Synthesis and Characterization of Osmium(II) Compounds of Stoichiometry (C₅Me₅)OsL₂Br, (C₅Me₅)OsL₂H, and (C₅Me₅)Os(NO)Br₂

Abstract

The preparations of several new (pentamethylcyclopentadienyl)osmium(II) complexes from the osmium(III) compound (C 5 Me 5 ) 2 Os 2 Br 4 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 Os 2 Br 4 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 PPh 3 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 NaBH 4 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 NaBH 4 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 to be one of the most basic transition metal complexes known. Finally, treatment of (C 5 Me 5 ) 2 Os 2 Br 4 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 13 C/ 12 C isotope effect on the 31 P 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.