Why Does CoII Work Better than NiII for Activation of CO2 in the Presence of Bipyridyl-N-Heterocyclic Carbene Framework? Expanding the Metal Scope with FeII.

Herein, we present a complete and comprehensive computational study, discovering the underlying reasons for remarkably different CO2-activation capability of NiII and CoII, observed by Jurss et al. (Chem. Commun., 2018, 54, 3351-3354 and ACS Catal., 2019, 9, 7398-7408). The origin of this reactivity difference was not properly understood, presenting a major challenge for development of efficient catalysts in the area of CO2 reduction. We have explored two different ligand architectures: a nonmacrocyclic and a macrocyclic network. In case of the nonmacrocyclic ligand, the 3dx2-y2 orbital is unoccupied in low-spin 3d8 NiII, and hence, the metal is unable to participate in electron transfer to CO2. This is in striking contrast to CoII, where the in situ-generated high-spin 3d8 CoI intermediate has 3dx2-y2 as a singly occupied molecular orbital, which can participate to reduce CO2. This is further amplified in the macrocyclic system where two singly occupied molecular orbitals, 3dx2-y2 and 3dz2, of cobalt can now participate in electron transfer, whereas none of the nickel orbitals are involved. To develop a more sustainable, cost-effective, and Earth-abundant solution, we have extended our metal scope to FeII which shows promising results with involvement of iron in electron transfer to CO2 in the case of the macrocyclic ligand.