American Chemical Society
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NHC Complexes of Osmium Clusters: A Structural and Reactivity Study

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posted on 2008-11-24, 00:00 authored by Craig E. Cooke, Michael C. Jennings, Michael J. Katz, R. K. Pomeroy, Jason A. C. Clyburne
A carbene transfer agent, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene-silver(I)chloride ([IMes]AgCl), was treated with various osmium clusters. Reaction of ([IMes]AgCl) with [Os3(μ-H)2(CO)10] gave [Os3(μ-H)(μ-Cl)(CO)9(IMes)] (1), an osmium N-heterocyclic carbene complex. The reaction of ([IMes]AgCl) with the cluster [Os3(CO)10(CH3CN)2] yielded two products, [Os3(μ-Cl)(CO)10(μ-Ag(IMes)] (2) and [(IMes-H)][Os3(μ-Cl)(CO)10](μ4-Ag)[Os3(μ-Cl)(CO)10] (3). Compound (2) is a heterobimetallic carbene complex resulting from the complete incorporation of ([IMes]AgCl). Transmetalation using [(IMes)AgCl] and the cluster [Os4(μ-H)4(CO)12] in the presence of trimethylamine-N-oxide resulted in the formation of [Os4(μ-H)4(CO)11(n·IMes)] (4), where (n denotes normal carbene bonding, i.e., through C2). [Os4(μ-H)4(CO)10(IMes)2] (5) was formed using the activated cluster [Os4(μ-H)4(CO)10(CH3CN)2] and [(IMes)AgCl]. Compound (5) was found to be sensitive to silica gel column chromatography in the presence of dichloromethane decomposing to [Os4(μ-H)3(μ-Cl)(CO)11(IMes)] (6), which possessed a butterfly metal skeleton. The ruthenium analogue of (4), [Ru4(μ-H)4(CO)11(IMes)] (7), was prepared and identified spectroscopically but was found to be thermally unstable. Thermolysis of (4) in benzene yielded four products. The abnormal carbene complex [Os4(μ-H)4(CO)11(a·IMes)] (10) (a denotes abnormal carbene bonding through C3 or C4), an isomer of (4), and {Os4(μ-H)(CO)10[(η-C)N(Mes)C2H2NC6H2 (CH3)22-C)(η-C)C4H42-C)]} (11), an unusual complex resulting from the activation of benzene and activation of three C−H bonds on IMes, were isolated in modest yields. Two higher-nuclearity clusters, {Os55-C)(CO)14[η-C-η-NC2H2(Mes)]} (8) and [Os6(μ-H)45-C)(CO)15(IMes)] (9), both of which contain carbide ligands, were also isolated. {Os4(μ-H)(CO)10[(η-C)N(Mes)C2H2NC6H2(CH3)22-C)(η-C)CHC(CH3)C2 H22-C)]} (12), an analogue of (10), resulted from the thermolysis of (4) in toluene indicates that the reaction is potentially general and exhibits some regioselectivity. Crystal structure determinations were completed on all 11 osmium cluster complexes.