Synthesis, crystal structure, magnetic properties and computational study of a series of cyano-bridged MnIII-FeIII complexes

Different derivatives of the Schiff base (SB) N,N′-bis(2-hydroxybenzylidene)-2,2,-dimethylpropane-1,3,-diamine, LH2, have been used to obtain a series of compounds of which we here report the synthesis, structural determination and magnetic study. [Mn(SB)(H2O)]ClO4 and [NEt4]3[Fe(CN)6] react in methanol to give the cis-cyano-bridged assemblies, 3D [NEt4][Mn(5-ClL)]2Fe(CN)6]·2(MeOH) 1, 3D [NEt4]3[Mn(5-BrL)]6[Fe(CN)6]3·11(MeOH) 2, and a trans-cyano-bridged trinuclear compound [NEt4][Mn(4-MeOL)(H2O)]2Fe(CN)6]·(EtOH)·(H2O) 3, depending on the Schiff-base used. Complexes 1, 2 and 3 have been characterized by X-ray analyses, magnetic measurements and DFT calculations. The four CN− in the equatorial plane of the [Fe(CN)6]−3 moiety bridge four Mn ions, each in the cis position, which results in a 3D neutral layered structure with a [–M–NC–Fe–CN–M–] linkage for 1 and 2. The two CN− in the equatorial plane of the [Fe(CN)6]−3 moiety bridge two Mn ions in the trans position, which results in a hydrogen bonded 2D neutral layered structure for 3. Magnetic studies reveal that complexes 1 and 2 exhibit antiferromagnetic coupling, while complex 3 displays ferromagnetic interactions between low-spin Fe(III) and high-spin Mn(III) through the cyanide bridges. Complex 3 further displays alternating current (AC) frequency dependent magnetic susceptibility, typical of the presence of slow magnetization relaxation. Density functional theory (DFT) combined with the broken symmetry state method were used to study the magnetic coupling behavior of complexes 1 and 3. DFT computations reveal that the calculated exchange coupling constants Jtheo were in good agreement with the experimental ones.