Supramolecular lattice-solvent control of iron(ii) spin transition parameters

Abstract

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.