Reduction of CO2 by a masked two-coordinate cobalt(i) complex and characterization of a proposed oxodicobalt(ii) intermediate

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C-O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(I) complex, (LCo)-Co-tBu (where L-tBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C-O cleavage, the cobalt(I) monocarbonyl complex (LCo)-Co-tBu(CO) and the dicobalt(II) carbonate complex ((LCo)-Co-tBu) 2(mu-CO3). Kinetic studies and computations show that the kappa N,eta(6)-arene isomer of (LCo)-Co-tBu rearranges to the kappa N-2,N' binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to (LCo)-Co-tBu. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C-O bond rupture. A plausible intermediate in the reaction of CO2 with (LCo)-Co-tBu is the oxodicobalt(II) complex (LCoOCoLtBu)-Co-tBu, which has been independently synthesized through the reaction of (LCo)-Co-tBu with N2O. The rapid reaction of (LCoOCoLtBu)-Co-tBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by (LCo)-Co-tBu. (LCoOCoLtBu)-Co-tBu is a novel example of a thoroughly characterized molecular cobalt-oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the m-oxo ligand between two antiferromagnetically coupled high-spin cobalt(II) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations