Structural, Electronic, and Catalytic Modulation of Chelating Pyridylideneamide Ruthenium(II) Complexes

A family of Ru(cym)-type complexes bearing different pyridylideneamide (PYA) ligands has been prepared. Incorporation of diverse potentially coordinating sites afforded a series of different chelating hybrid ligands containing a PYA nitrogen as one donor site and a second variable donor site constituted of a cyclometalated aryl ring (complex 3a), a pyridine (3b), a pyridylidene (3c), another PYA unit (3d), or a triazolylidene ligand (3e). Structural and electrochemical analyses indicate considerable electronic variation in this series with decreasing donor ability from phenyl > PYA triazolylidene pyridylidene > pyridine. This trend allows the electronic properties of the metal center to be tailored and reveals the strong donor properties of PYA ligands, surpassing those of pyridine-derived N-heterocyclic carbenes. The effect of these tunable donor properties was demonstrated in transfer hydrogenation catalysis, for which a direct correlation between donor properties and catalytic activity was established.