Resonance Effects from Substituents on L-Type Ligands Mediate Synthetic Control of Gold Nanocluster Frontier Orbital Energies

The ligands of metal nanoclusters exert a profound effect on their properties and reactivity, but a systematic explanation for these impacts comparable to that of mononuclear coordination complexes does not currently exist. We show that quantitative control of frontier orbital energies of prototypical Au-PPh3 nanoclusters is driven by resonance effects reaching from the exterior of the ligand shell to the HOMO and LUMO orbitals that are nominally located in the cluster core. Hammett studies of Au-PPh3 ligands with para- and meta- methyl and methoxy groups indicate that electronic resonance effects dominate electrostatic effects, but do not specify whether resonance effects stop at the Au-P bond or extend into the cluster core. Quantum chemical calculations show no significant trend in charge on the Au atoms, but do show a pattern of alternating shortened and lengthened bonds that includes the Au-P bond, consistent with resonance effects spanning the full cluster. Computed orbitals show a substantial contribution from atomic orbitals of the ligand substituent to the cluster-core-based HOMO orbital for the para- case, mediated by phenyl ring and phosphorous orbitals. These results suggest that a resonance picture must exist to rationalize ligand effects in nanoclusters and, potentially, nanoparticles, even in the ground state. These resonance effects can be used to engineer cluster properties and reactivity and modulate energy and charge transfer in and out of nanoclusters.