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 Ru^II→ Ru^III 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 Ru^IIRu^III species to be classified as a completely delocalized (Class III) mixed-valence compound.