coordinated by a EtOH molecule and a tridentate bpp ligand with three nitrogen donor atoms occupying meridional positions (Fig. 6). EPR spectroscopy gave an axial signal of g = 2.262 and g = 2.070, which is consistent with a metal-based radical that is a d9 Ni(I) complex. DFT calculations (B3LYP/def2-TZVP) also supported this oxidation state assignment, with a spin density of 1.08 on the nickel atom. Remarkably, this Ni(I) complex catalyzes the cross-coupling reaction of benzaldehyde dimethylacetal and iodobenzene to afford the dialkyl ether product (Scheme 2).
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Scheme 2 Comproportionation reaction of NiCl2(DME), Ni(COD)2, and bpp to yield (bpp)NiCl (top). Optimized catalytic reaction conditions (middle). Cross-coupling reaction using (bpp)NiCl as catalyst (bottom).40

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Fig. 6 Drawing (left) and the molecular structure (right) of [(bpp)Ni(μ-Cl)(EtOH)]2 with thermal ellipsoids set at 50%. Hydrogen atoms are not shown.40

4. Five-coordinate Ni(I)

4.1 Pentadentate ligands

Using a conformationally-flexible pentadentate ligand with a mixed hard–soft donor atom set, bis(5-diphenylphosphino-3-thiapentanyl)amine (psnet), Holm and coworkers characterized its Ni(II) and Ni(I) complexes and further showed that the Ni(I) complex generated H2 stoichiometrically via reaction with hydride reagents.41 Reduction of the Ni(II) precursor, [Ni(psnet)][BF4]2, with NaBH4 in THF led to the isolation of green crystals of the Ni(I) salt [Ni(psnet)][BF4] (Scheme 3). Its magnetic moment of 2.10 μB is consistent with the presence of one unpaired electron, indicating a d9 Ni(I) complex. X-ray crystallography showed that both [Ni(psnet)]2+ and [Ni(psnet)]+ are five-coordinate (Fig. 7), however, their geometries are quite different. For the Ni(II) complex, the P–Ni–N angles are 103.4(3)° and 112.3(3)°, the P–Ni–P angle is 144.3(1)° and the S–Ni–S angle is 176.1(1)°. Upon reduction, the P–Ni–N angles increase to 115.6(1) and 119.8(1) and the P–Ni–P angle narrows to 124.6(1)°, and the S–Ni–S angle also decreases to 166.5(1)°. The Ni–P and Ni–N distances remain almost unchanged upon reduction, however, the Ni–S distances increase by ca. 0.24–0.25 Å, which can be explained by an additional electron in the dz2 orbital which causes the elongation.41
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Scheme 3 Synthesis of [NiI(psnet)][BF4] via reduction of [NiII(psnet)][BF4]2 with NaBH4 and subsequent reaction with HCl to generate H2.41

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Fig. 7 The molecular structures of [NiII(psnet)][BF4]2 (left) and [NiI(psnet)][BF4] (right) (psnet = bis(5-diphenylphosphino-3-thiapentanyl)amine). Hydrogen atoms and the BF4 anion are not shown. Selected bond distances (Å) and angles (°): [NiII(psnet)][BF4]2: Ni(1)–S(1) 2.183(3), Ni(1)–S(2) 2.189(3), Ni(1)–P(1) 2.230(3), Ni(1)–P(2) 2.218(3), Ni(1)–P(1) 2.144(9), P(1)–Ni(1)–N(1) 103.4(3), P(2)–Ni(1)–N(1) 112.3(3). [NiI(psnet)][BF4]: Ni(1)–S(1) 2.429(1), Ni(1)–S(2) 2.426(1), Ni(1)–P(1) 2.245(2), Ni(1)–P(2) 2.242(2), Ni(1)–P(1) 2.128(4), P(1)–Ni(1)–N(1) 115.6(1), P(2)–Ni(1)–N(1) 119.8(1).41

4.2 Tetradentate ligands

4.2.1 Tripodal ligands. Polydentate ligands including those of a tripodal type have been shown to be effective in stabilizing numerous transition metal complexes in either low or high oxidation states. In 1977 Sacconi and coworkers reported42 that the tripodal ligand, NAs3Ph2, tris(2-diphenylarsinoethyl)amine (N[CH2CH2As(C6H5)2]3), stabilized a Ni(I) complex. Reaction of this ligand with NiBr2 or NiI2 in boiling butanol and subsequent addition of NaBPh4 gave the five-coordinate Ni(II) complexes, [(As3Ph2N)NiX][BPh4] (X = Br, I).42 Further reduction of [(As3Ph2N)NiX][BPh4] using NaBH4 in the presence of NaBPh4 afforded the dimeric, air-stable Ni(I) complex [(As3Ph2N)2Ni2X][BPh4]. X-ray crystallographic studies of the iodide complex revealed a well-separated, dinickel cation [(As3Ph2N)2Ni2I]+ and a BPh4 anion, with the iodine atom of the cation located on a crystallographic inversion center (Fig. 8). Each Ni(I) ion is five-coordinated with trigonal bipyramidal geometry and is coordinated to three arsenic atoms and one nitrogen atom of the NAs3Ph2 ligand in addition to the bridging iodide. The Ni–I and Ni–N distances of 2.994(4) Å and 2.31(2) Å are significantly longer than the distances found in similar five-coordinate Ni(II) complexes,43 which can be explained on the basis of one additional electron (d9 vs. d8) in the 3dz2 orbital that leads to elongation of the axial bond distances.44 The three N–As bond lengths of 2.337(5), 2.352(5) and 2.358(5) Å are comparable to those in the structurally similar Ni(II) complex, (As3Ph2N)Ni(Ph), with Ni–Asavg of 2.34 Å.45 In a related study, they reported the synthesis of the neutral NiI–iodide complexes supported by NAs3Ph2 and NP3Ph2 (tris(2-diphenylphosphinoethyl)amine) ligands. They were obtained from NaBH4 reduction of the reaction mixture of NiI2 and the free amine or phosphine ligand. X-ray structural analysis showed these two complexes consist of Ni(NAs3Ph2)I and Ni(NP3Ph2)I molecules.46,47 Cecconi, Midollini and Orlandini showed that two similar tripodal ligands, tris(2-diphenyphosphinoethyl)phosphine (PP3Ph2), and NAs3Ph2 are also effective in stabilizing complexes of Ni(I).48 The purple four-coordinate Ni(I) complex, [Ni(PP3Ph2)][ClO4], was synthesized from the reaction of Ni(C2H4)(PPh3)2 and PP3Ph2 in the presence of [C3Ph3][ClO4] as an oxidizing agent.48 Surprisingly, it has trigonal monopyramidal geometry with the Ni(I) atom coordinated solely to the four phosphorus atoms of the ligand. The five-coordinate complex, [(As3Ph2N)NiPPh3][ClO4], was prepared in a similar manner but the nickel atom is also coordinated by an additional PPh3 ligand.48
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Fig. 8 Drawing (left) and the molecular structure (right) of [(As3Ph2N)2Ni2I][BPh4]. Hydrogen atoms and the BPh4 anion are not shown.42

In pursuit of a synthetic model for carbon monoxide dehydrogenase (CODH), Holm and coworkers used a sulfur-rich tetradentate tripodal ligand, N(CH2CH2SBut)3 (NS3But), to enforce a five-coordinate sulfur-rich coordination environment at nickel.49,50 When carbon monoxide was bubbled through a THF solution containing the Ni(II) hydride complex, [Ni(NS3But)H][BPh4] (prepared by the reaction of [Ni(NS3But)Cl][BPh4] and NaBH4) at −20 °C, an unusual green paramagnetic species [Ni(NS3But)CO][BPh4] was isolated in 20% yield. The structure of the cation features a distorted trigonal bipyramidal metal geometry with Ni–C = 1.85(1) Å and slightly elongated Ni–S and Ni–N distances in comparison to those in the Ni(II) precursor (Fig. 9). The mechanism of the reduction is of interest. Analysis of the byproducts of this reaction showed that they are Ni(CO)4 and the protonated ligand (HNS3But)+. Furthermore, a different preparation of the Ni(II) hydride from [Ni(NS3But)Cl][BPh4] and EtMgBr produced just 3% of the Ni(I) species, [Ni(NS3But)][BPh4]. These results indicate that [Ni(NS3But)CO][BPh4] is formed from the reaction of [Ni(NS3But)][BPh4] and CO. It was proposed that two independent reactions occurred simultaneously as shown by:50

[Ni(NS3But)H]+ + 4CO → Ni(CO)4 + (HNS3But)+

[Ni(NS3But)]+ + CO → [Ni(NS3But)CO]+

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Fig. 9 The molecular structure of [Ni(NS3But)CO][BPh4]. Hydrogen atoms and the BPh4 anion are not shown. Selected bond distances (Å) and angles (°): Ni(1)–S(1) 2.353(2), Ni(1)–S(2) 2.351(3), Ni(1)–S(3) 2.384(3), Ni(1)–N(1) 2.353(2), Ni(1)–CO 1.85(1), C–O 1.15(1), Ni–C–O 177.5(9).50

It is worth noting that a similar reaction of a Ni(II) hydride, [Ni(NP3Ph2)H][BPh4], and CO resulted in intramolecular hydrogen transfer to the ligand N atom and reduction to Ni(0) to form [Ni(HNP3Ph2)CO][BPh4].51

4.2.2 Macrocyclic ligand complexes. A great number of Ni(II) complexes of macrocyclic ligands are known.2–4 Macrocyclic complexes of Ni(I) have also attracted considerable interest because Ni(I) has been proposed as an intermediate in several chemical and biological transformations and the macrocyclic complexes have been proposed as models for these systems.2–4 The majority were electrochemically generated but only a few X-ray crystal structures have been reported. The first five-coordinate Ni(I) macrocyclic complex, (R,S,R,S)-[Ni(L3){NHC(OH)Me}][ClO4] (L3 = 1,3,6,8,12,15-hexaazatricyclo[,12]icosane), was reported52 by Suh and coworkers in 1996. It was prepared by the reduction of the Ni(II) macrocyclic complex (R,R,S,S)-[Ni(L3)][ClO4]2·½H2O with sodium amalgam in acetonitrile under N2.53,54 (Note: the crystal structure of [Ni(L3)][ClO4] has also been determined,54 see Section 5.1 four-coordinate Ni(I) section.) The authors found that during the reaction, the solvent acetonitrile (MeCN) was hydrated to acetamide (MeCONH2) and coordinated to the Ni(I) center in the iminol form.52 The X-ray crystal structure showed a square-pyramidal coordination for the metal (Fig. 10). The Ni–Nmacrocycle distances vary, with distances to the tertiary nitrogen donor, 2.119(4) and 2.151(4) Å, being longer than those to the secondary nitrogen donor (2.081(5) and 2.099(4) Å). In addition, these bond distances are significantly longer than those in the parent Ni(II) complex, (R,R,S,S)-[Ni(L3)][ClO4]2·½H2O, which has average Ni–Nmacrocycle distances of 1.951 Å, and the four-coordinate square planar Ni(I) complex, (R,R,S,S)-[Ni(L3)][ClO4], of average Ni–Nmacrocycle distances of 1.878(4) and 1.978(3) Å.53 The EPR spectrum displayed a rhombic spectrum (powder (room temperature): g1 = 2.287, g2 = 2.183, g3 = 2.035; frozen MeCN solution (77 K): g1 = 2.264, g2 = 2.169, g3 = 2.027), which is indicative of a d9, five-coordinate Ni(I) ion.52
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Fig. 10 Drawing (left) and the molecular structure (right) of (R,S,R,S)-[Ni(L3){NHC(OH)Me}][ClO4] with thermal ellipsoids set at 50%. Hydrogen atoms (except N–H and O–H) and the ClO4 anion are not shown.52

Peters and Uyeda prepared55 a heterobimetallic Ni/Zn complex supported by a tetramethylfuran-derived diimine-dioxime N4-macrocyclic ligand, TMFdoen (bis(3-(oxyimino)butan-2-ylidene)ethane-1,2-diamine). The Ni(II) precursor, [Ni(TMFdoen)Zn(Me3-tacn)(MeCN)][BPh4]2, was obtained by the reaction of the TMFdoenH2 ligand and [Ni(H2O)6][ClO4]2 and subsequent reaction with Me3-tacn (1,4,7-trimethyl-1,4,7-triazacyclononane), [Zn(H2O)6][ClO4]2 and NaBPh4. Chemical reduction using CoCp2 afforded a dark green paramagnetic species that had an isotropic EPR signal with g = 2.02, indicating a ligand-centered radical rather than a Ni(I) complex. The addition of PPh3 to the solution led to color change from green to blue-green. The EPR spectrum revealed an axial signal with g = 2.208 and g = 2.044 as well as a 12-line hyperfine coupling from the four equatorial N atoms, indicating a metal-based radical. The X-ray crystal structure of the NiI–PPh3 complex (Fig. 11), [(Ph3P)Ni(TMFdoen)Zn(Me3-tacn)(MeCN)][BPh4], revealed Ni–N distances that are elongated by 0.099–0.157 Å in comparison to those in either [Ni(TMFdoen)Zn(Me3-tacn)(MeCN)][BPh4]2 or [Ni(TMFdoen)Zn(Me3-tacn)(MeCN)][BPh4], which is due to the population of the dx2y2 orbital of nickel. Furthermore, the C–N and C–C distances in the ligand backbone remained similar to the distances in [Ni(TMFdoen)Zn(Me3-tacn)(MeCN)][BPh4]2, indicating that the oxidation state of the ligand is unchanged. DFT calculations (B3LYP/6-31G(d)) supported the oxidation state assignments. The Mulliken spin population on the nickel atom in [Ni(TMFdoen)Zn(Me3-tacn)(MeCN)][BPh4] was 0.02, indicating a ligand-based reduction and a +2 oxidation state for nickel. The PPh3 complex had a spin population of 1.12 on nickel which was located in a dx2y2 orbital, and further corroborated the Ni(I) assignment and the elongation of the Ni–N distances.

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Fig. 11 Drawing (left) and the molecular structure (right) of [(Ph3P)Ni(TMFdoen)Zn(Me3-tacn)(MeCN)][BPh4] with thermal ellipsoids set at 50%. Hydrogen atoms, the BPh4 anion and cocrystallized THF molecules are not shown.55

Ray and coworkers described56 the redox properties of nickel complexes of a newly designed ligand, (DMC-nit)+ (17,23-dimethyl-1,4,11,14,17,23,27-heptaazatetracyclo[,11.05,10]heptacosa-5,7,9-trien-27-yl), which has a N-heterocyclic nitrenium group anchored on a macrocyclic ligand. Reaction of equimolar (DMC-nit)X (X = PF6, BPh4) with Ni(COD)2 afforded [Ni(DMC-nit)]X2 as purple crystals. X-ray crystallography of [Ni(DMC-nit)][PF6]2 revealed a distorted square pyramidal geometry (τ5 = 0.48) (Fig. 12).57 The Ni(I) center is coordinated by four nitrogens of the macrocyclic ligand and a nitrogen (Nnit) of the benzotriazolium group. An interesting structural feature is that Nnit atom has a pyramidal coordination (sum of interligand angles = 336.67°), which contrasts with a previously reported Rh(I) benzenetriazolium complex that has a planar coordination at Nnit.58 Thus, the Ni–Nnit bonding can be described as a mainly M→L+ π-interaction with little or no contribution from and the lone pair on Nnit to the bonding to the metal center.

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Fig. 12 The molecular structure of [Ni(DMC-nit)][PF6]2 with thermal ellipsoids set at 50%. Hydrogen atoms, the PF6 anion and cocrystallized solvent molecules are not shown. Selected bond distances (Å) and angles (°): Ni(1)–Nnit 1.986(2), Ni(1)–N(1) 2.081(3), Ni(1)–N(2) 2.137(3), Ni(1)–N(3) 2.174(3), Ni(1)–N(4) 2.141(2), Σ0 Nnit 336.67.56

4.3 Bidentate ligands

The bidentate phosphine dppm (bis(diphenylphosphino)methane) was used as a bridging ligand to support several dimeric Ni(I) complexes. The novel NiI–NiI dimer, [Ni2(μ-CNMe)(CNMe)3(dppm)2][PF6]2, was prepared by Kubiak and coworkers via the substitution reaction of [Ni2(CNMe)8][PF6]2 with dppm in CH2Cl2 solution.59 The crystal structure shows a dinickel core (Ni–Ni = 2.5924(9) Å) with two bridging dppm ligands and a CNMe ligand. The two nickel atoms are additionally coordinated by one or two CNMe ligands. A deep green-black complex, Ni2Cl2(μ-CO)(μ-dppm)2, was synthesized by Manojlovic-Muir, Puddephatt and coworkers from the comproportionation reaction of Ni2(CO)2(μ-CO)(μ-dppm)2 and NiCl2(dppm)2 or of Ni(CO)2(dppm-κP)2 and NiCl2·6H2O.60 The crystal structure showed two NiICl units connected by a Ni–Ni bond of length 2.617(1) Å, an unsymmetrically bridging CO ligand (Ni1–CO = 1.790(4) Å, Ni2–CO = 1.926(4) Å and Ni1–C–Ni2 = 89.5(2)°) as well as two bridging dppm ligands. In solution, the 31P NMR spectrum of Ni2Cl2(μ-CO)(μ-dppm)2 has only one signal even at very low temperature (−90 °C), which suggests a more symmetrical “A-frame” structure than in the solid state. The above result can be explained also by the facile fluxional behavior of the complex (Scheme 4).
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Scheme 4 Proposed fluxional behavior of Ni2Cl2(μ-CO)(μ-dppm)2.60

To investigate the key steps involving the formation of acrylic acid from CO2 and ethylene, Walther and coworkers treated two metallacyclic nickel carboxylates (TMEDA)Ni(C2H4COO) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) or (2-pyridine)2Ni(C2H4COO), also called “nickelalactones”, with a variety of N- and P-donor ligands (Scheme 5).61 Most of the ligands engaged in simple ligand exchange to form new nickelalactones, LNi(C2H4COO), and some of the ligands underwent reductive coupling to eliminate metallic nickel, CO2 and ethylene to form Ni(0) complexes, NiL2. However, addition of dppm afforded deep green crystals that were identified as the dimeric Ni(I) species, [(dppm-η1-P)Ni]2(μ-PPh2)(μ-dppm)(μ-O2C3H3), by X-ray crystallography. The Ni–Ni core (distance = 2.563(1) Å) was supported by two monodentate dppm ligands, a bridging dppm, PPh2 and most interestingly, a bridging acrylate ligand, which shows that β-hydride elimination from a nickelalactone to form acrylic acid is feasible under suitable conditions.

image file: c7cs00216e-s5.tif
Scheme 5 Left: Reaction of a “nickelalactone” with dppm to form [(dppm-η1-P)Ni]2(μ-PPh2)(μ-dppm)(μ-O2C3H3). Right: The molecular structure of [(dppm-η1-P)Ni]2(μ-PPh2)(μ-dppm)(μ-O2C3H3) with thermal ellipsoids set at 50%. Hydrogen atoms, and cocrystallized solvent molecules are not shown.61

Transition metal hydride species are of interest not only in their own right but also because they are often involved in transition metal catalysis.62,63 Wilke and Jonas synthesized the first NiI-hydride complex in 1970 by reacting (dcpe)NiCl2 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane) with two equivalents of NaBMe3H in toluene.64 Deep red crystals formed and mass spectroscopy and cryoscopic measurements in benzene indicated that the complex is dimeric. Its diamagnetic nature suggests a Ni–Ni bond, however, its molecular structure was not determined until 43 years later when Martin and coworkers showed it to be {(dcpe)Ni}2(μ-H)2 with two bridging hydrides (Fig. 13).65 The crystal structure showed two (dcpe)NiI fragments bridged by the two hydride ligands, forming a Ni2H2 core. The Ni–Ni distance of 2.4078(5) Å, together with its diamagnetism suggested that there was a Ni–Ni bond. Other nickel hydride dimers stabilized by various bidentate phosphine ligands were synthesized by several groups (Table 1).66–69 They were generally synthesized via reduction of the Ni(II) complexes, (diphosphine)NiCl2, with a hydride source such as KBEt3H, LiBEt3H (Scheme 6). Notably, Pörschke and coworkers used a bulky bidentate phosphine ligand dtbpe (1,2-bis(di-tert-butylphosphino)ethane) to stabilize a dimeric Ni(I) hydride complex, [(dtbpe)Ni]2(μ-H)2 (along with its isotopomers [(dtbpe)Ni]2(μ-H)(μ-D) and [(dtbpe)Ni]2(μ-D)2).68 These novel hydride/deuteride complexes were synthesized by reduction of (dtbpe)NiCl2 with activated magnesium under H2(g)/HD(g)/D2(g) in THF between −40 and 20 °C. Alternatively, the synthesis can be achieved by the reaction of the Ni(0) complex, (dtbpe)Ni(benzene), with H2(g)/HD(g)/D2(g) between −60 and 20 °C. Moreover, the reactivity of [(dippe)Ni]2(μ-H)2 (dippe = 1,2-bis(diisopropylphosphino)ethane) was further examined by several groups. For example, it was shown by Jones, García and coworkers to effect the desulfurization of dibenzothiophene and biphenyl-2-thiol at room temperature.70–72 A dimeric Ni(I) complex with bridging hydride and biphenyl-2-thiolate ligands, (dippe)2Ni2(μ-H)(μ-S-2-biphenyl), was isolated and the X-ray crystal structure featured a Ni–Ni bond distance of 2.5124(4) Å. Its catalytic dehydrocoupling reactivity towards phenylsilane to produce polysilanes was also demonstrated by Abu-Omar and coworkers.73

image file: c7cs00216e-f13.tif
Fig. 13 The molecular structure of [(dcpe)Ni]2(μ-H)2 with thermal ellipsoids set at 50%. Hydrogen atoms (except Ni–H) are not shown. Selected structural data are listed in Table 1.65
Table 1 Structurally characterized dimeric NiI–H complexes supported by chelating diphosphine ligands with selected bond distances (Å) and angles (°)
Complex Ni1–P Ni2–P Ni1–H Ni2–H Ni–Ni Ref.
a dcpe = 1,2-bis(dicyclohexylphosphino)ethane.b dippp = 1,3-bis(diisopropylphosphino)propane.c dcpp = 1,3-bis(dicyclohexylphosphino)propane.d dtbpe = 1,2-bis(di-tert-butylphosphino)ethane.e dippe = 1,2-bis(diisopropylphosphino)ethane.f Not applicable due to the presence of an inversion center.
[(dcpe)Ni]2(μ-H)2a 2.1350(7)








2.4078(5) 65
[(dippp)Ni]2(μ-H)2b 2.146(2)








2.438(1) 66
[(dcpp)Ni]2(μ-H)2c 2.129(2)








2.441(1) 67
[(dtbpe)Ni]2(μ-H)2d 2.165(1)


f 1.58(4) f 2.433(1) 68
[(dippe)Ni]2(μ-H)2e 2.1291(6)


f 1.64(6)


f 2.3737(5) 69

image file: c7cs00216e-s6.tif
Scheme 6 General synthetic route to Ni(I) hydride dimers.

