Selective Formation, Reactivity, Redox and Magnetic Properties of Mn(III) and Fe(III) Dinuclear Complexes with Shortened Salen-Type Schiff Base Ligands

The reactivity of the shortened salen-type ligands H(3)salmp, H(2)salmen and H(2)sal(p-X)ben with variable para-substituent on the central aromatic ring (X = tBu, Me, H, F, Cl, CF(3), NO(2)) towards the trivalent metal ions manganese(III) and iron(III) is presented. The selective formation of the dinuclear complexes [M(2)(μ-salmp)(2)], M = Mn (1a), Fe (2a), [M(2)(μ-salmen)(2)(μ-OR)(2))], R = Et, Me, H and M = Mn (3a–c) or Fe (4a–c), and (M(2)(μ-sal[p-X]ben)(2)(μ-OMe)(2)), X = tBu, Me, H, F, Cl, CF(3), NO(2) and M = Mn (5a–g) or Fe (6a–g), could be identified by reaction of the Schiff bases with metal salts and the base NEt(3), and their characterization through elemental analysis, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction of 2a·2AcOEt, 2a·2CH(3)CN and 3c·2DMF was performed. In the case of iron(III) and H(3)salmp, when using NaOH as a base instead of NEt(3), the dinuclear complexes [Fe(2)(μ-salmp)(μ-OR)(salim)(2)], R = Me, H (2b,c) could be isolated and spectroscopically characterized, including the crystal structure of 2b·1.5H(2)O, which showed that rupture of one salmp(3−) to two coordinated salim(−) ligands and release of one salH molecule occurred. The same hydrolytic tendency could be identified with sal(p-X)ben ligands in the case of iron(III) also by using NEt(3) or upon standing in solution, while manganese(III) did not promote such a C–N bond breakage. Cyclic voltammetry studies were performed for 3b, 4b, 5a and 6a, revealing that the iron(III) complexes can be irreversibly reduced to the mixed-valence Fe(II)Fe(III) and Fe(II)(2) dinuclear species, while the manganese(III) derivatives can be reversibly oxidized to either the mixed-valence Mn(III)Mn(IV) or to the Mn(IV)(2) dinuclear species. The super-exchange interaction between the metal centers, mediated by the bridging ligands, resulted in being antiferromagnetic (AFM) for the selected dinuclear compounds 3b, 4b, 5a, 5e, 5f, 6a and 6e. The coupling constants J (–2J Ŝ(1)·Ŝ(2) formalism) had values around −13 cm(−1) for manganese(III) compounds, among the largest AFM coupling constants reported so far for dinuclear Mn(III)(2) derivatives, while values between −3 and −10 cm(−1) were obtained for iron(III) compounds.