Ru(η6-1,3,5-cyclooctatriene)(η2-dimethyl fumarate)2: a novel, versatile zerovalent ruthenium complex with electron-deficient olefinic ligands

The reactivity of a novel zerovalent ruthenium complex, Ru(η6-cot)(η2-dmfm)2 (cot = 1,3,5-cyclooctatriene, dmfm, =dimethyl fumarate), which is readily prepared from Ru(η4-cod)(η6-cot) (cod = 1,5-cyclooctadiene) and dmfm was examined. The reaction with monodentate phosphine or amine ligands gave Ru(η6-cot)(dmfm)(L) (L = ligand) via dissociation of dmfm. Among bidentate phosphines, dppm (dppm = bis(diphenylphosphino)methane) reacted to give Ru(η4-cot)(dmfm)(dppm) along with releasing a dmfm ligand. In the case of dppe (dppe = 1,2-bis(diphenylphosphino)ethane), two types of complexes were obtained depending on the reaction conditions, Ru(dmfm)(dppe)2 and an alkyl alkenyl complex; in the formation of the latter complex, sp2 C–H bond activation of dmfm occurred. Ru(η4-cot)(dmfm)(N⌢N) and Ru(dmfm)2 (N⌢N⌢N) were formed by reacting with bidentate and tridentate nitrogen ligands. The reactions with arenes gave π-coordinated complexes, Ru(η6-arene)(dmfm)2. p-Quinones and a p-biqunone reacted to give Ru(η6-cot)(p-quinone) and {Ru(η6-cot)}2(p-biquinone), respectively, along with the dissociation of two dmfm ligands. It was found that low-valent ruthenium complexes preferably bear both electron-donating and accepting ligands simultaneously to be thermodynamically stable.