Formation of a tetranickel octacarbonyl cluster from the CO₂ reaction of a zero-valent nickel monocarbonyl species

Abstract

The reaction of a zerovalent nickel complex {Na}{(PNP)Ni(CO)} (1) with CO₂(g) results in the formation of a tetrameric cluster complex {(PN^COONaP)Ni(CO)₂}₄ (2) as the major product along with {(PNP)Ni}₂-μ-CO₃-κ²O,O (3), {(PNP)Ni}₂-μ-CO₂-κ²C,O (4) and (PNP)Ni(CO) (5). A labelling experiment with ¹³CO₂(g) reveals that not only the carbamate moeity but also the CO ligand in 2 originate from CO₂(g). The formation of both a carbonyl species 2 and a carbonate species 3 indicates that nickel-mediated reductive disproportionation of CO₂ occurs. Alternatively, a direct CO₂ oxidative reaction of 1 occurs to produce {(PNP)Ni}₂-μ-CO₂-κ²C,O (4) in a minor pathway. The synthesis and characterization of 2 and other products are described herein.

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Yunho Lee

Yunho Lee is an Associate Professor in the Department of Chemistry at the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea. He attended Chonbuk National University where he received a B.S. degree in Chemistry in 2000. Yunho left Korea to begin his doctoral studies under the guidance of Prof. Kenneth D. Karlin at Johns Hopkins University, Baltimore. After receiving his Ph.D. in 2007, Yunho was a postdoctoral fellow in the laboratory of Prof. Jonas C. Peters at the Massachusetts Institute of Technology and the California Institute of Technology. In the winter of 2010, Yunho returned to Korea and started his independent career as an Assistant Professor at KAIST. He received the Distinguished Lectureship Award from the Chemical Society of Japan in 2011 and the Young Inorganic Chemist Award from the Korea Chemical Society in 2015. He was promoted to associate professor in September 2015. His primary research interests focus on the fundamental transition metal coordination chemistry and its application in various small molecule conversions and catalysis inspired by metalloenzymes.

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Introduction

Carbon dioxide functionalization mediated by transition metal complexes has been widely explored in recent years to utilize CO₂ as a synthetic C1 source.¹ In particular, the carbon dioxide reactivity of a low-valent nickel species is receiving much attention due to its relevance to the biological CO₂ conversion to CO occurring at the active site of the CODH (carbon monoxide dehydrogenase).² At the C cluster of CODH, zerovalent nickel is hypothesized to be responsible for the initial binding of CO₂. Further activation of carbon dioxide occurs revealing the bond formation between nickel and carbon to produce a Ni–COO–Fe species.²

In organonickel chemistry, CO₂ activation mediated by divalent nickel commonly reveals the formation of a nickel(II)–formate species via an insertion reaction to a Ni–H bond.³ In the case of a monovalent nickel species, the Caulton group reported the reaction of ((^tBu₂PCH₂SiMe₂)₂N)Ni^I with CO₂ revealing unusual transposition between the amide nitrogen and one CO₂ oxygen to produce a nickel(I)–cyanate species.⁴ Recently, the Limberg group reported the reductive coupling of CO₂ at the monovalent nickel center supported by a β-diketiminato ligand to give an oxalate-bridged dinickel(II) complex.⁵ The corresponding anionic analogue K₂[L^tBuNi^I(N₂²⁻)Ni^IL^tBu] reveals the disproportionation of CO₂ to CO and CO₃²⁻ mediated by nickel(I) with an additional 1 electron donation from a bridging anionic N₂ ligand.⁵ Binding of CO₂ to a single transition metal ion was firstly demonstrated by Aresta and coworkers in the preparation of (PCy₃)₂Ni(η²-CO₂).⁶ A similar zero-valent nickel–CO₂ adduct was also reported by Hillhouse and coworkers using a bidentate phosphine ligand.⁷ Further reaction of these nickel–CO₂ adducts resulted in CO production with associated phosphine oxidation.^7,8 The Lee group reported a 5-coordinate nickel–η²-CO₂ complex (PP^MeP)Ni(CO₂) (PP^MeP = PMe[2-PⁱPr₂-C₆H₄]₂) and its reactivity toward triarylborane.⁹ Structural and theoretical analyses suggest that such a 5-coordinate nickel center supported by a tridentate ligand might significantly enhance the reactivity of a bound CO₂ ligand compared to an analogous 4-coordinate species.⁹ Recently, few nickel(0) species supported by N-heterocyclic carbene ligands were also reported. The reaction of [(IPr)Ni]₂ (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with CO₂ resulted in the generation of [(IPr)Ni]₂(μ-CO)(μ-η²,η²-CO₂) and (IPr)Ni(CO₃) by reductive disproportionation.¹⁰ The CO₂ reaction of a dinickel(0) complex supported by a binucleating bis(N-heterocyclic carbene) ligand revealed the formation of a dicarbonyl bridged dinickel(0) complex presumably with the formation of carbonate.¹¹

