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

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.