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

Reactions of nickel complexes supported by an anionic PNP pincer ligand (PNP– = N­[2-PiPr2-4-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-κC (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-κC (4) and {(PNP)­Ni}2-μ-CO2-κ2C,O (6); the latter was obtained from the reaction of 4 with a nickel hydroxyl complex. Both species possess a NiCOO-κC 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–.