Selective Formation, Reactivity, Redox and Magnetic Properties of MnIII and FeIII Dinuclear Complexes with Shortened Salen-Type Schiff Base Ligands

The reactivity of the shortened salen-type ligands H₃salmp, H₂salmen and H₂sal(p-X)ben with variable para-substituent on the central aromatic ring (X = tBu, Me, H, F, Cl, CF₃, NO₂) towards the trivalent metal ions manganese(III) and iron(III) is presented. The selective formation of the dinuclear complexes [M₂(μ-salmp)₂], M = Mn (1a), Fe (2a), [M₂(μ-salmen)₂(μ-OR)₂)], R = Et, Me, H and M = Mn (3a–c) or Fe (4a–c), and (M₂(μ-sal[p-X]ben)₂(μ-OMe)₂), X = tBu, Me, H, F, Cl, CF₃, NO₂ and M = Mn (5a–g) or Fe (6a–g), could be identified by reaction of the Schiff bases with metal salts and the base NEt₃, and their characterization through elemental analysis, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction of 2a·2AcOEt, 2a·2CH₃CN and 3c·2DMF was performed. In the case of iron(III) and H₃salmp, when using NaOH as a base instead of NEt₃, the dinuclear complexes [Fe₂(μ-salmp)(μ-OR)(salim)₂], R = Me, H (2b,c) could be isolated and spectroscopically characterized, including the crystal structure of 2b·1.5H₂O, which showed that rupture of one salmp³⁻ 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 NEt₃ 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^IIFe^III and Fe^II₂ dinuclear species, while the manganese(III) derivatives can be reversibly oxidized to either the mixed-valence Mn^IIIMn^IV or to the Mn^IV₂ 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 (–2JŜ₁·Ŝ₂ formalism) had values around −13 cm⁻¹ for manganese(III) compounds, among the largest AFM coupling constants reported so far for dinuclear Mn^III₂ derivatives, while values between −3 and −10 cm⁻¹ were obtained for iron(III) compounds.