Alternative synthetic route for the heterometallic CO-releasing [Sb@Rh12(CO)27]3− icosahedral carbonyl cluster and synthesis of its new unsaturated [Sb@Rh12(CO)24]4− and dimeric [{Sb@Rh12Sb(CO)25}2Rh(CO)2PPh3]7− derivatives

The hetero-metallic [Sb@Rh-12(CO)(27)](3-) cluster has been known as for over three decades thanks to Vidal and co-workers, and represents the first example of an E-centered (E=heteroatom) icosahedral rhodium carbonyl cluster. However, its synthesis required high temperature (140-160 degrees C) and elevated CO pressure (400 atm). Applying the redox condensation method for cluster preparation, we herein report a new synthetic, high-yield route for preparing [Sb@Rh-12(CO)(27)](3-) under much milder conditions of temperature and pressure. Notably, when the same synthesis was carried out under N-2 instead of CO atmosphere, the new isostructural but unsaturated derivative [Sb@Rh-12(CO)(24)](4-) was obtained, for which we report the full X-ray structural characterization. This species represents one of the few examples of an icosahedral cluster disobeying the electron-counting Wade-Mingos rules, possessing less than the expected 170 cluster valence electrons (CVEs). Judging from IR monitoring, the two species can be obtained one from the other by switching between N-2 and CO atmosphere, making [Sb@Rh-12(CO)(27)](3-) a spontaneous CO-releasing molecule. Finally, the study of the chemical reactivity of [Sb@Rh-12(CO)(27)](3-) with PPh3 allowed us to obtain the new [{Sb@ Rh12Sb(CO)(25)}(2)Rh(CO)(2)PPh3](7-) dimeric compound, for which we herein report the full X-ray structural and P-31 NMR analyses.