Complexes containing nickel and iron with thiolato ligands are of great interest due to their potential as synthetic models for NiFe-hydrogenase enzymes.8–10 The participation of Ni(I) in complexes involving the Ni(I)/Fe(II) oxidation state combination (in the absence of a μ-hydride ligand), has been proposed as an intermediate in these catalytic cycles.9,10 Many synthetic model complexes feature nickel as Ni(II) and examples of Ni(I) remain rare.10 It should be noted, however, that assignment of the oxidation states of nickel is nearly impossible for these heterobimetallic complexes solely on the basis of their crystal structures, complementary methods such as EPR spectroscopy and DFT calculations are often required for a more accurate assessment. In 2005, Schröder and coworkers reported a heterobimetallic Ni(I)/Fe(I) complex by reacting the square planar Ni(II) complex, (dppe)Ni(pdt) (dppe = bis(diphenylphosphino)ethane, pdt2− = 1,3-propanedithiolate), with an Fe(0) source such as Fe3(CO)12, Fe(CO)3(BDA) (BDA = benzylidene acetone) or Fe2(CO)9.74 The resulting green complex, (dppe)Ni(μ-pdt)Fe(CO)3, is diamagnetic which can be explained by the strong NiI–FeI bonding with a distance of 2.4666(6) Å (Fig. 14). The oxidation state assignment is also consistent with the population analysis. Later it was shown by Rauchfuss and coworkers that protonation of this complex with HBF4 resulted in the first NiIIFeII-hydride species, [(dppe)Ni(μ-pdt)(μ-H)Fe(CO)3][BF4], that could serve as a model complex for the Ni–R state (i.e. NiIIFeII) of the hydrogenase.75,76 Two similar heterobimetallic Ni(I)/Fe(I) complexes, (dppbz)Ni(μ-pdt)Fe(CO)3 and (dppbz)Ni(μ-pds)Fe(CO)3 (pds2− = 1,3-propanediselenolate, dppbz = 1,2-bis(diphenylphosphino)benzene), were synthesized under similar reaction conditions by Song and coworkers.77 The Ni–Fe distances of 2.4600(6) Å for (dppbz)Ni(μ-pdt)Fe(CO)3 and 2.5021(7) Å (dppbz)Ni(μ-pds)Fe(CO)3 are comparable to that in Schröder's (dppe)Ni(μ-pdt)Fe(CO)3.74 A heterobimetallic Ni(I)/Ru(II) species that serves as a model for Ni–L state (i.e. NiIFeII) was reported by Rauchfuss and coworkers. The cationic complex, [(cymene)Ru(μ-pdt)Ni(dppe)][BArF4] (BArF4 = B(3,5-C6H3(CF3)2)4), was prepared by one-electron oxidation of the neutral congener with [FeCp2][BArF4].78 The EPR spectrum of gx = 2.025, gy = 2.053 and gz = 2.240 is consistent with a Ni(I)/Ru(II) complex. Two structurally characterized mixed oxidation state complexes having Ni(I)/Fe(II) oxidation states, CpNi(μ-pdt)Fe(dppe)(CO) and CpNi(μ-SPh)2Fe(dppv)(CO) (dppv = cis-1,2-bis(diphenylphosphino)ethylene), as model complexes for Ni–L states, were reported by Rauchfuss and coworkers.79 Both complexes were prepared by the reduction of the parent cationic Ni(II)/Fe(II) complex, [CpNi(μ-pdt)Fe(dppe)(CO)][BF4] and [CpNi(μ-SPh)2Fe(dppv)(CO)][BF4], with CoCp2. DFT calculations of the spin density showed that the majority of the unpaired electron is located on the nickel atom, 0.71 for CpNi(μ-pdt)Fe(dppe)(CO) and 0.72 for [CpNi(μ-SPh)2Fe(dppv)(CO)][BF4], which indicated nickel-based reductions and the formation of Ni(I)/Fe(II) complexes. The rhombic EPR signals also support the assignment of a Ni(I) oxidation state.

image file: c7cs00216e-f14.tif
Fig. 14 Drawings (left) and the molecular structures (right) of (dppe)NiI(μ-pdt)FeI(CO)3 (left) and CpNiI(SPh)2FeII(dppv)(CO) (right) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown.74,79

4.4 Monodentate ligands

It was shown80 by Owston and coworkers that heating a dimeric Ni(I) precursor, Ni2(CO)6(PPh2)2, in mesitylene resulted in the formation of a mixture of green crystals of the diphosphine complex (OC)3NiPPh2PPh2Ni(CO)3 and black crystals of [Ni(μ-PPh2)(CO)2]2, which were characterized by X-ray crystallography. The Ni(I) atom in (OC)3NiPPh2PPh2Ni(CO)3 has distorted tetrahedral coordination with average Ni–C and Ni–P bond distances of 1.803(8) and 2.225(3) Å. The dimer [Ni(μ-PPh2)(CO)2]2 had two Ni(I) atoms with distorted tetrahedral coordination bridged by two phosphide groups. The average Ni–P distance is 2.192(2) Å and average Ni–C distance is 1.79(1) Å. The Ni–Ni separation is 2.510(1) Å, suggesting the presence of a Ni–Ni single bond.81 Jones, Atwood and coworkers reacted NiCl2(PMe3)2 with two equivalents of lithium di-tert-butylphosphide, LiPBut2, to afford red-brown crystals of [Ni(μ-PBut2)(PMe3)]2.82 In related studies, they also showed that the reaction of NiCl2(PMe3)2 with one equivalent of dilithium tert-butylphosphide, Li2PBut, afforded purple crystals of [Ni{μ-P(H)But}(PMe3)2]2.83 Both [Ni(μ-PBut2)(PMe3)]2 and [Ni{μ-P(H)But}(PMe3)2]2 displayed dimeric structures with two pseudotetrahedrally coordinated Ni(I) ions and two bridging phosphide ligands. For [Ni(μ-PBut2)(PMe3)]2, the Ni–Pphosphide and Ni–Pphosphine distances are 2.174(3) Å and 2.316(5) Å and there is a Ni–Ni single bond length of 2.375(3) Å. For [Ni{μ-P(H)But}(PMe3)2]2, the Ni–Pphosphide distance of 2.172(1) Å and the Ni–Pphosphine distance of 2.181(2) Å are similar to those in [Ni(μ-PBut2)(PMe3)]2. However, it has a longer Ni–Ni distance of 2.559(2) Å, which is within the range known for Ni–Ni single bonds (Table 5). The authors reasoned that the shorter Ni–Ni bond in [Ni(μ-PBut2)(PMe3)]2 is due to the lower electron count for each Ni(I) atom (16 e) compared to 18 e for each Ni(I) in [Ni{μ-P(H)But}(PMe3)2]2. In addition, they found that [Ni(μ-PBut2)(PMe3)]2 can undergo further substitution by reaction with CO. The X-ray crystallographic analysis of the crystalline products revealed the asymmetric Ni2(μ-PBut2)2(CO)2(PMe3) and Ni2(μ-PBut2)2(CO)3.84 A related compound [Ni(μ-PCy2)(PCy2Me)]2 was produced from the thermolysis or the reaction of Ni2Cl2(μ-dcpm)2(μ-H) (dcpm = bis(dicyclohexylphosphino)methane) with LiH.85 It has a similar Ni–Ni single bond distance of 2.3910(8) Å to that found in [Ni(μ-PBut2)(PMe3)]2 (2.375(3) Å).

Analogous dimeric Ni(I) arsenide complexes featuring Ni–Ni bonds were reported by Jones and Whittlesey.86 The black dimeric five-coordinate Ni(I) complex, [Ni(μ-AsBut2)(PMe3)]2, was synthesized by the reaction of NiCl2(PMe3)2 with two equivalents of LiAsBut2 in THF at −78 °C. The Ni2As2 core is essentially planar with two trigonal planar coordinated Ni(I) ions. The Ni–Ni distance of 2.429(1) Å is within the single bond range (Table 5) but is somewhat longer than the Ni–Ni bond distance in the structurally similar –PBut2 ligated congener [Ni(μ-PBut2)(PMe3)]2. This may be due to the larger covalent radius of arsenic (1.21 Å)87 in comparison to that of phosphorus (1.11 Å)87 at the bridging position. The PMe3 ligand can be exchanged with an excess of p-tolyl isocyanide to afford red-black crystals of [Ni(μ-AsBut2){NC(p-tol)}]2. The Ni2As2 unit remains planar but the coordination of each Ni(I) ion became distorted tetrahedral. The elongation of the Ni–Ni distance (2.693(2) Å) may be explained by the increased electron count (18 e) in each of the nickel ions in [Ni(μ-PBut2){NC(p-tol)}]2, similar to the phosphide-bridged Ni(I) analogue, [Ni{μ-P(H)But}(PMe3)2]2.83

A Ni0–aryne complex,88 (Et3P)2Ni(η2-C6H2-4,5-F2), was shown to transform to a dinuclear Ni(I) complex, [(Et3P)2Ni]2(2,3-C6H2F2-2′,3′-C6H2F2), upon treatment with a catalytic amount of NiBr2(PEt3)2 and Na/Hg (Scheme 7).89,90 X-ray crystallography of the dark brown crystals showed that the structure features a Ni–Ni unit with a bond distance of 2.3710(5) Å bridged by a biaryl fragment (Fig. 15). Each nickel atom was additionally coordinated by two PEt3 ligands. Remarkably, the two hydrogens of the aryl rings are ortho-disposed, as opposed to para-disposed in the precursor. They could also isolate a long-lived intermediate and showed it to be a Ni(0) dimer, [(Et3P)2Ni]2(μ-η22-C6H2-4,5-F2), by X-ray crystallography. Addition of one equivalent of a Lewis acid B(C6F5)3 to the aryne complex (Et3P)2Ni(η2-C6H2-4,5-F2) to precipitate Et3P·B(C6F5)3 afforded a mixed oxidation state trinuclear [(Et3P)Ni]33-4,5-C6H2F2)(μ3-4,5-C6H2F2-4′,5′-C6H2F2) as dark brown crystals. Reaction of this trinuclear complex with three equivalents of PMe3 or PEt3 afforded a similar dinuclear product [(Et3P)(R3P)Ni]2(3,4-C6H2F2-3′,4′-C6H2F2) (R = Me or Et), in which two Ni(I) atoms are bridged by a biaryl ligand with a NiI–NiI distance of 2.3079(8) Å for R = Me and two hydrogen atoms on the aryl ring are para-disposed (Fig. 15). Interestingly, both complexes are thermally unstable and slowly convert to [(Et3P)(R3P)Ni]2(2,3-C6H2F2-2′,3′-C6H2F2) over the course of two (R = Et) and four (R = Me) days (Scheme 7).

image file: c7cs00216e-s7.tif
Scheme 7 Formation of [(Et3P)2Ni]2(2,3-C6H2F2-2′,3′-C6H2F2) and the isomerization of [(Et3P)(R3P)Ni]2(3,4-C6H2F2-3′,4′-C6H2F2) (R = Me, Et).89,90

image file: c7cs00216e-f15.tif
Fig. 15 The molecular structures of [(Et3P)2Ni]2(2,3-C6H2F2-2′,3′-C6H2F2) (left) and [(Et3P)(Me3P)Ni]2(3,4-C6H2F2-3′,4′-C6H2F2) (right) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown.89,90

5. Four-coordinate Ni(I)

5.1 Macrocyclic ligands

Besides the above-mentioned original “Bellucci's salt”, Ni2(CN)64−, which has a dimeric structure and square planar coordinated Ni(I) ions, a number of Ni(I) complexes with tetrahedral or square planar coordination were reported in the 1970s.

A general method of synthesizing Ni(I) macrocyclic complexes is via the reduction (either chemically or electrochemically) of a parent Ni(II) species. Several four-coordinate Ni(I) complexes were characterized electrochemically and by EPR methods.2–4 However, the first structurally characterized macrocyclic Ni(I) complex was not reported until 1989 when Latos-Grażyński, Balch and Olmstead91,92 used the diphenyldi-p-tolyl-21-thiaporphyrin ((SDPDTP)H) ligand to prepare the five-coordinate NiII–chloride species, Ni(SDPDTP)Cl. Access to brown crystals of the Ni(I) complex, NiI(SDPDTP), was accomplished by reduction with an aqueous solution of sodium dithionite as reductant and subsequent crystallization by diffusion of acetonitrile into a benzene solution of Ni(SDPDTP) (Fig. 16).92 The rhombic EPR signal (g = 2.030, 2.040, 2.109 in frozen toluene) confirmed the +1 oxidation state of nickel.

image file: c7cs00216e-f16.tif
Fig. 16 The molecular structure of Ni(SDPDTP). Hydrogen atoms and a cocrystallized acetonitrile molecule are not shown. Selected bond distances (Å) and angles (°): Ni(1)–N(1) 2.015(11), Ni(1)–N(2) 1.912(14), Ni(1)–N(3) 2.014(12), Ni(1)–S(1) 2.143(5), N(1)–Ni(1)–N(2) 93.2(5), N(3)–Ni(1)–N(2) 93.0(5), N(1)–Ni(1)–N(3) 173.2(6), S(1)–Ni(1)–N(2) 170.6(4).92

Since the first structurally characterized example, several other four-coordinate macrocyclic Ni(I) species have been characterized. In 1991, Fujita and coworkers reported the synthesis and crystal structure of [Ni(diene)]ClO4 (diene = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) by the Na/Hg reduction of [Ni(diene)](ClO4)2 (Fig. 17).93 The average Ni–Namine distance of 2.066(6) Å and Ni–Nimine distance of 1.984(7) Å are 0.128 and 0.077 Å longer than those in its parent Ni(II) complex. This bond lengthening is due to the extra electron occupying the nickel dx2y2 orbital, which increases the effective size of the nickel ion and the metal–ligand distances. Three years later they reported the synthesis and structure of C-(R,S,S,R)-[Ni(HTIM)]ClO4 (HTIM = 2,3,9,10-tetramethyl-1,4,5,11-tetraazacyclotetradecane).94 Two almost equal Ni–N bond distances were observed (1.937(3) and 1.944(3) Å). Upon reduction, the Ni–N bond lengths increase by ca. 0.11 Å to accommodate the larger Ni(I) ion. Riordan and coworkers reported95 the synthesis of a Ni(I) tetramethylcyclam complex by reduction of [Ni(tmc)](OTf)2 (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) with Na/Hg and crystallization afforded blue-green crystals of [Ni(tmc)](OTf)·NaOTf. The authors noted that, upon reduction, the isomeric integrity of the nickel complex is not maintained. Chemical reduction of either (R,R,S,S)- or (R,S,R,S)-[Ni(tmc)](OTf)2 afforded both isomers. However, despite this, (R,R,S,S)-[Ni(tmc)](OTf)·NaOTf crystallized selectively and was structurally characterized. The nitrogen atoms are coplanar with the sum of N–Ni–N interligand angles = 360°. There are two sets of different Ni–N bond lengths (2.095(5) and 2.120(5) Å), which are ca. 0.12 Å longer than those in the Ni(II) precursor. The alkyl transfer reaction from a CoIII–methyl complex, MeCo(dmgBF2)2L (dmgBF2 = (difluoroboryl)dimethylglyoximato; L = py, PEt3), to [Ni(tmc)]+, which is relevant to the synthesis of acetyl-CoA by carbon monoxide-dehydrogenase, was also investigated.96,97

image file: c7cs00216e-f17.tif
Fig. 17 The molecular structure of the cation of [Ni(diene)]ClO4. Hydrogen atoms and the ClO4 anion are not shown. Selected bond distances (Å) and angles (°): Ni(1)–N(1) 2.064(6), Ni(1)–N(2) 1.988(8), N(1)–Ni(1)–N(2) 85.0(3).93

Suh and coworkers have reported several structurally characterized Ni(I) complexes stabilized by macrotricyclic ligands that have noncoordinating N atoms.52–54 They were prepared by Na/Hg reduction of the Ni(II) complexes, [Ni(L)](ClO4)2 (L = L1 = 8-methyl-1,3,6,8,10,13,15-heptaazatricyclo[,15]octadecane; L2 = 1,3,6,9,11,14-hexaazamacrotricyclo[,9]octadecane; L3 = 1,3,6,8,12,15-hexaazatricyclo[,12]icosane) in acetonitrile solution. X-ray structural studies showed that, like the previous Ni(I) macrocyclic complexes, they had square planar coordination (Fig. 18). However, unlike the ring expansion and the increased Ni–Nmacrocycle bond distances observed for the previous complexes, the ring size remained almost unchanged with comparable Ni–Nmacrocycle bond distances. These results may be due to the more rigid nature of L1, L2 and L3 compared to the more flexible diene, tmc and HTIM ligands, which can adapt to the larger size Ni(I) ion through ring expansion.

image file: c7cs00216e-f18.tif
Fig. 18 The schematic diagram (top) and molecular structures (below) of the Ni(I) cationic complexes Ni(L1)+, Ni(L2)+ and Ni(L3)+. Hydrogen atoms, the ClO4 anions and the cocrystallized solvent molecules are not shown.52–54

Catenane ligands have been shown to stabilize several low-oxidation state transition metal complexes.98,99 Compared to the less flexible cyclam type macrocyclic ligands, catenane ligands can accommodate tetrahedral coordination, which is the preferred coordination geometry for the Ni(I) complexes. In 1994, the flexibility of catenanes was demonstrated by Pascard, Sauvage and coworkers, who prepared the Ni(II) complex from excess Ni(NO3)2·6H2O and a [2]catenand, Cat30 (Fig. 19), followed by anion exchange with BF4.100 Dark orange crystals of [Ni(Cat30)](BF4)2 were crystallized from CH2Cl2/C6H6. Cyclic voltammetry of [Ni(Cat30)](BF4)2 showed a reversible Ni(II)/Ni(I) couple at −0.18 V, whereas the square planar cyclam-type macrocyclic Ni(II) complexes typically displayed a Ni(II)/Ni(I) couple at −1.3 V (vs. SCE in MeCN solution). These results showed that [Ni(Cat30)](BF4)2 is more readily reduced to an Ni(I) species than the cyclam-type macrocyclic Ni(II) complexes. Interestingly, it is also possible to reduce [Ni(Cat30)]ClO4 to an Ni(0) species electrochemically without any decomposition. The Ni(I) catenand complex, [Ni(Cat30)]ClO4, was prepared from electrochemical reduction of [Ni(Cat30)](BF4)2 (−0.45 V vs SCE in MeCN solution) in MeCN/LiClO4 solution and crystallized from CH2Cl2/C6H6 as blue-black crystals. The crystal structure of the Ni(II) complex showed very distorted four-coordination where the nickel atom is located on the plane formed by three of the four nitrogen donor atoms, which can be explained on the basis of maximizing π–π interactions of the ligands. The crystal structure of the Ni(I) complex showed that it was a rare (at the time) example of a tetrahedrally coordinated Ni(I), however, the π–π interaction observed in Ni(II) complex is lost upon reduction.

image file: c7cs00216e-f19.tif
Fig. 19 Drawing of the structure of [Ni(Cat30)]ClO4.100

5.2 Tridentate ligands

5.2.1 Tripodal ligands. In the early 1970s, Sacconi and coworkers used a tridendate tripodal ligand, 1,1,1-tris(diphenylphosphinomethyl)ethane (commonly known as triphos), MeC(CH2PPh2)3 to complex a Ni(I) ion.101 Reaction of triphos with NiI2 in ethanol/CH2Cl2 solution formed the four-coordinate Ni(I) complex, (triphos)NiI. The X-ray structure of crystals grown from a CH2Cl2 solution revealed a distorted tetrahedral Ni(I) geometry in which Ni(I) is coordinated by three phosphorus atoms of the triphos ligand and by iodide.102,103 The room temperature magnetic moment of 1.93–1.98 μB is consistent with the presence of one unpaired electron at the d9 Ni(I) ion. A noteworthy feature is that the Ni(I) species is formed directly from NiI2 and the triphos ligand. In comparison, the syntheses of (triphos)NiCl and (triphos)NiBr (note: their crystal structures were not reported) require one equivalent of reducing agent, NaBH4. McGrady and coworkers showed that by reacting (triphos)NiCl2 with excess NaBH4 (ca. seven equivalents) in dry THF, a yellow Ni(I) borohydride complex, (triphos)Ni(η2-BH4), could be isolated.104 The crystal structure showed that in addition to three phosphorus atoms, the nickel atom is coordinated by two hydrogens from the borohydride anion (Fig. 20). The two Ni–H distances are 1.59(5) and 1.83(5) Å. It is structurally similar to the Co(I) complex, (triphos)Co(η2-BH4).105 The magnetic moment of 2.47 μB (Gouy) and 2.57 μB (Evans’ method) indicate one unpaired electron with partially quenched spin–orbit coupling. The distorted tetrahedral Ni(I) triphos chalcogenolate complexes, (triphos)Ni(ER) (E = S, R = Ph, But; E = Se, R = Ph), were synthesized from the reactions of a Ni(0) complex, Ni3(triphos)4, with dichalcogenides REER.106 The room temperature magnetic moments range from 1.57–1.68 μB, indicating the presence of one unpaired electron. The EPR spectra of THF solutions showed coupling constant to the phosphorus nuclei of ca. 60 gauss. The giso values range from 2.1110–2.1263 and are higher than the ge value of 2.0023, suggesting the existence of spin–orbit coupling. The crystal structure of a closely related complex, (triphos)NiSH, has also been reported.107,108 A slightly modified triphos ligand, PhC(CH2PPh2)3 (Ph-triphos), was synthesized by Huttner and coworkers to complex to iron, cobalt and nickel. The complex (Bz-triphos)NiCl (Bz-triphos = PhCH2C(CH2PPh2)3), which was synthesized via a similar method to (triphos)NiX and was structurally characterized.109
image file: c7cs00216e-f20.tif
Fig. 20 The molecular structure of (triphos)Ni(η2-BH4) with thermal ellipsoids set at 50%. Hydrogen atoms (except B–H) are not shown. Selected bond distances (Å) and angles (°): Ni(1)–P(1) 2.2501(12), Ni(1)–P(2) 2.2475(13), Ni(1)–P(3) 2.2463(13), Ni(1)–H(A) 1.83(5), Ni(1)–H(B) 1.59(5), P(1)–Ni(1)–P(2) 93.87(5), P(1)–Ni(1)–P(3) 89.50(5), P(2)–Ni(1)–P(2) 94.90(5).104

Tripodal thioether ligands have also attracted interest because of their biological relevance. Riordan and coworkers used the monoanionic borato ligand, [PhB(CH2SR)3] (R = But, PhTtBut; R = 1-Ad, PhTtAd), in an attempt to stabilize tetrahedral late transition metals in a +2 oxidation state in a sulfur-rich environment.110,111 In the case of cobalt, they prepared (PhTtBut)CoMe from the corresponding (PhTtBut)CoCl and MgMe2 (or MeLi). In contrast, reactions of (PhTtR)NiCl (R = But or 1-Ad) with MgMe2 (or MeLi) afforded an unexpected Ni(I) species, (κ2-PhTtR)Ni(η2-CH2SR), which is formed by MeLi reduction and subsequent borato ligand alkylation (Scheme 8). The reaction of (PhTtR)NiCl and MeLi in the presence of a donor ligand such as CO or PMe3 trapped the tetrahedral, four-coordinate Ni(I) complexes in the form of (PhTtR)NiL (R = But or 1-Ad, L = CO, PMe3) (Scheme 8, Fig. 21). The Ni–Savg distance is 2.237 Å for (PhTtBut)NiCO, 2.269 Å (PhTtAd)NiCO, 2.298 Å for (PhTtBut)NiPMe3 and 2.281 Å for (PhTtAd)NiPMe3.109,110 The Ni–CO distance is 1.754(7) Å with a Ni–C–O angle of 171.0(8)° for (PhTtBut)NiCO and 1.815(4) Å and 175.0(4)° for (PhTtAd)NiCO. Alternatively, (κ2-PhTtBut)Ni(η2-CH2SBut) can be synthesized by Na/Hg reduction of (PhTtBut)NiCl, whereas performing the reduction under a CO atmosphere afforded (PhTtBut)NiCO, suggesting that reductive mechanism is the most likely route to the Ni(I) species. The dioxygen reactivity of the NiI–CO complexes was also explored.112–114

image file: c7cs00216e-s8.tif
Scheme 8 Reactions of (PhTtR)NiCl (R = But or 1-Ad).110,111

image file: c7cs00216e-f21.tif
Fig. 21 The molecular structures of (PhTtBut)NiPMe3 (left) and (PhTtBut)NiCO (right). Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): (PhTtBut)NiPMe3: Ni(1)–P(1) 2.211(2), Ni(1)–S(1) 2.305(2), Ni(1)–S(2) 2.2893(19), Ni(1)–S(3) 2.300(2), S(1)–Ni(1)–S(2) 93.08(7), S(2)–Ni(1)–S(3) 91.61(8), S(1)–Ni(1)–S(3) 98.94(8); (PhTtBut)NiCO: Ni(1)–C(22) 1.755(8), C(22)–O(1) 1.127(11), Ni(1)–S(1) 2.231(2), Ni(1)–S(2) 2.233(3), Ni(1)–S(3) 2.248(2), Ni(1)–C(22)–O(1) 170.9(7), S(1)–Ni(1)–S(2) 97.01(10), S(2)–Ni(1)–S(3) 96.29(8), S(1)–Ni(1)–S(3) 94.66(7).110

The monoanionic tris(phosphino)borate ligands, [PhB(CH2PPh2)3] (PhBPPh3) and [PhB(CH2PPri2)3] (PhBPPri3), were shown by Peters and coworkers to support a series of Ni(0), Ni(I) and Ni(II) complexes.115 Attempts to isolate the Ni(I) complex by reducing (PhBPPh3)NiCl with one-electron reductants such as sodium amalgam led to the formation of (PhBPPh3)Ni(η2-CH2PPh2), due to partial ligand degradation similar to that of Riordan's (PhTtBut)NiCl system. Access to stable Ni(I) species can be achieved by the reduction (KC8 or Na/Hg) of the Ni(II) halide precursors, (PhBPPh3)NiX, in the presence of a neutral donors (L-type) such as PPh3, PMe3 and CNBut (Scheme 9). The above synthetic route yielded four Ni(I) complexes, (PhBPPh3)NiPPh3, (PhBPPh3)NiCNBut, (PhBPPri3)NiPMe3 and (PhBPPri3)NiCNBut. X-ray crystallography showed that all have a distorted tetrahedral metal geometry (Fig. 22). The solution magnetic moments are indicative of the presence of one unpaired electron and Ni(I) oxidation state with moments of 1.91 μB for (PhBPPh3)NiPPh3, 1.74 μB for (PhBPPh3)NiCNBut, 1.82 μB for (PhBPPri3)NiPMe3 and 1.68 μB for (PhBPPri3)NiCNBut. Cyclic voltammetry of (PhBPPri3)NiPMe3 and (PhBPPri3)NiCNBut showed reversible reduction (Ni(0)/Ni(I)) waves at E1/2 = −1.945 V and −1.848 V (vs. Fc/Fc+ couple), suggesting that Ni(0) species may also be isolable. However, although complete consumption of reducing agent and the paramagnetic Ni(I) species was observed, the putative Ni(0) species were unstable in THF solution and were reoxidized to regenerate the original Ni(I) complexes.

