Mono- and di-nuclear ruthenium(II)complexes of the ambidentate ligand 3,3'-dihydroxy-2,2'-bipyridine: Spectroscopic, electrochemical and mixed-valence properties

The ambidentate, potentially dinucleating ligand 3,3'-dihydroxy-2,2'-bipyridine (H(2)L) has been shown to co ordinate to bis(2,2'-bipyridyl)ruthenium(II) fragments in two ways. In [Ru(bipy)(2)(HL)]X (X = BPh(4) 1a or PF6 1b) HL(-) co-ordinates as a bipy-type N,N'-chelate; this brings the two hydroxyl substituents of the ligand close together with the result that one of them loses a proton to allow formation of an intramolecular O-H ... O hydrogen bond reminiscent of those found in planar bis(dioxime) complexes. In [{Ru(bipy)(2)}(2)-(mu-L)][PF6](2) 2 in contrast the ligand binds in a 'turned-around' mode, acting as an N,O-chelate to each metal with the two metal centres chemically equivalent. Complex 2 shows two reversible Ru-II-Ru-III couples at +0.12 and +0.28 V vs. ferrocene-ferrocenium; the separation between these corresponds to a comproportionation constant K-c of ca. 650 for the mixed-valence state, indicative of a class II species,and the electronic spectrum of the Ru-II-Ru-III species (generated by chemical oxidation) is also consistent with a valence-trapped (class II) mixed-valence species. Dinuclear complexes in which L(2-) acts as an N,N'-donor to one metal (five-membered chelate ring) and an O,O'-donor (seven-membered chelate ring) to the other could not be prepared; molecular modelling studies suggest that this binding mode would involve considerable ligand strain.