Synthesis, Characterization, and Photochemistry of Bis(arene) and Acetonitrile Complexes of Osmium(II)

Osmium complexes of arenes, acetonitrile, and chloride have been studied. The yield has been significantly increased for the synthesis of (η⁶-C₆H₆)Os(NCCH₃)Cl₂, an excellent starting material for the synthesis of Os-benzene complexes. Reactivity studies with this complex show that one chloride is displaced by Ag⁺ in acetonitrile to give [(η⁶-C₆H₆)Os(NCCH₃)₂Cl]⁺ but both chlorides are displaced by Ag⁺ in acetone to give [(η⁶-C₆H₆)Os- (acetone)₃]²⁺. The acetone ligands of the latter complex can be displaced by acetonitrile to give [(η⁶-C₆H₆)Os-(NCCH₃)₃]²⁺ and by other arenes (1,3,5-triethylbenzene, hexamethylbenzene, durene, naphthalene) to give [(η⁶-C₆H₆)Os(arene)]²⁺ complexes. Studies of thermal ligand exchange with the [(η⁶-C₆H₆)Os(NCCH₃)_3-xCl_x]^(2-x)+ complexes indicate that chloride enhances the rate of thermal acetonitrile exchange. Photolysis of [(η⁶-C₆H₆)Os(NCCH₃)₃]²⁺ and [(η⁶-C₆H₆)Os(NCCH₃)₂Cl]⁺ in acetonitrile results in loss of benzene to produce the new complexes [Os(NCCH₃)₆]²⁺ and [Os(NCCH₃)₅Cl]+ in high yield. Photolysis of (η⁶-C₆H₆)Os(NCCH₃)Cl₂ initially results in the formation of Os(NCCH₃)₄Cl₂, which subsequently undergoes photochemical loss of chloride to give [Os(NCCH₃)₅Cl]Cl. Examination of this reaction by ^lH NMR spectroscopy indicates that the eis isomer of Os(NCCH₃)₄Cl₂ is produced. The photolysis of [(η⁶-C₆H₆)Os(P(Ph)₃)(NCCH₃)Cl]⁺ in acetonitrile also leads to release of benzene to form the cis product. Electrochemical data acquired for the new acetonitrile complexes ([Os(NCCH₃)₆]²⁺, [Os(NCCH₃)₅Cl]⁺, and Os(NCCH₃)₄Cl₂) show highly reversible, one-electron oxidations with the potential a linear function of the stoichiometry. The osmium complexes are about 0.5 V easier to oxidize than the corresponding ruthenium analogs.