Structure and characterization of a μ-phenoxo-bis μ-acetato dinuclear Mn(II,II) complex [Mn2(LO)(μ-OAc)2](ClO4) (LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl}-4-methylphenol): Substituent effects

The synthesis, structure and characterization of the dinuclear Mn(II) complex [Mn2(LO)(μ-OAc)2](ClO4) (1) where LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl)}-4-methylphenol are reported. The reaction of Mn(ClO4)2 · 6H2O with the dinucleating ligand LOH and H3CCOONa in the presence of NEt3 in dry, degassed methanol and under an argon atmosphere, yields 1 as a colorless powder. The crystal structure of 1, determined by X-ray diffraction methods, shows a dinuclear Mn(II) complex in which two Mn(II) ions, each in six-coordinate approximate octahedral coordination, are bridged by the phenolate oxygen of LO− and by two acetate ions in a syn,syn-1,3-bridging mode. The Mn–Mn distance is 3.557(1) Å and Mn–Ophenolate–Mn angle is 112.50(9)°. Cyclic voltammetry of 1 in acetonitrile solution shows a quasi-reversible wave at E1/2 = 0.65 V, for the Mn2(II,II)/Mn2(II,III) redox process, and an irreversible oxidation peak at Ep,c = 1.22 V versus Ag/AgCl for the Mn2(II,III) to Mn2(III,III) oxidation process. Controlled potential electrolysis of 1 in acetonitrile solution at 0.85 V (versus Ag/AgCl) takes up 1 F of charge per mole of 1 to yield a brown solution of the Mn2(II,III) state of the complex, which, however, is unstable and reverts back to the Mn2(II,II) state in solution at room temperature. Least square fitting of the variable temperature magnetic susceptibility measurements on powdered sample of 1 is obtained with g = 1.888, J = −2.75 cm−1, Par = 0.008, TIP = 0. The low −J value and the room temperature calculated magnetic moment of the complex (5.30 BM per Mn(II)), which is less than the spin-only moment of Mn(II), show that the two Mn(II) ions are weakly antiferromagnetically coupled.

Graphical abstractCH3 substitution on LOH in 1 (compared to 1∗) causes decreases in Mn–O–Mn angle (112.50(9)° versus 115.8(2)°), Mn–Mn distance (3.557(1) Å versus 3.589(3) Å), voltammetric redox potentials (E1/2 (V versus Ag/AgCl), Mn(II,II) ↔ Mn(II,III) (0.65 V versus 0.73 V) and the Mn(II,III) → Mn(III,III) (Ep,a = 1.22 V versus 1.28 V) and increase in the lengths of the Cph–Oph–Mn–Npy bond chain (ph = phenolate; py = pyridyl). Both complexes are stable only in the Mn(II,II) and unstable in higher oxidation states.Figure optionsDownload full-size imageDownload as PowerPoint slide