The mononuclear compound (1) [Fe^II(L)₂](BF₄)₂ (L = 4-ethynyl-2,6-bis(pyrazol-1-yl)pyridine) was prepared and structurally as well as magnetically characterised. The crystallisation revealed the formation of two polymorphs – the orthorhombic 1A and the tetragonal form 1B. A third, intermediate phase 1C was found exhibiting a different orthorhombic space group. Reversibility of the phase transition between 1A and 1C was studied by variable-temperature single-crystal and powder X-ray diffraction studies, while an irreversible phase transition was observed for the transition of 1B→1C. The magnetic studies show that the 1A↔1C transition is accompanied by a very abrupt spin transition (ST) with 8 K hysteresis width (T_1/2(↓) = 337 K, T_1/2(↑) = 345 K). The ST was confirmed by Mössbauer spectroscopy as well as by DSC studies. In contrast, the 1B polymorph remained low-spin up to 420 K. In conclusion, a full cycle of intertwined phase- and spin-conversions of three polymorphs could be proven following the general scheme 1B→1C↔1A.

We report on the synthesis of spin transition compounds 1, 2 of formula [Fe(L)₂](A)₂ (where L = 2′,6′-bis(pyrazol-1-yl)-3,4′-bipyridine, A = ClO₄ ⁻—compound 1; A = BF₄ ⁻—compound 2) and compound 3 of formula [Fe(L)(LH)](BF₄)₃·H₂O·CH₃CN (where LH = 3-(2,6-bis(pyrazol-1-yl) pyridine-4-yl)-pyridinium(+)). Compounds 1, 2 and 3 were characterized by single-crystal X-ray diffraction, ESI-ToF mass spectrometry, ¹H NMR and elemental analysis. The single-crystal X-ray diffraction study of the counter anion analogues 1 and 2 reveals almost identical molecular structures without any significant presence of intermolecular interactions. However, in the case of compound 3, the crystal structure reveals supramolecular interactions involving molecular cations, BF₄ ⁻ anions and, most importantly, lattice solvent molecules. The presence of solvent water molecules induces the presence of two different types of hydrogen bonding: (i) water molecules interacting with the fluorine atoms of BF₄ ⁻ anions and (ii) water molecules interconnecting protonated and nonprotonated nitrogens of pyridine-3-yl substituents of neighboring complex cations. These overall hydrogen bonding pattern between the neighboring iron(II) complex cation moieties is responsible for the formation of a one dimensional (1D) hydrogen bonded zig-zag chain. The magnetic investigations elucidate high temperature spin transition behavior for both anion analogues 1 and 2, while compound 3 exhibits a lattice-solvent dependency of the temperature-driven spin transition accompanied with stepwise solvent liberation above room temperature. After complete solvent removal the solvent-free compound 3d, [Fe(L)(LH)](BF₄)₃, shows an abrupt spin transition accompanied with thermal hysteresis loop; T_1/2(↑) = 240 K and T_1/2(↓) = 231 K, ΔT_1/2 = 9 K. The Ising-like model that includes two vibrational modes has been applied in a direct fitting of magnetic data. The model recovers the temperature evolution of the χT product functions for all compounds under study, involving also compound 3d with the thermal hysteresis.

Four pentacoordinate complexes, 1–4, of the type [Co(L1)X₂] and [Co(L2)X₂] (where L1 = 2,6-bis(1-octyl-1H-benzimidazol-2-yl)pyridine for 1 and 2, L2 = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)-pyridine for 3 and 4; X = Cl⁻ for 1 and 3, X = Br⁻ for 2 and 4) have been synthesized, and their structures have been determined by X-ray analysis. The DC magnetic investigation confirmed the high-spin and anisotropic behavior of the metal centers of the reported compounds, and magnetic data were analyzed with respect to the spin Hamiltonian and the Griffith–Figgis Hamiltonian. CASSCF-NEVPT2 analysis enabled us to identify triaxial magnetic anisotropy for 1 and 2 and axial anisotropy for 3 and 4, and we discuss thoroughly the relationship between geometry and orbital ordering. The g-tensor components of the ground Kramers doublet extracted from EPR spectroscopy are compatible with this prediction of magnetic anisotropy. Simulated FIRMS experimental data of 2, 3 and 4 show very good agreement with theoretical calculations and provide precise values of zero-field splitting. The AC susceptibility measurements confirmed that the reported complexes were field-induced single-ion magnets. The slow relaxation of magnetization in 1 and 2 is mediated through two relaxation channels that are unusually close to each other. On the other hand, complexes 3 and 4 show the single-channel relaxation of magnetization, and their isostructural characteristic allowed us to study the relaxation changes caused solely by the replacement of terminal halido ligands. Finally, a wet lithographic technique has been used to evaluate both the processability of the complexes in solution and the preparation of microstructured films.