Synthesis and Reactivity of Nickel(II) Hydroxycarbonyl Species, NiCOOH-κC

Reactions of nickel complexes supported by an anionic PNP pincer ligand (PNP- = N[2-(PPr2)-Pr-i-4-Me-C6H3](2)) toward CO2 and CO are investigated, particularly for interrogating their C-O bond formation/cleavage chemistry. The formation of a nickel formate species (2) was accomplished by the reaction of (PNP)NiH with CO2, while the structural isomer complex (PNP)NiCOOH-kappa C (4) was successfully produced from the corresponding nickel hydroxyl compound by exposing it to CO(g). Its structurally unique character was gleaned by obtaining two solid-state structures for (PNP)NiCOOH-kappa C (4) and {(PNP)Ni}(2)-mu-CO2-kappa C-2,O (6); the latter was obtained from the reaction of 4 with a nickel hydroxyl complex. Both species possess a NiCOO-kappa C binding mode, which is reminiscent of the binding mode found at the carbon monoxide dehydrogenase (CODH) active site with its Ni-COO-Fe fragment. The cationic species {(PNP)NiCO}(+) (7) was also prepared via the protonation of 4, which then led to the investigation of the C-O bond formation in 7 by adding a nucleophile such as OH-.