Ru-Arene Complexes Bearing Chelated Pyrrole-2-Aldiminate: Synthesis, Structure, and Catalytic Application for Transfer Hydrogenation of Carbonyl Compounds.

Reactions of [{Ru(p-cymene)Cl2}2] with the four chosen pyrrole-2-aldimine (HL-R; R = OCH3, CH3, H, and Cl) ligand precursors in ethanol, using triethylamine as base, furnished complexes of the expected type, [Ru(p-cymene)(L-R)Cl], in good yields. The individual complexes are depicted as complex 1 (R = OCH3), 2 (R = CH3), 3 (R = H), and 4 (R = Cl). Molecular structures of complexes 2, 3, and 4 were determined by X-ray diffraction method, while the structure of complex 1 was optimized with DFT method. In their electronic spectra, all the complexes exhibited intense absorptions in the visible and ultraviolet regions, which were analyzed by TDDFT calculations. Electrochemical redox properties of the complexes were studied by cyclic voltammetry. Presence of the Ru-Cl moiety in these complexes indicated their potential to function as a catalyst-precursor in transfer-hydrogenation of organic substrates, which was explored toward transfer-hydrogenation of carbonyl compounds to the corresponding alcohols. Using 2-propanol as the provider of hydrogen and complex 1 as catalyst, hydrogenation of a variety of aldehydes and ketones could be achieved under relatively mild conditions, furnishing primary and secondary alcohols in very good isolated yields. A plausible mechanism for the observed catalysis has been proposed.