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 [MnIII2O(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 MnIII 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 MnIII 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 dyz|π*|dyz interaction decreases as the dihedral angle increases leading to ferromagnetic coupling at larger angles.
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