Carbon Dioxide Insertion into Group 9 and 10 Metal-Element σ Bonds

Carbon dioxide (CO₂) is an appealing feedstock for the sustainable preparation of a variety of carbon-based commodity chemicals because of its high abundance, low cost, and nontoxicity. The high kinetic and thermodynamic stability of CO₂, however, means that there are currently only a limited number of practical catalytic systems for the conversion of CO₂ into more valuable chemicals, and continued research in this area is required. One promising approach for the eventual transformation of CO₂ is to initially insert the molecule into transition-metal-element σ bonds such as M-H, M-OR, M-NR₂, and M-CR₃ bonds to form products of the type M-OC(O)E (E = H, OR, NR₂, or CR₃). CO₂ insertion has been demonstrated in numerous stoichiometric reactions involving transition-metal complexes, but in cases where insertion results in the formation of strong M-O bonds, the products are often too stable to undergo further transformations. Group 9 and 10 transition-metal complexes (M = Ni, Pd, Pt, Co, Rh, or Ir) form relatively weak M-O bonds, and as a consequence, a number of group 9 and 10 transition-metal catalysts in which CO₂ insertion is proposed as an elementary step in catalysis have been developed. In this Award Article, we summarize group 9 and 10 transition-metal complexes in which CO₂ insertion into a metal-element σ bond to form a M-OC(O)E-type product has been observed. Mechanistic similarities and differences are highlighted by comparing CO₂ insertion reactions in different types of group 9 and 10 metal-element σ bonds, and a general trend for predicting the rate-determining step of the insertion process is described based on the nucleophilicity of the element in the σ bond. Although we focus on stoichiometric reactivity, the relevance of CO₂ insertion to catalytic reactions is also emphasized throughout the paper.