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

Abstract

The ambidentate, potentially dinucleating ligand 3,3′-dihydroxy-2,2′-bipyridine (H₂L) has been shown to co-ordinate to bis(2,2′-bipyridyl)ruthenium(II) fragments in two ways. In [Ru(bipy)₂(HL)]X (X = BPh₄1a or PF₆1b) 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(µ-L)][PF₆]₂2 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.