Ditopic ligand effects on solution-phase equilibria and electrochemistry of atomically precise copper chalcogenide nanoclusters

Atomically precise copper chalcogenide nanoclusters are an exceptionally diverse class of nanomaterials with potential applications in chemiluminescence, sensing, and catalysis. However, most previously reported nanoclusters have been characterized exclusively in the solid state, leaving open questions as to their stability and function in solution. In this work, we report the first detailed solution-phase investigation of speciation, electrochemistry, and decomposition of atomically precise [Cu12S6] supported by a series of mono- and ditopic alkyldiphenylphosphines PPh2R (R = Et, -(CH2)5-, -(CH2)8-). While electronically identical, this series of ligands features monotopic and ditopic binding modes spanning zero, one, or three axes of the [Cu12S6] core, facilitating an in-depth examination of the impact of phosphine binding topology on solution behaviour. We find that binding topology dictates the extent of speciation, with complete solution stability being afforded through use of the longer octane chelate dppo (1,8-bis(diphenylphosphino)octane). Thus, the 1H and DOSY NMR spectra of [Cu12S6(dppo)4] indicate a rigid, stereochemically locked ligand configuration in which substantial stabilizing CH---S interactions are present. Meanwhile, electrochemical analysis coupled with DFT calculations indicates that the [Cu12S6] core undergoes a quasireversible one-electron oxidation at –0.50 V vs Fc0/+. In contrast, prolonged air exposure or treatment with chemical oxidants results in cluster degradation with formation of phosphine sulfide byproducts. This work demonstrates both progress and challenges in controlling the solution-phase behaviour and redox chemistry of phosphine-supported copper chalcogenide nanoclusters.