ELECTROCHEMICAL CONTROL OF BRIDGING LIGAND CONFORMATION IN A BINUCLEAR COMPLEX - A POSSIBLE BASIS FOR A MOLECULAR SWITCH

Reaction of the binucleating bridging ligands 3,3',4,4'-tetrahydroxybiphenyl (H4L1) or 3,3'',4,4''-tetrahydroxy-p-terphenyl (H4L2) with 2 equivalents of [Ru(bipy)2Cl2].2H2O (bipy = 2,2'-bipyridine) results in the formation of binuclear complexes [{Ru(bipy)2}(mu-L){Ru(bipy)2}]2+ 1(L = L1) and 2 (L = L2) in which the ligand L4- has been oxidized to the semiquinone (sq) form L2-. In each complex the co-ordinated catecholate fragment (cat) may be oxidised reversibly to the semiquinone and quinone (q) redox states, giving the five-membered redox series cat-cat, cat-sq, sq-sq, sq-q and q-q for the bridging ligands. In the sq-sq state the bridging ligands are necessarily planar due to the presence of double bonds between the aromatic rings; in the cat-cat and q-q states there is formally no double-bond character between the aromatic rings and they are free to adopt a twisted conformation. Spectroelectrochemical measurements confirm that in the cat-cat and q-q states, 1 and 2 behave approximately like their mononuclear counterparts [Ru(bipy)2(cat)] and [Ru(bipy)2(bq)]2+ [H-2cat = catechol (benzene-1,2-diol); bq = o-benzoquinone]; for 1, the results also show that in the mixed-valence sq-q and sq-cat oxidation states the two halves of the ligand are electronically equivalent (i.e. valence delocalised), which is best explained by the bridging ligand retaining the planar conformation of the sq-sq state. The change from planar to twisted therefore occurs at the extremes of the redox series, on formation of the cat-cat and q-q oxidation states. This control of bridging ligand conformation with oxidation state may form the basis for a molecular switch.