Modulation of electronic coupling across dioxolene-bridged osmium and ruthenium dinuclear complexes

The extent of metal-ligand orbital mixing and the degree of electronic coupling between [M(bpy)(2)](2+) fragments linked through a redox active 1,4-dihydroxy-2,5-bis(pyrazol-1'-yl)benz (p-L) bridge is described, where M is Os or Ru and bpy is 2,2'-bipyridyl. This is the first reported example of osmium and ruthenium/osmium metals linked across a 1,4-dioxolene bridge. In;fast scan cyclic voltammetry, the redox processes are reversible and four distinct one-electron processes are observed for the bridge and metal centers in the homo- and heterodinuclear complexes. The potential at which the first oxidation step occurs does not depend on the identity of the metal center. UV/vis spectroelectrochemistry, together with resonance Raman spectroscopy, suggests that the first two oxidation steps occur on the bridging ligand. For all complexes, an orbital mixing gradient occurs; metal-ligand orbital mixing increases in the order HQ much less than SQ < Q (HQ is the reduced hydroquinone bridge, SQ the semiquinone, and Q the quinone), and M-L orbital mixing is enhanced in ruthenium compared with osmium. Analysis of the bipyridyl reductions shows that metal-metal coupling across the HQ bridge is essentially absent. For the first time, stable M(III) polypyridyl quinone complexes are reported. Electrochemical data suggest that communication across the quinone bridge is extensive, with substantial stabilization of the metal(II/III) mixed valence compounds. The results obtained are discussed with respect to the pi acceptor properties of the bridge, the extent of metal-ligand orbital mixing and the relative back-donating properties of the metal centers. These results demonstrate the feasibility of controlling the extent of intercomponent communication by changing the identity of the metal centers and the oxidation state of the complex.