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(L1)] (1; H2L1 = 7,8-dihydroxy-6-methoxycoumarin) and the dinuclear
complexes [{Ru(PPh3)2(CO)2}2(L2)][PF6] {[2][PF6]; H3L2 = 9-phenyl-2,3,7-trihydroxy-6-fluorone} and [{Ru(PBu3)2(CO)2}2(L3)] (3; H4L3 = 1,2,3,5,6,7-hexahydroxyanthracene-9,10-dione) have been prepared; all complexes contain
one or two trans,cis-{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 semiquinone 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 31P 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.