Ruthenium Styryl Complexes with Ligands Derived from 2-Hydroxy- and 2-Mercaptopyridine and 2-Hydroxy- and 2-Mercaptoquinoline

Abstract

A series of ruthenium styryl complexes with potentially noninnocent κ2[N,O]− or κ2[N,S]− ligands have been prepared by treatment of 5-coordinated 16-valence-electron ruthenium styryl complexes Ru(CO)Cl(PiPr3)2(CH═CH-C6H4-4R) with deprotonated bidentate 2-hydroxy- or 2-mercaptopyridines or 2-hydroxy- or 2-mercaptoquinolines. These 6-coordinated complexes have been characterized by NMR and IR spectroscopy and by cyclic voltammetry. Moreover, the structures of complexes 1d, 2a, 3c, 5b, and 6b have been established by X-ray crystallography. Our results indicate that the pyridine-derived complexes exist as two isomers that differ with respect to the orientation of the κ2[N,O]− or κ2[N,S]− donor ligands relative to the CO and alkenyl ligands in the equatorial plane. The equilibrium between the two isomers is thermodynamically controlled. Thus, the relative amount of the minor isomer increases at higher temperatures. With the 2-hydroxyquinoline- or 2-mercaptoquinoline-derived ligands only one isomer is observed. Electrochemical studies show that these complexes undergo one or two reversible consecutive one-electron oxidations, the potentials of which respond to the electronic properties of the 4-substituent at the styryl ligand and those of the ancillary chelate ligand. Strong ligand contributions to the first oxidation of the complexes were experimentally verified by IR and EPR spectroelectrochemistry. Quantum chemical calculations reproduce our experimental results, including the positions of the Ru(CO) vibrational bands of the neutral complexes and of their corresponding radical cations. Our combined results indicate that the oxidation of all complexes is dominated by the styryl ligand, irrespective of the electronic nature of the 4-substituent and of the [N,O]− or [N,S]− chelate ligand.