Preparation, reactivity, and structural peculiarities of hydroxyalkyl-functionalized second-generation ruthenium carbene complexes

The preparation, immobilization, and structure of second-generation ruthenium benzylidene metathesis catalysts bearing hydroxyalkyl chains on their N-heterocyclic carbene ligands are described. Complexes of this type are prone to rearrange their ligand sphere such that the neutral ligands get cis- rather than trans-disposed. Moreover, treatment of complex 7d with pyridine results in an unprecedented ionization by loss of one of its chloride ligands. The resulting cationic species 13, though devoid of catalytic activity, is the first member of a new type of ruthenium carbene complexes characterized by an octahedral coordination geometry and a donor/acceptor interaction between the metal center and the tethered hydroxyl group. The trans-configured hydroxyalkyl-functionalized complexes 8a and 8b, the cis-configured compound 10, and the ion pair 13 have been structurally characterized by X-ray crystallography.