Electronic structures of a new atomically precise Cu₁₄H nanocluster revealed by combined spectro-electrochemistry and...

Copper-based atomically precise nanoclusters with discrete and tunable electronic structures can be synthetically tailored to enable applications in heterogeneous catalysis, sensing, and energy conversion. Unlike the better-known gold or silver counterparts, Cu nanoclusters often exhibit subtle optical and complicated electrochemical properties, making it challenging to determine their intrinsic electronic structures. Here, we combine voltammetry, spectroscopy and spectroelectrochemistry to probe a new [Cu₁₄H(BzT)₁₂(TPP)₆]BF₄ nanocluster, with experimental observations supported by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Structure-wise, benzyl thiol (BzT) and triphenylphosphine (TPP) ligands form six S₂-Cu-P units bound to a distorted Cu8 cage, within which four Cu atoms coordinate in a near square planar geometry with a single deuteride (hydride) located exactly at the Wyckoff position as an inversion center. The high uniformity of nanoclusters redissolved from single crystals allows for rigorous correlation between ensemble-based experiments and DFT calculations. Cu₁₄H features an electrochemical band gap of 2.3 eV, with oxidation of the HOMO being quasi-reversible and reduction to LUMO chemically irreversible. The signature of quantized one-electron charging, featuring relatively uniform peak spacing in voltammograms, is obscured by the less reversible electron transfer and the interferences from intermediate species associated with electron transfer reactions. A general strategy for deconvoluting these complex redox features is demonstrated by varying the potential range, scan rate, and solvent in cyclic voltammetry, in comparison with those of precursor Cu complexes. Spectroelectrochemistry experiments resolve rich yet subtle features otherwise inaccessible. Through the comparisons of original and further oxidized nanoclusters, TD-DFT calculations successfully reproduce the experimental trends, which are explained by orbital reorganization and shifts in charge distribution. These findings demonstrate the efficacy and generalizability of combining electrochemical-spectroscopic and computational approaches to understand the redox properties and electronic structures of material systems that lack or display weak classical diagnostic features.