The reaction of a nickel monocarbonyl species with CO₂ is currently under investigation in our laboratory due to its relevance to CODH active site chemistry. Recently, we reported a series of mononuclear nickel complexes supported by an anionic PNP ligand (PNP⁻ = N[2-PⁱPr₂-4-Me-C₆H₃]₂⁻). In the (PNP)Ni system, a formal nickel(0) state cannot be achieved without a π-accepting ligand, such as CO.^12,13 Therefore, we have employed a zerovalent nickel monocarbonyl adduct, {Na}{(PNP)Ni(CO)} (1)¹³ to study CO₂ conversion reactions. According to the solid-state structure, 1 has two sodium interactions with both a zero-valent nickel ion (d_Ni–Na2: 2.8523(5) Å) and an amido nitrogen (d_N1–Na2: 2.463(1) Å) in its dimeric assembly (Fig. 1). This structural information suggests that the competition might occur between a nickel ion and an amido nitrogen during the reaction of 1 with CO₂. As anticipated, the CO₂ reaction showed an immediate transformation of 1 to multiple products (Scheme 1). The major product was identified as a tetrameric cluster {(PN^COONaP)Ni(CO)₂}₄ (2). Isotope labelling experiments with ¹³CO₂ revealed that 2 contains CO molecules originated from CO₂. The carbonate species, {(PNP)Ni}₂-μ-CO₃-κ²O,O (3), was also produced indicating that the reductive disproportionation of CO₂ occurs with a nickel(0) species. In fact, a reaction product mixture also contains additional portions of {(PNP)Ni}₂-μ-CO₂-κ²C,O (4)¹⁴ and (PNP)Ni(CO) (5) as minor products.¹³ Therefore, the CO₂ reaction of 1 involves multiple reaction pathways, vide infra.

Fig. 1 Displacement ellipsoid (50%) representations of {Na}{(PNP)Ni(CO)} (1) in dimeric assembly with two co-crystallized THF molecules. All hydrogen atoms are omitted for clarity. See the ESI† for details.

Scheme 1

Results and discussion

Reaction of a nickel(0) monocarbonyl species with CO₂(g)

By exposing CO₂(g) to a degassed solution of {Na}{(PNP)Ni(CO)} (1) in THF at room temperature, an immediate reaction occurs (<1 min) revealing a color change from orange to dark green. This solution was found to contain several diamagnetic species according to the ³¹P NMR spectrum (see the ESI†). A major pentane-insoluble product was isolated in 35.5% yield as a light green solid, vide infra. The remaining pentane soluble portion contains two known diamagnetic products, {(PNP)Ni}₂-μ-CO₂-κ²C,O (4, 8.8%)¹⁴ and (PN^HP)Ni(CO)₂ (6, 2.8%)^14,15 with several unidentified species according to the ³¹P NMR spectroscopic data (see the ESI†). In fact, there is an additional paramagnetic product (PNP)Ni^I(CO) (5).¹³ After the green species and 4 were removed from a product mixture, compound 5 was isolated as a green crystalline solid in >22% yield from the recrystallization of a cold diethyl ether solution at −35 °C. The identity of 5 was confirmed with IR (ν_CO 1931 cm⁻¹) and XRD crystallographic data (see the ESI†).¹³

The green unknown species exhibits a broad singlet at 35.5 ppm (ν_1/2 = ∼900 Hz, C₆D₆) in the ³¹P NMR spectrum. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of an ether solution of a green product. The solid-state structure reveals an unanticipated T_d symmetric tetrameric nickel cluster, {(PN^COONaP)Ni(CO)₂}₄ (2) in which a CO₂ molecule is incorporated into each central nitrogen atom of a PNP ligand (Fig. 2). The carbamate vibration in 2 detected at 1626 cm⁻¹ also supports its structural data.¹⁶ In the nickel cluster, each sodium ion binds to four oxygen atoms from carbamates (O3, O3′ and O4′′) and CO (O1) to construct a tetrameric structure (d_O1–Na1 = 2.455(6), d_O3–Na1 = 2.243(6), and d_O4–Na1 = 2.200(6) Å, Fig. 2). Two C–O bond distances for carbamate, d_C3–O3 and d_C3–O4 are 1.25(1) and 1.28(1) Å, respectively. The bond distance between Ni1 and N1 in 2 is significantly elongated (d_Ni1–N1 = 3.339(5) Å) compared with that of 1 (d_Ni1–N1 = 2.054(1) Å, Fig. 1). There are two CO ligands coordinated to each nickel center (d_C1–O1 = 1.132(8) and d_C2–O2 = 1.155(8) Å) along with two phosphorus atoms allowing for the formation of a four coordinate zero-valent nickel center.

