Ruthenium Alkylidene Complexes of Chelating Amine Ligands

A density functional theory-based computational comparison of various [Ru]−L bonds in L(PCy3)Cl2RuCH2 complexes, where L is a dative ligand, shows that similar bond strengths can be expected for ruthenium−imine and ruthenium−amine bonds. The similarity remains when comparing the corresponding bond strengths in ruthenium olefin metathesis catalysts coordinated by bidentate Schiff-base ligands with those of ruthenium complexes of corresponding chelating amino-benzyloxy ligands. This observation prompted us to synthesize two bidentate amino-benzyloxy ligands and one tridentate amino-bis(benzyloxy) ligand. Ruthenium alkylidene complexes coordinated by the bidentate ligands proved to be unstable and eluded isolation. Reaction of the potassium salt of the tridentate ligand with (PCy3)2Cl2RuCHPh and (PCy3)2Cl2Ru(CH-o-OiPrC6H4) resulted in two complexes, 10 and 11, respectively, that represent the first examples of ruthenium alkylidenes (arylidenes) coordinated by an amine. 10 and 11 contain an essentially symmetrically coordinated tridentate ligand and both are also coordinated by a tri-cyclohexyl-phosphine ligand. The isopropyloxy function of the alkylidene group in 11 is not coordinated to the metal. Complexes 10 and 11 are fairly stable to air, moisture, and protic solvents, but display low thermal stability. Only very low catalytic activity has been obtained in ring-closing metathesis of diethyl diallylmalonate using 10 or 11 dissolved in acid-free solvents. The presence of a Brønsted acid increases the activity dramatically, although at the expense of an increased decomposition rate of the catalyst.