Atomically precise rhodium–indium carbonyl nanoclusters: synthesis, characterization, crystal structure and electron-sponge features

In this paper we present the investigation of the reactivity of [Rh₇(CO)₁₆]^3- with InCl₃, with the aim of expanding the more general study that allowed us to obtain, among other species, the icosahedral [Rh₁₂E(CO)₂₇]^n- (n = 4 when E = Ge or Sn; n = 3 when E = Sb or Bi) family of clusters. Indeed, the study resulted in the isolation and characterization of the analogous In-centred icosahedral [Rh₁₂In(CO)₂₈]^3- nanocluster (1), which is isoelectronic and isostructural with the [Rh₁₂E(CO)₂₇]^n- congeners. During the course of the reaction two more new species, namely the octahedral [Rh₆(CO)₁₅InCl₃]^2- (2) and the dimeric [{Rh₆(CO)₁₅InCl₂}₂]^2- (3) have also been identified. The reaction between [Rh₇(CO)₁₆]^3- and InCl₃ proved to be poorly selective; nevertheless, by fine tuning some reaction parameters it was possible to drive the reaction more towards one product or the other. Alternatively, [Rh₆(CO)₁₅InCl₃]^2- can be more selectively prepared by reacting either [Rh₅(CO)₁₅]^- or, less efficiently, [Rh₆(CO)₁₅]^2- with InCl₃. As for the dimeric [{Rh₆(CO)₁₅InCl₂}₂]^2- species, this was only isolated by carrying out the reaction with [Rh₇(CO)₁₆]^3- under inert atmosphere, as opposed to under CO. All clusters were characterized by IR spectroscopy and ESI-MS, and their molecular structures were fully established by single-crystal X-ray diffraction studies. The [Rh₁₂In(CO)₂₈]^3- species was also analysed by EDS via SEM, and further investigated through in situ infrared spectroelectrochemistry and CV experiments to check its multivalence nature. Indeed, [Rh₁₂In(CO)₂₈]^3- can reversibly undergo two monoelectronic oxidation and one bi-electronic reduction processes, behaving like an electron sponge and, thus, giving rise to the further [Rh₁₂In(CO)₂₈]^n- derivatives (n = 1, 2 and 5). These results parallel the findings for the [Rh₁₂E(CO)₂₇]^n- series. The geometry variations of the metal framework associated with the changes in the cluster negative charge were investigated by means of DFT calculations.