Fig. 2 Displacement ellipsoid (50%) representations for (a) the cluster complex {(PN^COONaP)Ni(CO)₂}₄ (2), (b) the core structure of 2 with omission of ligand isopropyl and aromatic groups, and (c) a monomer unit of 2 composed of (PN^COOP)Ni(CO)₂ and a sodium ion. All hydrogen atoms are omitted for clarity. See the ESI† for details.

The nickel(0) center of 2 is reminiscent to the previously reported nickel(0) dicarbonyl species, (PN^HP)Ni(CO)₂ (6).¹⁴ The IR spectrum of 2 exhibits CO vibrations at 1976 and 1905 cm⁻¹, which were observed at 1994 and 1933 cm⁻¹ in 6.¹⁴

In order to comprehend the solution-state structure, the diffusion coefficient measurements of 2 with analogous compounds were conducted by pulse gradients spin echo nuclear magnetic resonance (PGSE-NMR) experiments.¹⁷ The diffusion coefficient for 2 was obtained as 4.877 × 10⁻¹⁰ m² s⁻¹ in a non-coordinating solvent, C₆D₆ at 25 °C. In comparison, the diffusion coefficients of a mononuclear complex (PN^HP)Ni(CO)₂ (6) and a dinuclear complex {(PNP)Ni}₂-μ-CO₂-κ²C,O (4) as reference compounds were also measured as 1.079 × 10⁻⁹ and 7.737 × 10⁻¹⁰ m² s⁻¹, respectively (Table 1). The hydrodynamic radius of 2 (r_Solution = 7.366 Å) derived from the Stokes–Einstein equation (eqn (1)) is similar to the solid-state radius estimated from XRD data (r_Solid = 8.8640 Å).¹⁸ Thus, the diffusion NMR spectroscopy experiments suggest that the tetrameric structure of 2 is maintained in a benzene solution.

Table 1 Diffusion constants and hydrodynamic radii for 2, 4 and 6

Compound	Diffusion constant (×10^−10 m^2 s^−1)	Hydrodynamic radius (Å)
2	4.877	7.366
4	7.737	4.643
6	10.79	3.329

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Origin of CO in the generation of {(PN^COONaP)Ni(CO)₂}₄ (2)

According to the structure of {(PN^COONaP)Ni(CO)₂}₄ (2), two CO molecules are coordinated to each nickel ion. One can anticipate that one of the CO ligands has originated from either CO₂(g) or another Ni–CO species during the reaction. From the reaction of 1 with a CO/CO₂ (1 : 1) gas mixture, the yield of 2 was dramatically increased to 66.6% (see the Experimental section). This experiment suggests that the formation of 2 requires free CO. Yet, we cannot unfortunately differentiate two different CO sources. We then utilized ¹³C-labelled CO₂(g) in the synthesis of 2 from the reaction of 1-¹²CO (see the Experimental section). The isolated ¹³C-labelled product 2-¹³CO₂ revealed three augmented ¹³C-NMR signals at 165.31, 203.64 and 205.88 ppm (see the ESI†). The signal at 165.31 ppm corresponds to ¹³C of carbamate and those at 203.64 (t, J = 5.3 Hz) and 205.88 ppm represent two ¹³CO ligands in 2. The intensity of two carbonyl peaks is similar to each other indicating that labelled ¹³CO has been incorporated over two different positions at a nickel(0) center. However, the ¹²CO coordination in 2-¹³CO₂ presumably remains since both carbonyl peak intensities are significantly lower than expected (see the ESI†). In fact, the infrared spectrum also supports that both ¹²CO (1976 and 1905 cm⁻¹) and ¹³CO (1967sh and 1884 cm⁻¹) are present in 2-¹³CO₂. The vibration for a carbamate group is shifted to 1579 cm⁻¹ from 1626 cm⁻¹ (Δ(¹³CO₂) = −47 cm⁻¹, see the ESI†). Thus, the labelled product can be formulated as {(PN^13COONaP)Ni(CO)_x(¹³CO)_2−x}₄ (2-¹³CO₂). This result indicates that new CO(g) might be generated from CO₂ conversion and the corresponding CO(g) participates in the formation of 2.

