The isolation and purification of tris-2,2′-bipyridine complexes of ruthenium(II) containing unsymmetrical ligands

Monomeric ruthenium(II) complexes [Ru(L)3]2+ containing unsymmetric bipyridine ligands [Where L = 5-methyl-2,2′-bipyridine (L1), 5-ethyl-2,2′-bipyridine (L2), 5-propyl-2,2′-bipyridine (L3), 5-(2-methylpropyl)-2,2′-bipyridine (L4), 5-(2,2-dimethylpropyl)-2,2′-bipyridine (L5) or 5-(carbomethoxy)-2,2′-bipyridine (L6)] have been studied and the meridional and facial isomers isolated by the use of cation-exchange column chromatography (SP Sephadex C-25) eluting with either sodium toluene-4-sulfonate or sodium hexanoate. The relative yield of the facial isomer was found to decrease with increasing steric bulk, preventing the isolation of fac-[Ru(L5)3]2+. The two isomeric forms were characterized by 1H NMR spectroscopy, with the complexes [Ru(L1–3)3]2+ demonstrating an unusually large coupling between the H6 and H4 protons. Crystals suitable for X-ray structural analysis of [Ru(L1)3]2+ were obtained as a mixture of the meridional and facial isomers, indicating that separation of this isomeric mixture could not be achieved by fractional crystallisation. The optical isomers of the complex [Ru(L3)3]2+ were chromatographically separated on SP Sephadex C-25 relying upon the inherent chirality of the support. It is apparent that chiral interactions can inhibit geometric isomer separation using this technique.