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Scheme 9 Reactions of (PhBPR3)NiCl (R = Ph or Pri).115

image file: c7cs00216e-f22.tif
Fig. 22 The molecular structures of (PhBPPh3)NiPPh3 (left), (PhBPPh3)NiCNBut (middle), (PhBPPri3)NiPMe3 (right) with thermal ellipsoids set at 50%. Hydrogen atoms and the cocrystallized solvent molecules are not shown. Selected bond distances (Å) and angles (°): (PhBPPh3)NiPPh3: Ni(1)–P(1) 2.28(2), Ni(1)–P(2) 2.28(2), Ni(1)–P(3) 2.27(2), Ni(1)–P(4) 2.25(2), P(1)–Ni(1)–P(2) 95.6(8), P(2)–Ni(1)–P(3) 96.8(8), P(1)–Ni(1)–P(3) 92.2(8); (PhBPPh3)NiCNBut: Ni(1)–C(46) 1.861(2), C(46)–N(1) 1.159(2), Ni(1)–P(1) 2.2291(6), Ni(1)–P(2) 2.2155(6), Ni(1)–P(3) 2.2515(6), Ni(1)–C(22)–O(1) 175.14(18), P(1)–Ni(1)–P(2) 91.86(2), P(2)–Ni(1)–P(3) 94.58(2), P(1)–Ni(1)–P(3) 95.00(2); (PhBPPri3)NiPMe3: Ni(1)–P(1) 2.2533(5), Ni(1)–P(2) 2.2764(5), Ni(1)–P(3) 2.2873(5), Ni(1)–P(4) 2.2631(5), P(1)–Ni(1)–P(2) 92.761(19), P(2)–Ni(1)–P(3) 98.240(19), P(1)–Ni(1)–P(3) 94.739(18).115

Meyer and coworkers developed a monoanionic N-anchored tripodal Tris–NHC ligand, tris(2-(3-tert-butylimidazol-2-ylidene)ethyl)amine (TIMENBut), to stabilize three-coordinate Ni(0) and four-coordinate Ni(I) complexes.116 The Ni(0) complex, Ni(TIMENBut), was synthesized from the reaction between Ni(COD)2 and the free carbene ligand TIMENBut. Cyclic voltammetry showed a reversible Ni(0)/Ni(I) couple at E1/2 = −2.5 V (vs. Fc/Fc+ couple) and a reversible Ni(I)/Ni(II) couple at −1.09 V, suggesting that a stable Ni(I) complex might be chemically accessible. Indeed, treatment with an oxidizing agent such as benzyl chloride in methylene chloride generated the yellow cationic complex Ni(TIMENBut)Cl in good yield. The X-ray crystal structure showed that the cation and anion are well separated with an approximate trigonal monopyramidal nickel geometry with three carbene C atoms forming the trigonal plane and the anchoring N atom occupying the axial position (Fig. 23). Surprisingly, the chloride anion did not coordinate to the Ni(I) ion. It is worth noting that the average Ni–N distance (2.229 Å) is ca. 1 Å shorter than the Ni–N distance in the neutral analog (3.204 Å), indicating the enhancement of a Ni–N interaction and the formation of a Ni–N bond. In addition, the Ni–C distances (avg. 1.996(4) Å) are longer than those in the neutral complex (avg. 1.892(1) Å) despite the fact that Ni(I) is smaller than Ni(0), which can be explained by the efficient π-backbonding capability of the NHC ligand.

image file: c7cs00216e-f23.tif
Fig. 23 The molecular structure of Ni(TIMENBut)Cl with thermal ellipsoids set at 50%. Hydrogen atoms and the Cl anion are not shown. Selected bond distances (Å) and angles (°): Ni(1)–C(3) 1.988(4), Ni(1)–C(12) 2.004(4), Ni(1)–C(21) 1.996(4), Ni(1)–N(1) 2.223(3), C(3)–Ni(1)–C(12) 114.68(17), C(3)–Ni(1)–C(21) 120.27(18), C(12)–Ni(1)–C(21) 123.26(17).116
5.2.2 Pincer ligands. Mindiola, Meyer and coworkers used an anionic PNP pincer ligand, N[2-PPri2-4-MeC6H3]2 (PNaminePPri), to stabilize the dimeric Ni(I) complex, [Ni(μ-PNaminePPri)]2, which was synthesized from a KC8 reduction of Ni(PNaminePPri)Cl in 62% yield along with a 5–15% yield of the Ni(II) hydride, Ni(PNaminePPri)H (Scheme 10).117 The solid state structure featured a Ni2N2 diamond core and a short Ni⋯Ni distance of 2.3288(7) Å, which is close to the sum of the covalent radii of 2.33 Å for nickel.87 The magnetic moment of 1.78(1) μB, determined by Evans’ method at 25 °C in toluene, is close to the 1.73 μB spin-only magnetic moment for one unpaired electron, which suggests that it exists as a monomer in solution. Although the complex has a short Ni⋯Ni distance that is within the known range for NiI–NiI bonds (Table 5), magnetic and EPR studies led to the conclusion that the dimer exists as a diradicaloid. In addition, they could isolate the three-coordinate Ni(I) monomer, Ni(PNaminePPri)[double bond, length as m-dash]NNCPh2, by reacting the dimer with N2CPh2. A four-coordinate NiI-CO complex, Ni(PNaminePPri)CO, was obtained by Lee and coworkers from the reduction of the NiII–CO complex, [Ni(PNaminePPri)CO](BF4).118 It can be further reduced to afford the Ni0–CO species, [Na(12-crown-4)2][Ni(PNaminePPri)CO], forming a rare Ni(II), Ni(I), Ni(0) redox series of Ni–CO complexes.
image file: c7cs00216e-s10.tif
Scheme 10 Synthesis of a Ni(I) dimer [Ni(μ-PNPPri)]2 and subsequent trapping of a monomeric Ni(I) species by N2CPh2.117

A monoanionic PNP pincer ligand with a pyrrole backbone 2,5-bis((diphenylphosphino)methyl)-1H-pyrrole (PN(H)PPh) was used by Gade and coworkers to stabilize a dimeric complex of Ni(I), [Ni(PNPPh)]2.119 Interestingly, this Ni(I) complex was synthesized by the reaction of the ligand with Ni(COD)2, in contrast to the commonly observed oxidative addition of the pincer N–H bond to Ni(COD)2, which forms a Ni(II) pincer complex with a Ni–H moiety.120–122 Alternatively, it can also be synthesized from the reaction of the Ni(II) halide complexes, Ni(PNPPh)X (X = Cl, I), with LiEt3BH. In this case the Ni(II) hydride complex, Ni(PNPPh)H, formed as an intermediate which can be detected by 1H NMR spectroscopy at −40 °C. The diamagnetic Ni(I) dimer has a Ni–Ni distance of 2.3259(2) Å, suggesting a strong Ni–Ni interaction (Fig. 24). Similarly, an anionic PSiP-type pincer ligand, –Si(Me)(2-PPh2-C6H4)2 (PSiPPh), was shown by Hazari and coworkers to support a dimeric Ni(I) complex, [Ni(PSiPPh)]2, which has a five-coordinate “hypervalent” silyl group.123 It was synthesized from the reaction of (PSiPPh)NiCl and LiEt3BH in toluene with concomitant H2 evolution. The X-ray crystal structure revealed a dimeric Ni2Si2 core with a short Ni–Ni bond distance of 2.3057(9) Å which was asymmetrically bridged by the hypervalent silicon atom which was confirmed by solution state 29Si NMR spectroscopy that showed two resonances at 16.62 and 15.81 ppm (Fig. 24). The hypervalent silicon atom has a distorted square pyramidal coordination with τ5 values57 of 0.13 and 0.21. NBO analysis revealed that a 4c–2e bond stabilizes the hypervalent silicon in this structure.

image file: c7cs00216e-f24.tif
Fig. 24 Drawings (left) and the molecular structures (right) of [Ni(PNPPh)]2 (left) and [Ni(PSiPPh)]2 (right) with thermal ellipsoids set at 50%. Hydrogen atoms and cocrystallized solvent molecules are not shown. Selected bond distances (Å) and angles (°): [Ni(PNPPh)]2: Ni(1)–Ni(2) 2.32589(17), Ni(1)–N(1) 1.9194(8), Ni(2)–N(2) 1.9273(8), Ni(1)–P(1) 2.2333(3), Ni(1)⋯P(2) 2.7805(3), Ni(2)–P(2) 2.1337(3), Ni(2)–P(3) 2.2386(3), Ni(2)⋯P(4) 2.6856(3), Ni(1)–P(4) 2.1240(3), N(1)–Ni(1)–P(3) 163.37(3), N(2)–Ni(2)–P(2) 162.79(3); [Ni(PSiPPh)]2: Ni(1)–Ni(2) 2.3057(9), Ni(1)–P(1) 2.1592(15), Ni(1)–Si(1) 2.3155(16), Ni(1)⋯Si(2) 2.5992(15), Ni(1)–P(3) 2.2116(15), Ni(2)–P(2) 2.2134(15), Ni(2)–P(4) 2.1576(14), Ni(2)⋯Si(1) 2.4985(16), Ni(2)–Si(2) 2.3462(15), Si(1)–Ni(1)–P(3) 164.30(6), P(2)–Ni(2)–Si(2) 162.95(6).119,123

A novel mercury bridged Ni(I) dimer, [(PNpyrrolePBut)Ni]2(μ-Hg) (PNpyrrole(H)PBut = 2,5-bis((di-tert-butylphosphino)methyl)-1H-pyrrole), supported by a bulkier pincer ligand was reported by Walter and coworkers.124 The diamagnetic dark red complex was synthesized by the reaction of (PNpyrrolePBut)NiBr with Na/Hg (Scheme 11). The crystal structure features two (PNpyrrolePBut)Ni unit with a bridging mercury atom, exhibiting a near-linear Ni–Hg–Ni array (Ni–Hg–Ni angle = 178.699(13)° and 175.056(13)°) (Fig. 25). The two (PNpyrrolePBut)NiI fragments are almost orthogonal to minimize steric repulsion. Exposure of [(PNpyrrolePBut)Ni]2(μ-Hg) to H2 homolytically cleaved the H2 molecule to afford the Ni(II) hydride complex, (PNpyrrolePBut)NiH (Scheme 11), suggesting that this complex could serve as a synthon for species containing the (PNpyrrolePBut)Ni fragment.

image file: c7cs00216e-s11.tif
Scheme 11 Synthesis of [(PNpyrrolePBut)Ni]2(μ-Hg) and subsequent H2 cleavage.124

image file: c7cs00216e-f25.tif
Fig. 25 The molecular structure of [(PNpyrrolePBut)Ni]2(μ-Hg) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown.124

To study the metal–ligand cooperation in H2 activation, Peters and coworkers developed three ambiphilic PBP-type ligands, PhB(o-PPh2C6H4)2 (PhDPBPh), MesB(o-PPh2C6H4)2 (MesDPBPh) and PhB(o-PPri2C6H4)2 (PhDPBPri), that feature P → Ni and Ni → B interactions in Ni(0) and Ni(I) complexes.125,126 The Ni(I) complexes were synthesized via the comproportionation reaction of Ni(COD)2, NiBr2 and the ligand in THF (Scheme 12). The solid state structures showed that the Ni(I) atom was coordinated to two phosphines and a bromide (Fig. 26). In addition, the complexes possessed short Ni–(η2-B,C) interactions. These complexes could be further reduced to Ni(0) by treatment with Na/Hg. Notably, two of the Ni(0) complexes, (MesDPBPh)Ni and (PhDPBPri)Ni(N2), displayed reactivity towards H2 that could either show reversible addition of H2 across the Ni–B unit or form Ni0-H2 complexes that could subsequently convert to NiII–dihydride species. A diamagnetic NiI–NiI dimer, [Ni(μ-PhPBP)]2, supported by two anionic bis(phosphino)boryl ligand, PhPB(H)P (1,3-bis((diphenylphosphino)methyl)-2-methyl-2,3-dihydro-1H-benzo[d][1,3,2]diazaborole), was synthesized via the reaction of Ni(COD)2 and PhPB(H)P.127 The synthetic method is reminiscent of that of Gade's Ni(I) dimer, [Ni(PNPPh)]2.119 The crystal structure showed a NiI–NiI core which is unsymmetrically bridged by two PhPBP ligands, the Ni–Ni distance of 2.2393(7) and 2.2449(7) Å being much shorter than that of a typical NiI–NiI bond (Table 5). Interestingly, this species also activates H2 reversibly, a phenomenon that was previously unknown for Ni(I) species and was attributed to the presence of the rare Ni–boryl moiety.

image file: c7cs00216e-s12.tif
Scheme 12 Synthesis of Ni(I) complexes supported by ArDPBR ligands.125,126

image file: c7cs00216e-f26.tif
Fig. 26 The molecular structures of (PhDPBPh)NiBr (left), (MesDPBPh)NiBr (middle) and (PhDPBPri)NiBr (right) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown.125,126
5.2.3 Terpyridine complexes. In 2004, Vicic and coworkers reported an interesting reaction between (TMEDA)NiMe2 (TMEDA = N,N,N′,N′-tetramethylethylenediamine) and a neutral tridentate tpy ligand (2,2′;6′,2′′-terpyridine) to afford (tpy)NiMe that appears to be a four-coordinate Ni(I) complex (Scheme 13).128 However, the follow-up EPR (isotropic g signal of 2.021 ± 0.002) and computational studies revealed that (tpy)NiMe is best described as a Ni(II) complex of a ligand-based radical.129 Later, they synthesized a four-coordinate (tpy)NiBr complex from the comproportionation reaction of Ni(COD)2, NiBr2(DME) and tpy ligand. A low-temperature powder EPR study revealed an axial signal of g = 2.256 and g = 2.091 that is consistent with a Ni(I) complex and a metal-centered dx2y2 ground state.130 DFT calculations further confirmed the metal-based radical character. The chloride analog, (tpy)NiCl, was reported by Wieghardt and coworkers by the reaction of NiCl2, tpy and two equivalents of Na/Hg.131 The structurally characterized iodide analog, (p-But-tpy)NiI (p-But-tpy = 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine), was synthesized by oxidative addition of MeI or EtI to the mixture of Ni(COD)2 and p-But-tpy.128 The different oxidation states of (tpy)NiMe and (tpy)NiX (X = Cl, Br) are explained on the basis of the stronger σ bonding –Me ligand raising the energy of dx2y2 orbital, thereby favoring the ligand-based radical character of the (tpy)NiMe complex.
image file: c7cs00216e-s13.tif
Scheme 13 Syntheses of (tpy)NiMe, (p-But-tpy)NiI and (tpy)NiX (X = Cl, Br).128–131

5.3 Bidentate ligands

Ni(I) α-diimine derivatives exhibit a rich chemistry. For example, tom Dieck and coworkers showed that reactions of (NNDipp)Ni(COD) (NNDipp = [(2,6-Pri2C6H3)NCH]2) with HC[triple bond, length as m-dash]CR (R = CH2OCH3) yielded blue crystals of dimeric [(NNDipp)Ni]2(μ-HC[double bond, length as m-dash]C(R)(R)C[double bond, length as m-dash]CH) in which two alkyne HC[triple bond, length as m-dash]CCH2OCH3 molecules are coupled to afford a dianionic butadienediyl ligand bridging the two nickel atoms (Scheme 14).132 Furthermore, they also synthesized a paramagnetic Ni(I) dimer stabilized by two α-diimine ligands and a bridging 2,2′-bisallyl ligand, {(NNMe,Dmp)Ni}2{μ-η33-(CH2)2CC(CH2)2} (NNMe,Dmp = [(2,6-Me2C6H3)NCMe]2), by treating the Ni(0) complex, (NNMe,Dmp)Ni(butadiene), with excess allene (Scheme 14).133 The X-ray crystal structure showed that the nickel had a square planar coordination in which each Ni(I) ion is coordinated by an α-diimine ligand and an additional bridging diallyl ligand. Upon treatment of Brookhart's α-diimine NiII–bromide complex, (NNMe,Dipp)NiBr2 (NNMe,Dipp = [(2,6-Pri2C6H3)NCMe]2),134 with AlMe3 or MAO, Rieger and coworkers obtained unexpected deep purple crystals that were later identified by X-ray crystallography to be those of a dimeric Ni(I) species, [(NNMe,Dipp)Ni]2(μ-Br)2 (Scheme 14).135 The Ni⋯Ni distance of 3.3009(11) Å is too long for significant Ni–Ni bonding. Similarly, the reaction of (NNMe,Dipp)NiBr2 with PhMgBr afforded [(NNMe,Dipp)NiBr]2[μ-MgBr2(THF)2] which has a (NNMe,Dipp)NiIBr unit coordinated to MgBr2(THF)2. These Ni(I) complexes displayed no polymerization activity towards alkenes such as ethylene, 1-hexene and cyclic olefins. An analogous Ni(I) dimer with bridging chlorides, [(NNMe,Dipp)Ni]2(μ-Cl)2, was obtained as purple crystals from the reaction of (NNMe,Dipp)NiCl2 with 2-ethylindenyllithium in THF solution.136
image file: c7cs00216e-s14.tif
Scheme 14 Synthesis of α-diimine stabilized Ni(I) complexes.132,133,135,136

The blue-violet four-coordinate neutral nickel bis-bipyridine complex, Ni(bpy)2 (bpy = 2,2′-bipyridine), was reported as early as 1962 by Taube and Herzog137 and later by Meyer and Behrens in 1966.138 They described it as a pseudotetrahedral Ni(0) complex in which the nickel atom is coordinated by two bpy ligands. However, its crystal structure was unknown until 2015 when Wieghardt and coworkers resynthesized it as dark blue crystals from the reaction of NiCl2, two equivalents of bpy and two equivalents of lithium metal.130 The coordination at nickel lies between square planar and tetrahedral (Fig. 27). The average Cpy–Cpy bond distances of 1.442(2) Å in the bpy ligand are shorter than those in the neutral free bpy ligand (1.490(3) Å)139 but longer than those in the singly140 (1.431(3) Å in [K(bpy)(en)], en = ethylenediamine) and doubly reduced140 (1.399(6) Å in [Rb2(bpy)(en)]) alkali metal bpy salts. Magnetic measurements gave a magnetic moment of ca. 0.25 μB at room temperature indicating almost diamagnetic character with a S = 0 ground state. Combining structural, magnetic, and computational data, they concluded that it is best described as a Ni(I) complex of a neutral bpy ligand and a singly reduced bpy radical anion, with the unpaired electron delocalized over both bpy ligands. In addition, the Ni(I) ion is antiferromagnetically coupled to the radical with a calculated J value of −1355 cm−1, resulting in a S = 0 ground state.

image file: c7cs00216e-f27.tif
Fig. 27 The molecular structure of Ni(bpy)2 with thermal ellipsoids set at 50%. Hydrogen atoms are not shown.131

Vicic and coworkers prepared a mixed oxidation state Ni(I)/Ni(II) hydride species, (NiBr)2(μ-dippm)(μ-H) (dippm = bis(diisopropylphosphino)methane), and showed that deprotonation with bases such as KOBut or LiN(SiMe3)2 in THF afforded dimeric Ni(I) complexes with Ni–Ni bonds (Scheme 15).141 Reactions of (NiBr)2(μ-dippm)(μ-H) with two equivalents of LiN(SiMe3)2 led to the cleavage of the methylene unit of the dippm ligand to form the bridged diisopropylphosphide complex. Reactions of the stronger base KOBut at room temperature led to deprotonation of the methylene hydrogen. A tetrahedrally coordinated, cationic Ni(I) supported by two bidentate 1,1′-bis((diphenylphosphino)ferrocene) (dppf) ligands, [Ni(dppf)2][PF6], was prepared by facile chemical oxidation of the Ni(0) complex, Ni(dppf)2, with [FeCp2][PF6].142 The rhombic EPR signal of gx = 2.098, gy = 2.113 and gz = 2.332 is indicative of a Ni(I) complex. Two Ni(I) complexes supported by two chelating cyclic diphosphine ligands, [Ni(PBut2NBn2)2][BF4] (PBut2NBn2 = 1,5-dibenzyl-3,7-di-tert-butyl-1,5,3,7-diazadiphosphocane)143 and [Ni(PCy2NBut2)2][BF4] (PCy2NBut2 = 1,5-di-tert-butyl-3,7-dicyclohexyl-1,5,3,7-diazadiphosphocane),144,145 were obtained and structurally characterized. The complex [Ni(PBut2NBn2)2][BF4] was prepared by the chemical oxidation of the Ni(0) complex, Ni(PBut2NBn2)2, with [FeCp*2][BF4] (Cp* = pentamethylcyclopentadienyl). [Ni(PCy2NBut2)2][BF4] was synthesized from the comproportionation reaction between Ni(PCy2NBut2)2 and [Ni(PCy2NBut2)2][BF4]2 (Scheme 16). The X-ray crystal structures showed that the nickel atom has a tetrahedral coordination with similar Ni–P bond distances which are slightly longer than those in the Ni(0) congeners, Ni(PBut2NBn2)2 and Ni(PCy2NBut2)2. This elongation upon oxidation is explained by the weakening of Ni → P π-bonding. The magnetic moments (Evans’ method) of 1.78 μB for [Ni(PBut2NBn2)2][BF4] and 1.90 μB for [Ni(PCy2NBut2)2][BF4] and the rhombic EPR signals (gx = 2.104, gy = 2.070, gz = 2.006 for [Ni(PBut2NBn2)2][BF4] and gx = 2.146, gy = 2.063, gz = 2.017 for [Ni(PCy2NBut2)2][BF4]) are consistent with the +1 oxidation state of nickel.

image file: c7cs00216e-s15.tif
Scheme 15 Reactions of Ni2Br2(μ-dippm)2(μ-H) with KOBut and LiN(SiMe3)2.141

image file: c7cs00216e-s16.tif
Scheme 16 Synthesis of [Ni(PBut2NBn2)2][BF4] and [Ni(PCy2NBut2)2][BF4] via oxidation and comproportionation.143–145

A new bis(diphenylphosphino)-1,2-diaminobenzene ligand (1,2-(NHPPh2)2C6H4) was used by Hey-Hawkins and coworkers to support a Zn(II) complex, [Zn(THF){1-N(PPh2)-2-N(μ-PPh2)C6H43N,N,P}]2 and a “Dewar-benzene-like” Ni(I) complex, [Ni{1-NH(PPh2)-2-N(μ-PPh2)C6H42N,P}]2, which was synthesized via the reaction of NiCl2 and the monolithiated ligand in a THF solution (Scheme 17).146 The solid state molecular structure features a Dewar-benzene-like Ni2N2P2 core with a Ni–Ni bond distance of 2.4152(6) Å. DFT calculations (B3LYP/LANL2DZ) also support the Dewar-benzene-type structure and further suggests that a two-electron reduction should convert it to a benzene-like structure. Although they were unable to explore the redox properties of this complex, the benzene-like core was observed also for the isoelectronic Zn(II) species.

image file: c7cs00216e-s17.tif
Scheme 17 The synthesis of [Ni{1-NH(PPh2)-2-N(μ-PPh2)C6H42N,P}]2. Selected bond distances (Å) and angles (°): Ni(1)–Ni(1A) 2.4152(6), Ni(1)–N(2) 1.896(2), Ni(1)–P(2A) 2.1150(7), Ni(1A)–Ni(1)–P(2A) 68.78(2), N(2)–Ni(1)–P(2A) 157.26(7).146

5.4 Monodentate ligands

In 1977 Dartiguenave, Klein and coworkers reported the isolation of an cationic, unusual tetrahedral Ni(I) complex, [Ni(PMe3)4]BPh4, which was obtained during the crystallization of the Ni(II) complex, [Ni(Me)(PMe3)4]BPh4, from THF solution (Scheme 18).147 An analysis of the crystal structure revealed that the bond angles lie between values expected for a trigonal bipyramid and a monocapped tetrahedron with the methyl substituent being in the capping position. This may explain its instability (CH4 and C2H6 were detected during the decomposition process by gas chromatography (GC)) and decomposition to [Ni(PMe3)4]BPh4.148 The crystal structure of [Ni(PMe3)4]BPh4 shows that it contains well-separated [Ni(PMe3)4]+ and BPh4 ions without any close Ni⋯Ph interactions. The Ni–P distances and P–Ni–P angles are very similar (2.213(3), 2.211(3), 2.221(3) and 2.221(3) Å and 106.0(1), 104.6(1), 108.0(1), 106.1(1), 119.9(1), 111.0(1)°). The room temperature magnetic moment of 2.40 μB indicates one unpaired electron and a significant orbital moment. Slightly bulkier monodentate phosphines such as triphenylphosphine can also stabilize four-coordinate Ni(I) complexes. For example, in 1964 Heimbach first showed that Ni(I) triphenylphosphine complexes can be obtained by oxidation, comproportionation or reduction (Scheme 19).149 However, the structural characterization of some of the NiI–PPh3 complexes, NiBr(PPh3)3,150 NiCl(PPh3)3·C7H8,151 NiCl(PPh3)3,152 was not achieved until later (Fig. 28). The data showed that they have monomeric, four-coordinate Ni(I) structures.
image file: c7cs00216e-s18.tif
Scheme 18 Formation of [Ni(PMe3)4]BPh4 from recrystallization of [Ni(Me)(PMe3)4]BPh4, from THF solution.147

image file: c7cs00216e-s19.tif
Scheme 19 Formation of Ni(PPh3)nX (X = Cl, Br, I, n = 2 or 3) from oxidation, comproportionation and reduction.149

image file: c7cs00216e-f28.tif
Fig. 28 The molecular structures of NiCl(PPh3)3 (left) and NiBr(PPh3)3 (right) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): NiCl(PPh3)3: Ni(1)–Cl(1) 2.2933(17), Ni(1)–P(1) 2.3082(9), P(1)–Ni(1)–P(1A) 108.21(3), P(1)–Ni(1)–Cl(1) 110.71(3); NiBr(PPh3)3: Ni(1)–Br(1) 2.4363(9), Ni(1)–P(1) 2.3137(5), P(1)–Ni(1)–P(1A) 108.87(2), P(1)–Ni(1)–Cl(1) 110.061(8).150,152

When investigating Ni(II) complexes containing the –P(SiMe3)2 ligand, Schäfer and Deppisch found that with two or more –P(SiMe3)2 groups, there was elimination of P(SiMe3)3 at low temperature (ca. 268–263 K) to yield complex mixtures.153 For Ni{P(SiMe3)2}2(PMe3)2, reduction from Ni(II) to Ni(I) occurred and dark violet crystals were isolated. X-ray crystallography afforded a structure involving the centrosymmetric, dimeric trigonal planar four-coordinate Ni(I) complex, [Ni(PMe3)]2[μ-P(SiMe3)2]2, with two Ni(I) ions coordinated to two PMe3 ligands and a bridging P(SiMe3)2 ligand. The Ni–Pphosphide distance of 2.186(1) Å, Ni–Pphosphine distance of 2.129(1) Å and Ni–Ni bond distance of 2.382(10) Å compare well with the previously discussed four- and five-coordinate dimeric NiI–phosphine/phosphide complexes (Table 5).