Carbon monoxide can be generated through the reductive disproportionation of CO₂, which gives CO and CO₃²⁻ from two CO₂ and two additional electrons, according to the literature.¹⁹ Nickel-mediated reductive disproportionation of CO₂ was demonstrated with nickel(I) and nickel(0) species.^5,10,11 Thus, the formation of carbonate can be expected. In order to confirm the generation of carbonate, a nickel carbonate species {(PNP)Ni}₂-μ-CO₃-κ²O,O (3) was independently prepared from the reaction of {(PNP)Ni^II(CO)}{BF₄} with Na₂CO₃ (see the Experimental section). Compound 3 exhibiting a singlet signal at 30.6 ppm in the ³¹P NMR spectrum was fully characterized by various spectroscopic methods. The solid state structure of 3 confirms a dinuclear nickel bridging carbonate species (see Fig. 3). The presence of 3 (5.3%) in a reaction product mixture of {Na}{(PNP)Ni(CO)} (1) with CO₂(g) was confirmed by ³¹P NMR spectroscopy (see the ESI†). Therefore, carbon monoxide along with CO₃²⁻ might be produced from the reductive disproportionation of CO₂.

Fig. 3 Displacement ellipsoid (50%) representations of {(PNP)Ni}₂-μ-CO₃-κ²O,O (3). A carbonate moiety was disordered over two distinctive positions. For clarity, only one component is shown. All hydrogen atoms are omitted for clarity. See the ESI† for details.

After CO generation via reductive disproportionation, {(PN^COONaP)Ni(CO)₂}₄ (2) is produced by accepting both CO and CO₂. Initially, the C–N bond between CO₂ and an amide moiety in a PNP ligand can be formed. Further Ni(0)–N bond cleavage and CO coordination occur to produce 2. However, we cannot rule out the other pathway that the addition of CO₂ to a nickel(0) center occurs to form (PNP)Ni–COONa¹⁴ followed by reductive elimination of a C–N bond. In an analogous PNP nickel system, C–N bond-forming reductive elimination reactions were reported.²⁰ [RC(O)N(o-C₆H₄PPh₂)₂]Ni(CO)₂ (R = Me or Et) can be generated from the reaction of (N(o-C₆H₄PPh₂)₂)NiCOR with addition of excess CO(g).²⁰ However, this reaction is unlikely since the CO(g) addition to {(PNP)NiCOONa}₂·(THF)¹⁴ does not give 2. Both CO(g) production mediated by Ni(0) and carbamate generation via nucleophilic addition of amide to CO₂(g) suggest that the two major reactions are competitive.

Formation of {(PNP)Ni}₂-μ-CO₂-κ²C,O (4) and (PNP)Ni(CO) (5)

From the CO₂ reaction, a dinuclear nickel carboxylate species {(PNP)Ni}₂-μ-CO₂-κ²C,O (4) was detected as a minor product (8.8%). According to a ¹³CO₂ labelling experiment, {(PNP)Ni}₂-μ-¹³CO₂-κ²C,O (4-¹³CO₂) was detected by its characteristic triplet of triplet signal at 192.40 ppm in the ¹³C NMR spectrum revealing that a CO₂ moiety of 4 comes from CO₂(g) (see the ESI†). This product can be generated from the direct addition of CO₂ to a zero-valent nickel center followed by stabilization with a nickel(II) source (Scheme 2). The yield of 4 from the reaction of {Na}{(PNP)Ni(CO)} (1) with CO₂(g) increases in the presence of a nickel(II) source such as {(PNP)Ni(CO)}{BF₄} (36%) or 3 (26%).²¹ In these reactions, the cluster 2 was not generated (see the Experimental section). These experiments suggest another pathway of the CO₂ reaction of 1 to generate a carboxylate species through a direct Ni–C bond formation. This reaction involves the elimination of a CO coordination from a nickel center. In fact, compound 4 can further react with CO. By exposing 4 toward CO, a monovalent nickel–CO adduct (PNP)Ni(CO) (5) was immediately produced in 87% yield, suggesting that the free CO(g) can be a directing agent to induce the inner-sphere electron transfer.

