Metal-centered control of carbon dioxide activation and C-O bond cleavage in [M(depe)2] complexes: a density functional theory study.

Understanding the role of metal identity in CO2 activation and C-O bond cleavage is critical to the rational design of effective molecular platforms for carbon valorization. Density functional theory (DFT) calculations were performed to investigate the reactivity of [M(depe)2] complexes (depe = 1,2-bis(diethylphosphino)ethane; M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Zr, Nb, Mo, Tc, Ru, Rh, and Pd) toward CO2 coordination, activation, and subsequent C-O bond cleavage. The results reveal dramatic changes in the preference for the spin state, the coordination geometry and the characteristics of the frontier orbital of the parent [M(depe)2] fragments that control their interaction with CO2. Early and relatively oxophilic metals favour formation of stabilizing η2-(C,O) adducts and increased substrate activation, as demonstrated by greater O-C-O bending, larger C-O bond elongation and greater metal-to-CO2 charge transfer, whereas later metals favour less perturbed η1-C coordination modes. The computed cleavage paths further indicate that strong activation alone is not enough for deeper reactivity, since C-O bond rupture also requires good stabilization of the highly polarized transition state. Of the systems studied, Ti, Zr and Nb provide the best compromise between CO2 activation and C-O bond breakage, providing both substantial substrate perturbation and low reaction barriers and favourable product energetics. In summary, this work provides a descriptor-based correlation between the electronic structure at the metal center and reactivity of CO2 in the [M(depe)2] series, which can be used as a useful guide towards the design of transition metal complexes for selective CO2 conversion.