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 6. Reaction of the PNP ligand (PNP
= 2,6-bis­(tBu2PCH2)­pyridine) with
RuCl3(NO)­(PPh3)2 yielded the Ru­(II)
complex 3. Chloride displacement by BArF– (BArF– = tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate)
gave the crystallographicaly characterized, linear NO Ru­(II) complex 4, which upon treatment with NaBEt3H yielded the
Ru(0) complexes 5. The crystallographically characterized
Ru(0) square planar complex 5·BF4 bears a linear NO ligand located trans to the pyridilic nitrogen.
Further treatment of 5·BF4 with excess LiOH gave the crystallographicaly characterized Ru(0)
square planar, linear NO complex 6. Complex 6 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 7a and 7b. Further treatment of 7a + 7b with 2 equivalent of sodium isopropoxide gave
the crystallographicaly characterized, bent-nitrosyl, square pyramidal
Ru­(II) complex 8. Complex 8 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 9, which upon treatment with 2 equiv
of sodium isopropoxide gave complex 10. Complex 10 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.