This work explores the reactivity of a reversible superbase–CO ₂ zwitterion, which can be used as a stoichiometric source of CO ₂ .

In transition metal-mediated carboxylation reactions, CO₂ inserts into a metal–nucleophile bond. At the carboxylation transition state (TS), CO₂ may interact with the metal (inner-sphere path) or may insert without being activated by the metal (outer-sphere path). Currently, there is no consensus as to which path prevails. In order to establish general predictions for the insertion of CO₂ into metal–carbon bonds, we computationally analyze a series of experimentally reported Cu, Rh, and Pd complexes. Our focus is on carboxylation of aromatic substrates, including C_sp³benzyl and C_sp²aryl and alkenyl nucleophiles. We observe clear trends, where the nature of the nucleophile determines the preferred path: benzylic C_sp³ nucleophiles favor outer-sphere and C_sp² systems favor inner-sphere CO₂ insertion into the metal–carbon bond. An exception are Cu–benzyl bonds, where inner- and outer-sphere CO₂ insertions are found to be competitive, highlighting the need to include both paths in mechanistic studies and in the rationalization of experimental results. For insertion into Pd–C_sp² bonds, we find that the metal–CO₂ interactions at the TS are weak and may be beyond 3 Å for sterically congested ligands. Nonetheless, on the basis of a comparison to other TSs, we argue that the CO₂ insertion into Pd–C_sp² bonds should be classified as inner-sphere.

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄ ⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using ( ^tBu PBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into ( ^tBu PBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.