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Keywords:

  • Ruthenium;
  • N ligands;
  • Hydrogen transfer;
  • Homogeneous catalysis;
  • Fluxionality

Abstract

Ruthenium derivatives of the type [RuCl(arene)(NN)][BPh4] (arene = benzene, 1; p-cymene, 2) with NN = 2-methoxyphenyl-bis(3,5-dimethylpyrazol-1-yl)methane (bpz*mArOCH3) have been synthesized and characterized. Two isomers were formed for both complexes and in each case the aryl ring was bonded to the methynic carbon with an axial disposition in the metallacycle. In the complexes either the chloro ligand (isomer A) or the arene (isomer B′) is oriented towards the pyrazolyl rings. Isomer B′ is reported here for the first time for these types of derivatives. The structure of 1 was determined by X-ray diffraction and corresponds to isomer A. The fluxional behaviour of the new derivatives, along with that of other compounds previously synthesized by us that contain similar ligands, has been studied to evaluate the influence of the substitution in the pyrazolyl groups and in the aryl substituent of the central methynic carbon. Restricted rotation of the rings was found in complexes 1 and 2 and atropisomers were detected at low temperature. Complexes 1 and 2 were active precursors for the catalytic transfer hydrogenation of benzophenone with 2-propanol as the hydrogen source in the presence of base and also under base-free conditions. The results were compared with those obtained for other related derivatives. It was concluded that the simultaneous presence of the methyl and methoxy groups on the pyrazolyl and aryl rings, respectively, gives rise to worse results for the reactions with a base but an improvement was found for the reactions carried out in the absence of base.