Stereoisomers of palladium(II) complexes with the dipyridyl ligands 2,3-di(2-pyridyl)pyrazine (dpp), 2,3-di(2-pyridyl)quinoxaline (dpq) and 6,7-dimethyl-2,3-di(2-pyridyl)quinoxaline (ddpq). Crystal structure of [Pd(dpq)2](PF6)2

A solid-state and solution study to characterize the possible configurations of the [Pd(L)(2)](2+) cations (L = dpp, dpq, ddpq) has been carried out. To accomplish this objective the homoleptic complexes [Pd(L)(2)](hfac)(2) (L = dpp, dpq, ddpq hfac hexafluoroacetylacetonate ion) were isolated. The syntheses were performed in diethyl ether by reacting [Pd(hfac),] with a two molar equivalent of the respective L ligand. Besides, X-ray quality crystals were obtained by metathesis with NH4PF6 to derive the hexafluorophosphate species [Pd(dpq)(2)](PF6)(2). The X-ray crystallographic study shows that the coordination sphere of the [Pd(dpq)(2)](2+) cation is square planar and that the bidentate chelating dpq ligands are linked via their N-pyridine atoms. Furthermore, the cation structure reveals an intramolecular pi-pi stacking between the two quinoxaline rings in a 'pincer-like' configuration. In CDCl3 solution, the H-1 NMR spectra of the [Pd(L)(2)](hfac)(2) complexes permit to infer the presence of a mixture of two isomeric forms with different populations. The results suggest that the predominant form being that of the solid-state 'pincer-like'. For the less abundant species a configuration with the quinoxaline groups (dpq, ddpq) or pyrazine groups (dpp) in opposite side respect to the plane of the Pd(II) centre, 'step-like', is suggested. The preponderance of the 'pincer-like' configuration was attributed to the influence of the aromatic pi-pi stacking interactions. (C) 2003 Elsevier Science B.V. All rights reserved.