Synthesis and characterization of osmium(II) compounds of stoichiometry (C(5)Me(5))OsL(2)Br, (C(5)Me(5))OsL(2)H, and (C(5)Me(5))Os(NO)Br-2

The preparations of several new (pentamethylcyclopentadienyl)osmium(II) complexes from the osmium(III) compound (C(5)Me(5))(2)Os2Br4 are described; among these are phosphine and alkene complexes of stoichiometry (C(5)Me(5))OsL(2)Br and (C(5)Me(5))OsL(2)H as well as the nitrosyl complex (C(5)Me(5))Os(NO)Br-2. Treatment of (C(5)Me(5))(2)Os2Br4 with PPh(3) in ethanol or PMe(3) in dichloromethane affords the osmium(II) complexes (C(5)Me(5))OsL(2)Br, where L = PPh(3) or PMe(3); the 1,5-cyclooctadiene complex (C(5)Me(5))Os(cod)Br can be made similarly in ethanol. Treatment of either the PPh3 or cod complex with other tertiary phosphines in refluxing heptane affords several other compounds of this class: (C(5)Me(5))OsL(2)Br, where L = PEt(3), 1/2 Me(2)-PCH(2)PMe(2), 1/2 Me(2)PCH(2)CH(2)PMe(2), or 1/2 Ph(2)PCH(2)PPh(2). These bromoosmium(II) species serve as excellent starting materials for the preparation of other osmium(II) complexes. For example, treatment with NaBH4 in ethanol or with NaOMe in methanol affords the hydrides (C(5)Me(5))OsL(2)H, where L = PMe(3), PEt(3), PPh(3), 1/2 cod, 1/2 Me(2)PCH(2)PMe(2), 1/2 Me(2)PCH(2)CH(2)-PMe(2), or 1/2 Ph(2)PCH(2)PPh(2). Interestingly, treatment of (C(5)Me(5))Os(PMe(3))(2)Br with NaBH4 in refluxing ethanol affords the dihydride cation [(C(5)Me(5))Os(PMe(3))(2)H-2(+)], which can be deprotonated with methyllithium in tetrahydrofuran to afford the electrically neutral hydride (C(5)Me(5))Os(PMe(3))(2)H. This hydride complex is expected ti, be one of the most basic transition metal complexes known. Finally, treatment of (C(5)Me(5))(2)Os2Br4 with nitric oxide in dichloromethane yields the osmium(II) complex (C(5)Me(5))Os(NO)Br-2. IR, NMR, and mass spectra of the new complexes are described. A secondary C-13/C-12 isotope effect on the P-31 NMR chemical shifts of ca. 0.025 ppm is noted in several compounds. Comparisons of these osmium(II) compounds with analogous ruthenium species suggests that the former have stronger metal-ligand bonds, are slower to undergo nucleophilic substitution reactions, and are stronger reducing agents.