Alcohol oxidation reactions catalyzed by ruthenium–carbonyl complexes of thioarylazoimidazoles

Alcohols are oxidized by N-methylmorpholine-N-oxide (NMO), ButOOH and H2O2 to the corresponding aldehydes or ketones in the presence of catalyst, [RuH(CO)(PPh3)2(SRaaiNR′)]PF6 (2) and [RuCl(CO)(PPh3)(SκRaaiNR′)]PF6 (3) (SRaaiNR′ (1) = 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole, a bidentate N(imidazolyl) (N), N(azo) (N′) chelator and SκRaaiNR′ is a tridentate N(imidazolyl) (N), N(azo) (N′), Sκ-R is tridentate chelator; R and R′ are Me and Et). The single-crystal X-ray structures of [RuH(CO)(PPh3)2(SMeaaiNMe)]PF6 (2a) (SMeaaiNMe = 1-methyl-2-{(o-thioethyl)phenylazo}imidazole) and [RuH(CO)(PPh3)2(SEtaaiNEt)]PF6 (2b) (SEtaaiNEt = 1-ethyl-2-{(o-thioethyl)phenylazo}imidazole) show bidentate N,N′ chelation, while in [RuCl(CO)(PPh3)(SκEtaaiNEt)]PF6 (3b) the ligand SκEtaaiNEt serves as tridentate N,N′,S chelator. The cyclic voltammogram shows RuIII/RuII (~1.1 V) and RuIV/RuIII (~1.7 V) couples of the complexes 2 while RuIII/RuII (1.26 V) couple is observed only in 3 along with azo reductions in the potential window +2.0 to −2.0 V. DFT computation has been used to explain the spectra and redox properties of the complexes. In the oxidation reaction NMO acts as best oxidant and [RuCl(CO)(PPh3)(SκRaaiNR′)](PF6) (3) is the best catalyst. The formation of high-valent RuIV=O species as a catalytic intermediate is proposed for the oxidation process. Copyright © 2014 John Wiley & Sons, Ltd.