Self-complementary nickel halides enable multifaceted comparisons of intermolecular halogen bonds: fluoride ligands vs. other halides

Abstract

The syntheses of three series of complexes designed with self-complementary motifs for formation of halogen bonds between an iodotetrafluorophenyl ligand and a halide ligand at square-planar nickel are reported, allowing structural comparisons of halogen bonding between all four halides C₆F₄I⋯X–Ni (X = F, Cl, Br, I). In the series trans-[NiX(2,3,5,6-C₆F₄I)(PEt₃)₂] 1pX and trans-[NiX(2,3,4,5-C₆F₄I)(PEt₃)₂] (X = F, Cl, Br, I) 1oX, the iodine substituent on the benzene ring was positioned para and ortho to the metal, respectively. The phosphine substituents were varied in the series, trans-[NiX(2,3,5,6-C₆F₄I)(PEt₂Ph)₂] (X = F, I) 2pX. Crystal structures were obtained for the complete series 1pX, and for 1oF, 1oCl, 1oI and 2pI. All these complexes exhibited halogen bonds in the solid state, of which 1pF exhibited unique characteristics with a linear chain, the shortest halogen bond d(C₆F₄I⋯F–Ni) = 2.655(5) Å and the greatest reduction in halogen bond distance (I⋯F) compared to the sum of the Bondi van der Waals radii, 23%. The remaining complexes form zig-zag chains of halogen bonds with distances also reduced with respect to the sum of the van der Waals radii. The magnitude of the reductions follow the pattern F > Cl ∼ Br > I, 1pX > 1oX, consistent with the halogen bond strength following the same order. The variation in the I⋯X–Ni angles is consistent with the anisotropic charge distribution of the halide ligand. The temperature dependence of the X-ray structure of 1pF revealed a reduction in halogen bond distance of 0.055(7) Å on cooling from 240 to 111 K. Comparison of three polymorphs of 1oI shows that the halogen bond geometry may be altered significantly by the crystalline environment. The effect of the halogen bond on the ¹⁹F NMR chemical shift in the solid state is demonstrated by comparison of the magic-angle spinning NMR spectra of 1pF and 1oF with that of a complex incapable of halogen bond formation, trans-[NiF(C₆F₅)(PEt₃)₂] 3F. Halogen bonding causes deshielding of δ_iso in the component of the tensor perpendicular to the nickel coordination plane. The results demonstrate the potential of fluoride ligands for formation of halogen bonds in supramolecular structures.

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Introduction

Metal fluoride complexes should provide the strongest halogen bond acceptors among metal halide complexes. Their halogen bonding ability has been addressed in solution, but supramolecular structures connecting metal fluoride complexes via halogen bonds are unknown according to a current crystallographic database search. This study reveals that they are indeed capable of forming halogen-bonded chains and allows comparisons of their structures to those of corresponding metal chlorides, bromides and iodides.

Halogen bonding (XB) interactions between the electropositive region of a covalently-bound halogen and a Lewis base are increasingly recognised as motifs in secondary bonding that bear comparison with hydrogen bonds.^1–5 They have been utilized in solution,^4,6,7 for example for anion recognition,^4,8 in catalysis,⁹ in medicinal chemistry,¹⁰ in biological systems including proteins and nucleic acid junctions,^6,7,11–14 in liquid crystals¹⁵ and especially in supramolecular chemistry⁸ and crystal engineering,^16,17 where halogen bonding can provide suitable motifs for self-assembly of molecules. The halogen bond is most commonly described as an electrostatically attractive interaction between a σ-hole generated on a covalently-bound halogen, archetypally iodine in an organo-iodine compound containing fluorine substituents, and a Lewis base.^18–20 The organo-iodine compound is described as the halogen-bond donor and the base as the halogen-bond acceptor.

Halogen bonding interactions involving metal complexes in the solid state have been demonstrated extensively.^16,21–34 Three approaches to investigating such interactions have been adopted, the design of self-complementary molecules,^30,34 co-crystallization of the halogen-bond donor with the halogen-bond acceptor³¹ or formation of halogen bonds between ion-pairs.^21,29,32,33 In a self-complementary molecule, the halogen-bond acceptor and the donor are both present in the same molecule. A series of such complexes exhibiting C–X⋯X′–M (M = Pt, Pd, X = Cl, Br, I; X′ = Cl, Br) halogen bonds was demonstrated by use of halopyridine ligands as the XB donors and halide ligands as the XB acceptors.^30,34 Among examples of halogen-bonded co-crystals is a series of pincer palladium halides PdX(PCP) (PCP = 2,6-bis[(di-tert-butylphosphino)methyl]phenyl; X = Br, Cl and I) co-crystallized with I₂, 1,4-C₆F₄I₂ and I(CF₂)₄I as XB donors.³¹ The metal–halide complexes showed the general trend of the strength of the C–I⋯X–Pd halogen-bonding interaction decreasing in the order X = Cl > Br > I, and of the three XB donors used, I₂ showed the strongest interaction.³¹ These halogen-bond interactions emphasize the different roles of the two halogens involved in C–X⋯X′–M interactions. The carbon-bound halogen serves the Lewis acidic role and adopts an approximately linear C–X⋯X′ interaction geometry, whereas the metal-bound halogen serves in a Lewis basic role and adopts a markedly bent X⋯X′–M geometry. These geometries maximize the interaction between the positive potential of the organic halogen and the most negative region of the inorganic halogen. A different class of co-crystals is illustrated by ruthenium halide complexes that form a halogen-bonded network with dihalogens (Br₂ or I₂).³⁵ In spite of these thorough studies, the behaviour of metal fluorides with respect to halogen bonding remains unknown in the crystalline state.

We have reported the hydrogen- and halogen-bonding properties of group 10 metal fluoride complexes in solution for complexes of the type trans-[MF(Ar^F)(PR₃)₂] (M = Ni, Pd, Pt, Ar^F = tetrafluoropyridyl and related fluoropyridyls, R = Et, Cy).^36–38 Measurement of the dependence of the chemical shift of the ¹⁹F NMR resonance of the metal fluoride moiety on the concentration of added iodopentafluorobenzene demonstrated the formation of the F₅C₆–I⋯F–Ni halogen bonds (Scheme 1a) and allowed us to determine their free energies and enthalpies of formation, and in so doing to establish metal–fluorides as among the strongest halogen bond acceptors. This conclusion is supported by related work showing that zinc and magnesium fluoride complexes behave analogously.^39,40

Scheme 1 (a) Formation of halogen bonds with nickel fluoride in solution and (b) structural motif with envisaged intermolecular halogen-bonding interaction.

We have attempted to co-crystallize the nickel fluoride complexes with halogen-bond donors in order to determine the geometry of the halogen bond but without success. We turned instead to the alternative strategy, design of self-complementary molecules. The feasibility of this approach is indicated by the crystallization of trans-[NiF{C₅NF₃(NH₂)}(PEt₃)₂] which shows a hydrogen bond between the amino group on the fluoropyridyl ligand and the nickel fluoride on the adjacent molecule.³⁷ We therefore set about the synthesis of a nickel fluoride complex with an iodine substituent on a fluorophenyl ligand, which would enable formation of an intermolecular halogen-bond network (Scheme 1b) between the iodine as halogen-bond donor and the nickel-fluoride as halogen-bond acceptor. We report the synthesis and structures of a series of Ni–X complexes (X = F, Cl, Br, I) with the halide ligand as halogen-bond acceptors, iodine at the 2- or 4-position on the tetrafluorophenyl ligand as halogen-bond donor and different phosphine groups. Their crystal structures reveal the changes in geometry and strength of the halogen-bonding interaction in the solid state. Geometric trends are determined as a function of halogen, regiochemistry, influence of crystalline environment and temperature. For examples with fluoride ligands (X = F) we also show that the chemical shift tensor in the ¹⁹F MAS NMR spectra is highly sensitive to halogen bond formation and strength.

Results

Complexes of the type illustrated in Scheme 1b were accessed by reaction of Ni(COD)₂ with 2 equiv. of trialkylphosphine⁴¹ to form Ni(COD)(PEt₃)₂ followed by reaction with 1,4-diiodo-2,3,5,6-tetrafluorobenzene or 1,2-diiodo-3,4,5,6-tetrafluorobenzene. This synthetic method is similar to that used by Bennett with 1,6-dibromo-2,3,4,5-tetrafluorobenzene.⁴² Oxidative addition of one of the carbon-iodine bonds of diiodotetrafluorobenzene gives rise to a nickel–iodide complex that may then be converted into a nickel fluoride, chloride or bromide. The resulting complexes contain a coordinated C₆F₄I ligand as a halogen bond donor and the nickel halide as a halogen-bond acceptor. The complexes and their labels are shown in Scheme 2. The labels 1pX and 1oX refer to the position of the iodine substituent on the ring, either para (p) or ortho (o) to nickel, and the halogen (X) bound directly to nickel.