Beginning in 1991, the introduction of N-heterocyclic carbenes (NHC) as ligands by Arduengo and coworkers154 made a large impact in both transition metal and main group coordination chemistry.155–161 When NHCs were introduced as ligands, they were first used to form group 11 metal complexes as in the two two-coordinate coinage metal species, [Cu(IMes)2]OTf (IMes = 1,3-bis(1,3,5-trimethylphenyl)imidazol-2-ylidene), [Ag(IMes)2]OTf, [Au(IMes)2]Cl, as well as complexes of Ni(0), Ni(IMes)2, and Pd(0), Pd(IMes)2.162–164 In 2004, an unexpected Ni(I) dimeric species was obtained by Caddick, Cloke and coworkers in an attempt to develop a conventional route to Ni(IBut)2 (IBut = 1,3-bis-tert-butylimidazol-2-ylidene) via a ligand substitution similar to that used for Ni(IMes)2.164 When Ni(COD)2 was reacted with excess IBut in THF in a Schlenk tube with a greased stopper, the grease-activated Ni(II) species, [Ni(IBut){O(Me2SiOSiMe2)-μ-O}]2 was obtained in low yield after two weeks (Scheme 20). Performing the reaction in a greaseless environment afforded the Ni(I) dimer, [Ni(IBut){μ-CN(CH)2NBut}]2, in 38% yield. A single crystal X-ray crystallographic analysis afforded an unusual Ni(I) structure with an Ni–Ni distance of 2.4354(9) Å. The application of N-heterocyclic carbenes to Ni(I) chemistry did not begin until the synthesis of the so-called “Sigman's dimer”, [(IPr)Ni(μ-Cl)]2 (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and [(SIPr)Ni(μ-Cl)]2 (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene). These derivatives were prepared from comproportionation of Ni(COD)2 and NiCl2(DME) in the presence of IPr or SIPr (Scheme 21).166 The crystal structure of [(IPr)Ni(μ-Cl)]2 displayed two trigonal planar coordinated Ni(I) ions complexed by a Ccarbene atom and two bridging chlorides and a Ni–Ni distance of 2.5194(5) Å (Fig. 29). Its diamagnetism is indicative of the presence of a Ni–Ni bond. The complex [(SIPr)Ni(μ-Cl)]2 is structurally similar to [(IPr)Ni(μ-Cl)]2.166 Later these complexes were shown to be extremely useful synthons for accessing –Ni(IPr) or –Ni(SIPr) fragments to afford a variety of two- and three-coordinate Ni complexes. A closely related diamagnetic Ni(I) dimer, [(IPr)Ni]2(μ-I)(μ-NO), was obtained by Warren and coworkers from the Na/Hg reduction of (IPr)Ni(NO)I (Scheme 21).167 The Ni–Ni distance of 2.314(1) Å is much shorter than [(IPr)Ni(μ-Cl)]2 (Fig. 29). To investigate a possible dinuclear mechanism in the catalytic Kumada coupling reaction, Matsubara and coworkers synthesized two IPr-stabilized Ni(I) dimers, [(IPr)Ni]2(μ-Cl)(μ-p-tol) and [(IPr)Ni]2(μ-p-tol)2.168 [(IPr)Ni]2(μ-Cl)(μ-p-tol) can be obtained from either a stoichiometric reaction of [(IPr)Ni(μ-Cl)]2 and p-tolMgCl in THF or an oxidative addition of p-tolCl with Ni(COD)2 in the presence of IPr. Subsequent addition of p-tolMgCl gave [(IPr)Ni]2(μ-p-tol)2 (Scheme 21). Both structures are similar to those of the dimers of Sigman's and Warren's with a Ni–Ni distance of 2.3954(5) Å for [(IPr)Ni]2(μ-Cl)(μ-p-tol) and 2.4067(8) Å for [(IPr)Ni]2(μ-p-tol)2.

image file: c7cs00216e-s20.tif
Scheme 20 Formation of a grease-activated Ni(II) complex, a carbene-stabilized Ni0(1,3-COD)2 complex and a carbene-stabilized Ni(I) dimer.165

image file: c7cs00216e-s21.tif
Scheme 21 (a) Synthesis of [(NHC)Ni(μ-Cl)]2 (NHC = IPr, SIPr). (b) Synthesis of [(IPr)Ni]2(μ-I)(μ-NO). (c) Reaction of [(IPr)Ni(μ-Cl)]2 with one or two equivalents of p-tolMgCl to form [(IPr)Ni]2(μ-Cl)(μ-p-tol) and [(IPr)Ni]2(μ-p-tol)2.166–168

image file: c7cs00216e-f29.tif
Fig. 29 The molecular structures of [(IPr)Ni(μ-Cl)]2 (left) and [(IPr)Ni]2(μ-I)(μ-NO) (right) with thermal ellipsoids set at 50%. Hydrogen atoms and cocrystallized Et2O molecule are not shown. Selected bond distances (Å) and angles (°): [(IPr)Ni(μ-Cl)]2: Ni(1)–Ni(2) 2.5194(5), Ni(1)–Cl(1) 2.2084(8), Ni(1)–Cl(2) 2.2212(9), Ni(1)–C(15) 1.871(3), Ni(2)–Cl(1) 2.2467(8), Ni(2)–Cl(2) 2.2254(8), Ni(2)–C(42) 1.885(3), C(15)–Ni(1)–Ni(2) 165.70(8), C(42)–Ni(2)–Ni(1) 174.63(8); [(IPr)Ni]2(μ-I)(μ-NO): Ni(1)–Ni(2) 2.3135(8), Ni(1)–C(1) 1.897(4), Ni(1)–N(5) 1.763(4), Ni(1)–I(1) 2.5375(7), Ni(2)–N(5) 1.758(4), Ni(2)–I(1) 2.5204(7), Ni(2)–C(28) 1.893(4), N(5)–O(1) 1.222(5), C(1)–Ni(1)–Ni(2) 169.14(3), C(28)–Ni(2)–Ni(1) 167.27(13).166,167

Bulky m-terphenyl isocyanide ligands were used by Figueroa and coworkers to mimic the electronic properties of the isoelectronic CO ligand. The four-coordinate Ni(I) complex, Ni(CNArMe6)3(OTf) (ArMe6 = C6H3-2,6(C6H2-2,4,6-Me3)2),169 was synthesized by one electron oxidation of the neutral, three-coordinate Ni0–tris-isocyanide complex, Ni(CNArMe6)3, with [FeCp2][OTf] (see below) in benzene solution. The product Ni(CNArMe6)3(OTf) displayed a distorted tetrahedral geometry (Fig. 30) with three similar Ni–C distances of 1.927(7), 1.948(7) and 1.962(6) Å that are longer than those in the Ni(0) complex, Ni(CNArMe6)3 (1.795(3), 1.805(3) and 1.817(3) Å) which may be a result of the higher coordination number of the metal or less metal-to-ligand backdonation. It is worth noting that Ni(CNArMe6)3(OTf) does not disproportionate in polar solvents such as THF. Instead, it formed the five-coordinate, trigonal bipyramidal Ni(I) complex, [(THF)2Ni(CNArMe6)3](OTf).

image file: c7cs00216e-f30.tif
Fig. 30 The molecular structure of Ni(CNArMe6)3(OTf) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): Ni(1)–C(1) 1.927(7), Ni(1)–C(2) 1.962(6), Ni(1)–C(3) 1.948(7), Ni(1)–O(1) 2.093(5), C(1)–N(1) 1.176(8), C(2)–N(2) 1.184(7), C(3)–N(3) 1.174(7), C(1)–Ni(1)–C(2) 108.3(3), C(2)–Ni(1)–C(3) 131.8(2), C(1)–Ni(1)–C(3) 99.7(3), C(1)–Ni(1)–O(1) 120.7(2).169

Fischer and coworkers investigated the oxidation chemistry of several closed-shell Ni(0) complexes of the monodentate metallanediyl ECp* (E = Al, Ga) ligands.170 Treatment of the complexes Ni(AlCp*)4, Ni(PPh3)2(AlCp*)2 and Ni(PPh3)2(GaCp*)2 with ferrocenium salts, (FeCp2)(BArF4) or (FeCp2)(OTf), in fluorobenzene afforded three structurally characterized Ni(I) species, [Ni(AlCp*)4][Cp*Al(OTf)3] (thermally unstable, only a few crystals could be isolated), [Ni(PPh3)2(AlCp*)2][BArF4] and [Ni(PPh3)2(GaCp*)2][BArF4] (Scheme 22). The solid state structures feature distorted tetrahedral coordination at nickel, which can be rationalized by Jahn–Teller distortion of the d9 tetrahedral species. The magnetic and EPR measurements (1.09 μB, g = 2.11 for [Ni(PPh3)2(AlCp*)2][BArF4] and 1.15 μB, g = 2.15 for [Ni(PPh3)2(GaCp*)2][BArF4]) are in accordance with the +1 oxidation state of nickel.

image file: c7cs00216e-s22.tif
Scheme 22 Synthesis of three Ni(I) complexes, [Ni(AlCp*)4][Cp*Al(OTf)3], [Ni(PPh3)2(AlCp*)2][BArF4] and [Ni(PPh3)2(GaCp*)2][BArF4].170

6. Three-coordinate Ni(I)

6.1 Tridentate ligands

Caulton and coworkers used the sterically tunable anionic tridentate PNP pincer ligand, [N(SiMe2CH2PBut2)2] ([PNaminePBut]), to study the effects of different d-electron counts on the binding and reactivity of CO and other small molecules with its nickel derivatives.171 The three-coordinate Ni(I) complex, Ni(PNaminePBut), was synthesized by the magnesium reduction of (PNaminePBut)NiCl in THF solution.171 The Fe(I)171 and Co(I)172,173 analogues were also synthesized in the same fashion. Surprisingly, none of these complexes binds a THF molecule or dimerizes like the previously discussed four-coordinate NiI–pincer complexes, although they are electronically and coordinatively unsaturated. This is presumably due to the steric bulk of the ligand substituents. The complex Ni(PNaminePBut) binds CO to form the four-coordinate Ni(I) complex, (PNaminePBut)NiCO (Scheme 23, Fig. 31), and the CO stretching frequency (νCO) of 1940 cm−1 indicates moderate Ni(I) backbonding (νfree[thin space (1/6-em)]CO = 2140 cm−1).170 The CO ligand binds reversibly to the Ni(I) atom, and can be removed under vacuum to reform the three-coordinate Ni(PNaminePBut). The planar, T-shaped geometry of Ni(PNaminePBut) is similar to that of its cobalt analog, Co(PNaminePBut). However, the CO complex, (PNaminePBut)NiCO (Fig. 31), which has a nonplanar coordination at nickel (sum of angles around Ni = 344.40°), is in marked contrast to the planar core of (PNaminePBut)CoCO. Interestingly, the reaction of Ni(PNaminePBut) with excess CO2 in benzene resulted in an unexpected three-coordinate NiI–isocyanate complex, (POPBut)Ni(NCO) (POPBut = O(SiMe2CH2PBut2)2), where the nitrogen atom of the pincer ligand and one of the oxygen atoms in CO2 have been transposed to form a new neutral POP-type pincer ligand (Scheme 23, Fig. 31).174
image file: c7cs00216e-s23.tif
Scheme 23 The synthesis of Ni(PNaminePBut) and its reactions with CO and CO2.171,174

image file: c7cs00216e-f31.tif
Fig. 31 The molecular structures of Ni(PNaminePBut) (left), (PNaminePBut)Ni(CO) (center) and (POPBut)Ni(NCO) (right) (cf. Scheme 23) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): Ni(PNaminePBut): Ni(1)–N(1) 1.9592(11), Ni(1)–P(1) 2.2031(4), Ni(1)–P(2) 2.2025(4), P(1)–Ni(1)–P(2) 196.996(14); (PNaminePBut)Ni(CO): Ni(1)–N(1) 1.990(3), Ni(1)–P(1) 2.3197(14), Ni(1)–P(2) 2.3664(12), Ni(1)–C(23) 1.766(4), C(23)–O(2) 1.150(5), P(1)–Ni(1)–P(2) 149.59(4), Ni(1)–C(23)–O(1) 175.6(4); (POPBut)Ni(NCO): Ni(1)–N(1) 1.9407(13), Ni(1)–P(1) 2.2660(4), Ni(1)–P(2) 2.2730(5), Ni(1)⋯O(1) 3.0447(10), N(1)–C(23) 1.160(2), C(23)–O(2) 1.2070(18), P(1)–Ni(1)–P(2) 149.745(14), Ni(1)–N(1)–C(23) 165.84(13), N(1)–C(23)–O(2) 179.68(19).171,174

Gade and coworkers reported two chiral bis(oxazolinylmethylidene)pyrrolidinido-nickel hydride species, (NNNR)NiH (R = Ph,175 Pri176), from the reaction of (NNNR)NiCl and LiEt3BH (Fig. 32), which were structurally characterized. In solution, this Ni(II) hydride complex was found to be in equilibrium with the planar T-shaped, three-coordinate Ni(I) species, Ni(NNNR) (Scheme 24). DFT calculations (B3LYP/6-311G(d,p)) indicated a mainly nickel-centered unpaired electron, which is consistent with a rhombic EPR signal (gx = 2.326, gy = 2.147, gz = 2.038). They determined the equilibrium constant (Keq) for (NNNPh)Ni to be 12.4 bar−1 at 295 K and the rate of interconversion to be second order with a rate constant (k) of 5.31 × 10−4 L mol−1 s−1 at 273 K. Remarkably, these three-coordinate Ni(I) complexes show diverse reactivity, for example, catalytic asymmetric hydrodehalogenation of germinal dihalogenides, O2 activation to generate a Ni(II) peroxo species, aryl halide activation and C–F bond activation (Scheme 25).175–180

image file: c7cs00216e-f32.tif
Fig. 32 Drawings (left) and the molecular structures (right) of (NNNPh)Ni and (NNNPri)Ni with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): (NNNPh)Ni: Ni(1)–N(1) 1.875(2), Ni(1)–N(2) 1.920(2), Ni(1)–N(3) 1.877(2), N(1)–Ni(1)–N(3) 169.44(9); (NNNPri)Ni: Ni(1)–N(1) 1.8955(19), Ni(1)–N(2) 1.9301(18), Ni(1)–N(3) 1.8903(18), N(1)–Ni(1)–N(3) 168.60(7).175,176

image file: c7cs00216e-s24.tif
Scheme 24 The equilibrium between three-coordinate Ni(I) species Ni(NNNPh) and NiII–hydride species (NNNPh)NiH.175

image file: c7cs00216e-s25.tif
Scheme 25 Selected catalytic and stoichiometric reactions of Ni(NNNR) species. (a) Bimolecular oxidative addition of aryl halides (ArX) to Ni(NNNPh).180 (b) Catalytic asymmetric hydrodehalogenation reactions by Ni(NNNR) (R = Ph, Pri, But, Ind).175 (c) Stoichiometric defluorination of perfluorodecalin using Ni(NNNPh).176

Lee and coworkers used a rigid acridane-based anionic pincer ligand, PNacriPPri, to support a T-shaped, three-coordination at Ni(I) in Ni(PNacriPPri) that has exceptional reactivity towards small molecules (Scheme 26).181 It was synthesized by the reaction of (PNacriPPri)NiCl with sodium naphthalenide (NaNaph) and isolated as brown crystals. The solid state molecular structure confirmed its monomeric identity, which is similar to Caulton's Ni(PNaminePBut) but different from Mindiola's dimeric Ni(I) complex, [Ni(μ-PNaminePPri)]2. The magnetic moment of 1.78 μB (Evans’ method, C6D6) and rhombic EPR resonance (g = 1.99, 2.22, 2.33) suggested a Ni(I) oxidation state. Furthermore, DFT calculations with Mulliken population analysis revealed that ca. 72.6% of the unpaired spin is located in the dx2y2 orbital of nickel. Ni(PNacriPPri) did not show any complexation with σ-donors such as THF, NMe3 and NEt3, presumably due to the metalloradical character of nickel. It did bind π-accepting ligands such as pyridine, PMe3 and CO, forming four-coordinate Ni(I) species, (PNacriPPri)Ni(pyridine), (PNacriPPri)NiPMe3 and (PNacriPPri)NiCO. The CO stretching frequency of 1936 cm−1 suggested slightly more efficient backbonding than that in Caulton's (PNaminePBut)NiCO. The reactivity of Ni(PNacriPPri) towards small molecules was also explored, remarkably, it doubly reduced CO2 and C2H4 (reversibly) to form the bimetallic Ni(II) species, [(PNacriPPri)Ni]2(μ-CO2) and [(PNacriPPri)Ni]2(μ-C2H4). Moreover, it homolytically cleaved σ-bonds in H2, RO–H (R = Me, Ph), PhS–SPh, Me–I, H2N–NH2 and Me–CN (Scheme 26).

image file: c7cs00216e-s26.tif
Scheme 26 Reactions of Ni(PNacriPPri).181

6.2 Bidentate ligands

In addition to bulky monodentate phosphine ligands, bulky chelating diphosphine ligands also stabilize a series of three-coordinate Ni(I) complexes (cf. Table 2). Schäfer and coworkers first showed that the bulky diphosphine ligand, dcpe, stabilized a three-coordinate Ni(I) phosphide complex, (dcpe)NiP(SiMe3)2.182 The X-ray crystal structure showed that the metal displayed three-coordinate distorted trigonal planar geometry with Ni–Pphosphine distances of 2.192(2) and 2.202(2) Å and Ni–Pphosphide distance of 2.225(2) Å. In related studies, Hillhouse and coworkers used the bulkier chelating diphosphine, dtbpe, to stabilize a series of three-coordinate Ni(I) phosphido,183 amido,184 alkyl,185 and silyl186 complexes. They were synthesized by salt elimination reactions from the dimeric red Ni(I) chloride complex, [(dtbpe)Ni(μ-Cl)]2,183,187 and the lithium phosphides, LiPBut2 or (Et2O)LiP(H)ArMe6, lithium anilide, LiN(H)Dipp, lithium alkyls, LiR (R = CH2But, CH2SiMe3, CH2CMe2Ph), and potassium silyl, KSi(H)Mes2. The isolated compounds, (dtbpe)NiPBut2,183 (dtbpe)NiP(H)ArMe6,183 (dtbpe)NiN(H)Dipp,184 (dtbpe)NiR (R = CH2But (Fig. 33), CH2SiMe3, CH2CMe2Ph),185 and (dtbpe)NiSi(H)Mes2 (Fig. 33)186 are rare examples of three-coordinate Ni(I) complexes with bidentate ligands (Scheme 27). The successful isolation of Ni(I) alkyls from a Ni(I) starting material is notable, since attempted alkylation of Ni(II) halides often leads to reduction. These complexes can be oxidized to cationic Ni(II) species with mild oxidizing agents such as ferrocenium or tropylium hexafluorophosphate. Deprotonation and salt elimination from these cationic complexes with NaN(SiMe3)2 afforded terminal Ni(II) phosphinidene or imide products.
Table 2 Structurally characterized three-coordinate NiI–bisphosphine (P2–Ni–L) complexes with selected bond distances (Å) and angles (°)
Complex Ni–P(1) Ni–P(2) P(1)–Ni–P(2) Ni–L Ref.
NiCl(PPh3)2·(THF) 2.2091(6) 2.2012(6) 111.52(2) 2.1481(6) 152
Ni(PNaminePBut) 2.2031(4) 2.2025(4) 169.996(14) 1.9592(11) 171
(POPBut)Ni(NCO) 2.2660(4) 2.2730(5) 149.745(14) 1.9407(13) 174
(dcpe)NiP(SiMe3)2 2.192(2) 2.202(2) 89.6(1) 2.225(2) 182
(dtbpe)NiPBut2 2.218(2) 2.235(3) 91.18(9) 2.2077(12) 183
(dtbpe)NiN(H)Dipp 2.2094(8) 2.2012(7) 91.91(3) 1.881(2) 184
(dtbpe)NiCH2But 2.2101(11) 2.2081(11) 91.02(3) 1.981(3) 185
(dtbpe)NiSi(H)Mes2 2.2502(10) 2.2507(9) 90.60(3) 2.3731(10) 186
(PriDPDBFphos)NiCl 2.209(1)








(dippf)NiCl 2.195(1) 2.211(1) 104.49(3) 2.1585(9) 191
(dppf)NiCl 2.2072(7) 2.2046(7) 106.13(3) 2.1668(7) 192 and 194
(dppf)NiBr 2.2085(10) 2.2094(9) 106.68(4) 2.2935(6) 193
[(dppf)Ni(μ-I)]2 2.204(1) 2.2244(9) 104.91(4) 2.6278(5)


NiCl(PPh3)2 2.2536(5) 2.2393(5) 114.94(2) 2.1666(6) 235
Ni{N(SiMe3)2}(PPh3)2 2.220(4) 2.213(4) 107.0(2) 1.88(1) 236
(Pri3P)2NiCl 2.2215(5)   119.44(2) 2.1761(7) 243
(Pri3P)2NiBr 2.2230(7)   120.18(3) 2.3033(7) 243
(Pri3P)2NiI 2.2336(10) 2.2402(10) 126.05(4) 2.4810(7) 243
(Pri3P)2Ni(C6F5) 2.2431(5) 2.2329(5) 145.22(2) 1.973(2) 249
Ni(PPh3)2(σ-C2B10Me8H3) 2.2586(9) 2.2696(8) 104.9(1) 1.998(3) 250

image file: c7cs00216e-f33.tif
Fig. 33 Drawings and the molecular structures of (dtbpe)Ni(CH2But) (upper) and (dtbpe)NiSi(H)Mes2 (lower) with thermal ellipsoids set at 50%. Hydrogens atoms (except Si–H) are not shown. Selected bond distances (Å) and angles (°): (dtbpe)Ni(CH2But): Ni(1)–P(1) 2.2101(11), Ni(1)–P(2) 2.2081(11), Ni(1)–C(19) 1.981(3), P(1)–Ni(1)–C(19) 110.97(8), P(2)–Ni(1)–C(19) 157.82(8); (dtbpe)NiSi(H)Mes2: Ni(1)–P(1) 2.2507(9), Ni(1)–P(2) 2.2502(10), Ni(1)–Si(1) 2.3731(10), P(1)–Ni(1)–Si(1) 138.75(4), P(2)–Ni(1)–Si(1) 126.68(4).185,186

image file: c7cs00216e-s27.tif
Scheme 27 Reactions of (dtbpe)NiCl2 and [(dtbpe)Ni(μ-Cl)]2 that provide access to a variety of Ni(I) complexes.68,183–187