Scheme 2

Conclusions

The reaction of a zerovalent nickel complex {Na}{(PNP)Ni(CO)} (1) with CO₂(g) results in the formation of an unexpected cluster complex {(PN^COONaP)Ni(CO)₂}₄ (2) as a major product along with {(PNP)Ni}₂-μ-CO₃-κ²O,O (3), {(PNP)Ni}₂-μ-CO₂-κ²C,O (4), (PNP)Ni(CO) (5) and other byproducts. The XRD data of 2 clearly show a T_d symmetric tetrameric nickel cluster structure, where a CO₂ molecule is incorporated into an amide moiety of a PNP ligand. The cluster structure of 2 is maintained in its solution state in benzene supported by its small diffusion coefficient measured by diffusion NMR spectroscopy. According to ¹³CO₂ labelling experiments, labelled carbon monoxide and carbamate moieties found in {(PN^13COONaP)Ni(CO)_x(¹³CO)_2−x}₄ (2-¹³CO₂) originate from ¹³CO₂. As a major CO production, reductive disproportionation of CO₂ occurs to produce both CO and {(PNP)Ni}₂(μ-CO₃) (3). In fact, a direct CO₂ oxidative reaction might occur with 1 to give {(PNP)Ni}₂-μ-CO₂-κ²C,O (4) as a minor reaction pathway. Therefore, the formation of 4 and 5 along with 2 and 3 indicates that the CO₂ reaction of 1 involves multiple reaction pathways.

Experimental

General consideration

All manipulations were carried out using standard Schlenk or glovebox techniques under a N₂ atmosphere. Solvents were deoxygenated and dried by thoroughly sparging with Ar gas followed by passage through an activated alumina column. Solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran in order to confirm effective oxygen and moisture removal. CO₂ gas (99.999% purity) was dried by storage in a column of activated 3-Å molecular sieves at a pressure of 20 bar. All reagents were purchased from commercial vendors and used without further purification unless otherwise stated. {Na}{(PNP)Ni(CO)} (1),¹³ {(PNP)Ni}₂-μ-CO₂-κ²C,O (4),¹⁴ (PNP)Ni(CO) (5),¹³ (PN^HP)Ni(CO)₂ (6),¹⁴ {(PNP)NiCOONa}₂·THF¹⁴ and {(PNP)Ni(CO)}{BF₄}¹³ were prepared according to literature procedures. Elemental analyses were carried out on a Thermo Scientific FLASH 2000 series instrument at the KAIST Research Analysis Center. Deuterated solvents were purchased from Euriso-top, degassed, and dried over activated 4-Å molecular sieves prior to use.

Spectroscopic measurements

A Bruker AVHD-400 spectrometer was used to measure ¹H and ³¹P NMR data. ¹³C NMR spectra were recorded on Bruker AVHD-400 and Agilent Technologies DD2 600 spectrometers. The chemical shifts for ¹H NMR and ¹³C NMR spectra were quoted in parts per million (ppm) referenced to residual solvent peaks. ³¹P NMR spectra were decoupled by broad band proton decoupling and chemical shifts were quoted in parts per million (ppm) referenced to external phosphoric acid as 0.0 ppm. The following abbreviations are used to describe peak splitting patterns when appropriate: s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, m = multiplet, dd = doublet of doublet, tt = triplet of triplet, bs = broad singlet. Coupling constants, J, were reported in hertz (Hz). Diffusion NMR experiments for 2, 4 and 6 were conducted using a Bruker AVANCE HD 800 spectrometer at Korea Basic Science Institute. The data were collected in C₆D₆ at 298K with the bipolar pulse pairs stimulated echo pulse sequence (ledbpgp2s). The pulse field gradient was incremented from 0 to 47 G cm⁻¹ with 16 steps. UV-vis spectra were recorded with an Agilent Cary 60 UV-vis spectrophotometer using a 1 cm two-window quartz cell sealed with a screw-cap purchased from Hellma Analytics (117.100-QS). Infrared spectra were recorded using KBr pellets by using a Bruker VECTOR 33. Frequencies are given in reciprocal centimeters (cm⁻¹) and only selected absorbances were reported.