Scheme 2 Key to complexes.

Synthesis and characterization of trans-[NiX(2,3,5,6-C₆F₄I)(PEt₃)₂], 1pX (X = I, F, Cl, Br)

The synthetic methods for the 1pX series are shown in Scheme 3. The initial reaction with 1,4-diiodo-2,3,5,6-tetrafluorobenzene resulted in red trans-[NiI(2,3,5,6-C₆F₄I)(PEt₃)₂] 1pI as the major product in 84% yield. The ³¹P{¹H}-NMR spectrum showed a singlet at δ 15.4 for the two phosphorus atoms in equivalent environments, consistent with a trans geometry. The fluorine atoms of the aromatic ring of 1pI appeared in the ¹⁹F-NMR spectrum as two second-order multiplets, AA′XX′ at δ −113.3 and δ −123.6, resulting from the coupling of the neighbouring magnetically inequivalent ortho and para fluorine atoms on the ring. The peak at m/z 695.93 (due to ⁵⁸Ni with corresponding peaks for other nickel isotopes) in the LIFDI mass spectrum was identified as the molecular ion [M]⁺ base peak (100%) of trans-[NiI(2,3,5,6-C₆F₄I)(PEt₃)₂]. In addition, the ³¹P{¹H}-NMR and ¹⁹F-NMR spectra showed singlets at δ 13.5 and δ −117.7, respectively, due to a minor product. The identity of this species as [trans-NiI(PEt₃)₂]₂(μ-2,3,5,6-C₆F₄) was demonstrated by the peak at 990.17 in the LIFDI mass spectrum which corresponds to the molecular ion [M]⁺. This complex arises from the oxidative addition of both the iodine atoms on the 1,4-diiodotetrafluorobenzene (Scheme 4).

Scheme 3 Synthetic routes to complexes 1pI, 1pF, 1pCl and 1pBr.

Scheme 4 Dinuclear product [trans-NiX(PEt₃)₂]₂(μ-2,3,5,6-C₆F₄).

The reaction of 1pI with [NMe₄]F in THF gave a colour change from red to yellow in 5 h at room temperature. After purification and removal of unreacted 1pI, a yellow solid was isolated in 71% yield. The major product of the reaction was identified as trans-[NiF(2,3,5,6-C₆F₄I)(PEt₃)₂] 1pF. The ³¹P{¹H}-NMR spectrum showed a resonance at δ 13.1 as a doublet with ²J_PF = 46 Hz.⁴¹ The ¹⁹F-NMR spectrum of the purified product showed two resonances at δ −113.8 and δ −124.9 for the four aromatic fluorine atoms with second-order multiplets AA′XX′ spin system. The nickel fluoride resonance of the complex was observed as a triplet at δ −387.9, J_PF = 46 Hz.⁴¹ The LIFDI mass spectrum showed a molecular ion [M⁺] base peak (100%) at 587.99 with appropriate isotopic peaks.

The reaction of 1pI with [NMe₄]Cl in THF at room temperature gave rise to a yellow solution of 1pCl. Alternatively, complex 1pCl could be synthesised rapidly and more efficiently by reacting a solution of 1pF in toluene with chlorotrimethylsilane for 15 min at room temperature (Scheme 3). The synthesis of 1pBr paralleled the chloride analogue and used either [NEt₄]Br in THF or bromotriethylsilane in toluene.

Synthesis of trans-[NiX(2,3,5,6-C₆F₄I)(PEt₂Ph)₂], 2pI (X = I) and 2pF (X = F)

The electronic structure of the spectator ligands on the metal centre may have a significant effect on the halogen-bonding interaction, as demonstrated by earlier work.³⁶ To investigate this effect on the solid-state structures, the triethylphosphine ligands on the complexes were replaced with diethylphenyl phosphine. The complex trans-[NiI(2,3,5,6-C₆F₄I)(PEt₂Ph)₂] 2pI was prepared in a similar manner to the PEt₃ analogue. The nickel iodide 2pI obtained was converted to the nickel fluoride 2pF by reaction with [NMe₄]F (64% yield). NMR data for 2pI and 2pF corresponded closely to those of 1pI and 1pF. The complex 2pF was also characterised by its LIFDI mass spectrum, which showed a molecular ion [M⁺] base peak (100%) at 684.00.

Synthesis of trans-[NiX(2,3,4,5-C₆F₄I)(PEt₃)₂], 1oX, X = I, F, Cl and Br

Our next target was to change the position of the halogen-bond donor on the fluoroaromatic ring in order to compare the effect of the electron-withdrawing fluorine atoms ortho to the iodine and to change the relative orientation of the halogen-bond donor and acceptor groups. The synthesis of complexes from 1,2-diiodotetrafluorobenzene proceeded similarly to those from 1,4-diiodotetrafluorobenzene. The reaction of Ni(PEt₃)₂(COD) with 1,2-diiodotetrafluorobenzene gave rise to trans-[NiI(2,3,4,5-C₆F₄I)(PEt₃)₂] 1oI in 67% yield. The ³¹P{¹H}-NMR spectrum of the product exhibited a singlet at δ 10.3 for the two equivalent phosphorus atoms. The ¹⁹F-NMR spectrum revealed four resonances for the four aromatic fluorine atoms that were assigned with the help of ¹⁹F–¹⁹F COSY spectroscopy.

The product of the reaction between 1oI and [NMe₄]F yielded trans-[NiF(2,3,4,5-C₆F₄I)(PEt₃)₂] 1oF. The fluoride resonance was observed in the ¹⁹F-NMR spectrum at δ −397.8 as a triplet of doublets as a result of phosphorus coupling (J_FP = 46 Hz), and a four-bond coupling to the aromatic fluorine ortho to the metal centre (J_FF = 9 Hz). Four further ¹⁹F NMR resonances were observed in the aromatic region. The ³¹P{¹H} NMR spectrum exhibited a doublet at δ 9.4 (d, J = 46 Hz). The synthesis of 1oCl and 1oBr followed the same pattern as for 1pCl and 1pBr.

A further minor product of reaction of 1oI with [NMe₄]F exhibited resonances at δ −131.4 and at δ −150.2 in the ¹⁹F-NMR spectrum and a peak at δ 27.3 in the ³¹P{¹H}-NMR spectrum. Single crystals of this species were obtained from hexane/benzene and allowed us to identify it as the η²-tetrafluorobenzyne complex of Ni(PEt₃)₂, Ni(PEt₃)₂(η²-C₆F₄) (see ESI‡). The spectra and structure are similar to benzyne complexes reported in the literature.^42–45

The energetics of halogen bond formation in solution have been reported for complexes such as trans-[NiF(C₅NF₄)(PEt₃)₂] with C₆F₅I.³⁸ We attempted to determine the energetics of the self-complementary interactions of complex 1pF or 1oF by measuring ¹⁹F-NMR spectra as a function of concentration. However, the solubility of both complexes 1pF and 1oF proved insufficient to obtain reliable data.

Crystal structures

Single crystals of complexes 1pX were obtained by solvent diffusion. Selected intramolecular bond lengths and angles for X-ray crystal structures of 1pF, 1pCl, 1pBr and 1pI are given in Table 1; intermolecular distances and angles are given in Table 2.

Table 1 Selected intramolecular bond lengths (Å), bond angles (°) and torsion angles (°) of 1pX (X = F, Cl, Br, I), 2pI and 1oX (X = F, Cl, I) determined at 110 K

Complex	I1–Caromatic	Ni–P1	Ni–X	Ni–C1
a Bond I2–C4. b Measurements are for the major component. c Bond I2–C2. d Metal plane Ni–P1–P2–C1–X. e Angle (P1–Ni–P1′).
1pF	2.096(11)	2.208(2)	1.837(5)	1.873(13)
1pCl	2.082(2)	2.2086(5)	2.2035(5)	1.881(2)
1pBr	2.082(2)	2.2176(6)	2.3385(3)	1.881(2)
1pI	2.092(6)^a	2.224(2)	2.5221(9)	1.877(6)
2pI	2.089(2)^a	2.2200(6)	2.5348(4)	1.902(2)
1oF	2.111(6)	2.218(2)	1.841(4)	1.890(6)
1oCl	2.095(4)	2.270(5)	2.201(5)	1.899(5)
1oI(α)	2.096(4)^c	2.225(1)	2.5332(6)	1.907(4)
1oI(β)	2.098(3)^c	2.234(1)	2.5358(4)	1.893(3)
1oI(γ)	2.088(5)^c	2.231(1)	2.5337(8)	1.908(5)

