From antiferromagnetic to ferromagnetic exchange in a family of oxime-based MnIII dimers: a magneto-structural study

Abstract

The reaction of Mn(ClO₄)₂·6H₂O, a derivatised phenolic oxime (R-saoH₂) 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^III₂O(R-sao)(tpa)₂](ClO₄)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-O²⁻ 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.