X-ray crystallography

The diffraction data of {Na}{(PNP)Ni(CO)} (1)¹³ were collected on an ADSC Quantum-210 detector at 2D SMC at the Pohang Accelerator Laboratory, Korea. Crystals of 1 suitable for X-ray diffraction were obtained by slow diffusion of pentane into a THF solution. A crystal of suitable size and quality was coated with Paratone-N oil and mounted on a Dual-Thickness MicroLoops LD purchased from MiTeGen. The data were collected with Si(111) double crystal monochromated synchrotron radiation (λ = 0.70000 Å) at 100 K. The ADSC Q210 ADX program²² was used for data collection and HKL3000sm (Ver. 703r)²³ was used for cell refinement, reduction and absorption correction. The diffraction data of 2, 3, 5 and {(PNP)NiCOONa}₂·THF¹⁴ were collected on a Bruker SMART APEX II instrument. Crystals of {(PNP)NiCOONa}₂·(THF) suitable for X-ray diffraction were obtained by slow diffusion of pentane into a THF solution. The data were collected with graphite-monochromated MoKα radiation (λ = 0.71073 Å) under a stream of N₂ (g) at 120 K. Cell parameters were determined and refined by the SMART program.²⁴ Data reduction was performed using the SAINT software.²⁵ An empirical absorption correction was applied using the SADABS program.²⁶ The structures were solved by direct methods and all non-hydrogen atoms were subjected to anisotropic refinement by full-matrix least-squares on F² by using the SHELXTL/PC package.²⁷ Hydrogen atoms were placed at their geometrically calculated positions and refined riding on the corresponding carbon atoms with isotropic thermal parameters. A highly disordered ether solvent molecule for 2 could not be appropriately modeled and was removed using the PLATON program, SQUEEZE function.²⁸

Reaction of 1 with CO₂(g). In a 40 mL Schlenk tube, the solution of {Na}{(PNP)Ni(CO)} (1, 300 mg, 0.557 mmol) in 10 mL of THF was degassed by three freeze–pump–thaw cycles on the Schlenk line. CO₂(g) was charged under ambient conditions resulting in an immediate color change from orange to dark green. After 1 min, all volatiles were removed under vacuum. The green solid residue was dissolved in 20 mL of toluene and stirred for 30 min at room temperature. The resulting solution was filtered through Celite, and all volatiles were removed under vacuum. The product, {(PN^COONaP)Ni(CO)₂}₄ (2, 121 mg, 0.0495 mmol, 35.5%) was obtained as a light green solid after washing with pentane and drying under vacuum. Corresponding characterization for 2 is separately discussed below. To isolate the other products soluble in pentane, the combined filtrate was collected and dried under vacuum. The green residue was washed with cold pentane (−35 °C) to give an orange solid of {(PNP)Ni}₂-μ-CO₂-κ²C,O (4, 25 mg, 0.025 mmol, 8.8%). The identity of 4 was established by comparison of its ³¹P NMR spectrum with its published ³¹P NMR data.¹⁴ A green crystalline solid of (PNP)Ni(CO) (5, 63 mg, 0.12 mmol, 22%) was isolated by recrystallization from a cold ether solution at −35 °C. The identity of 5 was established by comparison of both IR and XRD data with its previous report.¹³ Updated XRD data of 5 were obtained without a naphthalene molecule which was included in the original report.¹³

Characterization of 2. ¹H NMR (400 MHz, benzene-d₆) δ 7.48 (s, 8H), 7.25 (s, 8H), 7.01 (s, 8H), 3.3–2.2 (bs, 16H), 2.05 (s, 24H), 1.44 (q, J = 6.3 Hz, 24H), 1.4–0.2 (m, 72H). ¹³C NMR (101 MHz, benzene-d₆) δ 205.88 (s), 203.63 (t, J = 5.6 Hz), 165.30, 150.61, 134.72, 132.50, 131.17, 130.52, 27.87, 21.53, 21.46, 21.02, 19.89, 19.22, 19.15, 18.76. ³¹P NMR (162 MHz, benzene-d₆) δ 35.5 (bs). ¹H NMR (400 MHz, THF-d₈) δ 7.30 (d, J = 2.9 Hz, 8H), 7.20 (dd, J = 8.2, 3.7 Hz, 8H), 6.96 (d, J = 7.9 Hz, 8H), 2.91 (sept, J = 7.3 Hz, 8H), 2.36–2.28 (m, 8H), 2.27 (s, 24H), 1.24–1.13 (m, 48H), 0.99 (dd, J = 13.9, 6.8 Hz, 24H), 0.73 (dd, J = 13.5, 6.9 Hz, 24H). ¹³C NMR (101 MHz, THF-d₈) δ 204.34 (s), 203.80 (t, J = 7.0 Hz), 164.00, 152.53, 134.39, 134.18, 133.47, 132.52, 132.39, 129.92, 29.07 (t, J = 12.2 Hz), 27.18 (t, J = 6.9 Hz), 21.04, 20.96, 20.27 (t, J = 2.7 Hz), 19.60 (t, J = 4.7 Hz), 18.95. ³¹P NMR (162 MHz, THF-d₈) δ 34.7. Anal. Calcd for C₁₁₆H₁₆₀N₄Ni₄O₁₆P₈: C, 57.08; H, 6.61; N, 2.30. Found: C, 56.76; H, 6.61; N, 2.08. UV-vis [THF, nm (L mol⁻¹ cm⁻¹)]: 626 (282), 426sh (1520), 343 (20 510). IR (KBr pellet, cm⁻¹): ν_CO 1976, 1905, ν_C=O 1626. Crystals of 2 suitable for X-ray diffraction were obtained by slow evaporation of an ether solution.