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Complex	X–Ni–C1	P1–Ni–P2	P1–Ni–X	P1–Ni–C1–C2	Angle between metal plane and ring plane^d
1pF	180.0	177.1(1)^e	91.46(6)	85.2(3)	85.2(2)
1pCl	175.82(5)	175.65(2)	89.12(2)	87.9(1)	82.13(4)
1pBr	176.61(6)	176.94(2)	88.85(2)	86.2(2)	85.60(5)
1pI	178.3(2)	178.33(7)	89.55(5)	84.0(5)	85.14(16)
2pI	175.36(2)	175.36(2)	89.59(2)	88.7(2)	89.07(5)
1oF	179.3(3)	173.12(9)	88.60(16)	85.3(6)	84.4(2)
1oCl	178.1(3)	171.0(4)	89.67(16)	90.1(4)	85.1(1)
1oI(α)	173.8(1)	177.33(5)	88.87(3)	89.17(3)	90.0
1oI(β)	179.8(1)	174.72(3)	89.67(2)	89.6(3)	89.3(1)
1oI(γ)	178.1(1)	169.72(6)	92.67(4)	84.0(4)	86.8(1)

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Table 2 Intermolecular distances and the angles of 1pX, (X = F, Cl, Br, I), 2pI, 1oF, 1oCl, and 1oI determined at 110 K

Compound	Intermolecular I⋯X/Å	R ^AIX	R ^BIX	C–I⋯X/°	I⋯X–Ni/°	Angle/° between adjacent metal planes^b	Angle/° between adjacent ring planes	Ni⋯Ni⋯Ni/°
a The normalised distance, R, is defined according to Lommerse et al.^48R^AIX = d(I⋯X)/(r^AI + r^AX), where r^AI and r^AX are the Alvarez van der Waals radii^46 of the iodine atom, and halogen (F 1.46, Cl 1.82, Br 1.86 or I 2.04 Å), respectively, in the C–I⋯X–Ni halogen bond. R^BIX = d(I⋯X)/(r^BI + r^BX), where r^BI and r^BX are the Bondi van der Waals radii^47 of the iodine atom, and halogen (F 1.47, Cl 1.75, Br 1.83 or I 1.98 Å), respectively, in the C–I⋯X–Ni halogen bond. b Metal plane Ni–P1–P2–C1–X.
1pF	2.655(5)	0.76	0.77	180.0	180.00	0.0	0.0	180.00
1pCl	3.2414(5)	0.84	0.87	167.09(5)	146.83(2)	72.69(2)	88.01(8)	159.91(1)
1pBr	3.3320(3)	0.85	0.87	167.55(6)	143.31(1)	70.91(3)	87.8(1)	158.95(1)
1pI	3.4970(6)	0.86	0.88	168.0(2)	139.18(3)	67.41(8)	87.4(3)	156.79(2)
2pI	3.6791(4)	0.90	0.93	167.39(6)	158.22(1)	0.0	0.0	180.00
1oF	2.941(5)	0.84	0.85	173.2(2)	172.4(2)	72.3(3)	31.7(3)	113.55(2)
1oCl major	3.342(6)	0.87	0.90	169.4(2)	158.1(3)	54.5(2)	26.1(2)	112.1(1)
1oCl minor	3.26(2)	0.84	0.87	178.7(4)	171.8(8)	72.1(7)	26.1(2)	108.8(2)
1oI(α)	3.5961(4)	0.88	0.91	178.5(1)	144.16(2))	77.1(1)	0.0	97.00(1)
1oI(β)	3.7456(4)	0.92	0.95	151.7(1)	133.48(1)	45.1(1)	12.4(2)	124.80(1)
1oI(γ)	4.0752(7)	1.00	1.03	169.2(1)	161.63(2)	64.6(1)	39.5(2)	101.06(1)

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For 1pF the fluoroaromatic ring is oriented almost perpendicular to the square plane of the metal coordination centre, 85.2(2)°. The crystal structure comprises chains of the nickel complexes linked by short C–I⋯F(Ni) halogen bonds (I⋯F 2.655(5) Å, C–I⋯F 180.0°, I⋯F–Ni 180.0°, Fig. 1a). The parallel sets of linear chains align with the b-axis and form layers in the ab plane. Complexes 1pCl, 1pBr and 1pI (Fig. 1b–d, Table 1) crystallize as antiparallel zig-zag chains with molecules linked by a C–I⋯X–Ni halogen bonds. The Ni⋯Ni⋯Ni angle (Table 2) may be taken as an estimate of the zig-zag angle and indicates a slight zig-zag for X = Cl, Br and I (Ni⋯Ni⋯Ni 159.91(1), 158.95(1), 156.79(2)°, respectively, cf. 180° for X = F). The intermolecular distances I⋯X are substantially less than the sum of van der Waals radii (R^A_IX 0.84–0.86; R^B_IX 0.84–0.87; see Table 2 for definitions of distances, R^A_IX and R^B_IX, which represent the fraction of the sum of van der Waals radii^46,47 of the two interacting atoms I and X), but show smaller reductions than observed for 1pF (R^A_IF 0.76; R^B_IF 0.77). The C–I⋯X angles deviate slightly from linear (ca. 167°) while the I⋯X–Ni angles lie in the range 139–147°. The torsional angles between adjacent metal coordination planes in the chains are ca. 70°, while adjacent benzene ring planes are approximately orthogonal (ca. 88°, Table 2). In addition, the crystal structure of the minor dinuclear product, [trans-NiBr(PEt₃)₂]₂(μ-2,3,5,6-C₆F₄) was determined (see ESI‡ and Scheme 4).

Fig. 1 Molecular structures of 1pX (a) X = F, (b) X = Cl, (c) X = Br, (d) X = I, showing intermolecular halogen bonds. Displacement ellipsoids at 50% probability level. Hydrogen atoms not shown. Three-molecule chains are illustrated in the ESI.†

Single crystals of 2pI suitable for X-ray crystallography were grown as dark yellow blocks by solvent diffusion (Fig. 2). The three aromatic rings present in the structure orient themselves away from each other and there are neither inter- nor intramolecular π–π interactions. Unlike 1pI, the Ni coordination planes and the tetrafluoroiodophenyl rings are parallel to one another. The complexes pack with zig-zag halogen-bonded chains, which run parallel (and anti-parallel) to the b-axis. The C–I⋯I–Ni halogen bonds (I⋯I 3.6791(4) Å, C–I⋯I 167.39(6)°, I⋯I–Ni 158.22(1)°) are markedly longer than those of 1pI (Table 2). Attempts to obtain single crystals of 2pF suitable for X-ray crystallography were fruitless.

Fig. 2 Molecular structure of 2pI showing intermolecular C–I⋯I–Ni halogen bond. Displacement ellipsoids at 50% probability level. Hydrogen atoms not shown. A three-molecule chain is illustrated in the ESI.‡

Single crystals of 1oF, 1oCl and 1oI were obtained that were suitable for X-ray crystallography.|| Selected intramolecular distances and angles are given in Table 1 and intermolecular distances and angle in Table 2. The crystal structures are illustrated in Fig. 3. It might be expected that the Ni–C distances in the 1pX series would be shorter than those in the 1oX series because of the ortho-fluorine effect.⁴⁹ Although the Ni–C distances are consistently shorter for 1pX than for their 1oX analogues, the differences are of marginal significance. The differences in the C–I distances are insignificant. In all of the 1oX halogen-bonded complexes, the iodine on the ring in the ortho position causes a pronounced zig-zag of the complexes, as described by the Ni⋯Ni⋯Ni angle (96 ≤ Ni⋯Ni⋯Ni ≤ 125°, Table 2). The normalised distances of the I⋯X contacts R^B_IX are 0.85 for 1oF, 0.90 for 1oCl (major) and 0.91–1.03 for 1oI, much larger than for 1pX analogues.

Fig. 3 Molecular structures of 1oX (a) X = F, (b) X = Cl, (c) X = I, showing intermolecular C–I⋯X–Ni halogen bonds. The structure of 1oCl is disordered (see Table 2). Only the major component is shown. Hydrogen atoms and some methyl groups omitted for clarity. Displacement ellipsoids at 50% probability level. Three-molecule chains are illustrated in the ESI.‡

Single crystals of 1oI were obtained as three polymorphs, 1oI(α) crystalized from hexane, and the other two (1oI(β) and 1oI(γ)) from CHCl₃ layered with hexane. Polymorph 1oI(α) shows the typical C–I⋯I–Ni halogen-bonding contacts (3.6100(3) Å). The polymorphs from chloroform exhibited the same patterns of halogen bonding but with progressively longer contacts. The C–I⋯I–Ni contact is 3.7456(4) Å for 1oI(β), (major component), while 1oI(γ) has two independent molecules in the unit cell, one of which has a C–I⋯I–Ni distance of 4.0752(7) Å and the other shows no halogen bonding at all. The changes in halogen-bonding geometry accompany changes in orientation of molecules along the halogen-bonded chains, exemplified by the twisting of the ring planes of adjacent molecules with respect to one another for 1oI(β) and 1oI(γ), but coplanar arrangement in 1oI(α).

