Lowering Coordination Symmetry as a Strategy for Tuning Slow Magnetic Relaxation in Quasi-Isotropic Mn(II) Systems

Two pentacoordinate Mn(II) complexes containing tridentate 2.2′-[oxybis(methylene)] bis(1H-1,3-benzimidazole) (OMeBB) ligand and pseudohalide coligands, [Mn(OMeBB)(NCS)2] (1) and [Mn(OMeBB)(NCO)2] (2), exhibit a very rare {MnIIN4O} coordination mode involving semicoordinate bonds between the ether oxygen atom of the ligand and the Mn(II) center. Susceptibility, magnetization, EPR and ab initio calculations confirm only small magnetic anisotropy. The AC susceptibility measurements reveal field-induced slow magnetic relaxation over the broadest temperature range reported so far for Mn(II) systems, extending up to T ≈ 30 K, with two or three relaxation channels predominantly governed by the direct relaxation mechanism. These results demonstrate that low coordination symmetry and unconventional ligand topology provide an effective approach for tuning magnetic relaxation behavior in quasi-isotropic Mn(II) systems.