Electrochemical Control of Bridging Ligand Conformation in a Binuclear Complex-A Possible Basis for a Molecular Switch

Article abstract of DOI:10.1039/DT9940000799

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 *2H2O (bipy = 2,2'-bipyridine) results in the formation of binuclear complexes <(μ-L)>(2+) 1 (L = L¹) and 2 (L = L²) in which the ligand L(4-) has been oxidized to the semiquinone (sq) form L(2-).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 and (2+) ; 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 confirmation 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.

Full text of DOI:10.1039/DT9940000799

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