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, d⁵) 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 ≠ g_y ≠ 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/Ru^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.