Sterically Directed, Site-Preferential Selenization of Discrete Copper Sulfide Nanoclusters

The controlled incorporation of selenide ions into metal sulfide phases is of great interest to chemists and engineers because of the numerous potential uses of sulfoselenide materials for energy storage and catalysis. However, the need for high-performing devices places stringent demand on compositional and site control in these materials. Herein, we study selenide incorporation into the soluble phosphine-supported copper sulfide nanoclusters [Cu12S6(dppo)4] (1-S: dppo = 1,8-bis(diphenylphopshino)octane) using Se(SiMe3)2 as a selenide source. Selenization occurs preferentially at the [Cu4S4] “equator” sites of the prolate core, with favored retention of sulfides in the apical [Cu4S] positions as judged by X-ray crystallography studies. DFT studies indicate that the site preference arises from the greater steric accessibility of the equatorial sites located under the flexible octane bridges relative to the apical sites covered by four P–Ph moeities, which gives rise to thermodynamic energy differences of ~5 kcal/mol. Further solution studies of [Cu12E6(dppo)4] 1-E (E = S,Se) elucidate key differences between the core structures and ligand flexibility in solution. While 77Se NMR studies indicate that 1-Se maintains its prolate core solution, variable-temperature NMR analysis suggest that the dppo ligand is significantly more flexible in 1-Se than 1-S as a result of the larger size of Se, which results in inward puckering of the Se–Cu–Se linkages in the [Cu4Se4] equator. The puckering provides additional room for flexion and rotation of the octane arm and likely contributes to the thermodynamic site preference of selenization. These findings suggest powerful means of tuning metal chalcogenide structure and function via fine tailoring of selenide positioning.