Deciphering Charge Delocalization across Mercaptobenzonitrile Ligand Terminals and Inner Hydrides in Atomically Precise Cu14H10 Nanoclusters for Electrocatalytic CO2 Reduction

Abstract Cu hydride nanoclusters with atomic precision in composition and structure offer exciting catalytic activities and opportunities for rigorous structure–function correlations. Determination of their atomic structures, especially the hydride locations, hinges on the availability of high-quality single crystals and neutron beams, while their relatively weak UV–visible absorption and electrochemical instability challenge the analysis of their electronic properties. Herein, we synthesize a Cu14H10 nanocluster coordinated by three 4-mercaptobenzonitrile (MBN) and eight triphenylphosphine ligands. Two-dimensional NMR analyses combined with single crystal X-ray diffraction reveal unequivocally the locations of the ten hydrides in four groups (stoichiometry ratio of 1:3:3:3) with drastically different environments (chemical shifts 11–3 ppm): one H at the inversion center and three triangular layers along the C3 axial bridging together the Cu core. MBN serves two additional purposes besides stabilizing ligands: its electron-withdrawing nature and a highly sensitive vibrational reporter of its local charge density. The first property gives rise to a low-lying Cu-core-to-MBN metal-to-ligand charge transfer (MLCT) excited state, and the second property enables direct spectroscopic probing of both the charge transfer nature and dynamics of the MLCT excited state through the dynamic spectral shift of the MBN CN stretching mode. Through combined transient IR absorption spectroscopy and time-dependent density functional theory (TD-DFT) calculations, we obtained the complete atomic and electronic structure, revealing core-to-ligand charge transfers in these nanoclusters. Computed under identical conditions using an iso-structure with a mildly electron-donating ligand, the Cu14H10 featuring charge delocalization toward the electron-withdrawing MBN has a smaller HOMO–LUMO gap, which is deemed favorable for electrocatalytic CO2 reduction. The preferential formation of complex multielectron-hydrogenation products (8–12 e–, C1 hydrocarbons, and C2+ products) over two-electron pathways such as CO formation or the competing hydrogen evolution reaction highlights their promise as atomically precise catalysts for deep CO2 reduction.