Reactivity of [Ru₄(μ-H)₄(CO)₁₂] with N-Heterocyclic Carbenes

The tetraruthenium tetrahydrido cluster compound [Ru4(μ-H)4(CO)12] (1) reacts at room temperature with N-heterocyclic carbenes of the 1,3-disubstituted imidazolin-2-ylidene type (R1R2Im) to give the carbonyl substitution products [Ru4(μ-H)4(CO)11(R1R2Im)] (R1R2Im = 1,3-dimethylimidazolin-2-ylidene, 2; 1-phenyl-3-methylimidazolin-2-ylidene, 3; 1,3-diphenylimidazolin-2-ylidene, 4; and 1,3-dimesitylimidazolin-2-ylidene, 5). In solution, compounds 2−5 are fluxional in the NMR time scale and display the same pattern of ν(CO) IR absorptions. DFT calculations have shown that the CS arrangement of the Ru4H4 cluster core of these clusters is 0.2−1.7 kcal mol−1 more stable than the D2d cluster core, the smallest difference corresponding to the clusters with the very bulky 1,3-dimesitylimidazolin-2-ylidene ligand. Two conformers with CS Ru4H4 cluster core have been found by X-ray crystallography in the crystals of compound 3. The thermal stability of compounds 2−5 has also been studied. While the dimethyl derivative 2 is stable in refluxing toluene for 3 h, the dimesityl derivative 5 slowly decomposes in solution at room temperature. Two isostructural heptanuclear derivatives, [Ru7(μ3-H)(μ4-CO)(μ-CO)2(CO)14{μ-(η1-η6-C6H4)RIm}] (R = Me, 6; Ph, 7), which contain a quadruply bridging CO ligand and an orthometalated phenyl ring that is additionally coordinated as an η6-arene ligand, are the major products of the thermolysis of compounds 3 and 4 in toluene at reflux temperature.