Single-crystal to single-crystal transformation in a pentagonal bipyramidal Fe(II) complex with a N(3)S(2) chelating ligand: switching on single-ion magnet properties via axial ligand substitution.

A new pentagonal bipyramidal Fe(II) complex [Fe(HL)(MeOH)]Cl·MeOH (1) with a pentadentate NS chelating ligand 2,6-diacetylpyridine bis(4--(4-ethylphenyl)thiosemicarbazone) (HL) was synthesized. It was found that after exposure to air, desolvation of sample 1 occurs followed by a single-crystal to single-crystal (SC-SC) transformation into the [Fe(HL)Cl] complex (2). Investigation of the single crystal structure revealed the loss of both crystallized and coordinated methanol, accompanied by the replacement of the latter by chlorine, which was the outer-sphere anion in structure 1. Mössbauer spectroscopy confirmed a direct conversion of 1 to 2. Such SC-SC transformations with the substitution of neutral ligands by anions and changing both the composition and charge of the resulting complex are rarely observed. The transformation of 1 into 2 becomes reversible upon dissolving complex 2 in methanol with heating, when the axial Cl ligands are again replaced by MeOH and the growth of crystals of 1 is observed. The axial ligand substitution dramatically affects the magnetic properties of the complexes: only complex 2 demonstrates the single-ion magnet behavior in contrast to 1. The use of sulfur NS analogues of oxygen NO ligands leads to increasing the magnetization barrier. For a detailed analysis and comparison of magnetic properties, quantum chemical calculations were performed.