Synthesis, structures and reactivity of bis(diphenylphosphino)-methane (dppm)-substituted selenido osmium carbonyl clusters

The unsaturated cluster [(μ-H)Os3(CO)8{Ph2PCH2P(Ph)C6H4}] (2) reacts with elemental selenium at 110 °C to give two triosmium clusters [Os3(CO)7(μ3-Se)2(μ-dppm)] (3), [Os3(CO)7(μ3-CO)(μ3-Se)(μ-dppm)] (4) and the tetraosmium cubane-like cage cluster [Os4(CO)10(μ3-Se)4(μ-dppm)] (5) in 20, 47 and 5% yields, respectively. Treatment of the labile cluster [Os3(CO)10(MeCN)2] with dppmSe at ambient temperature gives the dinuclear compound [Os2(CO)6(μ-Se)(μ-dppm)] (6) along with 3 and 4 in 13, 24 and 4% yields, respectively, while with dppmSe2 at 80 °C it yields 3 and 5 in 27 and 7% yields, respectively, by oxidative transfer of selenium atoms to the metal center. The reaction of 3 with Me3NO at 80 °C leads to loss of 1 mol of CO and formation of the condensation derivative [Os6(CO)12(μ3-Se)4(μ-dppm)2] (7), containing a central 64-electron butterfly core, in 75% yield. Compound 7 reacts with CO at 98 °C to regenerate 4 by cleavage of the three unsupported metalmetal bonds. In the disubstituted square-pyramidal selenido cluster, 3, the dppm ligand bridges two bonded osmium atoms whereas in 7 both the dppm ligands bridge the open osmium–osmium edges. Treatment of 3 with PPh3 in the presence of Me3NO at room temperature gives the phosphine-substituted compound [Os3(CO)6(μ3-Se)2(μ-dppm)(PPh3)] (8) and the dimer 6 in 26 and 20% yields, respectively. The structure of 8 is comparable with that of 3 from which it is derived by replacing one equatorial carbonyl group with the PPh3 ligand. The molecular structures of the complexes 3, 4, 6, 7 and 8 have been fully elucidated by single-crystal X-ray diffraction studies.