Mono- and dinuclear ruthenium carbonyl complexes with redox-active dioxolene ligands: Electrochemical and spectroscopic studies and the properties of the mixed-valence complexes

The mononuclear complex [Ru(PPh3)(2)(CO)(2)(L-1)] (1; H2L1 = 7,8-dihydroxy-6-methoxycoumarin) and the dinuclear complexes [{ Ru(PPh3)(2)(CO)(2)}(2)(L-2)] [PF6] {[2][PF6]; H3L2 = 9-phenyl-2,3,7-trihydroxy-6-fluorone} and [{Ru(PBu3)(2)(CO)(2)}(2)(L-3)] (3; H4L3 = 1,2,3,5,6,7-hexahydroxyanthracene-9,10-dione) have been prepared; all complexes contain one or two trans,cis-f Ru(PR3)(2)(CO)(2)} units, each connected to a chelating dioxolene-type ligand. In all cases the dioxolene ligands exhibit reversible redox activity, and accordingly the complexes were studied by electrochemistry and UV/vis/NIR, IR, and EPR spectroscopy in their accessible oxidation states. Oxidation of 1 to [1](+) generates a ligand-centered serniquinone radical with some metal character as shown by the IR and EPR spectra. Dinuclear complexes [2](+) and 3 show two reversible ligand-centered couples (one associated with each dioxolene terminus) which are separated by 690 and 440 mV, respectively. This indicates that the mixed-valence species [2](2+) has greater degree of electronic delocalization between the ligand termini than does [3]+, an observation which was supported by IR, EPR, and UV/vis/NIR spectroelectrochemistry. Both [2](2+) and [3](+) have a solution EPR spectrum consistent with full delocalization of the unpaired electron between the ligand termini on the EPR time scale (a quintet arising from equal coupling to all four P-31 nuclei); [3](+) is localized on the faster IR time scale (four CO vibrations rather than two, indicative of inequivalent { Ru(CO)(2)} units) whereas [2](2+) is fully delocalized (two CO vibrations). UV/vis/NIR spectroelectrochemistry revealed the presence of a narrow, low-energy (2695 nm) transition for [3](+) associated with the catecholate --> semiquinone intervalence transition. The narrowness and solvent-independence of this transition (characteristic of class III mixed-valence character) coupled with evidence for inequivalent {Ru(CO)(2)} termini in the mixed-valence state (characteristic of class II character) place this complex at the class II-III borderline, in contrast to [2](2+) which is clearly class III.