Ján Pavlik

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

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; T1/2(↑) = 240 K and T1/2(↓) = 231 K, ΔT1/2 = 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.

The interplay of iron(II) spin transition and polymorphism

The mononuclear compound (1) [Fe(II)(L)(2)](BF(4))(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.

Interplay between spin crossover and exchange interaction in iron(III) complexes

In the dinuclear and polynuclear metal complexes exhibiting the low-spin (LS) to high-spin (HS) transition, the spin-crossover phenomenon interferes with the magnetic exchange interaction. The latter manifests itself in forming spin-multiplets, which causes a possible overlap of the band originating in different reference spin states (LL, LH, HL, and HH). A series of dinuclear Fe(III) complexes has been prepared; the iron centers are linked by a bidentate bridge (CN-, and diamagnetic metallacyanates {Fe(CN)5(NO)}, {Ni(CN)4}, {Pt(CN)4}, and {Ag(CN)2}). Magnetic measurements confirm that the spin crossover proceeds on the thermal propagation. This information has been completed also by the Mössbauer spectral (MS) data. A theoretical model has been developed that allows a simultaneous fitting of all available experimental data (magnetic susceptibility, magnetization, HS mole fraction) on a common set of parameters.

Spin crossover and high spin electroneutral mononuclear iron(III) Schiff base complexes involving terminal pseudohalido ligands

Investigations into a series of six novel mononuclear iron(III) Schiff base complexes with the general formula [Fe(L)X] (where H2L is a pentadentate Schiff-base ligand, X = pseudohalido ligand) are reported. Several different aromatic 2-hydroxyaldehyde derivatives were used in combination with N,N′-bis(2-aminoethyl)-1,3-propanediamine (compounds 1–5) and 2,2′-diaminodiethylamine (for 6) to synthesize the H2L Schiff base ligands. The consecutive reactions with iron(III) chloride resulted in the preparation of the [Fe(L)Cl] precursor complexes which were left to react with pseudohalido ligands (NCS− for 1, 2, 3, 4, 6; N3− for 4). Structural investigations revealed a usual coordination of the pentadentate Schiff base ligands via N3O2 donor atoms and the sixth coordination place is occupied by the N donor of the corresponding pseudohalido ligand. The spin crossover was observed in two cases with transition temperatures: Tc = 83 K (hysteresis width of ΔT = 2 K) for 1 and Tc = 174 K for 2. Magnetic investigations of compounds 3–6 revealed high spin behaviour. The magnetic data of all compounds were analysed using the spin Hamiltonian formalism including the zero-field splitting term and the molecular field effect. In the case of the spin crossover complexes 1 and 2, the Ising-like model was applied.

Series of high spin mononuclear iron(III) complexes with Schiff base ligands derived from 2-hydroxybenzophenones

The reaction of various phenols with benzoyl chloride afforded the derivatives of phenyl benzoate that subsequently underwent Fries rearrangement. The obtained 2-hydroxybenzophenone analogues were combined with linear aliphatic triamines, which afforded pentadentate Schiff base ligands. Moreover, nine new iron(III) complexes with the general formula [Fe(Ln)X] (where, Ln is the dianion of the pentadentate Schiff base ligand, N,N′-bis((2-hydroxy-5-methylphenyl)phenyl)methylidene-1,5-diamino-3-azapentane = H2L1, N,N′-bis((2-hydroxy-3,5-dimethylphenyl)phenyl)methylidene-1,5-diamino-3-azapentane = H2L2, N,N′-bis((2-hydroxy-5-chlorophenyl)phenyl)methylidene-1,5-diamino-3-azapentane = H2L3, N,N′-bis((2-hydroxy-4-methylphenyl)phenyl)methylidene-1,5-diamino-3-azapentane = H2L4, N,N′-bis((2-hydroxy-5-bromophenyl)phenyl)methylidene-1,7-diamino-4-azaheptane = H2L5, N,N′-bis((2-hydroxy-5-bromophenyl)phenyl)methylidene-1,7-diamino-4-methyl-4-azaheptane = H2L6 and X is the chlorido, azido or isocyanato terminal ligand) were synthesized and characterized via elemental analysis, and IR and UV-VIS spectroscopy; in addition, the crystal structures of all the complexes were determined by X-ray diffraction. Magnetic investigation reveals high spin state behaviour in all the reported compounds. DFT calculations and analysis of the magnetic functions allowed to extract absolute values of the zero field splitting parameters and exchange coupling constants.

Light-Induced Excited Spin State Trapping

This chapter offers reader an opportunity to become familiar with the contemporary view on the principles of spin crossover (SCO) with emphasis on its specific branch, the light-induced excited spin state trapping (LIESST) effect. It surveys the very basic physical models, which can be considered as standard working tools for simple quantitative characterization of finite-temperature behaviour of SCO or LIESST systems. It highlights the importance of structural changes arising upon the light irradiation, as well as the comparison of structural diversity between the thermally and photoinduced high spin states. The author believes that understanding of mysterious equilateral triangle consisting of thermal spin crossover on the first edge, photoinduced spin crossover on the second and structural diversity upon LS-HS transformation on the third edge can help approach the photoinduced spin transition under ambient conditions and so to their future technological applications. Keywords: finite-temperature behaviour; light-induced excited spin state trapping; LS-HS transformation; photoinduced spin transition; spin crossover