An octahedral rhodium cluster with six phosphine and 12 hydride ligands and 10 too few electrons.

Late-transition-metal (low-oxidation state) clusters, with p-acceptor ligands and early-transition-metal (high-oxidation state) clusters, with p-donor ligands, have been intensively studied for several decades. Apart from the fascinating structural aspects of cluster compounds and rationalization of their bonding, much motivation for their study has been driven by potential applications. It was suggested that transition-metal clusters might prove to be effective catalysts, filling the void between mononuclear species and colloidal (or heterogeneous) catalysts and open up new possibilities in catalysis and organic synthesis. Research at the interface of lateand early-transition-metal cluster chemistry has not been forthcoming although polyoxometalates have been combined with organometallic fragments and late–early-transition-metal bonds are well known. However, in a recent communication by Weller et al. , a latetransition-metal cluster based on an octahedral rhodium core was reported that has a structure and an electron count similar to that of an early-transition-metal cluster, and thus bridges the two largely separate and distinct regimes. The cluster was isolated after a remarkably simple synthesis involving reduction under hydrogen of the complex, [RhL2(nbd)] , where L is a phosphine and nbd is norbornadiene. The rhodium precursor represents a widely used class of hydrogenation catalyst, especially for asymmetric reductions where L2 is a chiral bisphosphine. [6] Therefore, apart from the absence of any substrate, what is so different about the synthesis which leads to such an intriguing cluster? The answer to this question remains to be confirmed, but three factors appear to be important. First, commencing with a rhodium salt comprising a weakly coordinating anion, that is, [1-H-closo-CB11Me11] or [B(ArF)4] (ArF= 3,5-bis(trifluoromethyl)phenyl). Second, the use of relatively weakly coordinating solvents, that is, C6H5F or CH2Cl2. Third, the utilization of a phosphine which appears to have the ideal topology to form a protective sheath over the cluster (see below). The cluster in question, [Rh6(PiPr3)6(m-H)12] , is prepared according to Scheme 1, and accounts for about 20% of the starting material—the other compounds formed in the reaction remain uncharacterized.