Variable-temperature X-ray crystallographic studies of 1pF′ and 1pF

The effects of temperature and pressure on C–Cl⋯X–M halogen bonds (X = Cl, Br) have been studied by Mínguez Espallargas et al., illustrating the compressibility of these non-covalent interactions.^16,50 In order to quantify the temperature dependence of our system, crystals of 1pF were selected as they showed the strongest X-bonding interaction of the series of complexes prepared. A crystal of 1pF was cooled to 111 K and the structure determined, warmed to 200 K and analysed again, and finally warmed to 240 K and the structure obtained a third time. At 111 K and 200 K, the space group was I2 whereas at 240 K the space group was Cc. The space group change was accompanied by a change in the unit cell, suggesting a phase change between 200 and 240 K. Overlaying the structures at 200 and 240 K revealed that the major difference in structures is the position of one of the triethylphosphine ligands (Fig. S41‡). At the higher temperature, the triethylphosphines are no longer related by a two-fold rotation and one is disordered in two positions (see ESI‡). The variable temperature X-ray crystallographic data of 1pF show that a 4.5% reduction in the volume per molecule (V/Z) is observed when the temperature is changed from 240 K to 200 K and ca. 2% reduction in the volume is observed from 200 K to 111 K (Table 3). The change in space group from I2 to Cc removes the linearity of the halogen-bonded chain. Nevertheless, the key angles remain close to 180°: the C–I⋯F angle is 177.8(8), I⋯F–Ni 164.8(7) and the Ni⋯Ni⋯Ni angle is 171.5(3)°. Although changes in bond lengths to nickel were insignificant, the intermolecular distance between the fluoride on the metal centre and the iodine on the fluoroaromatic ring decreased with temperature by 0.055(7) Å from 240 to 111 K. The normalised halogen-bonded distance R^B_IF remains below 0.80 throughout the temperature range (Table 2).

Table 3 Selected variable-temperature X-ray crystallographic data for complex 1pF

	Space group/temperature, K
	I2/111^a	I2/200	Cc/240
a Independent determination from 110 K study reported in Tables 1 and 2. b Metal plane Ni–P1–P2–C1–X.
V/Z/Å^3	577.09(5)	588.17(4)	614.7(1)
(C)I⋯F(Ni)/Å	2.652(3)	2.683(5)	2.707(7)
Ni–F/Å	1.839(4)	1.832(5)	1.855(10)
Ni–C/Å	1.872(10)	1.879(10)	1.88(2)
Ni–P/Å	2.211(1)	2.214(1)	2.209(5)
C–I⋯X/°	180.0	180.0	177.8(7)
I⋯X–Ni/°	180.0	180.0	164.8(7)
Angle/° between adjacent metal planes^b	0.0	0.0	26.3(7)
Angle/° between adjacent ring planes	0.0	0.0	4.9(10)
Ni⋯Ni⋯Ni/°	180.0	180.0	171.50(3)

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Solid-state NMR spectroscopic studies

Magic-angle spinning solid-state NMR (MAS SSNMR) spectroscopy offers an opportunity to learn more about the halogen-bonding interactions.^51–53 We have found one other example of a ¹⁹F-MAS SSNMR spectrum for a nickel fluoride complex⁵⁴ and one example of calculated ¹⁹F tensors for some cobalt fluorides.⁵⁵ The fluoride bound to the nickel centre can act as a ¹⁹F-MAS SSNMR spectroscopic handle because the chemical shift is very sensitive to the fluoride environment³⁶ and appears at high field with no overlap from other resonances. ¹⁹F-MAS SSNMR spectroscopy was used to analyse the effect on the fluoride resonance of halogen-bonding interactions in complexes 1pF and 1oF. The complex trans-[NiF(C₆F₅)(PEt₃)₂] 3F, which has no halogen-bond donor atom on its backbone was also analysed by ¹⁹F-MAS SSNMR spectroscopy for comparison.⁴¹ The spectrum of 1pF (Fig. 4) shows the isotropic chemical shift of the nickel fluoride at δ −359.8 with numerous spinning side bands (black dots in Fig. 4). The other peaks in the range of δ −100 to −160 arise from the resonances of the fluoroaromatic ring. Measurement at various spinning speeds allowed simulation of spectrum and determination of the components of the chemical shift anisotropy (CSA) tensor and identification of δ_iso of the fluoride resonance. Expansions of the isotropic region of the spectra are shown in Fig. 5 and the trends in the CSA components in Fig. 6.

Fig. 4 ¹⁹F-MAS SSNMR spectrum of 1pF. Top: best fit simulated spectrum for the Ni–F resonance with the positions of the shielding tensor marked. Bottom: experimental spectrum. Spinning speed 15.6 kHz. Signals arising from the fluorinated benzene ligand (−110 to −160 ppm) are truncated. The black dots indicate the NiF peak and its spinning side bands. The blue bar indicates δ_iso.

Fig. 5 Expansion of isotropic region of ¹⁹F-MAS SSNMR spectrum of (a) 1pF, (b) 1oF and (c) trans-[NiF(C₆F₅)(PEt₃)₂] 3F with no halogen bond, (asterisk solvated complex impurity <4%).

Fig. 6 Top: positions of the observed (¹⁹F-MAS SSNMR) principal tensor components (δ₁₁, δ₂₂, δ₃₃) and isotropic shift (δ_iso) for 3F, 1oF and 1pF. Middle: values of δ₁₁, δ₂₂, δ₃₃, and δ_iso determined by DFT calculations for trans-[NiF(2,3,5,6-C₆F₄I)(PMe₃)₂] and its halogen-bonded complex with C₆F₅I. Bottom: calculated structure of C₆F₅I⋯FNi(2,3,5,6-C₆F₄I)(PMe₃)₂ showing orientation of CSA tensor (δ₃₃ component is perpendicular to the plane of the page).

The components of the anisotropic chemical shift tensor are labelled as δ₁₁, δ₂₂ and δ₃₃, where δ₁₁, is the least shielded component and δ₃₃ is the most shielded component (δ₁₁ ≥ δ₂₂ ≥ δ₃₃).⁵⁶ For 1pF, the simulations show δ₁₁ at δ −165, δ₂₂ at δ −266 and δ₃₃ at δ −645. The ³¹P{¹H} spectrum of 1pF (see ESI‡) reveals a coupling ²J_PF = 42 ± 3 Hz, similar to the value observed in solution (46 Hz). We were unable to obtain the J_PF coupling in the ¹⁹F-MAS SSNMR spectra for 1oF and 3F.

In addition, the structure of the gas-phase model trans-[NiF(2,3,5,6-C₆F₄I)(PMe₃)₂] and its halogen-bonded complex with C₆F₅I were determined by DFT methods showing a (C)I⋯F(Ni) distance of 2.749 Å. DFT simulation of the ¹⁹F-MAS SSNMR spectra of both these species showed that δ₁₁ lies along the Ni–F bond, δ₂₂ lies parallel to the Ni–P bonds and δ₃₃ lies perpendicular to the coordination plane of nickel (Fig. 6, lower pane). Moreover, the CSA calculated by DFT methods for trans-[NiF(2,3,5,6-C₆F₄I)(PMe₃)₂] and trans-[NiF(2,3,5,6-C₆F₄I)(PMe₃)₂]⋯IC₆F₅ show that the changes in the shielding of the δ₁₁ and δ₂₂ components are small and of opposite sign, thus minimizing their effect on the isotropic shift.

The isotropic chemical shift δ_iso of the fluoride resonance of 3F was observed at δ −393.9 in the ¹⁹F-MAS SSNMR spectrum (Table 4, Fig. 5), in good agreement with the solution-state fluoride resonance at δ −394.3 (in C₆D₆) for the same complex. In contrast, the solution-state fluoride resonance of 1pF appeared ∼29 ppm to higher field at δ −388.3 than its solid state isotropic chemical shift (δ −359.8). Similarly, the solution-state fluoride resonance of 1oF was observed ∼25 ppm to higher field at δ −397.9 than its solid state counterpart. Thus the solid-state NMR spectra provide strong evidence of deshielding of δ_iso with increasing halogen-bond interaction. Table 4 gives values of the SSNMR parameters. The δ₃₃ component is systematically deshielded upon increasing halogen bond strength, mirroring what is observed in the isotropic chemical shift, and there is also a reduction in the span, Ω. Since the DFT calculations indicate a minimal contribution of δ₁₁ and δ₂₂ to the changes in δ_iso, we conclude that the changes in δ₃₃ are dominated by the effect of halogen bonding. The deshielding of the NiF resonance is also consistent with the effect of addition of C₆F₅I to trans-[NiF(C₅NF₄)(PEt₃)₂] in solution.^36,38 A very large chemical shift anisotropy was also reported for another nickel fluoride complex,⁵⁴ and very strong paramagnetic shielding was observed perpendicular to the metal phosphorus bond for terminal phosphido complexes in their ³¹P-MAS SSNMR spectra.⁵⁷ Large chemical shift anisotropies have recently been measured and calculated for metal hydrides.^58,59

