Polynuclear osmium-dioxolene complexes: comparison of electrochemical and spectroelectrochemical properties with those of their ruthenium analogues

Reaction of [Os(bipy)(2)Cl-2] (bipy=2,2'-bipyridine) with the poly-dioxolene ligands 3,4,3',4'-tetrahydroxybiphenyl (H4L1), 2,3,6,7-trihydroxy-9-phenylxanthen-3-one (H3L2) or 2,3,6,7,10,11-hexahydroxytriphenylene (H6L3) afforded the complexes [{Os(bipy)(2)}(2)(mu-L-1)][PF6](2), [{Os(bipy)(2)}(2)(mu-L-2)][PF6](3), and the trinuclear complex [{Os(bipy)(2)}(3)(mu-L-3)][PF6](3) abbreviated as [Os-2(L-1)](2+), [Os-2(L-2)](3+) and [Os-2(L-3)](3+), respectively. In these complexes two or three {Os-III(bipy)(2)(OO)} fragments are linked by the conjugated bridging ligands (where OO denotes a dioxolene binding site in any oxidation state). The complexes exhibit rich electrochemical behaviour, displaying a combination of metal-centred Os-III-Os-II couples (as reductions) and ligand-centred couples (as oxidations). UV/VIS/NIR spectroelectrochemical analysis was carried out on all three complexes in all accessible oxidation states, and the spectra were assigned with reference to the mononuclear model complex [Os-III(bipy)(2)(cat)][PF6] (H(2)cat=catechol) which was also examined spectroelectrochemically, as well as being crystallographically characterised. The comparison with the previously described ruthenium analogues is interesting due to their different internal oxidation state distribution. Whereas the mononuclear complex [Os-III(bipy)(2)(cat)](+) contains Os-III co-ordinated to a catecholate (oxidised metal, reduced ligand), the ruthenium analogue in the same oxidation state is [Ru-II(bipy)(2)(sq)](+) (sq=1,2-benzosemiquinone monoanion), i.e. reduced metal and oxidised ligand. The same pattern of behaviour persists in the dinuclear and trinuclear complexes, and leads to interesting differences in the electrochemical properties of the ruthenium and osmium congeners.