Synthesis and separation of the mer and fac isomers of [Rh(bpp)3][PF6]3 (bpp = 2,3-bis(2-pyridyl)pyrazine)

The new complex [Rh(dps)2Cl2]PF6 (1) (dps = di-2-pyridyl sulfide) was prepared by reacting RhCl3 with dps in an ethanol-water mixture (2:1). As shown by 1H NMR spectroscopy 1 exhibits a cis geometry. By refluxing 1 with bipy or phen (bipy = 2,2′-bipyridine, phen = 1,10-phenanthroline) in dimethylformamide the complexes [Rh(bipy)2Cl2]PF6 and [Rh(phen)2Cl2]PF6 were respectively obtained. On the contrary the reaction of 1 with bpp (bpp = 2,3-bis(2-pyridyl)pyrazine), carried out in the same experimental conditions, gave [Rh(bpp)3] [PF6]3 (2) in high yields. The meridional (mer) and facial (fac) isomers of 2 were separated by chromatography. Despite the complexity of the 1H and 13C NMR spectra of the mer isomer, almost complete assignment of the resonances was possible by employment of two-dimensional techniques and by comparison with the resonances of the fac isomer.