CO to CO2 Conversion at a Dinickel(II) Complex with Bridging Hydroxide─Emulating a Key Step of [Ni,Fe]-CO Dehydrogenase

Abstract

Late transition metal hydroxides are important intermediates in many catalytic processes. In [Ni,Fe] carbon monoxide dehydrogenase enzymes (CODHs), which catalyze the reversible CO/CO2 interconversion, CO insertion in the Ni–(μ–OH) bond of a Ni/Fe-bridging hydroxide is the proposed C–O bond-forming step during CO oxidation. Here, we report that CO reacts with the dinickel(II) complex LNi2(μ–OH) (L3– is a pyrazolato-centered ligand with two {N3} compartments), finally leading to HLNiII(CO); this is coupled to the formation of CO2 and Ni(CO)4, which is the driving force of the overall reaction. Mechanistic details have been investigated by 13CO labeling experiments and DFT calculations, and the product derivative HLNiII(CNBn) has been crystallographically characterized. The findings demonstrate that a nickel(II)-bound μ–OH can serve as a nucleophile toward CO and promote its oxidation to CO2, which is suggested to proceed via a metal-bridging O-protonated carbonite intermediate, emulating a key step of [Ni,Fe]-CODH reactivity. For CO2 release, in the present system, one of the nickel ions serves as a terminal electron sink and the corresponding ligand {N3} site of L3– as a proton acceptor.