Unsaturated platinum-rhenium cluster complexes. Synthesis, structures and reactivity

Two new compounds PtRe3(CO)(12)(PBu3t)(mu-H)(3), 9, and PtRe2(CO)(9)(PBu3t)(mu-H)(2), 10, were obtained from the reaction of Pt(PBu3t)(2) with Re-3(CO)(12)(mu-H-3), 8, at room temperature. Compound 9 contains a butterfly cluster of four metals formed by the insertion of the platinum atom from a Pt(PBu3t) group into one of the hydride-bridged metal-metal bonds of 8. The three hydrido ligands are bridging ligands across each of three new Pt-Re bonds. Compound 10 contains a triangular PtRe2 cluster with two hydrido ligands; one bridges a Pt-Re bond, and the other bridges the Re-Re bond. The new compound Pt2Re2(CO)(7)(PBu3t)(2)(mu-H)(2), 11, was obtained from the reaction of 8 with Pt(PBu3t)(2) in hexane at reflux. Compound 11 was also obtained from 10 by reaction with an additional quantity of Pt(PBu3t)(2). Compound 11 contains a tetrahedral cluster of four metal atoms with two dynamically active hydrido ligands. A CO ligand on one of the two platinum atoms also exchanges between the two platinum atoms rapidly on the NMR time scale. Compound 11 is electronically unsaturated and was found to add hydrogen at room temperature to form the tetrahydrido cluster complex, Pt2Re2(CO)(7)(PBu3t)(2)(mu-H)(4), 12. Compound 12 has a structure similar to 11 but contains one triply bridging hydrido ligand, two edge bridging hydrido ligands, and one terminal hydrido ligand on one of the two platinum atoms. A kinetic isotope effect D/H of 1.5(1) was determined for the addition of H-2 to 11. Hydrogen can be eliminated from 12 by heating to 97 degrees C or by the application of UV-vis irradiation at room temperature. Compound 12 adds CO at room temperature to yield the complex Pt2Re2(CO)(8)(PBu3t)(2)(mu-H)(4), 13, which contains a planar cluster of four metal atoms with a Pt-Pt bond and four edge bridging hydrido ligands. Compounds 11 and 12 react with Pt(PBu3t)(2) to yield the known five metal cluster complexes Pt3Re2(CO)(6)(PBu3t)(3)(mu-H)(2), 14, and Pt3Re2(CO)(6)(PBu3t)(3)(mu-H)(4), 15, respectively. Density functional calculations confirm the hydride positions in the lowest energy structural isomers of 11 and 12 and suggest a mechanism for H-2 addition to 11 that occurs on the Pt atom with the lower coordination number.