Synthesis, Single Crystal Structure And Efficient Catalysis For Alcohol Oxidation Of Novel Ru(II) Complexes With N,N,N-Tridentate Ligands

The oxidation of alcohols to the corresponding carbonyl compounds plays an important role in both laboratory and synthetic industrial applications. Traditional oxidation processes cause serious pollution to the environment, which doesn’t meet the requirements of "green chemistry". Hence, it would be of great economical and ecological importance to develop benign and clean processes.In this paper, a series of N, N, N-tridentate ligands have been synthesized. Their corresponding ruthenium(II) complexes(c1~c4) are obtained by coordination of the respective ligands with Ru Cl3 in combination with disodium pyridine-2, 6-dicarboxylate. The intermediates and ruthenium(II) complexes had been characterized by NMR, FTIR, UV-Vis and HR-MS. Crystal structure of the complex c1 have been analyzed by single crystal X-ray diffraction, which revealed that the N,N,N-tridentate ligand coordinates to Ru on one plane and the pyridine-2, 6-dicarboxylate on another plane. The two planes are nearly perpendicular to each other. The complex c1 has a distorted octahedral geometry. It was found that the imine group in the complex c1 was oxidized to amide during the single crystal growth under air atmosphere, and c1 was converted to a novel complex c1-B. In addition, a carboxylate group dissociated from the central ruthenium ion by replacement of one DMSO molecule to form another novel complex c1-C during the single crystal growth under DMSO atmosphere. The crystal structures of the c1-B and c1-C are similar to c1 having two planes which are nearly perpendicular to each other and presenting a distorted octahedral geometry. Then, a novel catalytic system is attempted to be developed using complex c1 as catalyst and 1-phenylethanol as the model substrate. The optimized conditions were obtained from a series of reactions, which were 1-phenylethanol 2 mmol, complex c1 0.002mmol(0.1mol%),molar ratio of oxidant TBHP to substrate 3:1, reaction temperature 40℃. Under these conditions, the conversion of 1-phenylethanol was 100% with selectivity toward acetophenone near 100% in 2.5h. The versatility of this novel catalytic system was evaluated under the optimized reaction conditions. The results revealed that most of secondary benzylic alcohols, including the ones with both an electron-withdrawing and an electron-donating group ortho or para to the hydroxymethyl, were selectively converted to their corresponding aromatic ketones in excellent yields under the optimal reaction conditions. Some secondary aliphatic alcohols were also oxidized to their corresponding ketones in moderate yields. The protocol was also applied to the oxidation of primary benzylic alcohols. However, the main products were carboxylic acids due to the deep oxidation of alcohols. The complexes c2~c3 also showed excellent catalytic activity in the same oxidation reaction.