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 κ2 N,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.
Highlights
Terminal oxo complexes of transition metals are o en invoked as intermediates in hydrocarbon activation,[1,2,3] oxygen atom transfer[4,5] and water oxidation.[6,7] Dinuclear oxo-bridged systems have garnered less attention, even though they have numerous important roles in reactive metallocofactors,[8,9] materials,[10] catalysts,[11] and physiological processes.[12]
We demonstrate that a likely intermediate is the Co2+–O2À–Co2+ complex LtBuCoOCoLtBu, which can be independently synthesized using N2O
The cobalt(I) source LtBuCo (1) was synthesized and characterized earlier,[32] and features a supporting b-diketiminate ligand that is bound in an unusual k1,h6 binding mode as shown at the le of Scheme 1
Summary
Terminal oxo complexes of transition metals are o en invoked as intermediates in hydrocarbon activation,[1,2,3] oxygen atom transfer[4,5] and water oxidation.[6,7] Dinuclear oxo-bridged systems have garnered less attention, even though they have numerous important roles in reactive metallocofactors,[8,9] materials,[10] catalysts,[11] and physiological processes.[12]. Carbon dioxide is a persistent environmental pollutant and a C1 feedstock for chemical industries, which has inspired a large amount of research on transition metal catalysts for CO2 reduction.[19,20,21] The practical motivations for CO2 transformation are accompanied by fundamental interest in the detailed mechanisms and charge localization in reduced CO2 intermediates.[19,22] Here, we focus on cobalt complexes, which are under active study because they are adept at catalytic reductions of CO2 to CO.[13,23,24,25,26,27,28,29,30,31]. We investigate the electronic structure and reactivity of this new oxodicobalt(II) complex in detail, including multi-reference complete active space selfconsistent eld (CASSCF) calculations
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