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