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

Abstract

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 ^<i>t</i>BuCo (where L ^<i>t</i>Bu = 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 ^<i>t</i>BuCo(CO) and the dicobalt(ii) carbonate complex (L ^<i>t</i>BuCo)₂(μ-CO₃). Kinetic studies and computations show that the κ<i>N</i>,η⁶-arene isomer of L ^<i>t</i>BuCo rearranges to the κ₂ <i>N</i>,<i>N'</i> binding mode prior to binding of CO₂, which contrasts with the mechanism of binding of other substrates to L ^<i>t</i>BuCo. 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 ^<i>t</i>BuCo is the oxodicobalt(ii) complex L ^<i>t</i>BuCoOCoL ^<i>t</i>Bu, which has been independently synthesized through the reaction of L ^<i>t</i>BuCo with N₂O. The rapid reaction of L ^<i>t</i>BuCoOCoL ^<i>t</i>Bu with CO₂ to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO₂ cleavage by L ^<i>t</i>BuCo. L ^<i>t</i>BuCoOCoL ^<i>t</i>Bu 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.