A new chiral dimanganese(iii) complex: synthesis, crystal structure, spectroscopic, magnetic, and catalytic properties

Abstract

Two enantiomeric complexes of formula [Mn^III₂(μ-OCH₃)₂(R-valBINAM)₂]·1.75DMF (1) and [Mn^III₂(μ-OCH₃)₂(S-valBINAM)₂]·2DMF (2) [valBINAM = 1,1′-binaphthalene-2,2′-bis(3-methoxysalicylideneiminate)] have been synthesized using as a ligand the chiral Schiff bases resulting from the condensation reactions between o-vanillin and the chiral 1,1′-binaphthyl-2,2′-diamine. The structures of 1 and 2 which have been solved by single crystal X-ray diffraction consist of neutral dimers, the manganese(III) ions being bridged by two methoxido anions, arising from the solvent, and by two valBINAM²⁻ ligands. Their circular dichroism spectra at room temperature emphasize the occurrence of the exciton coupling effect. The cryomagnetic measurements in the temperature range 2.0–300, for compound 2, reveal the occurrence of antiferromagnetic interactions between the two Mn^III ions (J = −35.5 cm⁻¹, the Hamiltonian being defined as H = −JS₁S₂). The catalytic behaviour of these complexes was checked in the epoxidation of methyl trans-cinnamate with N-methylmorpholine-N-oxide as an oxidant. The influence of the solvent was also investigated. The reactions in a CH₃CN/H₂O (5 : 1 v/v) solvent mixture afforded total chemo- and enantioselectivity in the desired epoxides.