NHC Complexes of Osmium Clusters: A Structural and Reactivity Study

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)₂(η²-C)(η-C)C₄H₄(η²-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, {Os₅(μ₅-C)(CO)₁₄[η-C-η-NC₂H₂(Mes)]} (8) and [Os₆(μ-H)₄(μ₅-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)₂(η²-C)(η-C)CHC(CH₃)C₂ H₂(η²-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.