Formation of Formic Acid Derivatives through Activation and Hydroboration of CO2 by Low-Valent Group 14 (Si, Ge, Sn, Pb) Catalysts

The chemistry of low-valent main group elements has attracted much attention in the past decade. These species are relevant because they have been able to mimic transition metal behavior in catalytic applications, with decreased material costs and diminished toxicity. In this contribution, we study the (LEH)-E-1 catalysts (E = Si(II), Ge(II), Sn(II), and Pb(II); L-1 = [ArNC(Me)CHC(Me)NAr] with Ar = 2,6-iPr(2)C(6)H(3)) for the formation of formic acid derivatives through hydroboration of CO2. Detailed characterization of relevant structures on the potential energy surface enabled us to rationalize different paths for the hydroboration of CO2. Interestingly, it was found that according to the activation energies for the whole catalytic cycle, the process of transformation of CO2 becomes more favored going down group 14. However, an effective energetic decrease for the process (taking as the reference the uncatalyzed reaction between CO2 and HBpin) is evidenced just from the germanium analogue. The trend reactivity found in the present study is a direct consequence of the change in the central main group element, enabling enhanced polar character of the E-H ((LEH)-E-1 in the CO2 activation step) and E-O (metal formates in the hydroboration step) bonds as the atomic radius increases. The transient stabilization of reaction intermediates found in the hydroboration step was rationalized through the non-covalent interaction index (NCI) and symmetry-adapted perturbation theory (SAPT). This computational study highlights the reactivity trends in group-14-based hydride catalysts in hydrometalation and posterior hydroboration to form formic acid intermediates. We hope that this study will motivate further experimental work in low-valent lead chemistry.