Metal-centered electronic effects in carborane single-site catalysts for CO2 reduction: A Cu–Ir comparative study

Designing efficient and selective electrocatalysts for CO 2 reduction remains a central challenge in sustainable chemistry. In this work, we present a comparative theoretical investigation of carborane-based single-site clusters, MC 2 B 9 H 11 (M = Cu, Ir), for the CO 2 reduction reaction (CO 2 RR) using detailed electronic structure analysis and Gibbs free energy profiling. Key electronic descriptors — including frontier molecular orbitals, spin density, density of states, and electrostatic potential maps — reveal two fundamentally distinct catalytic regimes dictated by the identity of the metal center. Both CuC 2 B 9 H 11 and IrC 2 B 9 H 11 efficiently catalyze CO 2 reduction, with CO identified as the most thermodynamically favorable product on both surfaces. CuC 2 B 9 H 11 further promotes selective formation of oxygenated products such as formaldehyde and methanol through stabilized intermediate pathways. In contrast, IrC 2 B 9 H 11 primarily favors CO evolution, with suppressed selectivity toward deeper hydrogenation and alternative reduction products. Solvent effects further differentiate the active metal center catalytic behavior. For the Ir system, intermediates are uniformly stabilized, leading to a nearly constant downward shift in free energies with minimal perturbation of relative reaction energetics, consistent with non-specific electrostatic stabilization. In contrast, the Cu system displays non-uniform solvation-induced reshaping of the potential energy surface, where polar intermediates are preferentially stabilized. This selective stabilization alters relative energetics and induces a solvent-driven shift in the preferred configuration of the *CHOH intermediate. Such behavior reflects a higher sensitivity of the Cu-centered electronic structure to the dielectric environment, arising from its more localized charge distribution and weaker metal–ligand delocalization.