Strategy for the stereochemical assignment of tris-heteroleptic Ru(II) complexes by NMR spectroscopy.

The relative stereochemistry of tris-heteroleptic ruthenium complexes [Ru(pp)(pp')(pp'')](PF(6))(2), where pp = 1,10-phenanthroline-4-carboxamide, pp' = 5,6-dimethyl-1,10-phenanthroline, and pp'' = 7,8-dimethyl dipyrido[3,2-a:2',3'-c]phenazine, was studied using NMR spectroscopy. The (1)H and (13)C spectra were assigned by using double-quantum-filtered correlation spectroscopy (DQF-COSY), heteronuclear single-quantum correlation (HSQC), and heteronuclear multiple-bond correlation (HMBC) experiments for the two diastereomers, each a pair of enantiomers. Nuclear Overhauser effect contacts between the neighboring ligands differentiated the two halves of each symmetrical ligand, thus enabling a full assignment of the NMR signals and an accurate determination of the relative stereochemistry of the complexes. The introduction of an additional chiral center to ligand pp by coupling it with L-lysine caused removal of the enantiomerism. Thus, four diastereomers were observed and their relative stereochemistry determined.