Table 4 Chemical shifts in solution in C₆D₆ (40 μM) and ¹⁹F-MAS SSNMR parameters for 1pF, 1oF and 3F^a

Sample	Solution δ	δ iso	Ω	κ	δ 11	δ 22	δ 33
a Chemical shift tensor parameters,^56δiso Isotropic chemical shift, δiso = (δ11 + δ22 + δ33)/3, Ω Span of the CSA powder pattern, Ω = δ11 – δ33, κ Skew, measures the asymmetry of the powder pattern, κ = (δ22 − δiso)/Ω.
1pF	−388.3	−359.8(2)	480(10)	0.58(3)	−165	−266	−645
1oF	−397.9	−373.0(2)	530(10)	0.40(3)	−143	−302	−673
3F	−394.3	−393.9(2)	575(10)	0.50(3)	−154	−298	−729

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The paramagnetic contribution to the components of the chemical shift tensor are understood via second order perturbation theory as being affected by the angular momentum operator L̂_i (i = x, y, z) which couples occupied orbitals to vacant orbitals after a 90° rotation about the direction i.^60,61 In this case δ₃₃ lies in the out-of-plane direction z and is rotated by L̂_y into the x direction that coincides with the vacant M–F σ* orbital. It is this orbital that is affected by the halogen bonding both through electrostatic effects and through interaction with the C–I σ and σ* orbitals.⁵² As the δ₃₃ component is deshielded upon formation of the halogen bond, the energy gap between these two orbitals must decrease.

Discussion of trends in halogen bonding

This study establishes that nickel halide complexes trans-[NiX(C₆F₄I)(PR₃)₂] form intermolecular C–I⋯X–M halogen bonds by self-recognition in the solid state, leading to halogen-bonded chain motifs. Their crystal structures provide extensive geometric data for these halogen bonds that demonstrate a marked difference between the fluoride complexes and those of the other halides. The strengths of halogen bonds may be compared by the normalised distances (here R^A_IX or R^B_IX, see Table 2).^16,48 As for other strong halogen bonds, C–X⋯X′–M halogen bonds adopt a characteristic angle C–X⋯X′ close to 180°; the X⋯X′–M angle, however, is typically in the 110–140° range for X′ = Cl, Br, I,^30,31,62 but can vary more widely^23,63 as the halide ligand exhibits a negative electrostatic potential attractive to halogen bond donors and other Lewis acidic moieties at all angles of approach.⁶⁴ Despite their established greater strength,^36–39 there have been no systematic crystallographic studies prior to this study that establish C–X⋯F–M halogen bond geometries and, in particular typical X⋯F–M angles.

In this study we examined the class of C–I⋯X–M halogen bonds through a series of systematic changes and perturbations, by monitoring the geometry as a function of: (i) halide ligand, X, (ii) regiochemistry of halogen-bond donor, I (1pXvs.1oX) (iii) influence of ligand sphere of the metal on halogen-bond acceptor, X, by changing PEt₃ for PPhEt₂, (iv) the influence of crystalline environment, by comparison of three polymorphs tnqh_x2026;of 1oI, and (v) the effect of temperature, in a study of 1pF.

Common features of halogen-bonded chain structures

1pF forms linear chains, whereas the structures of 1pX (X = Cl, Br, I) and 1oX (all X) form zig-zag chains. The zig-zag angles of the chains of 1pCl, 1pBr and 1pI are characterised by Ni⋯Ni⋯Ni angles in the 155–160° range (see figures in ESI‡). The angles between adjacent metal coordination planes and the angles between adjacent benzene ring planes also show marked differences between 1pF on the one hand and 1pCl, 1pBr and 1pI on the other (Table 2). The zig-zag in the ortho series is dominated by the 60° angle imposed by the ortho substitution, which causes a major reduction in the Ni⋯Ni⋯Ni angle compared to the para analogues (1pF 180° and 1oFca. 114°, 1pClca. 160° and 1oClca. 112°, 1pIca. 157° and 1oI(α)ca. 97°). Both the angle between adjacent ring planes and the Ni⋯Ni⋯Ni angle are significantly smaller for 1oI(α) than for the fluorine and chlorine analogues.

Trend with halide ligand for 1pX and 1oX: distance

The normalised distances (Table 2) range from 0.76 for 1pF to 0.90 for 2pI (Alvarez radii, R^A_IX) or 0.77 to 0.93 (Bondi radii, R^B_IX).^46,47 The normalised distance is markedly smaller for 1pF than for 1pCl, 1pBr and 1pI. Nevertheless, there is a small increase from Cl to Br and from Br to I. Thus the large negative electrostatic potential of fluorine is the dominant effect^38,64 and the changes between the other halogens are smaller in this series. The trend is consistent with the observations for hydrogen-bond acceptor behaviour of halide ligands, e.g. D–H⋯X–M, where the H-bond donor D = N, O (or even C). Normalised hydrogen bond distances follow the trend R^B_HF ≪ R^B_HCl ≤ R^B_HBr < R^B_HI.⁶⁴ Overall, the geometric results are consistent with fluoride ligands being much stronger halogen-bond and hydrogen-bond acceptors than their heavier congeners. Our results on 1pCl and 1pBr are also comparable to those recorded for the co-crystals of 1,4-(C₆F₄I₂) with PdX(PCP) (X = Cl, Br), but the Pd–F complexes were not synthesised.³¹

The normalised distances for the 1pX series are considerably smaller than for the 1oX series, especially for X = F (Table 2). Considering that the regiochemistry has a very minor effect on the Ni–C and the C–I distances, there is no evidence that the changes in the halogen-bond distance have an electronic origin. On the other hand, they can be rationalised on steric grounds since the σ-hole of the 1pX halogen-bond donor atom is more accessible than that in the 1oX series. The trend in normalised distances with halide ligand for the 1oX series, however, resembles that for 1pX.

Trend with halide ligand for 1pX and 1oX: angle

Halogen bonding interactions are directional forces because of the anisotropic charge distribution around halogen atoms. The charge distribution is compressed along the C–I axis, creating the positive potential trans to the C–I bond, leading to C–I⋯X angles that are close to linear.^16,18,48 Halide ligands (M–X) exhibit a similar anisotropy, albeit with a negative potential in all directions at separations appropriate for intermolecular interactions. Brammer et al.^30,64 reported the calculated electrostatic potential around the halogen in palladium halide complexes, trans-[PdX(CH₃)(PH₃)₂]. They showed that the most negative value in the molecular plane is consistent with I⋯X–M angles at 155° with a range of 130–180° range when X = F, whereas the other halogens exhibited minima at ca. 124° and a range of ca. 110–140°.⁶⁴ In the 1pX series of complexes, C–I⋯X angles are close to linear, consistent with typical halogen-bond behaviour (1pF 180°; 1pCl, 1pBr, 1pI all close to 167°, Scheme 5). The I⋯X–Ni angle of 1pF is 180°, exceeding the I⋯X–Ni angles for the other halides, which are in the range 139–147° (Table 2, Scheme 5). This result is in keeping with the analysis of angular variation of the halide negative electrostatic potential mentioned above, and is consistent with more restricted steric access to the halide ligand at lower I⋯X–Ni angles, since Ni–F and I⋯F separations are markedly shorter than for other Ni–X and I⋯X.

Scheme 5 Halogen bond length and C–I⋯Ni and I⋯X–Ni angles for the 1pX series (phosphines and aromatic fluorines omitted, for esd's see Table 2).

The C–I⋯X angles are again similar and close to linear across the 1oX series. The variation in the I⋯X–Ni angle is more marked, moving from 172.4(2)° to 158.1(3)° and 143.66(1)° along the series 1oF, 1oCl, 1oI(α) (Scheme 6, Table 2), which is qualitatively consistent with the trend in location of electrostatic potential minima at the halide ligand.

Scheme 6 Halogen bond length and C–I⋯Ni and I⋯X–Ni angles for 1oF, 1oCl and 1oI(α) (phosphines and aromatic fluorines omitted, for esd's see Table 2).

Effect of phosphine substituents

The change in phosphine from 1pI with PEt₃ to 2pI with PEt₂Ph causes a marked increase in the halogen-bond distance, indicating the sensitivity of the halogen bond to the ligand sphere of the metal. There are no solution-phase measurements on PEt₂Ph complexes for comparison although an increase in enthalpy of C–I⋯F–Ni halogen bonding when replacing PEt₃ ligands by PCy₃ has been reported.³⁸

Trend with polymorph for 1oI

The crystallisation of three solvent-free polymorphs of 1oI allowed the effect of crystalline environment on formation and geometry of the C–I⋯I–Ni halogen bond to be examined. The three polymorphs provide four different crystalline environments as there are two crystallographically independent molecules in 1oI(γ). The I⋯I distances vary from 3.5961(4) Å in 1oI(α) to 3.7456(4) in 1oI(β) to 4.0752(7) Å 1oI(γ), whereas the second independent molecule exhibits no halogen-bonding interaction. The different crystalline environments also lead to changes in halogen-bond angles C–I⋯I and I⋯I–Ni, and to changes in relative orientation of neighbouring molecules in the halogen-bonded chains (Table 2).

