Influence of sterically demanding carbene ligation on catalytic behavior and thermal stability of ruthenium olefin metathesis catalysts

The exchange reaction of one phosphine ligand in Cl-2(PCy3)(2)Ru=CHPh (1; Cy = cyclohexyl, C6H11 with the sterically demanding carbene ligands 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), 1,3-bis(4-methylphenyl)imidazol-2-yliden (ITol), and 1,3-bis(4-chlorophenyl)imidazol-2-ylidene (IpCl) leads to the isolation of the new complexes (PCy3)(IMes)Cl2Ru=CHPh (2), (PCy3)(ITol)Cl2Ru=CHPh (3), and (PCy3)(IpCl)Cl2Ru=CHPh (4). Similarly, one IMes ligand can be substituted for one PCyp(3) ligand in Cl-2(PCyp(3))(2)Ru=CHCH=C(CH3)(2) (5; Cyp = cyclopentyl, C5H9) to produce (PCyp(3))(IMes)Cl2Ru=CHPh (6) in high yield. X-ray structure analysis of 6 confirmed a near-square-pyramidal coordination sphere around the metal center. Improved catalytic properties and thermal stability are observed for 2 and 6 in comparison to the parent 1 and 5.