Binuclear cyclopentadienylosmium hydride chemistry: A stable quadruply bridged structure

Abstract

The pentamethylcyclopentadienylosmium (Cp*Os) system is of interest in forming a stable binuclear hydride Cp*2Os2(μ-H)4 with four bridging hydrogen atoms as well as a stable mononuclear hydride Cp*OsH5. Density functional theory (DFT) studies on the corresponding unsubstituted systems Cp2Os2Hn (n=8, 6, 4, 2, 0) and CpOsHn (n=5, 3, 1) are reported. The quadruply bridged structure Cp2Os2(μ-H)4 is shown to be the lowest energy Cp2Os2H4 structure. For the other Cp2Os2Hn systems (n=8, 6, 2) the lowest energy structures contain doubly bridged Cp2Os2(μ-H)2 units with decreasing OsOs distances as the numbers of hydride ligands is decreased. A related doubly bridged Cp2Os2(μ-H)2H2 structure is predicted to lie only 2.5kcal/mol above the quadruply bridged Cp2Os2(μ-H)4 isomer. The hydrogen-richer structures Cp2Os2Hn (n=8 and 6) are predicted to lose H2 easily to give Cp2Os2H4 in reactions within ∼8kcal/mol of being thermoneutral. However, the H2 dissociation energy of Cp2Os2H4 to give Cp2Os2H2+H2 is relatively high at ∼42kcal/mol consistent with its stability. Bridging cyclopentadienyl rings are found in low-energy Cp2Os2H2 and Cp2Os2 structures. For Cp2Os2, two structures, each with a bridging μ-Cp ring, are shown to lie more than 14kcal/mol in energy below the singlet and triplet unbridged isomers, which were previously shown by frontier molecular orbital analyses to have formal OsOs quintuple bonds.