Trend with temperature for 1pF

The study of the structure of 1pF using one single crystal at three temperatures** enabled the perturbing effect of temperature on the geometry of the C–I⋯F–Ni halogen bond to be examined. The halogen-bonded distance remains well over 20% shorter than the sum of the van der Waals radii (R^A_IF, R^B_IF < 0.8), and C–I⋯F and I⋯F–Ni angles remain close to linearity, across the temperature range studied (111–240 K). Two previous studies of halogen-bonding as a function of temperature also reveal only very small changes in halogen bond angles. Forni et al. reported changes in halogen-bond distance of 0.052(2) Å (C–I⋯N), 0.030(2) Å (C–I⋯O) and 0.059(2) Å (C–Br⋯N) over the temperature range 90–292 K.⁶⁵ Brammer et al. reported changes of 0.070(2) Å (C–Cl⋯Cl(M)) and 0.079(1) Å (C–Cl⋯Br(M)) over the temperature range 30–300 K.^16,50 These values are consistent with the changes observed here, viz. 0.055(7) Å. Such changes are greater than observed for intramolecular distances, but smaller than those for other intermolecular separations, which reflects the strength of these halogen bonds and, in conjunction with their directionality, underlines their utility in supramolecular assembly.

Conclusions

Our design strategy for a full range of structures of the type trans-[NiX(C₆F₄I)(PR₃)₂], for all halogens X, reveals supramolecular chain structures containing self-complementary halogen-bonding motifs linked by C–I⋯X–Ni halogen bonds. This methodology fulfils the objectives for thorough geometric comparisons and provides a platform for development of supramolecular assemblies with metal–fluoride complexes, which form the strongest halogen bonds. Moreover, the methods offer an entry into solid-state NMR studies of halogen bonding by use of the ¹⁹F nucleus of the fluoride ligand as a spectroscopic probe that is not complicated by nuclear quadrupole effects. Such NMR studies will also enable improved comparison between halogen bonds in the solid state and solution phase. This type of comparison is rarely accessible, but is important because the solid state provides the most accurate geometric information through crystallography, while the solution phase provides the most direct experimental means of determining interaction energies through spectroscopic titrations.

The strongest halogen bonds are formed by trans-[NiF(p-C₆F₄I)(PEt₃)₂], judged both by the reduction in I⋯X distance compared to the sum of the van der Waals distances and by the ¹⁹F SSNMR chemical shift. This system contains a linear chain. Other complexes of the type trans-[NiX(p-C₆F₄I)(PEt₃)₂] (1pX, X = Cl, Br, I) exhibit zig-zag chain structures with angles C–X⋯I and I⋯X–Ni, and angles between Ni coordination planes and between ring planes that are similar for X = Cl, Br and I. The corresponding complexes trans-[NiX(o-C₆F₄I)(PEt₃)₂] (1oX, X = F, Cl, Br, I) all form chains in the solid state with more pronounced zig-zags and weaker halogen bonds than their p-C₆F₄I counterparts (1pX). The relative strengths of the halogen bonds, as judged by smaller normalised C–I⋯X halogen-bond distances (R^A_IX, R^B_IX), follow the pattern 1pX > 1oX and F > Cl ∼ Br > I, consistent with observations of hydrogen bond acceptor capabilities of halide ligands, X. The variation in the I⋯X–Ni angles is consistent with the location of the electrostatic potential minima associated with anisotropic charge distribution of the halide ligand, X, which have a directing effect on the halogen bond.

We have shown by comparison of three polymorphic forms of 1oI that the crystalline environment can have a marked effect on halogen bond geometry and indeed on whether a (weaker) halogen bond is formed at all. These results emphasise the importance of examining trends in intermolecular interaction geometries in the solid state and avoiding overinterpretation of the link, for example, between intermolecular distances and interaction strength based on individual solid-state observations. The study of the crystal structure of 1pF over the temperature range 111–240 K reveals only a small change in halogen bond length, consistent with the C–I⋯F–Ni halogen bond being a strong intermolecular interaction.

Finally, we have shown that it is possible to demonstrate the existence of halogen bonding in the fluoride complexes with ¹⁹F-MAS SSNMR spectroscopy. With a suitable reference, trans-[NiF(C₆F₅)(PEt₃)₂] 3F, the presence of the halogen bonds is manifest through downfield shifts in the resonance of the Ni–F as has previously been reported in solution. The ¹⁹F SSNMR isotropic chemical shifts follow the order 3F < 1oF < 1pF in agreement with the crystal structures, where 1pF shows the strongest halogen-bonding interactions (smallest R_IX values). Comparisons between solution and solid-state NMR indicate that C–I⋯F–Ni halogen bonding causes deshielding of δ_iso by 25–30 ppm; examination of the chemical shift tensor indicates that by far the biggest contributor to the deshielding is δ₃₃, the component perpendicular to the metal coordination plane. This shift is consistent with coupling of the occupied F(2p_z) orbital with the vacant M–F σ* orbital; it is this orbital that is influenced by the halogen bonding.

Experimental

All experiments involving oxygen- and water-sensitive materials were performed under an argon or nitrogen atmosphere, in an argon-filled glove box or standard Schlenk (10⁻² mbar) techniques or high vacuum lines (10⁻⁴ mbar). Solvents (AR grade) for general use were dried over sodium, distilled and stored under argon. Solvents such as hexane and THF were collected from the Innovative Technology solvent purification system and were further dried and distilled. Deuterated solvents were dried over potassium and distilled prior to use.

All standard NMR spectra were recorded on Bruker AV I 500 or AV III 500 spectrometers, unless otherwise stated, in tubes fitted with Young's PTFE stopcocks. All ¹H and ¹³C chemical shifts are reported in ppm (δ) relative to tetramethylsilane and referenced using the chemical shifts of residual protio solvent resonances (benzene, δ 7.16), unless otherwise stated. The ³¹P{¹H} NMR spectra were referenced to external H₃PO₄. ¹⁹F NMR spectra were referenced to external CFCl₃.

LIFDI mass spectra were recorded on a Waters Micromass GCT Premier orthogonal time-of-flight instrument set to 1 scan per s with resolution power of 6000 FWHM and equipped with a LIFDI probe from LINDEN GmbH. Toluene was used for tuning the instrument.^66–68 The polyethylene glycol probe was kept at ambient temperature with the emitter potential at 12 kV. Activated tungsten wire LIFDI emitters (13 μm W from LINDEN) were ramped manually up to 100 mA for the emitter heating current during the experiment. The m/z values are accurate to 0.01 Da. Masses are quoted for ³⁵Cl, ⁵⁸Ni, ⁸¹Br.

X-ray diffraction data were collected on an Oxford Diffraction SuperNova diffractometer with MoKα radiation (λ = 0.71073 Å) at 110 K unless otherwise noted. Data collection, unit cell determination and frame integration were carried out with the program CrysAlisPro.⁶⁹ Absorption corrections were applied using crystal face-indexing and the ABSPACK absorption correction software within CrysAlisPro. Structures were solved and refined using Olex2 ⁷⁰ implementing SHELX algorithms. Structures were solved by either Patterson or direct methods using SHELXS-97 and refined by full-matrix least squares using SHELXL-97.⁷¹ All non-hydrogen atoms were refined anisotropically. Carbon-bound hydrogen atoms were placed at calculated positions with fixed isotropic displacement parameters and refined using a “riding model”. Detailed crystallographic data are provided in the ESI.‡

The ¹⁹F-MAS NMR spectra were acquired at 9.4 T (376.48 MHz) using a Bruker Avance III HD console and a Bruker 2.5 mm H(F)/X double-resonance probe. The MAS rotors and caps were pumped under high vacuum for 24 hours then stored in the glove box where they remained for at least a week before use. A 30° tip-angle pulse was used to acquire the spectra to ensure sufficient radio frequency bandwidth to yield undistorted spinning sideband intensities. Relaxation delays of 4–7 s were used. Spectra were collected at 295 K unless stated otherwise. All shifts are reported relative to CFCl₃ and were calibrated using solid sodium fluoride (δ_iso = −224.2) as a secondary external reference. Isotropic chemical shifts were determined by comparison of three or more spectra acquired over a range of rotation frequencies (9–30 kHz). Simulation of the shielding tensors was performed using the CSA tensor module⁷² within the WSolids NMR Simulation Package (Ver. 1.20.18).

