Ivan Šalitroš

Spin transition in arrays of gold nanoparticles and spin crossover molecules.

We investigate if the functionality of spin crossover molecules is preserved when they are assembled into an interfacial device structure. Specifically, we prepare and investigate gold nanoparticle arrays, into which room-temperature spin crossover molecules are introduced, more precisely, [Fe(AcS-BPP)2](ClO4)2, where AcS-BPP = (S)-(4-{[2,6-(dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)ethanethioate (in short, Fe(S-BPP)2). We combine three complementary experiments to characterize the molecule-nanoparticle structure in detail. Temperature-dependent Raman measurements provide direct evidence for a (partial) spin transition in the Fe(S-BPP)2-based arrays. This transition is qualitatively confirmed by magnetization measurements. Finally, charge transport measurements on the Fe(S-BPP)2-gold nanoparticle devices reveal a minimum in device resistance versus temperature, R(T), curves around 260-290 K. This is in contrast to similar networks containing passive molecules only that show monotonically decreasing R(T) characteristics. Backed by density functional theory calculations on single molecular conductance values for both spin states, we propose to relate the resistance minimum in R(T) to a spin transition under the hypothesis that (1) the molecular resistance of the high spin state is larger than that of the low spin state and (2) transport in the array is governed by a percolation model.

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.

Spin crossover in a heptanuclear mixed-valence iron complex

The complex [Fe(II){(CN)Fe(III)L(5)}(6)]Cl(2) consists of a mixed-valence heptanuclear cyanide-bridged unit formed of a Schiff-base pentadentate ligand L(5) and it shows a spin crossover of the peripheral Fe(III) centres.

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.

Anion driven modulation of magnetic intermolecular interactions and spin crossover properties in an isomorphous series of mononuclear iron(III) complexes with a hexadentate Schiff base ligand

A series of spin crossover iron(III) complexes with the general composition [Fe(4OH-L6)]X (H2-4OH-L6 = 1,8-bis(4-hydroxysalicylaldiminato)-3,6-diazaoctane; X = Cl, 1a; Br, 1b; I, 1c) was prepared. A combination of the results following the single crystal X-ray analysis, infrared and EPR spectroscopy, and temperature dependent magnetic experiments revealed that the Fe(III) atoms occur in the low-spin state below room temperature and the crystal structures of the complexes involve rich networks of non-covalent intermolecular contacts resulting in two-dimensional supramolecular structures. Alterations in the halide anions influence the strength of the non-covalent contacts and affect the magnetic properties of the studied complexes. The antiferromagnetic exchange interaction between the non-covalently bound cations is the most obvious in the case of 1a and it weakens with the growing anionic volume of X. The 1D and 2D spin Hamiltonian models were applied to quantitatively extract the information about the intermolecular magnetic exchange (fit on 1D infinite chain gives J(1a) = −2.86 cm−1, J(1b) = −2.02 cm−1, J(1c) = −1.16 cm−1). Furthermore, gradual spin crossover behaviour for all of the compounds of the series was observed above room temperature in the solid state. Spin crossover accompanied by thermochromism was also demonstrated by EPR experiments in solution.

Solvent-Induced Polymorphism of Iron(II) Spin Crossover Complexes

Two new mononuclear iron(II) compounds (1) and (2) of the general formula [Fe(L)2](BF4)2·nCH3CN (L = 4-(2-bromoethyn-1-yl)-2,6-bis(pyrazol-1-yl)pyridine, n = 1 for (1) and n = 2 for compound (2)), were synthesized. The room temperature crystallization afforded concomitant formation of two different solvent analogues: compound (1) exhibiting triclinic P-1 and compound (2) monoclinic C2/c symmetry. Single-crystal X-ray studies confirmed the presence of the LS (low-spin) state for both compounds at 180 K and of the HS (high-spin) state for compound (2) at 293 K, in full agreement with the magnetic investigations for both solvent polymorphs. Compound (1) exhibits spin transition above 293 K followed by subsequent solvent liberation, while the spin transition of (2) takes already place at 237 K. After complete solvent removal from the crystal lattice, compound (1d) (the desolvated polymorph derived from (1)) exhibits spin transition centered at 342 K accompanied by a thermal hysteresis loop, while the analogous compound (2d) (the desolvated derivate of compound (2)) remains blocked in the HS state over all the investigated temperature range.

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.

A charge neutral iron(II) complex with an above room temperature spin crossover (SCO) and hysteresis loop

We report on the unusually abrupt spin crossover (SCO) behaviour of a tridentate-nitrogen pyrazole–pyridine–tetrazole (L1H) based charge-neutral [Fe(L1)2] complex. Different reaction conditions were utilized to prepare the complex in crystalline and powder forms. X-ray crystallographic analysis of the complex at 180 K revealed distorted tetragonal bipyramidal geometry around the Fe(II) coordination center with Fe–N bond lengths and angles indicative of the low spin state of the complex. Investigation of the magnetic behaviour of the powder and crystalline forms of the complex yielded an abrupt and above room temperature first order SCO (T1/2↓ = 346.3 K and T1/2↑ = 349 K) with an ∼2.6 K hysteresis loop for the powder sample, whereas the crystalline form remained in the low spin state throughout the measurement temperature range. Upon irradiation with red or green light (λ = 637 nm or 532 nm, 10 mW cm−2) the powder form of the complex showed a light-induced excited spin state trapping (LIESST) effect with a T(LIESST) value of 63 K, and no LIESST effect was observed for the crystalline complex. Reversible phase transition and large enthalpy (ΔH) and entropy (ΔS) changes associated with SCO of [Fe(L1)2] were inferred from differential scanning calorimetry (DSC) experiments. This was corroborated by variable temperature small angle X-ray scattering (SAXS) measurements wherein different crystalline phases associated with LS and HS [Fe(L1)2] complexes and their reversible inter-conversion upon SCO were unambiguously observed.

Divergent Coordination Chemistry: Parallel Synthesis of [2×2] Iron(II) Grid‐Complex Tauto‐Conformers

Abstract The coordination of iron(II) ions by a homoditopic ligand L with two tridentate chelates leads to the tautomerism‐driven emergence of complexity, with isomeric tetramers and trimers as the coordination products. The structures of the two dominant [FeII 4 L 4]8+ complexes were determined by X‐ray diffraction, and the distinctness of the products was confirmed by ion‐mobility mass spectrometry. Moreover, these two isomers display contrasting magnetic properties (FeII spin crossover vs. a blocked FeII high‐spin state). These results demonstrate how the coordination of a metal ion to a ligand that can undergo tautomerization can increase, at a higher hierarchical level, complexity, here expressed by the formation of isomeric molecular assemblies with distinct physical properties. Such results are of importance for improving our understanding of the emergence of complexity in chemistry and biology.