Ruthenium(III) mediated C-H activation of azonaphthol: Synthesis, structural characterization and transfer hydrogenation of ketones

Treatment of [RuCl3(PPh3)(3)] with 1-(arylazo)naphthol ligands in benzene under reflux afford air-stable new organoruthenium(III) complexes with general composition [Ru(an-R)Cl(PPh3)(2)] (where, R = H, Cl, CH3, OCH3, OC2H5) in fairly good yield. The 1-(arylazo)naphtholate ligands behave as dianionic tridentate C, N, O donors and coordinates to ruthenium through phenolic oxygen, azo nitrogen and ortho carbon generate two five-membered chelate rings. The composition of the complexes have been established by analytical (elemental analysis and magnetic susceptibility measurement) and spectral (FT-IR, UV-Vis, EPR) methods. The complexes are paramagnetic (low-spin, d(5)) in nature and in dichloromethane solution show intense d-d transitions and ligand-to-metal charge transfer (LMCT) transitions in the visible region. The solution EPR spectrum of complex [Ru(an-CH3)Cl(PPh3)(2)] (3) in dichloromethane at 77 K shows rhombic distortion around the ruthenium ion with three different 'g' values (g(x) not equal g(x) not equal g(z)). The single crystal structure of the complex [Ru(an-OCH3)Cl(PPh3)(2)] (4) has been characterised by X-ray crystallography, indicates the presence of a distorted octahedral geometry in these complexes. All the complexes exhibit one quasi-reversible oxidative response in the range 0.60-0.79 V (Ru-IV/Rul(III)) and two quasi-reversible reductive responses (Ru-III/Ru-II; Ru-II/Ru-I) within the range -0.50 to -0.62 V and -0.93 to -0.98 V respectively. The formal potential of all the couples correlate linearly with the Hammett constant of the para. substituent in arylazo fragment of the 1-(arylazo)naphtholate ligand. Further, the catalytic efficiency of one of the ruthenium complexes (4) was determined for the transfer hydrogenation of ketones with an excellent yield up to 99% in the presence of isopropanol/KOH. (c) 2007 Published by Elsevier BN.