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 [Os 3 (µ-H) 2 (CO) 10 ] gave [Os 3 (µ-H)(µ-Cl)(CO) 9 (IMes)] (1), an osmium N-heterocyclic carbene complex. The reaction of ([IMes]AgCl) with the cluster [Os 3 (CO) 10 (CH 3 CN) 2 ] yielded two products, [Os 3 (µ-Cl)(CO) 10 (µ-Ag(IMes)] (2) and [(IMes-H)][Os 3 (µ-Cl)(CO) 10 ](µ 4 -Ag)[Os 3 (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 [Os 4 (µ-H) 4 (CO) 12 ] in the presence of trimethylamine-N-oxide resulted in the formation of [Os 4 (µ-H) 4 (CO) 11 (n • IMes)] (4), where (n denotes normal carbene bonding, i.e., through C 2 ). [Os 4 (µ-H) 4 (CO) 10 (IMes) 2 ] (5) was formed using the activated cluster [Os 4 (µ-H) 4 (CO) 10 (CH 3 CN) 2 ] and [(IMes)AgCl]. Compound (5) was found to be sensitive to silica gel column chromatography in the presence of dichloromethane decomposing to [Os 4 (µ-H) 3 (µ-Cl)(CO) 11 (IMes)] ( 6), which possessed a butterfly metal skeleton. The ruthenium analogue of ( 4), [Ru 4 (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 [Os 4 (µ-H) 4 (CO) 11 (a • IMes)] (10) (a denotes abnormal carbene bonding through C 3 or C 4 ), an isomer of (4), and {Os 4 (µ-H 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, {Os 5 (µ 5 -C)(CO) 14 [η-C-η-NC 2 H 2 (Mes)]} ( 8) and [Os 6 (µ-H) 4 (µ 5 -C)(CO) 15 (IMes)] ( 9), both of which contain carbide ligands, were also isolated. {Os 4 (µ-H)(CO) 10 [(η-C)N(Mes)C 2 H 2 NC 6 H 2 (CH 3 ) 2 (η 2 -C)(η-C)CHC(CH 3 )-C 2 H 2 (η 2 -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.

The product of this reaction is belieVed to be the first tetranuclear NHC cluster compound. Thermolysis of this complex in benzene at reflux resulted in exhaustiVe dehydrogenation of an o-methyl group on IMes, partial dehydrogenation of benzene, and subsequent coupling of the two unsaturated fragments to produce 2. The abnormal NHC complex (1‚aIMes) was also isolated. All compounds were characterized by spectroscopic studies and confirmed by X-ray crystallography.In recent years 1 there has been much interest in N-heterocyclic carbenes (NHCs), both as free carbenes, where they behave as organocatalysts, 2 and as essential components of transition metal complexes, which have a wide range of reactivity in either stoichiometric or catalytic reactions. 3 NHCs play a role broadly similar to that of phosphines, but they are considered to be much more strongly bound to a metal center than phosphines and, hence, less likely to participate in coordination sphere rearrangements. Recent studies show that NHC-metal bonds are not inert, and NHCs participate in various inter-and intramolecular reactions. 4 Several reports illustrate that NHCs are susceptible to single C-H activation on the N-substituents. 5 Reductive eliminations have also been identified. 6 Our studies of the reactions of imidazolium ions and/or NHCs with simple reagents such as the electron, the proton, and the hydrogen atom 7 For decomposition reactions of NHC-metal complexes see: Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247 (5) For recent examples of C-H activation, see: (a) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan, S. P. Angew. Chem., Int. Ed. 2005, 44, 2512. (b) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516. (c) Cabeza, J. A.; del Río, I.; Moguel, D.; Sánchez-Vega, M. G. Chem. Commun. 2005, 3956. (d) Corberán, R.; Sanaú, M.; Peris, E.

N-Heterocyclic carbene transfer reactions were attempted using IMesAgCl and two osmium clusters. The products isolated from these reactions suggest that NHC transfers can be unpredictable.