Olefin metathesis-active ruthenium complexes bearing a nucleophilic carbene ligand

The reaction of [Cp*RuCl](4) (1, Cp* = eta(5)-C5Me5) with the carbene ligan d 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) affords a coordin atively unsaturated Cp*Ru(IMes)Cl (3) complex in 86% isolated yield. Soluti on calorimetric results in this system provide information concerning the e lectron donor properties of the carbene ligand, which are very similar to t hose of PCy3. Structural information from single-crystal X-ray studies for complex 3 allows the determination of steric parameters associated with thi s ligand. The thermochemical information is used to predict the magnitude o f the enthalpic driving force behind substitution reactions involving RuCl2 (=C(H)Ph)(PCy3)(2) (1) and the carbene ligand, IMes, affording the RuCl2(=C (H)Ph)(PCy3)(IMes) (6) complex in high yield. A similar mixed carbene/phosp hine ruthenium complex, RuCl2(=C(H)Ph)(PPh3)(IMes), can also be isolated fr om RuCl2(=C(H)Ph)(PPh3)(2) and the IMes ligand. A single-crystal X-ray diff raction study has been performed on 6. The thermal stability of these mixed phosphine/carbene ruthenium carbene complexes has been studied at 60 degre es C in toluene. Their catalytic activity has been evaluated for the ring c losing metathesis of diethyldiallylmalonate.