μ-Chlorido-Bridged Dimanganese(II) Complexes of the Schiff Base Derived from [2+2] Condensation of 2,6-Diformyl-4-methylphenol and 1,3-Bis(3-aminopropyl)tetramethyldisiloxane: Structure, Magnetism, Electrochemical Behaviour, and Catalytic Oxidation of Secondary Alcohols

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Abstract

The reaction of 2,6-diformyl-4-methylphenol with 1,3-bis(3-aminopropyl)tetramethyldisiloxane in the presence of MnCl2 in a 1:1:2 molar ratio in methanol afforded a dinuclear μ-chlorido-bridged manganese(II) complex of the macrocyclic [2+2] condensation product (H2L), namely, [Mn2Cl2(H2L)(HL)]Cl·3H2O (1). The latter afforded a new compound, namely, [Mn2Cl2(H2L)2][MnCl4]·4CH3CN·0.5CHCl3·0.4H2O (2), after recrystallisation from 1:1 CHCl3/CH3CN. The co-existence of the free and complexed azomethine groups, phenolato donors, μ-chlorido bridges, and the disiloxane unit were well evidenced by ESI mass spectrometry and FTIR spectroscopy and confirmed by X-ray crystallography. The magnetic measurements revealed an antiferromagnetic interaction between the two high-spin (S = 5/2, g = 2) manganese(II) ions through the μ-chlorido bridging ligands. The electrochemical behaviour of 1 and 2 has been studied, and details of their redox properties are reported. Both compounds act as catalysts or catalyst precursors in the solvent-free low-power microwave-assisted oxidation of selected secondary alcohols, for example, 1-phenylethanol, cyclohexanol, 2- and 3-octanol, to the corresponding ketones in the absence of solvent. The highest yield of 72 % was achieved for 1-phenylethanol by using a maximum of 1 % molar ratio of catalyst relative to substrate.

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