Evaluation of diamagnetic product yields of the reaction of 1 with CO₂(g). In a 25 mL Schlenk tube, the solution of {Na}{(PNP)Ni(CO)} (1, 100 mg, 0.186 mmol) in 5 mL of THF was degassed by three freeze–pump–thaw cycles on the Schlenk line. CO₂(g) was charged under ambient conditions resulting in an immediate color change from orange to dark green. The reaction mixture was stirred for 1 min at room temperature and all volatiles were removed under vacuum. The green residue was dissolved in THF/C₆D₆ (5 : 1) and triphenylphosphine oxide (26 mg, 0.093 mmol) was added as an internal integration standard. The corresponding yields for 2 (0.022 mmol, 48%), 3 (0.0049 mmol, 5.3%), 4 (0.0077 mmol, 8.3%) and 6 (0.051 mmol, 2.8%) were determined by ³¹P NMR integration.

Labelling experiments; the reaction of 1 with ¹³CO₂(g). In a NMR tube equipped with a J.Young valve, the solution of {Na}{(PNP)Ni(CO)} (1, 30 mg, 0.056 mmol) in 0.7 mL of THF/C₆D₆ (5 : 1) was degassed by three freeze–pump–thaw cycles on the Schlenk line. ¹³CO₂(g) was charged under ambient conditions resulting in an immediate color change from orange to dark green. After shaking for 10 min at room temperature, compounds 2, 3 and 4 were detected by using the ¹³C NMR spectrum (see the ESI†). ¹³C NMR (101 MHz, THF/benzene-d₆) δ 164.92 (s) for 3 and 192.40 (tt, J = 26.2, 7.3 Hz) for 4. After additional shaking for 1 h at room temperature, all volatiles were removed under vacuum. The green solid residue was dissolved in 5 mL of toluene and stirred for 30 min at room temperature. The resulting solution was filtered through Celite, and all volatiles were removed under vacuum. The product, {(PN^13COONaP)Ni(CO)_x(¹³CO)_2−x}₄ (2-¹³CO₂, 7.0 mg, 0.0029 mmol, 20%) was obtained as a light green solid after washing with pentane and drying under vacuum. ¹³C NMR (101 MHz, benzene-d₆) δ 205.89 (s), 203.64 (t, J = 5.3 Hz), 165.31. IR (KBr pellet, cm⁻¹): ν_CO 1976, 1905, ν_¹³CO 1967sh, 1884, ν_¹³C=O 1579.

Synthesis of 2 from the reaction of 1 under CO/CO₂ conditions. In a 40 mL Schlenk tube, the solution of {Na}{(PNP)Ni(CO)} (1, 200 mg, 0.372 mmol) in 10 mL of THF was degassed by three freeze–pump–thaw cycles on the Schlenk line. After CO(g) was charged at 0.5 atm, the reaction mixture was exposed to CO₂(g) at 1 atm resulting in an immediate color change from orange to dark green. The reaction mixture was stirred for 10 min at room temperature and all volatiles were removed under vacuum. The green solid residue was dissolved in 20 mL of toluene and stirred for 30 min at room temperature. The resulting solution was filtered through Celite, and all volatiles were removed under vacuum. The product, {(PN^COONaP)Ni(CO)₂}₄ (2, 151 mg, 0.0619 mmol, 66.6%) was obtained as a light green solid after washing with pentane and drying under vacuum.

