Ru(0) and Ru(II) Nitrosyl Pincer Complexes: Structure, Reactivity, and Catalytic Activity

Despite considerable interest in ruthenium carbonyl pincer complexes and their substantial catalytic activity, there has been relatively little study of the isoelectronic ruthenium nitrosyl complexes. Here we describe the synthesis and reactivity of several complexes of this type as well as the catalytic activity of complex <b>6</b>. Reaction of the PNP ligand (PNP = 2,6-bis­(^tBu₂PCH₂)­pyridine) with RuCl₃(NO)­(PPh₃)₂ yielded the Ru­(II) complex <b>3</b>. Chloride displacement by BAr^F– (BAr^F– = tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate) gave the crystallographicaly characterized, linear NO Ru­(II) complex <b>4</b>, which upon treatment with NaBEt₃H yielded the Ru(0) complexes <b>5</b>. The crystallographically characterized Ru(0) square planar complex <b>5·BF</b>_<b>4</b> bears a linear NO ligand located trans to the pyridilic nitrogen. Further treatment of <b>5·BF</b>_<b>4</b> with excess LiOH gave the crystallographicaly characterized Ru(0) square planar, linear NO complex <b>6</b>. Complex <b>6</b> catalyzes the dehydrogenative coupling of alcohols to esters, reaching full conversion under air or under argon. Reaction of the PNN ligand (PNN = 2-(^tBu₂PCH₂)-6-(Et₂NCH₂)­pyridine) with RuCl₃(NO)­(H₂O)₂ in ethanol gave an equilibrium mixture of isomers <b>7a</b> and <b>7b</b>. Further treatment of <b>7a</b> + <b>7b</b> with 2 equivalent of sodium isopropoxide gave the crystallographicaly characterized, bent-nitrosyl, square pyramidal Ru­(II) complex <b>8</b>. Complex <b>8</b> was also synthesized by reaction of PNN with RuCl₃(NO)­(H₂O)₂ and Et₃N in ethanol. Reaction of the “long arm” PN²N ligand (PN²N = 2-(^tBu₂PCH₂−)-6-(Et₂NCH₂CH₂)­pyridine) with RuCl₃(NO)­(H₂O)₂ in ethanol gave complex <b>9</b>, which upon treatment with 2 equiv of sodium isopropoxide gave complex <b>10</b>. Complex <b>10</b> was also synthesized directly by reaction of PN²N with RuCl₃(NO)­(H₂O)₂ and a base in ethanol. A noteworthy aspect of these nitrosyl complexes is their preference for the Ru(0) oxidization state over Ru­(II). This preference is observed with both aromatized and dearomatized pincer ligands, in contrast to the Ru­(II) oxidation state which is preferred by the analogous carbonyl complexes