Lu and coworkers employed the wide bite angle diphosphine ligand 4,6-bis(3-diisopropylphosphinophenyl)dibenzofuran, PriDPDBFphos, to support complexes of four-coordinate Ni(II) and three-coordinate Ni(I), (PriDPDBFphos)NiCl2 and (PriDPDBFphos)NiCl.188 Together with the results on the corresponding complexes of Co(II) and Zn(II), they showed that PriDPDBFphos is flexible enough to support a wide range of bite-angles (P–M–P angles) that range from 115 to 180°. (PriDPDBFphos)NiCl was obtained from the reduction of the Ni(II) precursor with KC8 (Scheme 28). X-ray crystallographic analysis of (PriDPDBFphos)NiCl showed that there are two independent molecules having trigonal planar nickel coordination but with slightly different bond parameters in a unit cell. The Ni–P bond distances of 2.209(1) and 2.217(1) Å in one crystallographically independent molecule are ca. 0.03 Å shorter than those in the other molecule where the distances are 2.233(1) and 2.243 Å. Small differences were also observed in the Ni–Cl bond distance (2.163(1) Å and 2.180(1) Å) and the diphosphine (P–Ni–P) bite angle (115.53(4)° and 119.49(4)°). EPR spectroscopy of a frozen toluene solution at 20 K afforded a slightly rhombic signal (g = 2.09, 2.14 and 2.37) and is indicative of a Ni(I) species. Further investigation showed that when (PriDPDBFphos)NiCl was reacted with HCl·(dioxane) a mixture containing the Ni(II) complexes (PriDPDBFphos)Ni(H)Cl and (PriDPDBFphos)NiCl2 was formed. The Ni(II) hydride species released H2 upon photolysis and regenerated the Ni(I) complex (PriDPDBFphos)NiCl. These results suggest that simple H2 production involving a Ni(II)/Ni(I) redox couple is possible.189

image file: c7cs00216e-s28.tif
Scheme 28 Synthesis of (PriDPDBFphos)NiCl.188

Hor and coworkers reported the synthesis and structure of the three-coordinate Ni(I) complex, (FcNP)NiCl, supported by the ferrocendiyl iminophosphane ligand, {η5-C5H4CH[double bond, length as m-dash]N(C6H5)}Fe{η5-C5H4PBut2} (FcNP), which was synthesized by the reaction of (FcNP)NiCl2 with MeLi.190 This Ni(I) complex, along with a Ni(II) complex, (FcNP)NiCl2 and a Ni(0) complex, (FcNP)Ni(CNBut)3, catalyze ethylene oligomerization with MAO (methylaluminoxane) as a cocatalyst. Remarkably, the Ni(I) complex has the highest catalytic activity, although the 1-butene selectivity for the Ni(II) complex is higher. A related Ni(I) complex, (η2-CN-FcNP)NiCl, can also be synthesized from the reaction of either (FcNP)NiCl2 or (FcNP)NiCl complex with AlMe3 or MAO (Scheme 29). The solid state molecular structure showed a similar trigonal planar geometry, however, the N coordination has shifted to C[double bond, length as m-dash]N and thereby lengthened the C[double bond, length as m-dash]N distance (1.288(3) to 1.435(9) Å) (Fig. 34). Similarly, Walther and coworkers reported the synthesis and characterization of two three- and four-coordinate Ni(I) complexes stabilized by 1,1′-bisphosphinoferrocene ligands, (dippf)NiCl (dippf = 1,1′-bis(diisopropylphosphino)ferrocene) and (dtbpf)Ni(acac)2 (dtbpf = 1,1′-bis(di-tert-butylphosphino)ferrocene).191 The dppf ligand was independently used by the groups of Schoenebeck192,193 and Hazari194,195 as a supporting ligand for the investigation of nickel-catalyzed organic transformations such as trifluoromethylthiolation of aryl chlorides and Suzuki–Miyaura cross-coupling reactions. The dppf-supported NiICl complex, (dppf)NiCl (dppf) = 1,1′-bis(diphenylphosphino)ferrocene), could be synthesized either by an oxidative addition reaction of Ni(COD)2, dppf (generating (dppf)Ni(COD)) and chlorobenzene in toluene or from a comprotionation reaction between (dppf)Ni(C2H4) and (dppf)NiCl2.192,194 In a follow-up study, the monomeric (dppf)NiBr and dimeric [(dppf)Ni(μ-I)]2 were prepared in an analogous manner by Schoenebeck and coworkers.193 Notably, (dppf)NiCl was found to be catalytically active in a Suzuki–Miyaura cross-coupling reaction between 2-chloronaphthalene and 4-methoxyphenylboronic acid (Scheme 30).

image file: c7cs00216e-s29.tif
Scheme 29 Synthesis of (FcNP)NiCl and (η2-CN-FcNP)NiCl.190

image file: c7cs00216e-f34.tif
Fig. 34 The molecular structures of (FcNP)NiCl and (η2-CN-FcNP)NiCl with thermal ellipsoids set at 50%. Hydrogens atoms are not shown. Selected bond distances (Å) and angles (°): (FcNP)NiCl: Ni(1)–N(1) 1.995(2), Ni(1)–P(1) 2.2251(8), Ni(1)–Cl(1) 2.1929(9), N(1)–C(1) 1.288(3), N(1)–Ni(1)–P(1) 112.34(6); (η2-CN-FcNP)NiCl: Ni(1)–C(1) 1.887(8), Ni(1)–N(1) 1.963(6), Ni(1)–P(1) 2.169(2), Ni(1)–Cl(1) 2.231(2), N(1)–C(1) 1.435(9), N(1)–Ni(1)–P(1) 145.91(18).190

image file: c7cs00216e-s30.tif
Scheme 30 Suzuki–Miyaura cross-coupling reaction catalyzed by (dppf)NiCl.194

Another commonly employed class of bidentate ligands in inorganic and organometallic chemistry is that of the bulky β-diketiminates (Fig. 35).196,197 They have been shown to stabilize numerous low oxidation state transition metal complexes. The first Ni(I) complex of a bulky β-diketiminate ligand, NNBut,Pri (HC(CButNC6H3-2,6-Pri2)2), was reported by Holland and coworkers. It arose during attempts to synthesize a three-coordinate Ni(II) alkyl complex from NNBut,PriNiCl and MeLi (or MeMgCl).198 Instead of the desired methyl derivative, reduction of Ni(II) to Ni(I) occurred and X-ray crystallography revealed the structure of the product to be NNBut,PriNi(THF) (Scheme 31), which is in sharp contrast to the facile syntheses of the corresponding iron and cobalt methyl species, NNBut,PriMMe (M = Fe, Co), via the same route.199 The solution magnetic moment of 2.0 μB and a rhombic X-band EPR signal (g = 2.07, 2.11, 2.51) support a +1 oxidation state for nickel. An analogous reaction using a less bulky ligand, NNMe,Pri (HC(CMeNC6H3-2,6-Pri2)2), between NNMe,PriNiCl and MeLi in the presence of excess CO in Et2O afforded three-coordinate complex NNMe,PriNiCO with T-shaped geometry at nickel (Scheme 31).200 The EPR spectrum is slightly rhombic with g = 2.19, 2.17 and 2.01. The striking T-shaped coordination features very different N–Ni–C angles of 104.6(1)° and 158.9(1)°, in contrast to the Y-shaped geometry of NNBut,PriNi(THF), which should be favored because of steric bulk of the β-diketiminate ligand. However, DFT calculations (ROB3LYP/CEP-31G(d)) showed that the T-shaped geometry is energetically favored by 7.5 kcal mol−1 over the Y-shaped geometry.200 Jin and coworkers described a similar β-diketiminate Ni(I) complex, NNMe,PriNiPPh3, which was synthesized by a one pot reaction of NNMe,PriLi (from NNMe,PriH and BunLi) and trans-Ni(PPh3)2(Ph)Cl (Scheme 31).201 Two similar complexes, NNMe,PriNi(DMAP) and NNMe,PriNi(NCEt) were obtained from the reaction between the Ni(II) hydride complex, [NNMe,PriNi(μ-H)]2 (from NNMe,PriNiBr2Li(THF)2 and NaBEt3H), and DMAP or EtCN with the elimination of H2(g) (Scheme 31).202 Warren and coworkers also reported the syntheses of a variety of Ni(I) complexes stabilized by NNMe,Me2 (HC(CMeNC6H3-2,6-Me2)2) and NNMe,Me3 (HC(CMeNC6H2-2,4,6-Me3)2).203 The three-coordinate NiI–lutidine complexes, NNMe,Me2Ni(2,4-lutidine) and NNMe,Me3Ni(2,4-lutidine), were prepared by H2 reduction (80 psi) of the NiII–ethyl complex, NNMe,Me2(Et)(2,4-lutidine) and NNMe,Me3Ni(Et)(2,4-lutidine) (Scheme 31). Alternatively, the complexes could be obtained from NiCl2(2,4-lutidine) and TlNNMe,Me2 (or TlNNMe,Me3) followed by reduction with 0.5% Na/Hg. The solution magnetic moment of NNMe,Me2Ni(2,4-lutidine) was 1.8 μB and EPR spectroscopy indicated a rhombic environment (g = 2.435, 2.133, 2.068), both suggest a Ni(I) complex. Addition of an organonitroso molecule, (3,5-Me2-C6H3)NO, to two equivalents of NNMe,Me2Ni(2,4-lutidine) afforded the dimeric Ni(I) complex, (NNMe,Me2Ni)2[μ-η22-ON(3,5-Me2-C6H3)].203 Reactions of NNMe,Me2Ni(2,4-lutidine) with 1-adamantylazide (N3Ad) yielded the dimeric bridged imido complex, [NNMe,Me2Ni]2(μ-NAd), whereas a monomeric Ni(III)-imide species, (NMe,Me3Ni[double bond, length as m-dash]NAd) was obtained from the reaction of the bulkier NNMe,Me3Ni(2,4-lutidine) with AdN3.204

image file: c7cs00216e-f35.tif
Fig. 35 β-Diketiminate (Nacnac) ligands used in stabilizing Ni(I) complexes.196,197

image file: c7cs00216e-s31.tif
Scheme 31 Synthesis of three-coordinate Ni(I) complexes supported by β-diketiminate ligands.199–203

Stephan and coworkers reported the synthesis of a dimeric half-sandwich Ni(I)–toluene complex, (NNMe,PriNi)2(μ-η33-C7H8) (Fig. 36), by the reduction of the “ate” complex NNMe,PriNiBr2Li(THF)2 with K/Na alloy or MeMgBr.205 Performing the reduction of NNMe,PriNiBr2Li(THF)2 in the absence of toluene solvent yielded different results. Reaction of NNMe,PriNiBr2Li(THF)2 or NNMe,EtNiBr2Li(THF)2 with KC8 in hexane afforded two dimeric Ni(I) complexes, [NNMe,PriNi]2 and [NNMe,EtNi]2 (Fig. 36).206 In contrast, performing the same reaction in Et2O afforded the dimeric Ni(I) species, [NNMe,PriNi(μ-Br)Li(THF)2]2. Of note, when dissolving [NNMe,PriNi(μ-Br)Li(THF)2]2 in hexane it slowly converts to [NNMe,PriNi]2 with the formation of a white solid, presumably LiBr. Dissolving [NNMe,PriNi]2 or [NNMe,PriNi(μ-Br)Li(THF)2]2 in toluene immediately formed the original half-sandwich NiI–toluene complex. In contrast, Driess, Limberg and coworkers showed that the use of the bulky NNBut,Pri ligand allowed the isolation of a monomeric NiI–toluene complex, (NNBut,Pri)Ni(η2-C6H5Me) (Fig. 36).206 (NNMe,PriNi)2(μ-η33-C7H8) was described as having two Ni(II) ions and a reduced dianionic toluene ligand based on its structural parameters and diamagnetism. However, in frozen toluene solution (77 K) its EPR spectrum exhibited a rhombic signal (g = 2.14, 2.17, 2.46) that is indicative of a Ni(I) species. This result suggests that Ni(I) is accessible in solution, and this possibility is further supported by the formation of a series of Ni(I) species and when (NNMe,PriNi)2(μ-η33-C7H8) was reacted with a variety of donor ligands (PhCN, CH2(PPh2)2, PCy3, PH2Ph, PHPh2, OCPh2, PhC[triple bond, length as m-dash]CPh, Me3SiC[triple bond, length as m-dash]CSiMe3, Ph2CCH2, C5H4CMe2) (Scheme 32), a diverse array of complexes was formed.205,207,208 As a result, it can be alternatively described as a toluene-masked Ni(I) complex. Reactions of (NNMe,PriNi)2(μ-η33-C7H8) with two aryl azides, DippN3 and DmpN3, formed putative Ni(III)–imide intermediates that subsequently converted to Ni(II)–ketimide species.208 This result is in sharp contrast to Warren's isolable Ni(III)–imide complex, NNMe,Me3Ni[double bond, length as m-dash]NAd.203 Furthermore, this Ni(I) species was used by the groups of Driess and Limberg as a precursor for reactions with H2, N2, P4, O2, S8, Se and Te (Scheme 32).206,209–222 For example, two heterobinuclear P4 complexes with a side-on coordination of a P–P bond to Ni(I) center were prepared by heating (NNMe,RNi)2(μ-η33-C7H8) (R = Pri, Et) with (NN′Me,PriSi)P4 (NN′Me,Pri[double bond, length as m-dash]CH[(C[double bond, length as m-dash]CH2)CMe][N-(2,6-Pri2C6H3)]2) (Fig. 37 and Scheme 33).209 Addition of P4 to (NNMe,RNi)2(μ-η33-C7H8) (R = Pri, Et) afforded dimers that contain the neutral η3-coordinated P4 ligand, (NNMe,RNi)2(μ-η33-P4) (Fig. 37, Scheme 33).216 The first NiI–dinitrogen complex, (NNBut,Pri)Ni(μ-η11-N2)Ni(NNBut,Pri), was obtained from the reduction of NNBut,PriNiBr with KHBEt3, presumably via a NiII–hydride intermediate (Scheme 34).210 The N–N bond distance of 1.120(4) Å is indicative of a slightly weakened N–N bond (N[triple bond, length as m-dash]N bond distance in free N2 is 1.0975(1) Å).223 Alternatively, the N2 complex can be prepared with KC8 as a reducing agent under N2. In contrast, the use of the less bulky NNMe,Pri ligand under the similar reaction conditions yielded the bridged NiII–hydride dimer [NNMe,PriNi(μ-H)]2. The N2 ligand is labile, since dissolution of [NNBut,PriNi]2(μ-η11-N2) in Et2O quickly formed NNBut,PriNi(OEt2) with N2 evolution. Two dimeric anionic NiI–alkyl and aryl thiolates, (K·OEt2)(K)[NNBut,PriNi(SEt)2] and (K·OEt2)2[NNBut,PriNi(SPh)2], were isolated via chemical reduction with KC8 of their parent Ni(II) thiolates.221

image file: c7cs00216e-f36.tif
Fig. 36 The molecular structures of (NNMe2,PriNi)2(μ-η33-C7H8) (top-left), NNBut,PriNi(η2-C7H8) (top-right), [NNMe,PriNi]2 (bottom-left) and [NNMe,EtNi]2 (bottom-right) with thermal ellipsoids set at 50%. Hydrogens atoms and disordered toluene methyl group are not shown.205,206

image file: c7cs00216e-s32.tif
Scheme 32 Diverse reactivity of Stephan's (NNMe,PriNi)2(μ-η33-C7H8) complex that provide access to a range of Ni(I) and Ni(II) complexes.205,207,208

image file: c7cs00216e-f37.tif
Fig. 37 The molecular structures of (NNMe,PriNi)(μ-η22-P4)(SiNN′Me,Pri) (left) and (NNMe,RNi)2(μ-η33-P4) (right) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): (NNMe,PriNi)(μ-η22-P4)(SiNN′Me,Pri): Ni(1)–P(3) 2.255(4), Ni(1)–P(4) 2.277(2), Si(1)–P(1) 2.215(3), Si(1)–P(2) 2.205(3), Si(1)–N(1) 1.800(3), Si(1)–N(2) 1.752(4), Ni(1)–N(3) 1.877(3), Ni(1)–N(4) 1.904(3), P(3)–Ni(1)–P(4) 62.03(9), N(3)–Ni(1)–N(4) 96.6(2), P(1)–Si(1)–P(2) 89.87(9), N(1)–Si(1)–N(2) 101.3(2).; (NNMe,RNi)2(μ-η33-P4): Ni(1)–P(1) 2.195(1), Ni(1)–P(2) 2.339(1), Ni(1)–P(2A) 2.217(1), Ni(1)–N(1) 1.947(3), Ni(1)–N(2) 1.968(3), N(1)–Ni(1)–N(2) 95.0(1).209,216

image file: c7cs00216e-s33.tif
Scheme 33 Reactions of (NNMe,PriNi)2(μ-η33-C7H8) or (NNMe,EtNi)2(μ-η33-C7H8) with (NN′Me,PriSi)P4 or P4.209,216

image file: c7cs00216e-s34.tif
Scheme 34 Synthesis of the first NiI–N2 complex, (NNBut,Pri)Ni(μ-η11-N2)Ni(NNBut,Pri).210

The related bulky anilide–imine, sulfonamide–imine, amindinate, guanidinate and α-diimine ligands can also stabilize three-coordinate Ni(I) species. Two Ni(I) complexes, supported by bulky anilide–imine ligands, (DippN[double bond, length as m-dash]CHC6H4NDipp)NiIPPh3 and (DmpN[double bond, length as m-dash]CHC6H4NDipp)NiIPPh3, were reported by Jin and coworkers.224 They were synthesized using the in situ generated lithium salt, (DippN[double bond, length as m-dash]CHC6H4NDipp)Li, which was subsequently reacted with trans-Ni(PPh3)2PhCl (Scheme 35). Instead of the expected Ni(II) complex, reduction occurred and the corresponding Ni(I) complex was isolated in ca. 60% yield. Notably, biphenyl was formed (detected by GC-MS) during the reaction. They also showed that these complexes could catalyze norbornene polymerization with high catalytic activity. A closely related Ni(I) complex supported by a sulfonamide–imine ligand, (o-C(H)NDipp-C6H4NSO2Mes)NiIPPh3, was synthesized in a similar fashion (Scheme 35).225

image file: c7cs00216e-s35.tif
Scheme 35 Synthesis of three Ni(I) complexes supported by anilide–imine and sulfonamide–imine ligands.224,225

Eisen and coworkers reacted the nickel(II) N,N′-bis(trimethylsilyl)benzamidinate acetylacetonate, {PhC(NSiMe3)2}Ni(acac), with MeLi/LiBr in Et2O at −78 °C.226 However, reduction occurred and orange crystals of amidinate-bridged Ni(I) dimer were formed in almost quantitative yield. The crystal structure of the dimer showed that the amidinate ligand changed from a chelating mode in {PhC(NSiMe3)2}Ni(acac) to a bridging mode in [Ni{PhC(NSiMe3)2}]2 which has a short Ni–Ni bond distance of 2.2938(12) Å (Fig. 38). However, the dimer [Ni{PhC(NSiMe3)2}]2 was found to be unstable in solution and it slowly disproportionated to metallic nickel and the four-coordinate Ni(II) complex [Ni{PhC(NSiMe3)2}]2. In a related study, a structurally similar bulky guanidinate-bridged orange Ni(I) dimer, [Ni(μ-κ2-N,N′-Priso)]2 (Priso = (DippN)2CNPri2), along with a isomeric brown Ni(I) dimer, [Ni(μ-κ1-N-,η2-Dipp-Priso)]2, were isolated as mixtures by Jones and coworkers from the reduction with potassium and the guanidinato-NiII-bromide dimer, [Ni(Priso)(μ-Br)]2 in cyclohexane (Scheme 36).227 Attempted reduction using MeLi in a similar way to the reactions of β-diketiminato-NiII-halide and amidinato-NiII-acetoacetonate did not afford an isolable Ni(I) complex. It is noteworthy that [Ni(μ-κ1-N-,η2-Dipp-Priso)]2 slowly converts to [Ni(μ-κ2-N,N′-Priso)]2 in solution, suggesting that the former is the kinetic product and the latter is the thermodynamic product. The Ni–Ni bond distance of 2.2908(11) Å is almost identical to that in [Ni{PhC(NSiMe3)2}]2 (Fig. 38). Interestingly, reduction of [Ni(Priso)(μ-Br)]2 in benzene or toluene afforded the arene-bridged Ni(I) dimer, [Ni(Priso)]2(arene) (arene = benzene, toluene). Similar to Stephan's β-diketiminate Ni(I) toluene complex, it is also labile and is converted to mixtures of [Ni(μ-κ1-N-,η2-Dipp-Priso)]2 and [Ni(μ-κ2-N,N′-Priso)]2 upon standing in hexane solution in less than 24 h. [Ni(μ-κ2-N,N′-Priso)]2 is stable in both solution and solid state (up to 220 °C), in sharp contrast to the instability of Eisen's Ni(I) complex, [Ni{PhC(NSiMe3)2}]2. The reactivity of [Ni(μ-κ2-N,N′-Priso)]2 was explored, treatment of this complex with N3SiMe3 or CO afforded the dimeric NiII–azide complex, [Ni(Priso)]2(μ-N3)2, and the dimeric NiI–CO complex, [Ni(Priso)]2(μ-CO)2, which has a NiI–NiI distance of 2.437(1) Å. The α-diimine ligand, [(2,6-Pri2C6H3)NCMe]2 (NNMe,Dipp), can be used as a neutral ligand to support two four-coordinate Ni(I) complexes, [(NNMe,Dipp)Ni]2(μ-Br)2 and [(NNMe,Dipp)NiBr]2[μ-MgBr2(THF)2], that were described earlier, and it can also be reduced to a radical anion (NNMe,Dipp˙) to stabilize a dimeric Ni(I) complex. Yang and coworkers reduced Brookhart's complex (NNMe,Dipp)NiBr2134 with four equivalents of sodium to obtain [Ni(μ-NNMe,Dipp˙)]2, which features a short Ni–Ni bond length of 2.2957(6) Å. It can be further reduced with one, two or three equivalents of Na to afford mixed oxidation state Ni0/NiI complexes.228 They also showed that (NNMe,Dipp)NiBr2 reacted with two equivalents of NaH to yield the NiII–hydride complex, (NNMe,Dipp˙)Ni(μ-H)2Ni(NNMe,Dipp˙). Further reduction with one, two or four equivalents of Na or K yielded mixed oxidation state Ni(I)/Ni(II)–hydride or dimeric Ni(II)–hydride complexes.229

image file: c7cs00216e-f38.tif
Fig. 38 Drawing (left) and the molecular structures (right) of the Ni(I) amidinate dimer [Ni{PhC(NSiMe3)2}]2 and the guanidinate dimer [Ni(μ-κ2-N,N′-Priso)]2 with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): [Ni{PhC(NSiMe3)2}]2: Ni(1)–Ni(2) 2.2938(12), Ni(1)–N(1) 1.876(5), Ni(1)–N(3) 1.874(5), Ni(2)–N(2) 1.868(5), Ni(2)–N(4) 1.876(5), N(1)–Ni(1)–N(3) 179.6(2), N(2)–Ni(2)–N(5) 179.3(2); [Ni(μ-κ2-N,N′-Priso)]2: Ni(1)–Ni(2) 2.2908(11), Ni(1)–N(4) 1.858(4), Ni(1)–N(1) 1.873(4), Ni(2)–N(5) 1.866(5), Ni(2)–N(2) 1.873(4), N(1)–Ni(1)–N(4) 179.17(18), N(2)–Ni(2)–N(5) 179.32(18).226,227

image file: c7cs00216e-s36.tif
Scheme 36 Synthesis of [Ni(Priso)]2(arene) (arene = benzene, toluene), [Ni(μ-κ2-N,N′-Priso)]2 and [Ni(μ-κ1-N-,η2-Dipp-Priso)]2.227

Eaborn, Smith, and coworkers used the bidendate alkyl ligand –C(SiMe3)2(2-SiMe2C5H4N) to provide extra stability for the alkyl complexes of a number of s-, p- and f-block elements. Reaction of NiCl2(PPh3)2 and two equivalents of LiC[(SiMe3)2(2-SiMe2C5H4N)] provided access to the first Ni(I) alkyl complex, (Ph3P)Ni[C(SiMe3)(2-SiMe2C5H4N)].230 In this case, the lithium alkyl reagent both acts as a ligand and a reducing agent. The crystal structure revealed a planar (sum of angles at Ni = 360°), distorted T-shaped nickel geometry with a wide C–Ni–P angle of 155.67(11)°, with Ni–C and Ni–P distances of 2.025(4) Å and 2.200(1) Å (Fig. 39). The magnetic moment of 1.55 μB is consistent with one unpaired electron. The isolation of a Ni(I) complex from a Ni(II) starting material indicated that reduction occurred during the reaction, which is often observed in the reactions of organolithium reagents with transition metal halides.231

image file: c7cs00216e-f39.tif
Fig. 39 Drawing (left) and the molecular structure (right) of (Ph3P)Ni[C(SiMe3)(2-SiMe2C5H4N)] with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): Ni(1)–C(1) 2.025(4), Ni(1)–P(1) 2.220(1), Ni(1)–N(1) 2.007(3), P(1)–Ni(1)–C(1) 155.67(1), N(1)–Ni(1)–C(1) 98.28(14), N(1)–Ni(1)–P(1) 105.81(10).230

