Neutral half-sandwich η 6 -p-cymene ruthenium(II) complexes of general formula [Ru (η 6 -p-cymene)Cl(L)] (HL = monobasic O, N bidendate benzoylhydrazone ligand) have been synthesized from the reaction of [Ru(η 6 -p-cymene)(μ-Cl)Cl] 2 with acetophenone benzoylhydrazone ligands. All the complexes have been characterized using analytical and spectroscopic (Fourier transform infrared, UV-visible, 1 H NMR, 13 C NMR) techniques. The molecular structures of three of the complexes have been determined using single-crystal X-ray diffraction, indicating a pseudooctahedral geometry around the ruthenium(II) ion. All the ruthenium(II) arene complexes were explored as catalysts for transfer hydrogenation of a wide range of aromatic, cyclic and aliphatic ketones with 2-propanol using 0.1 mol% catalyst loading, and conversions of up to 100% were obtained. Further, the influence of other variables on the transfer hydrogenation reaction, such as base, temperature, catalyst loading and substrate scope, was also investigated. KEYWORDS benzoylhydrazone, crystal structure, transfer hydrogenation, η 6 -p-cymene ruthenium (II) complex
| INTRODUCTIONIn recent years, (η 6 -arene) ruthenium(II) derivatives have attracted considerable interest because of their potential roles as catalysts for a number of organic reactions, such as hydrogen transfer, [1] ring-closing metathesis, [2] stoichiometric C─C couplings, [3] catalytic oxidative Heck reactions, [4] allyl alcohol isomerization to alkyne hydration, [5] asymmetric catalysis in Diels-Alder reactions, [6] asymmetric transfer hydrogenation of ketones, [7] hydration of nitriles [8] and 1,3-dipolar cycloaddition reactions of nitrones with methacrolein. [9] In addition, the [Ru(η 6 -arene)(chelating-ligand)Cl]-type complexes reveal the characteristic 'piano stool' structure, with the unreactive arene as a 'spectator ligand' in the metal coordination sphere and the chloride as a suitable 'leaving group'. [10] These structural features appear favourable for providing sequential reactions involved in catalysis. Today most transfer hydrogenations are performed using transition metal complexes, predominantly based on Rh(I), [11] Ru (II) [12] and Ir(III) complexes, [13] which have been used and proved to be effective catalysts. As ligands of the catalysts for transfer hydrogenation, arenes such as p-cymene [14] and hexamethylbenzene [15] for ruthenium(II) complexes have been the most popular. Furthermore, Ru, Rh and Ir complexes bearing N, P, O, S, C element-based ligands with various forms (such as metal-N-heterocyclic carbenes, half sandwich, multidentate metal complexes and their combinations) are perhaps the most classic and popular catalysts for transfer hydrogenations.Aroylhydrazones, R─CO─NH─N═CH─R′, are an important class of Schiff base ligands, coordinating through protonated/deprotonated amide oxygen and the imine nitrogen of hydrazone moiety; very often an additional donor site (usually N or O) is provided by the aldehyde or ketone forming the hydrazone Schiff base. [16][17][18] Moreove...
