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

摘要

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, L^tBuCo (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 L^tBuCo(CO) and the dicobalt(ii) carbonate complex (L^tBuCo)2(μ-CO3). Kinetic studies and computations show that the κN,η⁶-arene isomer of L^tBuCo rearranges to the κ2N,N′ binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to L^tBuCo. 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 L^tBuCo is the oxodicobalt(ii) complex L^tBuCoOCoL^tBu, which has been independently synthesized through the reaction of L^tBuCo with N2O. The rapid reaction of L^tBuCoOCoL^tBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by L^tBuCo. L^tBuCoOCoL^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 μ-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.