6.3 Monodentate ligands

The development of the chemistry of three-coordinate Ni(I) complexes can be viewed in the context of three-coordinate transition metal species in general as discussed in a number of reviews.232–234 In the early years, the development of low-coordinate Ni(I) complexes was dominated by the use of the moderately bulky triphenylphosphine ligand, PPh3. Heimbach reported the synthesis of a three-coordinate NiI–bromide complex stabilized by triphenylphosphine, NiBr(PPh3)2, along with the four-coordinate Ni(I) complexes discussed in the previous section.149 However, its molecular structural type was unknown until 2000, when X-ray crystallography showed that the related chlorides NiCl(PPh3)2·(THF)152 and NiCl(PPh3)2235 are monomeric three-coordinate Ni(I) complexes with distorted trigonal planar geometries. The first isolation and structural characterization of a three-coordinate Ni(I) complex, Ni{N(SiMe3)2}(PPh3)2 (Fig. 40), were reported by Bradley and coworkers in 1972.236 Unexpectedly, it was obtained from the reaction of LiN(SiMe3)2 with NiCl2(PPh3)2. A room temperature magnetic susceptibility value of 1.91 μB indicated the presence of one unpaired electron confirming its +1 oxidation state. A cationic Ni(I) complex, [Ni(PPh3)3](BF4)·BF3·OEt2, was obtained by Saraev and coworkers from the reaction of Ni(PPh3)4 with four equivalents of BF3·OEt2.237 The dynamic magnetic properties of NiCl(PPh3)2·(THF) and Ni{N(SiMe3)2}(PPh3)2 were investigated later by Fink, Eichhöfer and coworkers and they showed that these three-coordinate Ni(I) complexes exhibit slow magnetic relaxation under an applied magnetic field at low temperature.238
image file: c7cs00216e-f40.tif
Fig. 40 The molecular structure of Ni{N(SiMe3)2}(PPh3)2 of Bradley and coworkers with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): Ni(1)–P(1) 2.220(4), Ni(1)–P(2) 2.213(4), Ni(1)–N(1) 1.88(1), P(1)–Ni(1)–P(2) 107.0(2).236

The bulkier phosphine PPri3 used by Johnson and coworkers also results in interesting Ni(I) chemistry.239 They showed that a novel asymmetric mixed oxidation state NiI/NiIII complex, [(Pri3P)Ni]2(μ-η2-C12H8)(μ-C12H8), could be synthesized from the reaction of Ni(COD)2, PPri3 and biphenylene.240 This complex underwent reductive C–C coupling to form the green NiI–NiI complex, [(Pri3P)Ni]2(μ-η22-C24H16), at 25 °C, presumably through reductive elimination at the Ni(III) center and subsequent C–C bond formation to form the quaterphenyldiyl ligand (Scheme 37). The related [(Et3P)Ni]2(μ-η22-C24H16)241 and [(Et3P)Ni]2(μ-η44-C36H20)242 have also been synthesized and characterized. The three-coordinate Ni(I) complexes, (Pri3P)2NiX (X = Cl, Br, I), were synthesized in high yield from the comproportionation reaction of (Pri3P)2NiX2 and (Pri3P)2Ni(η2-C2H4) (Scheme 38).243 They showed trigonal planar geometry with shorter Ni–P and Ni–X distances in comparison to those of the corresponding four-coordinate Ni(II) species, (Pri3P)2NiX2, as a result of their lower coordination number in which steric congestion is lowered. It is worth noting that this synthetic strategy is reminiscent of the first syntheses of Ni(PPh3)nX (n = 2 or 3) complexes reported by Heimbach in 1964 (Scheme 19).149 The room temperature magnetic moment (Evans’ method) of 2.05 μB (Cl), 2.14 μB (Br) and 2.19 μB (I) indicate that they are complexes of Ni(I). They were shown to display a rich reaction chemistry. Reduction of (Pri3P)2NiX with magnesium under a N2 atmosphere afforded the Ni(0) dinitrogen complex, [(Pri3P)2Ni]2(μ-η11-N2), which can further react with CO2, forming the Ni0–CO2 complex, (Pri3P)2Ni(η2-CO2) (Scheme 38). The N–N bond distance of 1.158(5) Å is consistent with a somewhat weakened N–N bond. In addition, the Ni0–N2 complex is a useful synthon to access numerous other Ni(I) species (Scheme 38). For example, it reacts with silanes such as Ph2SiH2 and Ph3SiH to form the dinuclear Ni(I) complexes, [(Pri3P)Ni]2(μ-SiHPh2)2 and [(Pri3P)Ni]2(μ-Ph)(μ-SiHPh2), which have agostic Ni–H–Si interactions (Scheme 39).244 Two similar complexes, [Ni(PCy3)]2(μ-SiHPh2)2 and [Ni(dmpe)]2(μ-SiHPh2)2 (dmpe = 1,2-bis(dimethylphosphino)ethane), were synthesized by Osakada and coworkers via mixing Ni(COD)2, H2SiPh2 and the phosphines in equimolar amounts in toluene or hexane.245 Replacing the phosphine with a N-heterocyclic carbene permits the isolation of [Ni(IPri2)]2(μ-SiHPh2)2 (IPri2 = 1,3-diisopropylimidazol-2-ylidene) from the reaction of Ni2(IPri2)4(COD), with two equivalents of Ph2SiH2.246 (Pri3P)2Ni(μ-η11-N2) also reacts with Ph2PH and PhSH to form the dinuclear four-coordinate Ni(I) complexes, [(Pri3P)Ni(μ-PPh2)]2 and [(Pri3P)Ni(μ-SPh)]2 (Scheme 38). Both display trigonal planar geometries at the metal atoms with Ni–Ni bond distances of 2.3702(4) and 2.3660(10) Å, respectively.243 A mixed oxidation states pentanuclear Ni(I)/Ni(II)–hydride cluster, [(Pri3P)2Ni]5H6, was obtained from the reaction of (Pri3P)2Ni(μ-η11-N2) with H2. When dissolved in C6D6, this cluster catalyzes H/D exchange with C6D6 to generate the isotopologue of this cluster, [(Pri3P)2Ni]5HnD6−n (n = 1–6).247 [(Pri3P)2Ni]5H6 also underwent room temperature hydrodesulfurization of dibenzothiophenes to form mixed oxidation state clusters, [(Pri3P)Ni]4(μ-H)44-S).248 The reaction of a Ni0–anthracene complex, (Pri3P)2Ni(η2-C14H10), with C6F6 afforded a mixture containing not only the oxidative addition Ni(II) product, trans-(Pri3P)2Ni(C6F5)(F), but also a unique paramagnetic three-coordinate Ni(I) aryl complex, (Pri3P)2Ni(C6F5).249

image file: c7cs00216e-s37.tif
Scheme 37 Formation of [(Pri3P)Ni]2(μ-η22-C24H16) from a mixed oxidation state NiI/NiIII complex.240

image file: c7cs00216e-s38.tif
Scheme 38 Synthesis and reactions of [(Pri3P)2Ni]2(μ-η11-N2).243,247

image file: c7cs00216e-s39.tif
Scheme 39 Synthesis of dimeric Ni(I) complexes bridged by two silyl ligands.244–246

A unique, yellow three-coordinate Ni(I) carboranyl complex Ni(PPh3)2(σ-C2B10Me8H3) was obtained in 25% yield by Xie and coworkers during attempts to synthesize metal–carboryne complexes from a salt elimination reaction between dilithiocarborane, Li2C2B10Me8H2, and NiCl2(PPh3)2. Instead, reduction occurred to generate mixtures of Ni(I) and Ni(0) species, presumably as a result of the decomposition of the Ni–carboryne species.250 Similarly, the authors noted that reactions of PdCl2(PPh3)2 or PtCl2(PPh3)2 with Li2C2B10Me8H2 led to the formation of C2B10Me8H4 and M(0).250

With regard to the four-coordinate Sigman's dimer, [(IPr)Ni(μ-Cl)]2, the related three-coordinate, monomeric Ni(I) bis-carbene chloride complex, Ni(IPr)2Cl was prepared by Matsubara and coworkers, who reacted the two-coordinate Ni(0) precursor, Ni(IPr)2, with p-chlorotoluene.251 They found that one of the IPr ligands dissociates in solution to form the dimer [(IPr)Ni(μ-Cl)]2 and free IPr. They exchanged one of the IPr ligands with PPh3, to form Ni(IPr)(PPh3)Cl as a monomeric, planar Y-shaped three-coordinate Ni(I) complex with Ni–Cl = 2.179(9) Å, Ni–P = 2.20(1) Å and Ni–C = 1.930(3) Å. The difference in geometry between Ni(IPr)2Cl (T-shaped) and Ni(IPr)(PPh3)Cl (Y-shaped) is probably due to electronic properties of the ligand rather than steric effects.252 Similarly, Louie and coworkers showed that the less bulky carbene IMes yielded similar three-coordinate Ni(I) complexes, Ni(IMes)2X (X = Cl, Br, I), via the reactions of Ni(IMes)2 and aryl halides (Scheme 40, Table 3).253 Around the same time, Ni(IMes)2Cl and Ni(IMes)2Br were prepared by Nocera and coworkers by photochemical H2 elimination (λexcitation > 295 nm) from Ni(IMes)2(H)Cl and Ni(IMes)2(H)Br, which activated the Ni–H bond via the population of the Ni–H σ* orbital (Scheme 40).254 The use of a smaller carbene IMe2Me2 (IMe2Me2[double bond, length as m-dash]1,2,3,4-tetramethylimidazol-2-ylidene) afforded the Ni(II) oxidative addition product Ni(IMe2Me2)2(Ar)Br from the reaction of Ni(COD)2 with aryl bromide and then with the IMe2Me2 ligand.253 Furthermore, the three-coordinate Ni(I) mono-carbene halide complexes could be obtained via addition of Lewis bases to Sigman's dimer, [(IPr)Ni(μ-Cl)]2. For example, addition of PPh3, POPh3 and pyridine to [(IPr)Ni(μ-Cl)]2 afforded Ni(IPr)Cl(PPh3), Ni(IPr)Cl(POPh3) and Ni(IPr)Cl(py), respectively.255 On the other hand, addition of a bidentate phosphine such as dppe and dppp (1,3-bis(diphenylphosphino)propane) did not afford the desired Ni(I) complexes. Instead, the Ni(0) complexes, Ni(dppe)2 or Ni(dppp)2, along with the Ni(II) complex Ni(IPr)2Cl2 (detected by NMR) were obtained. When dppb (1,4-bis-(diphenylphosphino)butane) was used, a dimeric Ni(I) complex, [Ni(IPr)Cl]2(μ-dppb), could be isolated, although it slowly disproportionated into Ni(dppb)2 and Ni(IPr)2Cl2. In the case of less bulky IMes, comproportionation reaction between Ni(COD)2 and Ni(PPh3)2X2 (X = Cl, Br) was used to obtain Ni(IMes)(PPh3)X. The crystal structures of these three-coordinate carbene complexes displayed monomeric, planar, T-shaped nickel geometries with similar Ni–C and Ni–Cl distances, indicating that, within limits, the size and shape of the ligand have little effect on the structure. Furthermore, the catalytic ability of these three-coordinate Ni(I) species was explored. Ni(IPr)2Cl was found to catalyze cross-coupling reactions of aryl halides with phenylmagnesium chloride.251 Ni(IPr)(PPh3)Cl was also shown to be a catalyst for Buchwald–Hartwig amination of aryl halides and diarylamines under mild conditions.252 The Ni(IMes)2X (X = Cl, Br) species were also shown to be effective in catalyzing Kumada and Suzuki coupling reactions (Scheme 41).253

image file: c7cs00216e-s40.tif
Scheme 40 Reactions of Ni(IMes)2 with halobenzenes and lutidinium halides and subsequent photolysis to generate Ni(I) complexes.253,254
Table 3 Structurally characterized three-coordinate NiI(NHC)2X complexes with selected bond distances (Å) and angles (°)
Complex Ni–C(1) Ni–C(2) C–Ni–C Ni–L Ref.
Ni(IPr)2Cl 1.952(3) 1.943(3) 168.18(5) 2.2966(12) 251
Ni(IMes)2Cl 1.911(2) 1.922(2) 166.47(10) 2.192(9) 253 and 254
Ni(IMes)2Br 1.913(4) 1.929(4) 166.46(16) 2.4428(6) 253 and 254
Ni(IMes)2I 1.924(3) 1.923(3) 168.26(12) 2.6084(1) 253

image file: c7cs00216e-s41.tif
Scheme 41 Selected (NHC)NiI-catalyzed organic transformations. (a) Kumada–Corriu cross-coupling catalyzed by Ni(IPr)2Cl (b) Buchwald–Hartwig amination of aryl halides catalyzed by Ni(IPr)(PPh3)Cl (c) Kumada cross-coupling reactions catalyzed by Ni(IMes)2X (X = Cl, Br) (d) Suzuki cross-coupling reactions catalyzed by Ni(IMes)2X (X = Cl, Br).251–253

Whittlesey and coworkers employed several six-, seven- and eight-membered ring NHCs (ring-expanded NHCs, RE-NHCs, Fig. 41) to support three-coordinate Ni(I) halide complexes, (RE-NHC)Ni(PPh3)X (X = Cl or Br).256,257 These complexes were prepared by similar comproportionation reaction of Ni(PPh3)2Br2 and Ni(COD)2 in the presence of two equivalents of the carbene (Scheme 42). X-ray crystal structures of the eight complexes that were synthesized showed that, although the carbenes have different ring sizes, they adopt similar trigonal planar geometries. Their catalytic abilities towards Kumada cross-couplings of aryl chlorides and aryl fluorides were also examined, which showed highest activity for smaller ring carbene systems.

image file: c7cs00216e-f41.tif
Fig. 41 RE-NHCs used to stabilize Ni(I) complexes.256,257

image file: c7cs00216e-s42.tif
Scheme 42 A representative synthesis of (RE-NHC)Ni(PPh3)X (X = Cl, Br).256,257

7. Two-coordinate Ni(I)

Two-coordination is the lowest coordination number generally known for transition metal complexes.258–260 Extremely bulky ligands are usually required to stabilize such complexes in order to avoid their association or decomposition. Structurally characterized two-coordinate Ni(I) species (Table 4) have generally been much rarer than their higher coordinate counterparts and only four stable Ni(I) complexes were listed in a 2012 review259 of two-coordinate, open-shell (d1–d9) transition metal complexes (cf. 18 complexes in Table 4). However, they are of increasing interest for several reasons, that include small molecule activation, catalysis, magnetism. Some newer species are complexed by neutral bulky donor ligand such as carbenes and phosphines in addition to a monoanionic amido, thiolato, alkyl and aryl ligand.
Table 4 Structurally characterized two-coordinate Ni(I) complexes with selected bond distances (Å), angles (°), and magnetic moments
Complex M–L (Å) L–M–L (°) μeff Ref.
(IPr)Ni{N(H)Dipp} 1.831(4), 1.806(4) (N); 1.878(5), 1.860(5) (C) 163.2(2), 167.4(2) 2.3 261
(IPr)Ni{N(SiMe3)2} 1.865(2) (N), 1.879(2) (C) 178.7(8) 1.9 261
(IPr)NiArMe6 1.923(2) (CNHC), 1.944(2) (Caryl) 175.97(8) 1.80 262
(IPr)NiCH(SiMe3)2 1.910(2) (CNHC), 1.968(2) (Calkyl) 174.8(1) 1.9 262
Ni(IMes)SArMe6 2.2424(7) (S), 1.935(2) (C) 163.27(16) 263
Ni(PPh3)SArMe6 2.2378(7) (S), 2.2034(9) (P) 107.30(3) 1.93 263
[Ni(μ-NPBut3)]4 1.864(4)–1.876(4) 178.31(1)–179.6(3) 4.40 266
[NBun4][Ni{N(SiMe3)Dipp}2] 1.8437(17), 1.8516(17) 176.51(8) 2.03 267
K[Ni{N(SiMe3)Dipp}2] 1.8436(15) 178.05(9) 1.66 267
(IPr)Ni{N(SiMe3)Dipp}2 1.8271(2) (N), 1.9123(2) (C) 173.01(7) 2.12 267
[K(18-crown-6)][Ni{N(SiMe3)Dipp}2] 1.8493(9) 180 2.14 268
(But3P)Ni{N(SiMe3)Dipp}2 1.8250(2) (N), 2.2006(1) (P) 165.6(1) 2.35 269
(Pri3P)Ni{N(SiMe3)Dipp}2 1.8407(2) (N), 2.1992(7) (P) 164.09(6) 2.55 269
(IPr)Ni(BHT) 1.863(3), 1.875(3), 1.880(2) (C); 1.7612(19), 1.778(2), 1.8374(17) (O) 162.19(9), 168.99(11), 173.3(1) 1.80 269
[Ni{N(SiMe3)2}]4 1.9127(2), 1.9151(2), 1.9166(2), 1.9189(2) 168.80(4), 168.90(4) 2.70 272
[Ni(6-Mes)2]Br 1.939(3), 1.941(3) 179.27(13) 2.2, 2.99 291
(IPr)Ni[N(But)CO(But)] 1.9149(8) (N), 1.9119(9) (C) 172.12(4) 1.87 292
(IPr)Ni[N(Pri)CO(Pri)] 1.85(2) (N), 1.894(4) (C) 169.0(5) 2.20 292

In 2008 Hillhouse and coworkers pioneered this area by reporting the first two-coordinate Ni(I) complexes via the reaction of Sigman's dimer, [(IPr)Ni(μ-Cl)]2, with either two equivalents of NaN(SiMe3)2 or LiN(H)Dipp which afforded the analytically pure, yellow Ni(I) complexes, (IPr)Ni{N(SiMe3)2} and (IPr)Ni{N(H)Dipp}.261 The magnetic moments determined by the Evans’ method were 1.9 μB and 2.3 μB, respectively, and indicate the presence of one unpaired electron and therefore, a Ni(I) oxidation state. (IPr)Ni{N(SiMe3)2} has an almost linear geometry with C–Ni–N = 178.7(8)°, whereas (IPr)Ni{N(H)Dipp} displays significant bending with C–Ni–N = 163.2(2)° and 167.4(2)°. These compounds were the first structurally characterized two-coordinate Ni(I) complexes. Later they reported the corresponding two-coordinate Ni(I) alkyl and aryl complexes by reacting [(IPr)Ni(μ-Cl)]2 with two equivalents of ClMgCH(SiMe3)2 or LiArMe6 to afford (IPr)NiCH(SiMe3)2 and (IPr)NiArMe6 (Scheme 43).262 The nickel atoms have a slightly bent coordination with N–Ni–C of 174.8(1)° for (IPr)NiCH(SiMe3)2 and 175.97(8)° for (IPr)NiArMe6 (Fig. 42). Solution magnetic properties (1.90 μB for (IPr)NiCH(SiMe3)2 and 1.80 μB for (IPr)NiArMe6) also indicate a +1 oxidation state for the metal. DFT calculations (B3LYP/LANL2DZ) confirmed the +1 oxidation state assignment, with the unpaired electron localized on the 3dz2–4s hybrid orbital of the nickel atom. Reaction of a Lewis base such as tert-butylisocyanide (ButNC) with (IPr)NiCH(SiMe3)2 afforded the orange three-coordinate Ni(I) complex, (IPr)Ni(CNBut){CH(SiMe3)2}.

image file: c7cs00216e-s43.tif
Scheme 43 Synthesis of carbene-stabilized Ni(I) complexes.261,262

image file: c7cs00216e-f42.tif
Fig. 42 The molecular structures of (IPr)NiCH(SiMe3)2 (left) and (IPr)NiArMe6 (right) with thermal ellipsoids set at 50%. Hydrogen atoms (except C–H of CH(SiMe3)2) are not shown. Selected structural data are listed in Table 4.262

Tatsumi and coworkers synthesized two-coordinate heteroleptic Ni(I) complexes supported by m-terphenyl thiolate and carbene/phosphine ligands. Ni(PPh3)SArMe6 was synthesized via the reaction of a three-coordinate Ni(I) starting material, Ni(PPh3)2{N(SiMe3)2}, with a bulky m-terphenyl thiol, ArMe6SH.263,264 A subsequent exchange reaction with a bulky NHC yielded (IMes)NiSArMe6. On the other hand, an exchange reaction with a less bulky carbene, IMe2Me2, yielded the dinuclear four-coordinate Ni(I) complex, [(IMe2Me2)Ni]2(μ-SArMe6)2, with a NiI–NiI distance of 2.3941(13) Å. The bulkier carbene ligand affords a more linear geometry with C–Ni–S = 163.27(16)° and a short Ni–Caryl interaction of 2.090(5) and 2.098(5) Å, whereas the less bulky PPh3 has a bent structure with C–Ni–P = 107.30(3)° and also a strong Ni–Caryl interaction of 2.129(3) and 2.147(3) Å. The use of less bulky thiols led to dimeric Ni(I) complexes with higher coordination numbers. For example, reactions of Ni(PPh3)2{N(SiMe3)2} with TripSH (Trip = 2,4,6-triisopropylphenyl), 1-AdSH or ArMe4SH (ArMe4 = C6H3-2,6(C6H3-2,6-Me2)2) afforded symmetric [(Ph3P)Ni(μ-STrip)]2, [(Ph3P)Ni{μ-S(1-Ad)}]2 and asymmetric (ArMe4S)Ni(μ-SArMe4)Ni(PPh3) complexes with NiI–NiI bond lengths of 2.3353(4), 2.3510(3) and 2.4238(4) Å, respectively. In addition, Ni(PPh3)SArMe6 is a good synthon that provides entry into Ni(I) complexes with a (ArMe6S)NiI fragment. The reaction between Ni(PPh3)SArMe6 and CNBut led to the formation of the dimeric Ni(I) complex, [(ButNC)Ni]2(μ-SArMe6)2, with a NiI–NiI distance of 2.3354(8) Å. Furthermore, the two-coordinate Ni(PPh3)SArMe6 was shown to be a useful synthon as a low-coordinate (ArMe6S)NiI source for bioinorganic chemistry. To model the active site of acetyl CoA synthase (ACS, Fig. 4), a complex, Ni(dadtEt)Ni(SArMe6)(PPh3) (dadtEt = N,N′-diethyl-3,7-diazanonane-1,9-dithiolate), that has mixed Ni(II)/Ni(I) oxidation state, was prepared by the reaction of Ni(dadtEt) with Ni(PPh3)SArMe6 (Scheme 44).265 The X-ray crystal structure showed that the Ni(I) atom has a distorted tetrahedral coordination. The magnetic moment of 2.21–2.47 μB is consistent with one unpaired electron and the rhombic EPR signal (g1 = 2.62, g2 = 2.12, g3 = 2.00) at 77 K in frozen toluene solution is consistent with a nickel-based radical.

image file: c7cs00216e-s44.tif
Scheme 44 Synthesis of Ni(dadtEt)Ni(SArMe6)(PPh3). The molecular structure of Ni(dadtEt)Ni(SArMe6)(PPh3) with thermal ellipsoids set at 50%. Hydrogen atoms and cocrystallized toluene molecule are not shown.265

In search of hydrocarbon-soluble metal complexes with catalytic ability, Stryker and his group used the bulky tris(tert-butyl)phosphoranimido ligand to support tetrametallic Co(I) and Ni(I) complexes, [M(μ-NPBut3)]4 (M = Co, Ni), which were synthesized in high yields by the chemical reduction of the metal(II) halide dimer, [MX(μ-NPBut3)(THF)]2 (M = Co, X = Cl; M = Ni, X = Br).266 The solid state structure of [Ni(μ-NPBut3)]4 has a coplanar tetrametallic framework, with each Ni(I) atom bonded to two bridging –NPBut3 ligands with almost linear N–Ni–N angles ranging from 178.31(18) to 179.6(3)° (Fig. 43). The Ni–N bond distances of 1.864(4)–1.876(4) Å are comparable to those in other structurally characterized Ni(I) complexes. The magnetic susceptibility measured at 27 °C was 4.40 μB, which suggests that it is a 3.50-electron paramagnet, though a full electronic and magnetic studies would be required to better understand these results. Moreover, they found that this complex and the cobalt analog catalyze (0.5 mol %) hydrogenation reactions of a terminal alkene and an internal alkyne under 1 atm H2 to afford alkanes in quantitative yield (Scheme 45).

image file: c7cs00216e-f43.tif
Fig. 43 The molecular structures of the tetrameric [Ni(μ-NPBut3)]4 (left) and [Ni{N(SiMe3)2}]4 (right) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown.266,272

image file: c7cs00216e-s45.tif
Scheme 45 Catalytic hydrogenation reactions using [M(μ-NPBut3)]4 (M = Co, Ni) as catalyst.266

Another route to two-coordinate Ni(I) complexes is the chemical reduction of the neutral two-coordinate Ni(II) complexes. Tilley and coworkers discovered that when heating Ni{N(SiMe3)Dipp}2 in the presence of IPr, a two-coordinate Ni(I) complex, (IPr)Ni{N(SiMe3)Dipp}2 could be isolated.267 This result suggested interesting redox properties for Ni{N(SiMe3)Dipp}2. Reduction of the strictly linear two-coordinate Ni{N(SiMe3)Dipp}2 with KC8 afforded K[Ni{N(SiMe3)Dipp}2], in which the K+ ion is sandwiched between two aryl (Dipp) groups.267 Subsequent cation exchange with NBun4Br cleanly generates [NBun4][Ni{N(SiMe3)Dipp}2] (Scheme 46). Around the same period, Power and coworkers reduced M{N(SiMe3)Dipp}2 (M = Fe, Co, Ni) with KC8 in the presence of 18-crown-6 to isolate crystals of [K(18-crown-6)][M{N(SiMe3)Dipp}2] (Scheme 46).268 X-ray crystallography revealed that the N–Ni–N angles in these anions are almost or strictly linear (180° in [K(18-crown-6)][Ni{N(SiMe3)Dipp}2], 176.51(8)° in [NBun4][Ni{N(SiMe3)Dipp}2], and 178.05(9)° in K[Ni{N(SiMe3)Dipp}2]). Magnetic moments, determined by the Evans’ method in solution or by SQUID magnetometry, confirmed the reduction from Ni(II) to Ni(I). It is worth noting that conformations of the anions are different, the two aryl groups are cis (in K[Ni{N(SiMe3)Dipp}2] and [NBun4][Ni{N(SiMe3)Dipp}2]) rather than trans (in [K(18-crown-6)][Ni{N(SiMe3)Dipp}2]) with respect to each other, suggesting that the conformation and the N–M–N angle in the solid state is flexible and also cation-dependent.

