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 (L-1). 5-ethyl-2.2'-bipyridine (L-2). 5-propyl-2,2'-bipyridine (L-3), 5-(2-methylpropyl)-2,2'-bipyridine (L-4). 5-(2.2-dimethylpropyl)-2,2'-bipyridine (L-5) or 5-(carbomethoxy)-2,2'-bipyridine (L-6)] 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(L-5)(3)](2-). The two isomeric forms were characterized by H-1 NMR spectroscopy. with the complexes [Ru(L1-3)(3)](2+) demonstrating an unusually large coupling between the H-6 and H-4 protons. Crystals suitable for X-ray structural analysis of [Ru(L-1)(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(L-3)(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.