Ditopic ligand effects on solution structure and redox chemistry in discrete [CuS] clusters with labile Cu-S bonds.

Copper chalcogenide nanoclusters (Cu-S/Se/Te NCs) are a broad and diverse class of atomically precise nanomaterials that have historically been studied for potential applications in luminescent devices and sensors, and for their beautiful, mineral-like crystal structures. By the "cluster-surface" analogy, Cu-S/Se NCs are prime candidates for the development of nanoscale multimetallic catalysts with atomic precision. However, the majority of studies conducted to date have focused exclusively on their solid-state structures and physical properties, leaving open questions as to their solution stability, dynamics, and reactivity. Herein, we report the first detailed interrogation of solution structure, dynamics, electrochemistry, and decomposition of Cu-S NCs. Specifically, we report the detailed NMR spectroscopy, diffusion-ordered spectroscopy, MALDI mass spectrometry, electrochemical and stoichiometric redox reactivity studies, and DFT studies of a series of [CuS] clusters with labile Cu-S bonds supported by monodentate phosphines and ditopic bis(diphenylphosphino)alkane ligands PPhR (R = Et, -(CH)-, -(CH)-). We find that the ligand binding topology dictates the extent of speciation in solution, with complete stability being afforded by the longer octane chelate in dppo (1,8-bis(diphenylphosphino)octane) according to H and DOSY NMR and MALDI-MS studies. Furthermore, a combined electrochemical and computational investigation of [CuS(dppo)] reveals that the intact [CuS] core undergoes a quasireversible one-electron oxidation at mild applied potentials ([CuS]: -0.50 V . Fc). In contrast, prolonged air exposure or treatment with chemical oxidants results in cluster degradation with S atom extrusion as phosphine sulfide byproducts. This work adds critical new dimensions to the stabilization and study of atomically precise metal chalcogenide NCs with labile M-S/Se bonds, and demonstrates both progress and challenges in controlling the solution behaviour and redox chemistry of phosphine-supported copper chalcogenide nanoclusters.