Synthesis and structural characterization of mixed-metal Pt–Rh clusters. Assembly of tetra- and tetra-hexanuclear clusters from smaller metal fragments

Abstract

The tetranuclear cluster [Rh2Pt2(CO)6(dppm)2] (1) has been obtained in good yield by redox condensation of [Rh(CO)4]− with [PtCl(dppm)]2 and also by substitution of two rhodium atoms in [Rh4(CO)12] for the binuclear [Pt2(CO)3(dppm)2] fragment. The cluster 1 crystallizes in two isomeric forms, 1a and 1b, the former prevails both in solution and in the solid state. The molecular structures of both isomers have been established by X-ray analysis, which showed the presence of a tetranuclear butterfly framework with platinum atoms at the wingtip positions in the both clusters. The isomers differ from each other in the mode of dppm coordination on the cluster core. Reaction of the labile cluster [Rh6(CO)15(NCMe)] with 1 results in formation of [{Rh6(μ3-CO)4(CO)10}(μ2-CO)2{Pt2Rh2(μ2-CO)3(CO)2(dppm)2}] (2), containing two cluster frameworks linked by a dative metal–metal bond. This cluster can also be obtained in slightly lower yield by reaction of [PtCl(dppm)]2 with either [Rh7(CO)16]3− or [Rh6(CO)15]2−. Treatment of [Rh6(CO)15(NCMe)] with two equivalents of [Pt2(CO)3(dppm)2] affords [{Rh6(μ3-CO)4(CO)10}(μ2-CO)2{Pt4(dppm)3}] (3), which displays another structural pattern containing the hexa- and tetranuclear cluster frameworks linked by a dative interaction. 31P spectroscopic studies of 1a and 2, and simulation of the corresponding spectroscopic patterns showed that the structures of the “Pt2Rh2(dppm)2” fragments found in the solid state remain unchanged in solution.