image file: c7cs00216e-s46.tif
Scheme 46 Synthesis and reactions of Ni(I) complexes supported by –N(SiMe3)Dipp ligand.267–269

The relative scarcity of two-coordinate Ni(I) complexes prompted the development of a generic method to prepare a series of Ni(I) compounds. Protonation of K[Ni{N(SiMe3)Dipp}2] using NEt3HCl in the presence of a neutral donor ligand L = PBut3, PPri3, dppe leads to the elimination of HN(SiMe3)Dipp, KCl and NEt3 and to form the neutral, two-coordinate Ni(I) complexes, (IPr)Ni{N(SiMe3)Dipp}, (But3P)Ni{N(SiMe3)Dipp}, (Pri3P)Ni{N(SiMe3)Dipp} and a three-coordinate Ni(I) complex, (dppe)Ni{N(SiMe3)Dipp} (Scheme 46).269 Furthermore, a simple ligand exchange reaction between a sufficiently bulky phenol, 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT(H)), and (IPr)Ni{N(SiMe3)Dipp} and (But3P)Ni{N(SiMe3)Dipp} resulted in the isolation of two-coordinate (IPr)Ni(BHT) and (But3P)Ni(BHT), with a dearomatized and η5-coordinated phenol ligand (Scheme 46). The two-coordinate Ni(I) species features slightly bent L–Ni–L coordination of 173.01° for (IPr)Ni{N(SiMe3)Dipp}2, 165.6(1)° for (But3P)Ni{N(SiMe3)Dipp}2, 164.09(6)° for (Pri3P)Ni{N(SiMe3)Dipp}2 and 162.19(9)–173.3(1)° for (IPr)Ni(BHT). Although magnetic susceptibilities by the Evans’ method afforded significant variations from the spin-only value (1.73 μB), they were consistent with the presence of one unpaired electron. The authors suggested that unquenched spin–orbit coupling might be the factor affecting the magnetic properties, which is common in two-coordinate transition metal complexes.260

In 1960s Bürger and Wannagat reported the synthesis of the first two- (putative) and three-coordinate transition metal complexes in their +2 oxidation states using the bulky bis(trimethylsilyl)amido ligand, –N(SiMe3)2.270,271 The Ni(II) complex, Ni{N(SiMe3)2}2, which was synthesized from NiI2 and two equivalents of NaN(SiMe3)2 in THF, was reported to be unstable and decomposed to a black solid at room temperature. In 2015, a reinvestigation of the reaction showed that the solid material produced almost black crystals from toluene which were shown to be the tetrameric Ni(I) complex, [Ni{N(SiMe3)2}]4 by X-ray crystallography (Scheme 47).272 Alternatively, this species can also be synthesized from the reaction between LiN(SiMe3)2 and NiCl2(DME) in Et2O. The structure of [Ni{N(SiMe3)2}]4 has four Ni(I) ions bridged by four –N(SiMe3)2 ligands, in an approximate square plane. The Ni(I) atoms were displaced toward each other to give Ni⋯Ni distances of 2.4328(4) and 2.4347(5) Å and non-linear N–Ni–N angles of 168.80(4) and 168.90(4)° (Fig. 43). The average Ni–N distance of 1.916 Å is longer than those in the two-coordinate Ni(I) complexes, Ni{N(SiMe3)Dipp}2 (1.8436(15)–1.8516(17) Å)267,268 and two-coordinate Ni(II) amido complexes, Ni{N(SiMe)3Dipp}2 (1.8029(9) Å),273 Ni{N(H)ArPri6}2 (1.8284(15) Å),274 Ni{N(H)ArPri4}2 (1.818(3) Å),275 Ni{N(H)ArMe6}2 (1.812(3), 1.819(3) Å),274 Ni{N(Ph)BMes2}2 (1.885(4) Å),276 Ni{N(Mes)BMes2}2 (1.865(2), 1.867(2) Å).277 But it is about the same as the bridging distance (avg. 1.911 Å) in the closely related three-coordinate Ni(II) amido dimer, [Ni(NPh2)2]2.278 SQUID magnetic measurements gave a magnetic moment of 2.70 μB at 300 K that suggested antiferromagnetic exchange between the Ni(I) ions. Magnetic data analysis afforded a value of J = −102(2) cm−1. DFT calculations (CASSCF/NEVPT2) revealed a singlet ground state, but with low-lying triplet and quintet states also populated.

image file: c7cs00216e-s47.tif
Scheme 47 Synthesis of Ni{N(SiMe3)2}2, [Ni{N(SiMe3)2}]4 and the decomposition of Ni{N(SiMe3)2}2.272

Two-coordinate transition metal complexes have also generated interest as potential candidates as single molecule magnets.260,279–290 The cationic, two-coordinate Ni(I) complex, [Ni(6-Mes)2]Br (6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene), was synthesized by Whittlesey and coworkers from the reaction between the free carbene 6-Mes and the three-coordinate Ni(6-Mes)(PPh3)Br.291 [Ni(6-Mes)2]Br showed an almost linear C–Ni–C angle of 179.27(13)° and Ni–C distances of 1.939(3) and 1.941(3) Å (Fig. 44). DFT calculations (BP86) suggested a degenerate ground state, indicating possible orbital magnetism. Room temperature magnetic moment measurements in solution (Evan's method, 2.2 μB) and solid state (Guoy balance, 2.7 μB; SQUID, 2.99 μB) gave values higher than the spin-only value (1.73 μB), which is due to the unquenched orbital angular momentum. Encouraged by these results, ac magnetic measurements were carried out to probe the magnetic relaxation dynamics. These data and the Arrhenius equation, τ = τ0[thin space (1/6-em)]exp(Ueff/kT), were used to reveal the spin-reversal barrier (Ueff) of 17 cm−1. Although it is not as large as those observed in other two-coordinate complexes,279–287 this was the first demonstration of NiI-based single molecule magnet behavior.

image file: c7cs00216e-f44.tif
Fig. 44 The molecular structure of the cation [Ni(6-Mes)]Br with thermal ellipsoids set at 50%. Hydrogen atoms, the Br counteranion and two cocrystallized CH2Cl2 molecules are not shown. Selected bond distances (Å) and angles (°): Ni(1)–C(1) 1.939(3), Ni(1)–C(2) 1.941(3), C(1)–Ni(1)–C(2) 179.27(13).291

Several two- and three-coordinate Ni(I) complexes supported by an IPr or an amidate ligand, (IPr)Ni{N(R)CO(But)} (R = Pri, But), were synthesized by Love, Schafer, and coworkers.292 They are accessed from the reduction by Na/Hg of a three-coordinate Ni(II) complex in Et2O, (IPr)NiCl{N(R)CO(But)}, synthesized from the salt elimination reaction in dry THF solution of Ni(IPr)(PPh3)Cl2 and sodium amidate, Na{N(R)CO(But)} (Scheme 48). Alternatively, these Ni(I) complexes can be synthesized from the reaction of Sigman's dimer, [(IPr)Ni(μ-Br)]2, and two equivalents of the corresponding sodium amidate. Interestingly, the coordination mode of the amidate ligand changes upon reduction. For the three-coordinate (IPr)Ni{N(Dipp)CO(But)} complex, X-ray crystallography shows that the amidate ligand has a κ2-N,C coordination mode with a Ni–arene backbonding interaction (Fig. 45). A solution magnetic susceptibility of 1.71 μB is consistent with a Ni(I) complex. For the two-coordinate (IPr)Ni{N(Pri)CO(But)} species, the amidate ligand adopts a κ1-N coordination mode (Fig. 45). Notably, it exhibits a δ-bis(C–H) agostic interaction between the Ni(I) center and the But hydrogen atoms. The agostic Ni⋯H interactions with distances of (2.024(15) Å and 2.159(15) Å) and C–H–Ni angles (101.8(10)° and 94.3(10)°) are within the ranges of the previously reported transition metal agostic complexes. It is noteworthy that if the But substituent is replaced by Pri, the structure of the isolated two-coordinate complex (IPr)Ni{N(Pri)CO(Pri)} no longer features an agostic interaction.293 Surprisingly, the two-coordinate complex, (IPr)Ni{N(Pri)CO(But)}, does not form Lewis base adducts with THF, MeCN, pyridine, PPh3 or PCy3. However, the addition of CNDmp allowed the isolation of a three-coordinate Ni(I) complex, (IPr)Ni[N(Pri)CO(But)-κ1N](CNDmp).

image file: c7cs00216e-s48.tif
Scheme 48 Synthesis of two- and three-coordinate Ni(I) complexes supported by IPr and amidate ligands.292

image file: c7cs00216e-f45.tif
Fig. 45 Drawings (left) and the molecular structures (right) of (IPr)Ni[N(Dipp)CO(But)] (top) and (IPr)Ni[N(But)CO(But)] (bottom) with thermal ellipsoids set at 50%. Hydrogen atoms (except agostic C–H), and the cocrystallized toluene molecule are not shown. Selected bond distances (Å) and angles (°): (IPr)Ni[N(Dipp)CO(But)]: Ni(1)–C(33) 2.195(2), Ni(1)–C(34) 2.166(2), Ni(1)–O(1) 1.9633(18), C(1)–Ni(1)–C(33) 171.82(9), C(1)–Ni(1)–C(34) 141.93(10), C(1)–Ni(1)–O(1) 107.90(8); (IPr)Ni[N(But)CO(But)]: Ni(1)–C(1) 1.9119(9), Ni(1)–N(1) 1.9149(8), Ni(1)–C(30) 2.4476(10), Ni(1)–H(1) 2.024(15), Ni(1)–H(2) 2.159(15), N(1)–Ni(1)–C(1) 172.12(4), C(30)–H(1)–Ni(1) 101.8(10), C(30)–H(2)–Ni(1) 94.3(10).292

8. Ni(I) olefin complexes

In search of better transfer hydrogenation or dehydrogenation catalysts and encouraged by previous reports on the Rh(I) complex of an amino olefin ligand, bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine (trop2NH), [Rh(trop2NH)(PPh3)](OTf), which catalyzes transfer hydrogenation,294–296 Grützmacher and coworkers reported the synthesis of a dark green Ni(I) complex, Ni(trop2NH)(OOCCF3), by a one step reaction of trop2NH and nickel(II) trifluoroacetate in the presence Zn powder as a reducing agent (Fig. 46).297 Ni(trop2NH)(OOCCF3) can be reduced further to a Ni(0) complex with Zn powder in the presence of PPh3. SQUID measurements afforded a magnetic moment of 1.81 μB at room temperature which is indicative of a Ni(I) oxidation state. The rhombic EPR signal (gx = 2.303, gy = 2.223 and gz = 2.012) and the DFT calculated nickel-centered spin density are also consistent of its +1 oxidation state. Remarkably, Ni(trop2NH)(OOCCF3) displayed very high catalytic activity in the dehydrogenation of Me2HN–BH3 (Scheme 49).
image file: c7cs00216e-f46.tif
Fig. 46 Drawing (left) and the molecular structure of Ni(trop2NH)(OOCCF3) with thermal ellipsoids set at 50%. Hydrogen atoms (except N–H), and the cocrystallized THF molecule are not shown. Selected bond distances (Å) and angles (°): Ni(1)–N(1) 2.061(2), Ni(1)–O(1) 2.176(1), Ni(1)–O(2) 2.265(1), Ni(1)–centroid(C(4),C(5)) 1.993, Ni(1)–centroid(C(19),C(20)) 2.001, N(1)–Ni(1)–O(1) 104.91(6), N(1)–Ni(1)–O(2) 164.94(5).297

image file: c7cs00216e-s49.tif
Scheme 49 Catalytic dehydrogenation of Me2HN–BH3 by Ni(trop2NH)(OOCCF3) and K(Me2NBH3) as cocatalyst.297

To isolate stable Ni(I) complexes coordinated solely by olefin ligands, Krossing and coworkers investigated the neutral Ni(0) complex, Ni(COD)2 (COD = 1,5-cyclooctadiene) as a metal source. Oxidation using the silver salt of a very weakly-coordinating anion, Ag[Al(ORF)4] (–ORF = –OC(CF3)3), in CH2Cl2 allowed the isolation of orange crystals of [Ni(COD)2][Al(ORF)4].298 Surprisingly, the powdered Ni(I) complex showed remarkable stability in the presence of air and moisture, however, it is O2-sensitive in solution. In THF solution, it disproportionates to Ni metal and a Ni(II) complex [Ni(THF)6][Al(ORF)4]2. The coordination geometry of nickel in [Ni(COD)2]+ is between square planar and tetrahedral (Fig. 47). Upon oxidation, the Ni–C bond distances increase by ca. 0.06–0.15 Å and the C[double bond, length as m-dash]C bond distances decrease by ca. 0.03–0.04 Å and are similar to the C[double bond, length as m-dash]C bond lengths (1.34 Å) in free COD,299 which can be explained on the basis of negligible π-backbonding in [Ni(COD)2]+ in comparison to that in Ni(COD)2. EPR spectroscopy yielded a rhombic signal (gx = 2.047, gy = 2.061 and gz = 2.390) that indicates a Ni(I) complex. X-ray absorption near-edge spectroscopy (XANES) revealed an edge inflection energy of ca. 8341 eV, which also supports the Ni(I) assignment. The room-temperature magnetic moment of 1.86 μB is in accordance with a Ni(I) ion. Furthermore, [Ni(COD)2][Al(ORF)4] serves as a suitable synthon for other Ni(I) complexes. For example, the reaction of [Ni(COD)2][Al(ORF)4] with PPh3 and dppp afforded [Ni(PPh3)3][Al(ORF)4] and [Ni(dppp)2][Al(ORF)4].

image file: c7cs00216e-f47.tif
Fig. 47 The molecular structure of the Ni(I) cationic species, [Ni(COD)2][Al(ORF)4], with thermal ellipsoids set at 50%. Hydrogen atoms are not shown for clarity.298

9. Cyclopentadienyl (Cp) and related complexes of Ni(I)

Cyclopentadienide (Cp) and related ligands are ubiquitous in transition metal coordination chemistry.62,300 In 1958, five years after the discovery of nickelocene by Fischer and Pfab in 1953,301 Fischer and Palm published an article titled “Cyclopentadienyl-metall-carbonyle des Nickels” (cyclopentadienyl-metal carbonyls of nickel) describing the first synthesis of a cyclopentadienyl–NiI complex, (NiCp)2(μ-CO)2.302 The diamagnetic dark red crystals could be obtained from the comproportionation reaction of Ni(CO)4 and NiCp2. Its solid state structure, however, remained unknown until 1980 when it was determined essentially simultaneously by the groups of Dahl303 (who also synthesized and characterized (MeCpNi)2(μ-CO)2) and Madach.304 Its structure was redetermined in 2003 at low temperature (Fig. 48).305 The crystal structure showed that there is a short NiI–NiI distance of 2.3691(3) and 2.3575(3) Å (two crystallographically independent molecules) with a non-planar Ni2(CO)2 unit. The remarkable hexameric octahedral Ni65-Cp)6 cluster was isolated as one of the products by Dahl and Paquette from the reaction of NiCp2 and sodium naphthalenide (NaNaph).306 Interestingly, one-electron oxidation using [FeCp2][PF6] yielded the cationic cluster [Ni65-Cp)6][PF6]. Schnöckel and coworkers used the then-new Al(I) species (AlCp*)4 to reduce NiCp2 in toluene to obtain the diamagnetic reddish brown Ni(I) compound (NiCp)2(AlCp*)2 (Scheme 50). Its structure is very similar to that of (NiCp)2(μ-CO)2, but with a longer NiI–NiI bond of 2.486(2) Å (Fig. 48).307 Figueroa and coworkers reacted nickelocene with a bulky m-terphenyl isocyanide ligand, ArMe6NC, in equimolar amounts and obtained isocyanide bridged dimeric Ni(I) complex, [CpNi(μ-CNArPri6)]2 with the elimination of cyclopentadiene dimer (Scheme 50).308 The Ni–Ni separation is 2.3797 (10) Å, which is close to the related [(CpNi)(μ-CNMe)]2309 of 2.3217(8) Å. The short Ni–Ni distances and the diamagnetic property suggest the existence of Ni–Ni bonding. The structure of the dimer of the bulkier isocyanide ligand, [CpNi(CNArPri6)]2 (ArPri6 = C6H3-2,6(C6H2-2,4,6-Pri3)2) featured an unsupported Ni–Ni bond (2.3453(16) Å) with two terminally coordinated isocyanide ligands. The reaction of nickelocene by Pietrzykowski and coworkers of organolithium reagents afforded numerous Ni(I), Ni(II) and mixed Ni(I)/Ni(II) oxidation state polynuclear clusters.310–313 Notably, reacting NiCp2 with one equivalent of phenyllithium in the presence of a slightly excess (1.1 equivalents) bis(trimethylsilyl)acetylene in THF solution afforded mixtures of products that contain (NiCp)2(μ-η22-Me3SiC[triple bond, length as m-dash]CSiMe3), (NiCp)2(μ-η22-PhC[triple bond, length as m-dash]CSiMe3), (NiCp)4(μ-μ-η2222-PhC[triple bond, length as m-dash]C–C[triple bond, length as m-dash]CSiMe3).311 A nickelocene derivative, Ni(C5H4SiMe2CH[double bond, length as m-dash]CH2)2, was synthesized and used to react with 1.3 equivalents MeLi in THF to form a dimer, [Ni(C5H4SiMe2CH[double bond, length as m-dash]CH2)]2, with a Ni–Ni bond distance of 2.5152(1) Å (Scheme 50).314
image file: c7cs00216e-f48.tif
Fig. 48 The molecular structures of (NiCp)2(μ-CO)2 (left) and (NiCp)2(AlCp*)2 (right), with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å): (NiCp)2(μ-CO)2: Ni(1)–Ni(2) 2.3691(3), Ni(1)–C(1) 1.8681(15), Ni(1)–C(2) 1.8807(15), Ni(2)–C(1) 1.8738(15), Ni(2)–C(2) 1.8679(15). (NiCp)2(AlCp*)2: Ni(1)–Ni(2) 2.486(2), Ni(1)–Al(1) 2.278(2), Ni(1)–Al(2) 2.274(3), Ni(2)–Al(1) 2.284(3), Ni(2)–Al(2) 2.274(2).303,305

image file: c7cs00216e-s50.tif
Scheme 50 Synthesis of dimeric Ni(I) complexes having Cp-related ligands.302,307–309,314

(NiCp)2(μ-CO)2 could serve as a useful synthon for compounds that contain the CpNiI fragment. For example, Fedushkin and coworkers showed that addition of a α-diimine ligand dpp-BIAN (1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) to 0.5 equivalents of (NiCp)2(μ-CO)2 in toluene afforded (dpp-BIAN)NiCp. The rhombic EPR signal (g1 = 2.183, g2 = 2.109, g3 = 1.993) confirms its +1 oxidation state.315 Barefield and coworkers reported the crystal structure of (bpy)NiCp from the reaction of (bpy)NiX2 (X = Cl, Br) with NaCp. EPR studies revealed a rhombic signal (g1 = 2.184, g2 = 2.080, g3 = 2.033) that is consistent with a Ni(I) oxidation state.316 A dimeric species with an unsupported NiI–NiI bond, [CpNi(PEt3)]2, was obtained by Wilke and coworkers via the reduction of CpNi(PEt3)Cl with activated magnesium.317 The complex features a rare unsupported NiI–NiI bond which has a length of 2.407(1) Å, which was attributed to the steric protection of the ligands. They also showed that [CpNi(PEt3)]2 is a useful synthon in the preparation of several Ni(I), Ni(II) complexes with –NiCp fragments (Scheme 51).317,318

image file: c7cs00216e-s51.tif
Scheme 51 Synthesis and reactivity of [CpNi(PEt3)]2.317,318

A boratabenzene derivative, di-tert-butylphosphidoboratabenzene (DTBB), was shown by Fontaine and coworkers to be an effective ligand for the stabilization of electronically unsaturated nickel and platinum complexes.319 In the case of Ni, treatment of NiBr2(PPh3)2 with one equivalent of K(DTBB) in THF solution afforded the emerald green diamagnetic Ni(I) dimer, [Ni(DTBB)]2, in 25% yield. The crystal structure features a Ni–Ni core, with a bond distance of 2.605(1) Å, supported by two η6-coordinated boratabenzenes with η1-coordinated phosphine ligands (Fig. 49).

image file: c7cs00216e-f49.tif
Fig. 49 The molecular structure of [Ni(DTBB)]2, with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): Ni(1)–Ni(1A) 2.605(1), Ni(1)–P(1) 2.173(7), P(1)–Ni(1)–Ni(1A) 92.9(3).319

An unexpected zwitterionic Ni(I) dimer, Ph3PNi(μ-PPh2)[syn,μ-{(η22-Ph)BPh3}]NiPPh3, was isolated as dark brown crystals by Zargarian and coworkers from the reaction of (1-SiMe3-Ind)Ni(PPh3)Cl and NaBPh4 during attempts to isolate a cationic Ni(II) species (Scheme 52).320 They could identify this cationic species [(1-SiMe3-Ind)Ni(PPh3)2][BPh4] by 31P and 1H NMR spectroscopy, however, the ready decomposition of this complex prevented its isolation. The solid state molecular structure of Ph3PNi(μ-PPh2)[syn,μ-{(η22-Ph)BPh3}]NiPPh3 displayed a dinuclear zwitterionic structure composed of two (Ph3P)NiI units bridged by μ-PPh2 and syn,μ-[(η22-Ph)BPh3] (Fig. 50). The authors proposed that the unusual bonding mode of BPh4 minimizes the steric interactions between PPh3 and BPh4. The Ni–Ni distance of 2.4471(11) Å is within the range of the typical Ni–Ni bond distances (cf. Table 5).

