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

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 Co (where L = 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 Co(CO) and the dicobalt(ii) carbonate complex (L Co)₂(μ-CO₃). Kinetic studies and computations show that the κN,η⁶-arene isomer of L Co rearranges to the κ₂ N,N' binding mode prior to binding of CO₂, which contrasts with the mechanism of binding of other substrates to L Co. 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 Co is the oxodicobalt(ii) complex L CoOCoL , which has been independently synthesized through the reaction of L Co with N₂O. The rapid reaction of L CoOCoL with CO₂ to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO₂ cleavage by L Co. L CoOCoL 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.