From antiferromagnetic to ferromagnetic exchange in a family of oxime-based Mn~(III) dimers: A magneto-structural study

摘要

The reaction of Mn(ClO_4)_2·6H_2O, a derivatised phenolic oxime (R-saoH_2) and the ligand tris(2-pyridylmethyl)amine (tpa) in a basic alcoholic solution leads to the formation of a family of cluster compounds of general formula [Mn ~(III) _2O(R-sao)(tpa)_2](ClO_4) _2 (1, R = H; 2, R = Me; 3, R = Et; 4, R = Ph). The structure is that of a simple, albeit asymmetric, dimer of two Mn~(III) ions bridged through one μ-O~(2-) ion and the -N-O- moiety of the phenolic oxime. Magnetometry reveals that the exchange interaction between the two Mn ~(III) ions in complexes 1, 3 and 4 is antiferromagnetic, but that for complex 2 is ferromagnetic. A theoretically developed magneto-structural correlation reveals that the dominant structural parameter influencing the sign and magnitude of the pairwise interaction is the dihedral Mn-O-N-Mn (torsion) angle. A linear correlation is found, with the magnitude of J varying significantly as the dihedral angle is altered. As the torsion angle increases the AF exchange decreases, matching the experimentally determined data. DFT calculations reveal that the d_(yz)π*d_(yz) interaction decreases as the dihedral angle increases leading to ferromagnetic coupling at larger angles.