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Transfer Hydrogenation of Ketones and Catalytic Oxidation of Alcohols with Half-Sandwich Complexes of Ruthenium(II) Designed Using Benzene and Tridentate (S, N, E) Type Ligands (E = S, Se, Te)

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posted on 13.12.2010, 00:00 by Pradhumn Singh, Ajai K. Singh
The complexes of composition fac-[(η6-C6H6)Ru(L)][PF6][X] (16; X = PF6 or Cl), formed by reacting 2-MeSC6H4CHNCH2CH2E-C6H4-4−R (L1L3) and 2-MeSC6H4CH2-NHCH2CH2E-C6H4-4−R (L4L6) (where E = S or Se, R = H; E = Te, R = OMe) with [{(η6-C6H6)RuCl(μ-Cl)}2] and NH4PF6, have been characterized by 1H, 13C{1H}, 77Se{1H}, and 125Te{1H} NMR spectroscopy and X-ray crystallography. The Ru−Se and Ru−Te bond lengths are in the ranges 2.4837(14)−2.4848(14) and 2.6234(6)−2.6333(7) Å, respectively. Complexes 16 have been found to be efficient catalysts for catalytic oxidation of alcohols with N-methylmorpholine-N-oxide, tBuOOH, NaOCl, and NaIO4 and transfer hydrogenation reaction of ketones with 2-propanol. The TON values are up to 9.9 × 104 and 9.8 × 104 for two catalytic processes, respectively. The oxidation probably involves the formation of intermediate species having Ru(IV)O. Complexes 13 are as efficient as 46 for transfer hydrogenation of ketones. In transfer hydrogenation, the mechanism does not appear to be dependent on the availability of hydrogen on nitrogen and probably involves Ru−H bond formation. The catalytic efficiency for both processes follows the order Te > Se > S, which may be due to the presence of a MeO group on Te.