Synthesis of 3. To a mixture of {(PNP)Ni(CO)}{BF₄} (86 mg, 0.14 mmol) and Na₂CO₃ (15 mg, 0.14 mmol) in 10 mL of THF, 1 mL of degassed water was added. The green reaction mixture was stirred for 24 h at 60 °C. After the volatiles were removed under vacuum, the remaining green solid was dissolved in pentane. The resulting green solution was filtered through Celite, and all volatiles were removed under vacuum. A green solid of {(PNP)Ni}₂-μ-CO₃-κ²O,O (3, 39 mg, 0.038 mmol, 54%) was isolated from recrystallization from a cold pentane solution at −35 °C. ¹H NMR (400 MHz, benzene-d₆) δ 7.43 (d, J = 8.5 Hz, 4H), 7.02 (s, 4H), 6.68 (d, J = 8.1 Hz, 4H), 2.52 (bs, 8H), 2.16 (s, 12H), 1.65 (q, J = 7.3 Hz, 24H), 1.36 (q, J = 6.5 Hz, 24H). ¹³C NMR (151 MHz, benzene-d₆) δ 164.26, 162.41 (t, J = 13.1 Hz), 132.34, 131.94, 128.35, 124.46 (t, J = 3.1 Hz), 121.22 (t, J = 18.0 Hz), 116.98 (t, J = 4.9 Hz), 24.18 (t, J = 10.9 Hz), 20.55, 19.37 (t, J = 2.8 Hz), 18.13. ³¹P NMR (162 MHz, benzene-d₆) δ 30.6. Anal. Calcd for C₅₃H₈₀N₂Ni₂O₃P₄: C, 61.53; H, 7.79; N, 2.71. Found: C, 61.36; H, 7.98; N, 2.44. UV-vis [THF, nm (L mol⁻¹ cm⁻¹)]: 622 (656), 348 (4180), 310 (3310), 274 (3410). IR (KBr pellet, cm⁻¹): ν_Ar 1595, ν_CO3 1537. Crystals of 3 suitable for X-ray diffraction were obtained by slow evaporation of an ether solution.

Reaction of 1 with CO₂ in the presence of {(PNP)Ni(CO)}{BF₄}. In a 25 mL Schlenk tube, the green solution of {(PNP)Ni(CO)}{BF₄} (20 mg, 0.033 mmol) in 3 mL of THF was layered on top of the frozen orange solution of {Na}{(PNP)Ni(CO)} (1, 18 mg, 0.033 mmol) in 3 mL of THF. The layered solution mixture was taken out of the glovebox, degassed while keeping the solution frozen and then exposed to CO₂(g). The reaction mixture was stirred for 10 min at −78 °C. After the volatiles were removed under vacuum, the remaining green solid was dissolved in pentane. The resulting green solution was filtered through Celite, and all volatiles were removed under vacuum. An orange solid of {(PNP)Ni}₂-μ-CO₂-κ²C,O (4, 12 mg, 0.012 mmol, 36%) was isolated from recrystallization from a cold pentane solution at −35 °C.

Reaction of 1 with CO₂ in the presence of 3. In a 25 mL Schlenk tube, the green solution of 3 (14 mg, 0.014 mmol) in 3 mL of THF was layered on top of the frozen orange solution of {Na}{(PNP)Ni(CO)} (1, 14 mg, 0.026 mmol) in 3 mL of THF. The layered solution mixture was taken out of the glovebox, degassed while keeping the solution frozen and then exposed to CO₂(g). The reaction mixture was stirred for 10 min at −78 °C. After the volatiles were removed under vacuum, the remaining green solid was dissolved in pentane. The resulting green solution was filtered through Celite, and all volatiles were removed under vacuum. An orange solid of {(PNP)Ni}₂-μ-CO₂-κ²C,O (4, 7.0 mg, 0.0069 mmol, 26%) was isolated from recrystallization from a cold pentane solution at −35 °C.

Reaction of 4 with CO. In a 25 mL Schlenk tube, a solution of {(PNP)Ni}₂-μ-CO₂-κ²C,O (4, 100 mg, 0.0982 mmol) in 10 mL of THF was degassed by three freeze–pump–thaw cycles on the Schlenk line. CO(g) was charged under ambient conditions resulting in an immediate color change from orange to green. The reaction mixture was stirred for 10 min at room temperature and all volatiles were removed under vacuum. The green residue was dissolved in pentane and filtered through Celite. The product, (PNP)Ni(CO) (5, 88 mg, 0.17 mmol, 87%), was obtained as a green solid after drying under vacuum.

Acknowledgements

This work was supported by the Korea CCS R&D Center Grant funded by the Korea government (Ministry of Science, ICT & Future Planning; no. NRF-2014M1A8A1049353).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Figures, tables giving characterization data for 1–5 and X-ray crystallographic data. CCDC 1446704-1446707 and 1453323. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00011h

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