image file: c7cs00216e-s52.tif
Scheme 52 Synthesis of cyclopentadienyl-, indenyl-, and tetraphenylborate-bridged Ni(I) dimers.320–322

image file: c7cs00216e-f50.tif
Fig. 50 The molecular structure of the zwitterionic Ni(I) dimer, Ph3PNi(μ-PPh2)[syn,μ-{(η22-Ph)BPh3}]NiPPh3, with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): Ni(1)–Ni(2) 2.4471(11), Ni(1)–P(1) 2.1397(16), Ni(2)–P(1) 2.1395(13), Ni(2)–P(3) 2.1841(16), Ni(1)–C(2) 2.120(4), Ni(1)–C(3) 2.094(4), Ni(2)–C(5) 2.075(5), Ni(2)–C(6) 2.125(5), P(1)–Ni(1)–P(2) 116.27(6), P(3)–Ni(2)–Ni(1) 165.66(5), P(2)–Ni(1)–Ni(2) 168.82(5), Ni(1)–P(1)–Ni(2) 69.76(5).320
Table 5 Complexes with NiI–NiI bonds344
Complex Ni–Ni (Å) Ref.
[Ni(μ-PhPBP)]2 2.2393(7) 127
Rb4[Ni2(CN)6] 2.29, 2.32 39
[Ni(μ-κ2-N,N′-Priso)]2 2.2908(11) 227
[Ni{PhC(NSiMe3)2}]2 2.2938(12) 226
[Ni(μ-NNMe,Dipp˙)]2 2.2957(6) 228
[(Et3P)(Me3P)Ni]2(3,4-C6H2F2-3′,4′-C6H2F2) 2.3079(8) 90
[(IPr)Ni]2(μ-I)(μ-NO) 2.314(1) 167
[(Et3P)Ni]2(μ-η22-C24H16) 2.314(4), 2.323(4) 241
K4[Ni2(CN)6] 2.32 37 and 38
(p-Ar′P2)Ni2(μ-Cl)2 2.3201(2) 332
[(CpNi)(μ-CNMe)]2 2.3217(8) 309
[Ni(PNPPh)]2 2.3259(2) 119
[(Pri3P)Ni]2(μ-η22-C24H16) 2.3352(6) 240
[(Ph3P)Ni(μ-STrip)]2 2.3353(4) 263
[(ButNC)Ni(μ-SArMe6)]2 2.3354(8) 263
[CpNi(CNArPri6)]2 2.3453(16) 308
(PriNDI)Ni2(μ-Cl) 2.3367(8) 337
[(Ph3P)Ni{μ-S(1-Ad)}]2 2.3510(3) 263
[(Et3P)Ni]2(μ-η44-C36H20) 2.3524(8) 242
(NiCp)2(μ-CO)2 2.3575(3), 2.3691(3) 302
(p-ArP2)Ni2(μ-Cl)2 2.36580(16) 330
[(Pri3P)Ni(μ-SPh)]2 2.3660(10) 243
[(Pri3P)Ni(μ-PPh2)]2 2.3702(4) 243
[(Et3P)2Ni]2(2,3-C6H2F2-2′,3′-C6H2F2) 2.3710(5) 89
(PriNDI)Ni2(μ-Br) 2.371(1), 2.378(1) 338
[(dippe)Ni]2(μ-H)2 2.3737(5) 69
[Ni(μ-NPBut3)]4 2.3745(9), 2.3752(8) 266
[Ni(μ-PBut2)(PMe3)]2 2.375(3) 82
[CpNi(μ-CNArPri6)]2 2.3797(10) 308
[Ni(PMe3)]2[μ-P(SiMe3)2]2 2.382(10) 153
Mes2But2(S)2Ni2(PCy3) 2.3844(3) 333
Mes2But2(S)2Ni2(PPh3) 2.3877(3) 333
Ni2Cl2(μ-dcpm)2(μ-H) 2.3910(8) 85
[(Pri3P)Ni]2(μ-Cl)(μ-Ind) 2.3918(8) 321
(DIPr)Ni2(μ-Cl)2 2.3936(3) 331
[(IMe2Me2)Ni]2(μ-SArMe6)2 2.3941(13) 263
[(IPr)Ni]2(μ-Cl)(μ-p-tol) 2.3954(5) 168
[(Pri3P)Ni]2(μ-Cl)(μ-Cp) 2.3995(10) 313
[(IPr)Ni]2(μ-Cp)(μ-Cl) 2.4015(3) 322
[(IPr)Ni]2(μ-p-tol)2 2.4067(8) 168
[CpNi(PEt3)]2 2.407(1) 317
[(dcpe)Ni]2(μ-H)2 2.4078(5) 65
[BrNi2](μ-dppm)(μ3-dppm-H) 2.408(2) 141
Ni2(μ-PBut2)2(CO)3 2.414(2) 84
[Ni{1-NH(PPh2)-2-N(μ-PPh2)C6H42N,P}]2 2.4152(6) 146
(ArMe4S)Ni(μ-SArMe4)Ni(PPh3) 2.4238(4) 263
[(Pri3P)Ni]2(μ-Cl)(μ-η22-PhBPh3) 2.4255(9) 321
[Ni(μ-AsBut2)(PMe3)]2 2.429(1) 86
[(dtbpe)Ni]2(μ-H)2 2.433(1) 68
[Ni(IBut)(μ-CN(CH)2NBut)]2 2.4354(9) 165
[(dippp)Ni]2(μ-H)2 2.438(1) 66
[(dcpp)Ni]2(μ-H)2 2.441(1) 67
[(SIPr)Ni]2(μ-Ind)(μ-Cl) 2.4425(6) 322
(p-ArP2)Ni2(μ-η11-C10H8) 2.44266(19) 330
Ni2(μ-PBut2)2(CO)2(PMe3) 2.446(2) 84
(Ph3PNi)2(μ-PPh2)(μ-η22-PhBPh3) 2.4471(11) 320
(PriNDI)Ni2(C3H5)Cl 2.4570(9) 339
(NiCp)2(Cp*Al)2 2.486(2) 307
[Ni(PCy3)]2(μ-SiHPh2)2 2.495(1) 245
(PriNDI)Ni2(C6H6) 2.496(1) 337
[(SIPr)Ni(μ-Cl)]2 2.5099(6) 166
[Ni(μ-PPh2)(CO)2]2 2.510(1) 80
(dippe)2Ni2(μ-H)(μ-S-2-biphenyl) 2.5124(4) 72
[Ni(IPri2)]2(μ-SiHPh2)2 2.5136(12) 246
(BrNi)(μ-Pri2P)(μ-dippm)[Ni(PMePri2)] 2.515(1) 141
Ni(C5H4SiMe2CH[double bond, length as m-dash]CH2)2 2.5152(1) 314
[(IPr)Ni(μ-Cl)]2 2.5194(5) 166
(ArP3)Ni2(μ-Cl)(OTf) 2.5248(3) 336
(PriNDI)Ni2(μ-Br)2 2.5316(7), 2.5399(7) 338
(PriNDI)Ni2(Et2SiH2) 2.5545(6) 340
[Ni{μ-P(H)But}(PMe3)2]2 2.559(2) 83
(PriNDI)Ni2(Ph2SiH2) 2.5625(9), 2.6284(9) 340
[(dppm-η1-P)Ni]2(μ-PPh2)(μ-dppm)(μ-O2C3H3) 2.563(1) 61
[Ni2(μ-CNMe)(CNMe)3(dppm)2][PF6]2 2.5924(9) 59
[Ni(DTBB)]2 2.605(1) 319
(PriNDI)Ni2(μ-Cl)2 2.6088(8) 337
Ni2Cl2(μ-CO)(μ-dppm)2 2.617(1) 60
[Ni(μ-κ1-N-,η2-Dipp-Priso)]2 2.6338(9) 227
{Ni(dmpe)}2(μ-SiHPh2)2 2.6417(5) 245
Mes2But2(S)2Ni2(PMe3)2 2.659(1) 333
[Ni(μ-AsBut2){NC(p-tol)}]2 2.693(2) 86

The dinuclear Ni(I) species, [(Pri3P)Ni]2(μ-Cl)(μ-Cp) and [(Pri3P)Ni]2(μ-Cl)(μ-Ind), which are bridged by cyclopentadienyl or indenyl (Ind) anions and bulky phosphine ligand, PPri3, were prepared by Johnson and coworkers by reacting (Pri3P)2NiCl with LiCp or LiInd (Scheme 52).321 Both of these complexes displayed short Ni–Ni bond distances of 2.3995(10) Å for [(Pri3P)Ni]2(μ-Cl)(μ-Cp) and 2.3918(8) Å for [(Pri3P)Ni]2(μ-Cl)(μ-Ind). Alternatively, these Ni(I) complexes can be synthesized via a comproportionation route with (Pri3P)Ni(Cl)(Cp) or (Pri3P)Ni(Cl)(Ind) and [(Pri3P)Ni]2(μ-N2). Furthermore, the reaction of [(Pri3P)Ni]2(μ-Cl) and 0.5 equivalents of NaBPh4 in Et2O led to the isolation of [(Pri3P)Ni]2(μ-Cl)(μ-η22-PhBPh3). Its crystal structure showed that it has a Ni–Ni bond distance of 2.4255(9) Å and a syn,μ-η22 bridging mode of the BPh4 ligand similar to Zargarian's Ni(I) dimer, Ph3PNi(μ-PPh2){syn,μ-[(η22-Ph)BPh3]}NiPPh3.320

Similarly, Hazari and coworkers used Sigman's dimer, [(NHC)Ni(μ-Cl)]2 (NHC = IPr, SIPr), to prepare the NHC-stabilized half-sandwich NiI–Cp and NiI–Ind complexes by the reaction of the dimers with NaCp (or LiInd) (Scheme 52).322 When 0.5 equivalents of NaCp or LiInd were employed, the Cp- or Ind-bridged dimers, [(NHC)Ni]2(μ-Cp)(μ-Cl), [(NHC)Ni]2(μ-Ind)(μ-Cl) (NHC = IPr, SIPr), were obtained. Using one equivalent of NaCp or LiInd afforded the momomeric Ni(I) complexes, (NHC)NiCp, (NHC)Ni(Ind) (NHC = IPr, SIPr). Some stoichiometric and catalytic transformations were explored and they further found that [(NHC)Ni]2(μ-Cp)(μ-Cl), (NHC)Ni(Cp) and (NHC)Ni(Ind) (NHC = IPr, SIPr) displayed catalytic activity in Suzuki–Miyaura cross-coupling reactions. Wolf, Whittlesey and coworkers showed that the (NHC)NiI and (RE-NHC)NiI complexes (IPr)NiCp, (IPr)NiCp*, (IMes)NiCp, (6-Mes)NiCp, (7-Mes)NiCp and (6-MesDAC)NiCp were also accessible from chemical (KC8) reduction of the Ni(II) chloride or bromide precursors.323,324 They found that some of these Ni(I) complexes reacted with P4, S8, Se and Te, to form the Ni(II) tetraphosphide complex, {(IPr)NiCp}2(μ-η11-P4), and several chalcogenide complexes, {(IPr)NiCp}2(μ-E2) (E = S, Se, Te) and {(IPr)NiCp}2(μ-E3) (E = S, Se) (Scheme 53).323,325 Furthermore, by using an extremely bulky cyclopentadienyl ligand, CpAr (CpAr = C5(4-Et-C6H4)5), they trapped the “CpArNi(I)” synthon with a variety of donor ligands such as IPr, IPri2Me2 (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, (Fig. 51)), a Ga(I) carbenoid (Fig. 51), Ga(NNMe,Pri),326 TEMPO, S8 and Se.327–329 The reduction (KC8) of the Ni(II) starting material, [CpArNi(μ-Br)]2 afforded a dark green solution which, presumably, contained the “CpArNi(I)” species. Subsequent addition of suitable donors afforded a number of Ni(I) or Ni(II) complexes containing the “CpArNi” fragment (Scheme 54).

image file: c7cs00216e-s53.tif
Scheme 53 Reactions of (IPr)NiCp with various substrates.323,325

image file: c7cs00216e-f51.tif
Fig. 51 The molecular structures of CpArNi(IPri2Me2) (left), CpArNi{Ga(NNMe,Pri)} (right) with thermal ellipsoids set at 50%. Hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): CpArNi(IPri2Me2): Ni(1)–C(1) 1.910(2), Ni(1)–CpAr(centroid) 1.793(1), C(1)–Ni(1)–CpAr(centroid) 168.932(1); CpArNi{Ga(NNMe,Pri)}: Ni(1)–Ga(1) 2.2914(3), Ni(1)–CpAr(centroid) 1.7922(7), Ga(1)–Ni(1)–CpAr(centroid) 164.6(1).328,329

image file: c7cs00216e-s54.tif
Scheme 54 The diverse reactivity of Wolf's “CpArNi” (CpAr = C5(4-Et-C6H4)5) synthon.327–329

Agapie and coworkers designed a p-terphenyl diphosphine ligand, p-(o-PPri2Ph)2-C6H4 (p-ArP2), to stabilize a bimetallic Ni(I) complex, (p-ArP2)Ni2(μ-Cl)2.330 This dark green species was synthesized via comproportionation of the Ni(0) complex, Ni(p-ArP2), with NiCl2(DME) in THF. Alternatively, it can be synthesized in a one-pot reaction by mixing the ligand, Ni(COD)2 and NiCl2(DME) in THF. X-Ray crystallographic analysis of the dark green crystal confirmed a Ni2Cl2 moiety stabilized by two phosphine ligands in addition to the arene unit with a NiI–NiI bonding distance of 2.36580(16) Å and short NiI–Carene distances of 2.05–2.17 Å (Fig. 52). A related dinickel complex with a NiI–NiI bond supported by dinucleating bis-NHC, (DIPr)Ni2(μ-Cl)2, was synthesized by a similar comproportionation reaction, which features similar NiI–NiI (2.3936(3) Å) and NiI–Carene (2.05–2.10 Å) distances (Fig. 52).331 Addition of PhMgBr to the THF solution of (p-ArP2)Ni2Cl2 afforded biphenyl, presumably by reductive elimination. Reaction of o,o′-biphenyldiylmagnesium bromide and (p-ArPri3)Ni2(μ-Cl)2 led to the isolation of the desired species, (p-ArPri3)Ni2(μ-η11-C10H8), which is probably stabilized due to the lack of accessible decomposition pathways such as β-hydride or reductive elimination. Stoichiometric reaction of (p-ArP2)Ni2(μ-η11-C10H8) with a geminal dichloroalkane, CRHCl2 (R = Me or H), afforded fluorene derivatives and the reformation of (p-ArP2)[Ni2(μ-Cl)2]. Reaction of (p-ArP2)Ni2(μ-η11-C10H8) and CO afforded fluorenone and the dinuclear Ni0-carbonyl species, (p-ArP2)Ni2(CO)n (n = 4 or 6) (Scheme 55). A new dimeric NiI–NiI species, (p-Ar′P2)Ni2(μ-Cl)2, along with the free ligand p-ArP2, were formed when heating a toluene solution of (p-ArP2)Ni(H)Cl at 70 °C (Scheme 56 and Fig. 52). In this case, the central arene unit of the ligand was partially hydrogenated by the nickel-hydride to form the cyclohexadiene unit. Isotopic labeling experiments showed that this metal-to-arene hydride migration is reversible.332

image file: c7cs00216e-f52.tif
Fig. 52 Drawings (left) and the molecular structures (right) of (p-ArP2)[Ni2(μ-Cl)2] (top), (DIPr)[Ni2(μ-Cl)2] (middle) and (p-Ar′P2)[Ni2(μ-Cl)2] (bottom) with thermal ellipsoids set at 50%. Hydrogen atoms and cocrystallized solvent molecules are not shown. Selected bond distances (Å) and angles (°): (p-ArP2)[Ni2(μ-Cl)2]: Ni(1)–Ni(2) 2.36580(16), Ni(1)–P(1) 2.1715(2), Ni(2)–P(2) 2.1793(2), P(1)–Ni(1)–Ni(2) 173.245(7), P(2)–Ni(2)–Ni(1) 175.427(7); (DIPr)[Ni2(μ-Cl)2]: Ni(1)–Ni(2) 2.3936(3), Ni(1)–C(1) 1.9097(17), Ni(2)–C(2) 1.9097(17), C(1)–Ni(1)–Ni(2) 173.29(2), C(2)–Ni(2)–Ni(1) 170.80(5); (p-Ar′P2)[Ni2(μ-Cl)2]: Ni(1)–Ni(2) 2.3201(2), Ni(1)–P(1) 2.1618(3), Ni(2)–P(2) 2.1551(3), P(1)–Ni(1)–Ni(2) 172.987(9), P(2)–Ni(2)–Ni(1) 174.104(9).330–332

image file: c7cs00216e-s55.tif
Scheme 55 Synthesis of (p-ArP2)Ni2(μ-η11-C10H8) and subsequent reaction with CO and chloroalkanes to generate fluorenone and fluorene derivatives.330

image file: c7cs00216e-s56.tif
Scheme 56 Heating a toluene solution of (p-Ar′P2)Ni2(μ-Cl)2 to form a dinuclear Ni(I) complex, (p-Ar′P2)Ni2(μ-Cl)2.332

Two similar dianionic p-terphenyl dithiols, (Mes2But2S2)H2 (5′,5′′′-di-tert-butyl-2,2′′′′,4,4′′′′,6,6′′′′-hexamethyl-[1,1′:3′,1′′:4′′,1′′′:3′′′,1′′′′-quinquephenyl]-2′,2′′′-dithiol)333 and (But4S2)H2 (3,3′′,5,5′′-tetra-tert-butyl-[1,1′:4′,1′′-terphenyl]-2,2′′-dithiol),334 were used by Berkefeld and coworkers to generate the corresponding dithiolate salts which were shown to support dinuclear Ni(I) thiolate species. The Ni(II) precursors were synthesized by the salt elimination reaction of NiCl2(L)n (L = PMe3, n = 2; L = py, n = 4) with the potassium salt of the dithiophenol ligand, K2(Mes2But2S2) and K2(But4S2), in toluene solution. Access to four dinuclear Ni(I) complexes, Mes2But2(S)2Ni2(L)2 (L = PMe3, PCy3 and PPh3) was achieved by comproportionation reactions of the Ni(II) complexes, (Mes2But2S2)Ni(L)n (L = PMe3, n = 2; L = PCy3, n = 1) and (But4S2)Ni(PMe3)2, with Ni(COD)2 (Scheme 57). The solid state structures revealed a dinuclear Ni2 core supported by two thiophenolate ligands and two phosphines, however, their structures are distinctly different. For (Mes2But2S2)Ni2(PMe3)2 and (But4S2)Ni2(PMe3)2, strong nickel–arene interactions (Ni–Carene = 1.949(4), 2.035(4) and 2.149(4) Å) were observed with the central arene unit deviating from planarity, which is due to strong backbonding from the d orbital of nickel to a π* orbital of the arene ligand. Thus, (Mes2But2S2)Ni2(PMe3)2 can also be described as a dinuclear Ni(II) complex with a doubly reduced central arene. Although the Ni–Ni separation is somewhat long (2.659(1) Å), the delocalization index (δ(Ni–Ni)) of 0.225 can be viewed as a weak bonding. On the other hand, the crystal structures of both (Mes2But2S2)Ni2(PCy3)2 and (Mes2But2S2)Ni2(PPh3)2 revealed a long average Ni–arene distances of 2.569(6) and 2.443(1) Å. Thus, the nickel and arene interaction can be considered minimal. The Ni–Ni bond distances of 2.3844(3) and 2.3877(3) Å indicated Ni–Ni bonding, which is confirmed by a delocalization index of 0.548 for (Mes2But2S2)Ni2(PPh3)2. Moreover, these complexes could be oxidized by one electron to form mixed oxidation state cationic NiI/NiII species.

image file: c7cs00216e-s57.tif
Scheme 57 Synthesis of dinuclear Ni(I) complex supported by p-terphenylthiophenolate ligands.333,334

A m-terphenyl diphosphine ligand, m-(o-PPri2Ph)2-C6H4 (m-ArP2), was used by Agapie and coworkers to stabilize several Ni(0), Ni(I), and Ni(II) complexes. Access to the Ni(I) complex, (m-ArP2)NiCl, was achieved by a comproportionation reaction of m-ArP2, Ni(COD)2 and NiCl2(DME) in THF solution.335 Subsequent salt elimination reactions between (m-ArP2)NiCl and lithium and sodium salts of various anions (NaN3, NaBF4, LiNHPh, LiNHDipp) afforded Ni(I) complexes stabilized by anions with different electronic properties, (m-ArP2)Ni(N3), (m-ArP2)Ni(BF4), [(m-ArP2)Ni(N3)][B(C6F5)3], (m-ArP2)Ni[N(H)Ph] and (m-ArP2)Ni[N(H)Dipp] (Scheme 58). The Ni–Carene distances for these complexes ranges from 2.399(2) to 2.621(3) Å, indicating different, but weaker, Ni–arene interactions. Indeed, a computational study on the truncated molecule (m-ArP2)NiCl revealed little contribution from the central arene. The reactivity of these Ni(I) species was also explored. Notably, oxidation of (m-ArP2)Ni[N(H)Ph] with 2,4,6-tri-tert-butylphenoxyl radical led to the conversion to 2,4,6-tri-tert-butylphenol and the Staudinger oxidation of the ligand phosphine arms, presumably through the generation of the putative nickel-imido species, (m-ArP2)Ni[double bond, length as m-dash]NPh (Scheme 58).

image file: c7cs00216e-s58.tif
Scheme 58 Synthesis of (m-ArP2)NiCl and subsequent salt elimination reactions and hydrogen atom abstraction of (m-ArP2)Ni{N(H)Ph} to generate the Staudinger oxidation product.335

Based on their work on p- or m-terphenyl Ni(I) chemistry and triarylbenzene architecture with pyridine and alkoxide donors, they further introduced a triphosphine ligand, 1,3,5-(o-PPri3Ph)3-C6H3 (ArP3), to obtain dinuclear Ni(I) complexes.336 Entry into the dinuclear Ni(I) species was achieved via a comprotionation reaction between Ni(COD)2, NiCl2(DME) and ArP3. The crystal structure of (ArP3)Ni2Cl2 showed that the two nickel atoms are on the opposite side of the central arene unit (Scheme 59). Abstraction of a chloride was achieved by the addition of TlOTf to afford (ArP3)Ni2(μ-Cl)(OTf). Interestingly, the solid state structure revealed that the two nickel atoms are on the same side of the central arene with a NiI–NiI bond distance of 2.5248(3) Å (Scheme 59). Treatment of (ArP3)Ni2(μ-Cl)(OTf) with LiCl regenerated (ArP3)Ni2Cl2, indicating a reversible Ni–Ni bond cleavage/formation induced by a halide. This molecular dynamic was thus colloquially referred to as “molecular hinges” and may find application in some molecular devices.

image file: c7cs00216e-s59.tif
Scheme 59 Synthesis of Ni(I) complexes from comproportionation reaction of Ni(COD)2, NiCl2(DME) and ArP3. The halide-mediated reversible transformation of (ArP3)Ni2Cl2 (left) and (ArP3)Ni2(μ-Cl)(OTf) (right).336

Uyeda and coworkers used a redox-active naphthyridine diimine-type ligand, PriNDI ((1E,1′E)-1,1′-(1,8-naphthyridine-2,7-diyl)bis(N-(2,6-diisopropylphenyl)ethan-1-imine)), to obtain dinuclear nickel complex which can be converted to five redox states (Scheme 60).337 Reactions of PriNDI with two equivalents of Ni(COD)2 in benzene yielded brown crystals of (PriNDI2−)Ni2I(C6H6) after workup and crystallization from pentane. The X-ray crystal structure revealed a dinuclear structure with Ni–Ni bond distances of 2.496(1) Å, with each nickel atom coordinated to a pyridiyl nitrogen atom, an imine nitrogen atom, and a bridging benzene molecule. Comparison of the metrical parameters and 1H NMR resonances of the coordinated ligand with free ligand PriNDI indicated a doubly-reduced ligand, PriNDI2−. In addition, DFT calculations (BP86/6-311G(d,p)) also supported this NiI–NiI oxidation state assignment. Remarkably, cyclic voltammetry of (PriNDI2−)Ni2I(C6H6) in THF showed rich electrochemistry with two reversible oxidation events (−1.04, −0.43 V vs. Cp2Fe/Cp2Fe+) and two reversible reduction events (−2.24, −2.86 V vs. Cp2Fe/Cp2Fe+). They could chemically oxidize and reduce to isolate complexes with different oxidation states of nickel and the ligand, in which reduction populated the σ* orbital of the dinickel unit and oxidation removed the electron from the ligand. For example, reduction of (PriNDI2−)Ni2I(C6H6) with one equivalent KC8 in THF gave K(THF)3(PriNDI2−)Ni0/I2(C6H6) with mixed Ni0/NiI oxidation state and an elongated Ni–Ni distance of 2.5947(7) Å. One or two electron oxidation of (PriNDI2−)Ni2I(C6H6) with 0.5 or one equivalents of (Bun4N)Br3 afforded (PriNDI)Ni2I(μ-Br) and (PriNDI)Ni2I(μ-Br)2, respectively, which the PriNDI ligand lost one or two electrons upon oxidation and the nickel oxidation state remained unchanged. The chloride analogues, (PriNDI)Ni2I(μ-Cl)2 and (PriNDI)Ni2I(μ-Cl), could be obtained by comproportionation reaction and subsequent one electron reduction of NiCl2(DME), Ni(COD)2 and PriNDI.338 Similarly, reaction of (PriNDI2−)Ni2I(C6H6) and allyl chloride afforded the oxidative addition product, (PriNDI)Ni2I(C3H5)Cl, and NBO analysis revealed that the charge on the metals remained virtually unchanged (+0.92, +0.92 in (PriNDI2−)Ni2I(C6H6) and +0.99, +0.99 in (PriNDI)Ni2I(C3H5)Cl) and a similar Ni–Ni distance of 2.4570(9) Å.339 Addition of silanes R2SiH2 (R = Et, Ph) to (PriNDI2−)Ni2I(C6H6) also afforded the dinuclear Ni(I) complexes, (PriNDI2−)Ni2I(R2SiH2).332 The catalytic reactivity of (PriNDI)Ni2(C6H6) towards organic substrates was also investigated and it was found to be active in reactions such as hydrosilylation, alkyne cyclotrimerizations and reductive cyclopropanations (Scheme 61).338,340–343

image file: c7cs00216e-s60.tif
Scheme 60 Synthesis of five dinuclear nickel complexes supported by PriNDI ligand with five different redox states.337

image file: c7cs00216e-s61.tif
Scheme 61 Selected catalytic transformations of organic substrates by (PriNDI)Ni2(C6H6).338–341

10. Summary and outlook

While Ni(I) has been generally considered to be a relatively rare and unexplored oxidation state in comparison to the other oxidation states of nickel (for example, Ni(0) and Ni(II)), numerous Ni(I) compounds have now been isolated and over 280 complexes have been structurally characterized (as of June 2017). It is an irony that in some cases, the Ni(I) complexes form from unstable Ni(II) precursors. The variety of ligands that support Ni(I) and the number of structures now available clearly show that Ni(I) complexes are no longer rare. Despite this, Ni(I) chemistry remains underdeveloped and there are obviously very many avenues open for further development.


BDABenzylidene acetone
18-crown-618-Crown-6 ether
NN′Me,PriHC[(C[double bond, length as m-dash]CH2)CMe][N(2,6-Pri2C6H3)]2
RERing expanded


We thank the National Science Foundation (Grant No. CHE-0840444, 1263760, 1531193 and 1565501) for support of this work and two Dual source X-ray diffractometers. We also thank Dr Kamran B. Ghiassi (Air Force Research Laboratory) for careful reading of this manuscript, Prof. David J. Liptrot (University of Bath) for insightful discussion and anonymous reviewers for their valuable comments and suggestions. Molecular graphics were performed with the UCSF Chimera27 and Olex2345 software package. Chimera is developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco (supported by NIGMS P41-GM103311).


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