Carbon monoxide coordination and its reversible conversion to CO2 occurs at the nickel site of CODH, which is important to understand the biological CO/CO2 management. To mimic the coordination environment of its active site, our group focuses specifically on the chemistry of four-coordinate nickel monocarbonyl species. A series of nickel carbonyl complexes supported by tridentate pincer ligands such as PNP (PNP− = bis(2-(diisopropylphosphino)-4-methylphenyl)amide), acriPNP (acriPNP− = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) and PPMeP (PPMeP = bis(2-(diisopropylphosphinophenyl))-methylphosphine) were synthesized to explore their coordination chemistry as well as reactivity towards CO2. The CO substitution of (PPMeP)Ni(CO) with CO2 does not occur suggesting its stability, which is different from the carboxylation of (acriPNP)Ni(CO). The tendency of CO2 reactivity of the nickel species of the both anionic PNP ligands and the neutral PPMeP ligand has been investigated. The single crystal X-ray diffraction studies reveal that their structural differences are predominant due to the ligand planarity. The PPMeP ligand prefers a distorted tetrahedral geometry, but the acriPNP ligand adopts a planar structure. According to the DFT calculations, the substitution of a CO ligand by CO2 in (PPMeP)NiCO to give (PPMeP)Ni(µ-CO2), was found to be thermodynamically unfavorable, while the same reaction of the nickel monocarbonyl species supported by bisphosphino amido ligands is more favorable. The reactions of (PEP)Ni(CO) with O2 and ROOH were also conducted to comprehend the oxidative reactivity of the four coordinate nickel monocarbonyl species. Upon addition of O2(g) to the solution of (PPMeP)Ni(CO) in toluene at −15 °C, a low-temperature stable intermediate was detected by UV-Vis spectroscopy, which was further converted to a nickel(II)-carbonate species (PPMeP=O)Ni(CO3). Additional product analysis and related reactions relevant to the reactivity of the nickel site of CODH will be discussed.

Nickel monocarbonyl species related to the CO coordination at the nickel site of CODH

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