Synthesis of the new binucleating compound pyrazin-2-yl 2-pyridyl sulfide for stepwise or direct approach to asymmetric binuclear ruthenium(II) complexes

Abstract

Pyrazin-2-yl 2-pyridyl sulfide (pzpys) has been prepared by reaction of 2-sulfanylpyridine with chloropyrazine. As shown by 1H NMR spectroscopy, its major isomer has two binding sites which are inequivalent, one chelating through the N1-pyrazine and N-pyridine atoms, the other monodentate through the N4 atom of the pyrazine ring. By reaction of pzpys with [RuL2(NO)(NO2)][PF6]2[(L = 2,2′-bipyridine (bipy) or di-2-pyridyl sulfide (dps)] the mononuclear complexes [RuL2(pzpys)(NO2)][PF6] have been obtained in which pzpys is the monodentate. The reactions of pzpys with cis-[RuL2Cl2] complexes [L = bipy, 1,10-phenanthroline (phen) or dps] have been studied. The dps derivative reacts with a large excess of pzpys affording [Ru(dps)2(pzpys)Cl][PF6] in which pzpys is monodentate. On the contrary the bipy and phen (L) derivatives, under the same experimental conditions, undergo substitution of both chloride ligands givings, as major products, [RuL2(pzpys)][PF6]2 in which pzpys is chelated. When the reactions were carried out in the presence of a stoichiometric amount of pzpys the binuclear complexes [L2Ru(pzpys)RuL2Cl][PF6]3(L = bipy or phen) were slowly formed. The mononuclear complexes have been used as ligands in the reactions with [Ru(bipy)2Cl2] giving the new binuclear species [L2Ru(pzpys)RuL2(NO2)][PF6]3(L = bipy or phen). The compounds have been fully characterized by infrared, UV/VIS, 1H and 13C NMR spectroscopies. In acetonitrile solution [Ru(bipy)2(pzpys)Cl][PF6] undergoes a reversible RuII→ RuIII electron transfer. In contrast, the one-electron oxidation of [Ru(dps)2(pzpys)Cl][PF6], [Ru(bipy)2(pzpys)(NO2)][PF6] and [Ru(bipy)2(pzpys)][PF6]2 is complicated by subsequent chemical reactions. The binuclear complex [(bipy)2Ru(pzpys)Ru(bipy)2Cl][PF6]3 undergoes two consecutive one-electron oxidations (ΔE°′= 0.55 V), which allow the corresponding RuIIRuIII species to be classified as a completely delocalized (Class III) mixed-valence compound.