Syntheses

Ni(COD)₂ was purchased from Sigma-Aldrich. 1,4-C₆F₄I₂, 1,2-C₆F₄I₂ (Fluorochem), Me₃SiCl and Et₃SiBr (Sigma-Aldrich) were distilled and were stored over molecular sieves. Triethylphosphine was purchased from Fluorochem. Commercially available tetramethylammonium fluoride tetrahydrate (Sigma-Aldrich) was dried following the literature method.^73,74 Complex 3F was synthesized as previously described.⁴¹

¹⁹F NMR aromatic resonances were assigned based on their chemical shifts and coupling constants, and ¹⁹F–¹⁹F COSY spectra. Note that ¹⁹F nuclei ortho to the metal always resonate to lower field than the meta or para¹⁹F nuclei. Fluorine atoms are labelled as below:

trans-[NiI(2,3,5,6-C₆F₄I)(PEt₃)₂] (1pI). A solution of Ni(PEt₃)₂(COD) was prepared by adding PEt₃ (69 μL, 0.36 mmol) to a stirred suspension of Ni(COD)₂ (50 mg, 0.18 mmol) in hexane (10 mL), in a 25 mL Schlenk flask inside the glove box. The resulting mixture was stirred further to obtain a clear orange solution. A solution of 1,4-C₆F₄I₂ (106 mg, 0.26 mmol) was prepared separately in hexane (5 mL) in a 50 mL Schlenk flask. To this the Ni(PEt₃)₂(COD) solution was added dropwise while stirring. The resulting red-brown solution was left stirring further for 15 min and then allowed to stand for 1 h upon which a red-brown solid precipitated. The solution was decanted and the residue was retained. The resulting solid was washed with cold pentane twice and dried under vacuum. Yield 105.1 mg, 84%. Single crystals for X-ray crystallographic studies were grown by layering of a saturated solution of 1pI in THF with hexane, giving dark red crystals at the interface of the two solvents at room temperature. NMR (C₆D₆, 298 K). ¹H NMR δ 0.84 (apparent quin, J 8.0 Hz, 18H, CH₃), 1.35 (m, 12H, CH₂). ³¹P{¹H} NMR: δ 15.4 (s). ¹⁹F-NMR: δ −113.3 (AA′XX′, 2F, F^b), −124.1 (AA′XX′, 2F, F^a). Mass spectrum (LIFDI, m/z⁺) 695.93 (100% M⁺): anal. calcd for C₁₈H₃₀F₄I₂NiP₂ C, 31.02; H, 4.34; found: C, 31.12; H, 4.32.

trans-[NiF(2,3,5,6-C₆F₄I)(PEt₃)₂] (1pF). To a THF solution (50 mL) of 1pI (100 mg, 0.14 mmol) in a 250 mL Schlenk flask was added an excess of anhydrous tetramethylammonium fluoride (150 mg, 1.6 mmol). The solution was stirred at room temperature for 5 h giving a red-brown precipitate which was discarded. The reaction mixture was allowed to settle for 1 h and the yellow solution was decanted and passed through a short column of Celite into another Schlenk flask and the solvent was evaporated under vacuum. The residue from this evaporation was extracted with benzene as far as possible and undissolved material was allowed to settle for 30 min. The yellow solution was decanted and passed through a short column of Celite into another Schlenk flask. The solvent was evaporated under vacuum. The resulting yellow solid product was washed with cold pentane twice and dried under vacuum. Yield 60.1 mg, 71.1% yield. Single crystals of 1pF suitable for X-ray crystallographic studies were obtained by slow diffusion of hexane into a saturated solution of 1pF in THF/benzene at room temperature.

NMR (C₆D₆, 298 K). ¹H NMR δ 0.97 (apparent quin, J 7.7 Hz, 18H, CH₃), 1.06–1.14 (m, 12H, CH₂). ³¹P{¹H} NMR: δ 13.1 (d, J_PF 46 Hz). ¹⁹F NMR: δ −113.8 (AA′XX′, 2F, F^b), −123.6 (AA′XX′, 2F, F^a), −387.9 (t, J_PF 46 Hz, 1F, Ni–F). Mass spectrum (LIFDI, m/z⁺) 587.99, (100% M⁺). Anal. calcd for C₁₈H₃₀F₅INiP₂ C, for 36.71; H, 5.13. Found: C, 36.59; H, 5.06.

trans-[NiCl(2,3,5,6-C₆F₄I)(PEt₃)₂] (1pCl). (a) From 1pI and [Me₄N]Cl. Method as for 1pF. Yield 55.2 mg, 63.5%. (b) From 1pF and Me₃SiCl. Chlorotrimethylsilane (13 μL, 0.1 mmol) was added to a solution of THF (10 mL) of 1pF, (50 mg, 0.08 mmol) and stirred for an hour. The solvent and excess reagents and by-products were removed under vacuum. The product was washed with pentane and dried. Yield 47 mg, 92%. Single crystals of 1pCl suitable for X-ray crystallography were obtained by slow diffusion of hexane into a saturated solution of 1pCl in THF/benzene and slow evaporation at room temperature over two weeks, yielding bright yellow blocks. NMR (C₆D₆, 298 K), ¹H NMR δ 0.89 (apparent quin, J 7.8 Hz, 18H, CH₃), 1.21–1.13 (m, 12H, CH₂). ³¹P{¹H} NMR: δ 14.3 (s). ¹⁹F NMR: δ −113.1 (AA′XX′, 2F, F^b), δ −124.1 (AA′XX′, 2F, F^a). Mass spectrum (LIFDI, m/z⁺) 604.00 (100% M⁺): anal. calcd for C₁₈H₃₀ClF₄INiP₂ C, 35.71; H, 4.99; found: C, 36.29; H, 4.87.

trans-[NiBr(2,3,5,6-C₆F₄I)(PEt₃)₂] (1pBr). (a) From 1pI and [Et₄N]Br. Method as for 1pF except that solid was extracted with toluene instead of benzene. Yield 64.3 mg, 70.7%. (b) From 1pF and Et₃SiBr. Bromotriethylsilane (17 μL, 0.1 mmol) was added into a solution of THF (10 mL) containing 2F (50 mg, 0.08 mmol) and stirred for 1 h. The solvent and excess reagents and by-products were removed under vacuum. The product was washed with pentane and dried. Yield 50 mg, 90%. Single crystals of 1pBr suitable for X-ray crystallographic studies were grown at room temperature by slow diffusion of hexane into a saturated solution of 1pBr in benzene, yielding bright yellow blocks. NMR (C₆D₆, 298 K). ¹H: δ 0.87 (m, 18H, CH₃), 1.20–1.26 (m, 12H, CH₂). ³¹P{¹H} NMR: δ 14.4 (s). ¹⁹F NMR: δ −112.9 (AA′XX′, 2F, F^b), −123.8 (AA′XX′, 2F, F^a). Mass spectrum (LIFDI, m/z⁺) 649.94 (100% M⁺). Anal. calcd for C₁₈H₃₀BrF₄INiP₂ C, 33.27; H, 4.65. Found: C, 33.41; H, 4.56.

trans-[NiI(2,3,5,6-C₆F₄I)(PEt₂Ph)₂] (2pI). A solution of Ni(PEt₂Ph)₂(COD) was prepared by adding PEt₂Ph (62.7 μL, 0.36 mmol) to a stirred suspension of Ni(COD)₂ (50 mg, 0.18 mmol) in hexane (10 mL) in a 25 mL Schlenk flask inside the glove box. The resulting mixture was stirred further to obtain a clear orange solution. A solution of 1,4-C₆F₄I₂ (106 mg, 0.26 mmol) was prepared separately in hexane (5 mL) in a 50 mL Schlenk flask. To this solution the Ni(PEt₂Ph)₂(COD) solution was added drop-wise while stirring. The resulting red-brown solution was left stirring further for 15 min and then allowed to stand for 1 h upon which a red-brown solid precipitated from the reaction mixture. The solution was decanted and the residue was retained. The resulting solid was washed with cold pentane twice and dried under vacuum. Yield 92 mg, 64%. Single crystals of 2pI suitable for X-ray crystallographic studies were obtained by slow diffusion of hexane into a saturated solution of 2pI in THF/benzene yielding dark blocks. NMR (C₆D₆, 298 K), ¹H NMR: δ 0.85 (apparent quin, J 7.5 Hz, 12H, CH₃), 1.59 (m, 4H, CH₂), 2.03 (m, 4H, CH₂), 6.98 (broad, 6H, Ph), 7.28 (broad, 4H, Ph). ³¹P{¹H} NMR: δ 16.2 (s). ¹⁹F NMR: δ −114.0 (AA′XX′, 2F, F^b), −123.6 (AA′XX′, 2F, F^a). Mass spectrum (LIFDI, m/z⁺) 791.90 (100%, M⁺).

