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, CF3, NO2) towards the trivalent metal ions manganese(III) and iron(III) is presented. The selective formation of the dinuclear complexes [M-2(mu-salmp)(2)], M = Mn (1a), Fe (2a), [M-2(mu-salmen)(2)(mu-OR)(2))], R = Et, Me, H and M = Mn (3a-c) or Fe (4a-c), and (M-2(mu-sal[p-X]ben)(2)(mu-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 center dot 2AcOEt, 2a center dot 2CH(3)CN and 3c center dot 2DMF was performed. In the case of iron(III) and H(3)salmp, when using NaOH as a base instead of NEt3, the dinuclear complexes [Fe-2(mu-salmp)(mu-OR)(salim)(2)], R = Me, H (2b,c) could be isolated and spectroscopically characterized, including the crystal structure of 2b center dot 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 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 (FeFeIII)-Fe-II and Fe-2(II) dinuclear species, while the manganese(III) derivatives can be reversibly oxidized to either the mixed-valence (MnMnIV)-Mn-III or to the Mn-2(IV) 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 S-1 center dot S-2 formalism) had values around -13 cm(-1) for manganese(III) compounds, among the largest AFM coupling constants reported so far for dinuclear Mn-2(III) derivatives, while values between -3 and -10 cm(-1) were obtained for iron(III) compounds.