trans-[NiF(2,3,5,6-C₆F₄I)(PEt₂Ph)₂] (2pF). Method as for 1pF. Yield 55.4 mg, 64.1%. NMR (C₆D₆, 298 K). ¹H NMR: δ 0.92 (overlapping apparent quin, J 7.7 Hz, 12H, CH₃), 1.41 (m, 4H, CH₂), 1.72 (m, 4H, CH₂), 7.00 (broad, 6H, Ph), 7.38 (m, 4H, Ph). ³¹P{¹H} NMR: δ 11.9 (d, J_PF 45 Hz). ¹⁹F NMR: δ −115.2 (AA′XX′, 2F, F^b), −126.5 (AA′XX′, 2F, F^a), −389.7 (t, J_PF 45 Hz, 1F, Ni–F). Mass spectrum (LIFDI, m/z⁺) 684.00 (100% M⁺).

trans-[NiI(2,3,4,5-C₆F₄I)(PEt₃)₂] (1oI). A solution of Ni(PEt₃)₂(COD) was prepared by adding PEt₃ (69 μL, 0.36 mmol) to a stirred suspension of Ni(COD)₂ (50 mg, 0.18 mmol) in hexane (10 mL) in a 25 mL Schlenk flask inside the glove box. The resulting mixture was stirred further to obtain a clear orange solution. A solution of 1,2-C₆F₄I₂ (106 mg, 0.26 mmol) was prepared separately in hexane (5 mL) in a 50 mL Schlenk flask. To this, the Ni(PEt₃)₂(COD) solution was added drop-wise while stirring. The resulting black-brown solution was left stirring further for 15 min. The reaction mixture was left in the freezer at −30 °C overnight for crystallization. The dark solution was decanted and the red crystals were washed with cold pentane twice and dried under vacuum. Yield 85.4 mg, 67.4%. Single crystals of 1oI(α) were obtained overnight from a saturated hexane solution of 1oI at −30 °C. The deep red blocks were washed with cold pentane and analysed by X-ray crystallography. Crystals grown from CHCl₃ layered with hexane yielded two further polymorphs, 1oI(β) (orange blocks) and 1oI(γ) (yellow plates). NMR (C₆D₆, 298 K). ¹H NMR δ 0.89 (apparent quin, J 7.8 Hz, 18H, CH₃), 1.41 (broad, 6H, CH₂), 1.51 (broad, 6H, CH₂). ³¹P{¹H} NMR: δ 10.3 (s). ¹⁹F NMR: δ −114.4 (dd, J_FF 32.0, 12.4 Hz, 1F, F^b), −115.4 (dd, J_FF 22.1, 12.1 Hz, 1F, F^e), −156.9 (dd, J_FF 31.7, 19.1 Hz, 1F, F^a), −159.1 (ddt, J_FF 22.19, 4 Hz, 1F, F^d). Mass spectrum (LIFDI, m/z⁺) 695.90 (100%, M⁺). Anal. calcd for C₁₈H₃₀F₄I₂NiP₂ C, 31.02; H, 4.34; found: C, 31.01; H, 4.22.

trans-[NiF(2,3,4,5-C₆F₄I)(PEt₃)₂] (1oF). Method as for 1pF. Yield 52 mg, 63%. Single crystals suitable for X-ray crystallography were obtained by slow diffusion of hexane into a saturated solution of 1oF in benzene in the freezer at −30 °C over two weeks. NMR (C₆D₆, 298 K). ¹H NMR δ 1.00 (apparent quin, J = 7.63 Hz, 18H, CH₃), 1.10–1.2 (m, 6H, CH₂), 1.21–1.33 (m, 6H, CH₂). ³¹P{¹H} NMR: δ 9.4 (d, J_PF 46 Hz). ¹⁹F NMR: δ −115.3 (dd, J_FF 20, 11 Hz, 1F, F^b), −116.2 (dd, J_FF 31, 11 Hz, 1F, F^e), −158.2 (dd, J_FF 31, 19 Hz, 1F, F^d), −160.2 (apparent t, J_FF 19 Hz, 1F, F^a), −397.9 (td, J_PF 46, J_FF 9 Hz, 1F, Ni–F). Mass spectrum (LIFDI, m/z⁺) 588.01, (100% M⁺). Anal. calcd for C₁₈H₃₀F₅INiP₂C, 36.71; H, 5.13. Found: C, 36.69; H, 5.13.

trans-[NiCl(2,3,4,5-C₆F₄I)(PEt₃)₂] (1oCl). (a) From 1oI and [Me₄N]Cl. Method as for 1pF. Yield 59 mg, 69% yield. Single crystals suitable for X-ray crystallography were obtained by slow diffusion of hexane into a saturated solution of 4-Cl in THF/toluene in the freezer at −30 °C over two weeks. (b) From 1oF and Me₃SiCl. Chlorotrimethylsilane (13 μL, 0.1 mmol) was added into a solution of THF (10 mL) containing 1oF (50 mg, 0.08 mmol) and stirred for 1 h. The solvent and excess reagents and by-products were removed under vacuum. The product was washed with pentane and dried. Yield 48 mg, 93%. NMR (C₆D₆, 298 K). ¹H NMR δ 0.93 (apparent quin, J 7.6 Hz, 18H, CH₃), 1.24 and 1.31 (m, 12H, CH₂). ³¹P{¹H} NMR: δ 9.5 (s). ¹⁹F NMR: δ −114.8 (two overlapping m, 1F^e + F^b), −157.2 (dd, J_FF 28.7, 18.1 Hz, 1F^a), −159.5 (apparent t of m, J_FF 22.2, 1F^d). Mass spectrum (LIFDI, m/z⁺) 603.97, (100% M⁺). Anal. calcd for C₁₈H₃₀ClF₄INiP₂, C, 35.71; H, 4.99. Found: C, 36.68; H, 4.87.

Synthesis of trans-[NiBr(2,3,4,5-C₆F₄I)(PEt₃)₂] (1oBr). (a) From 1oI and [Et₄N]Br. Method as for 1pF but extracted with toluene instead of benzene. Yield 62 mg, 68%.

(b) From 1oF and Et₃SiBr. Bromotriethylsilane (17 μL, 0.1 mmol) was added into a solution of THF (10 mL) containing 1oF, (50 mg, 0.08 mmol) and stirred for 1 h. The solvent and excess reagents and by-products were removed under vacuum. The product was washed with pentane and dried. Yield 53 mg, 96%. NMR (C₆D₆, 298 K). ¹H NMR δ 0.92 (apparent quin J 7.7 Hz, 18H, CH₃), 1.30 (m, 6H, CH₂), 1.38 (m, 6H, CH₂). ³¹P{¹H} NMR: δ 9.52 (s). ¹⁹F NMR: δ −114.5 (dd, J_FF 31.2, 12.2 Hz, 1F, F^b), −115.0 (dd, J_FF 21.8, 11.9 Hz, 1F, F^e), −157.0 (dd, J_FF 32.0, 19.3 Hz, 1F, F^a), −159.3 (t of m, J_FF 20.8 Hz, 1F, F^d). Mass spectrum (LIFDI, m/z⁺) 649.91, (100% M⁺). Anal. calcd for C₁₈H₃₀BrF₄INiP₂ C, 33.27; H, 4.65. Found: C, 33.36; H, 4.54.

DFT calculations

Calculations were performed using Gaussian 09 Revision B.01.⁷⁵ Geometries optimisations were performed using BH and HLYP hybrid functional^38,76,77 with 6-31+G(d,p) for H, C, P, and F atoms and SDD effective core potentials for Ni and I. Calculation of the shielding tensors was done using BH and HLYP functional with 6-311+G(d,p) for H, C, P, F and Ni and SDD effective core potential for I. The computed tensors from Gaussian were symmetrised, diagonalised and the direction cosines visualised using either home-written Matlab scripts or the program EFGShield.⁷⁸

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank Dr Iain Oswald (University of Strathclyde) for determining the crystal structure of [trans-NiBr(PEt₃)₂]₂(μ-2,3,5,6-C₆F₄) and thank Professors Christopher Hunter (University of Cambridge) and Odile Eisenstein University of Montpellier for useful discussions. We acknowledge an Overseas Research Scholarship from the University of York to VT. We also acknowledge support from EPSRC (grants EP/J012955/1 and EP/J012998/1).

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Footnotes

† Experimental datasets associated with this paper were deposited with the University of York library.

‡ Electronic supplementary information (ESI) available. CCDC 1825404, 1825439–1825452. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc00890f

§ Present addresses: CatScI Ltd, CBTC2, Capital Business Park, Wentloog, Cardiff, CF3 2PX, UK.

¶ Present addresses: School of Chemistry and Bioscience, Richmond Road, University of Bradford, Bradford, BD1 1DP, UK.

|| The crystal structure of 1oBr exhibited unresolved twinning and poorly resolved disorder and the geometric information was considered not of high enough quality for inclusion. Nevertheless, the major component showed the zig-zag halogen-bonded motif present in the other 1oX structures.

** The separate crystal structure determination at 110 K (Tables 1, 2 and S6) is consistent with that presented here, determined at 111 K (Tables 3 and S15).

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