Ivan Nemec

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.

Solvent-induced structural diversity in tetranuclear Ni(II) Schiff-base complexes: the first Ni4 single-molecule magnet with a defective dicubane-like topology

Two tetranuclear NiII complexes, namely [Ni4(L)4(CH3OH)3(H2O)]·CH3OH (1) and (Pr3NH)2[Ni4(L)4(CH3COO)2] (2, Pr3N = tripropylamine), were synthesized from a tridentate Schiff base ligand H2L (2-[(E)-(2-hydroxybenzylidene)amino]phenol) and Ni(CH3COO)2·4H2O, using different solvents and their ratios (CH3OH and/or CH2Cl2). The prepared Ni4 complexes are of different structural types, involving an Ni4O4 cubane-like core (1) and Ni4O6 defective dicubane-like core (2), with all the Ni atoms hexacoordinated. The complexes were characterized by elemental analysis, FT-IR spectroscopy, variable temperature and field magnetic measurements, and single crystal X-ray analysis. The DFT and CASSCF/NEVPT2 theoretical calculations were utilized to reveal information about the isotropic exchange parameters (Jij) and single-ion zero-field splitting parameters (Di, Ei). The variable temperature magnetic data suggested the competition of the antiferromagnetic and ferromagnetic intracluster interactions in compound 1, which is in contrast to compound 2, where all intracluster interactions are ferromagnetic resulting in the ground spin state S = 4 with an easy-axis type of anisotropy quantified by the axial zero-field splitting parameter D = -0.81 cm-1. This resulted in the observation of a field-induced slow-relaxation of magnetization (U = 3.3-6.7 K), which means that the complex 2 represents the first Ni4 single-molecule magnet with the defective dicubane-like topology.

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.

Suppressing of slow magnetic relaxation in tetracoordinate Co(II) field-induced single-molecule magnet in hybrid material with ferromagnetic barium ferrite

The novel field-induced single-molecule magnet based on a tetracoordinate mononuclear heteroleptic Co(II) complex involving two heterocyclic benzimidazole (bzi) and two thiocyanido ligands, [Co(bzi)2(NSC)2], (CoL4), was prepared and thoroughly characterized. The analysis of AC susceptibility data resulted in the spin reversal energy barrier U = 14.7 cm−1, which is in good agreement with theoretical prediction, Utheor. = 20.2 cm−1, based on axial zero-field splitting parameter D = −10.1 cm−1 fitted from DC magnetic data. Furthermore, mutual interactions between CoL4 and ferromagnetic barium ferrite BaFe12O19 (BaFeO) in hybrid materials resulted in suppressing of slow relaxation of magnetization in CoL4 for 1:2, 1:1 and 2:1 mass ratios of CoL4 and BaFeO despite the lack of strong magnetic interactions between two magnetic phases.

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.

Magnetic Anisotropy and Field‐induced Slow Relaxation of Magnetization in Tetracoordinate CoII Compound [Co(CH3‐im)2Cl2]

Static and dynamic magnetic properties of the tetracoordinate CoII complex [Co(CH3-im)2Cl2], (1, CH3-im = N-methyl-imidazole), studied using thorough analyses of magnetometry, and High-Frequency and -Field EPR (HFEPR) measurements, are reported. The study was supported by ab initio complete active space self-consistent field (CASSCF) calculations. It has been revealed that 1 possesses a large magnetic anisotropy with a large rhombicity (magnetometry: D = −13.5 cm−1, E/D = 0.33; HFEPR: D = −14.5(1) cm−1, E/D = 0.16(1)). These experimental results agree well with the theoretical calculations (D = −11.2 cm−1, E/D = 0.18). Furthermore, it has been revealed that 1 behaves as a field-induced single-ion magnet with a relatively large spin-reversal barrier (Ueff = 33.5 K). The influence of the Cl–Co–Cl angle on magnetic anisotropy parameters was evaluated using the CASSCF calculations.

Tetranuclear Ni(II) and Co(II) Schiff-base complexes with an M4O6 defective dicubane-like core: zero-field SMM behavior in the cobalt analogue

Two isostructural tetranuclear M(II)4 complexes with the general formula [M4(L)4(CH3OH)2] were prepared by the reaction of M(CH3COO)2·4H2O and H2L in the presence of Pr3N (M = Ni in 1, Co in 2, H2L = 2-{(E)-[(2-hydroxyphenyl)imino]methyl}phenol, Pr3N = tripropylamine). The crystal structure of 1 was determined by single-crystal X-ray diffraction and it was revealed that it possesses the defective dicubane {Ni4O6}2− core with two penta- and two hexacoordinate Ni atoms. The isostructurality of both complexes was confirmed by powder X-ray diffraction measurements. The analysis of the static magnetic data revealed that the prevailing antiferromagnetic interaction leads to the diamagnetic ground state in 1, whereas the ferromagnetic interactions dominate in 2. The analysis of magnetic data was supported by broken-symmetry DFT and CASSCF/NEVPT2 calculations, where the latter disclosed large magnetic anisotropy in both compounds. Moreover, the measurements of ac susceptibility in the zero-applied magnetic field confirmed the presence of slow-relaxation of magnetization in 2 and thus, this compound behaves as a single-molecule magnet.

Field-induced slow relaxation of magnetization in dinuclear and trinuclear CoIII⋯MnIII complexes

Two new dinuclear, [Co(3EtO-L5)(μ-CN)Mn(L4a)Cl] (1) and [Co(3EtO-L5)(μ-CN)Mn(L4a)Br] (2), and two trinuclear [{Co(3EtO-L5)(μ-CN)}2Mn(L4a)]I (3) and [{Co(3EtO-L5)(μ-CN)}2Mn(L4a)](NO3) (4) complexes were prepared and thoroughly characterized (H23EtO-L5 = N,N′-bis(3-ethoxy-2-hydroxybenzylidene)-1,6-diamino-3-azahexane, L4a2− = N,N′-ethane-bis(salicylideneiminate)dianion). The crystal structures were determined for all four compounds, while the static and dynamic magnetic properties were studied only for compounds 1–3. It has been revealed by simultaneous fitting of temperature and field dependent magnetic data and by using the spin Hamiltonian formalism involving the axial anisotropy term that the manganese(III) atoms possess relatively large and negative axial magnetic anisotropy in 1–3, with D = −3.9(2) cm−1 in 1, −4.9(2) cm−1 in 2, and −4.1(1) cm−1 in 3. These results were supported by ab initio CASSCF calculations which were in good agreement with the experimental ones, however, a small rhombicity was calculated contrary to the experimental evaluations: Dcalc(Ecalc/Dcalc) = −3.2 cm−1 (0.04) in 1, −3.0 cm−1 (0.03) in 2, and −3.6 cm−1 (0.04) in 3. The measurements of dynamic magnetic data confirmed that compounds 1–3 represent a new type of Mn(III) field-induced single-ion magnet. The peak maxima of the frequency dependent out-phase susceptibility were below the lowest accessible temperature in all the three cases (i.e. below 1.9 K) and this prevented us from the construction of the Argand diagram. Nevertheless, the approximate procedure for extracting the spin-reversal barrier (Ueff) and relaxation time (τ0) was used, and the values of Ueff ranging from 11 to 20 K and t0 from 0.1 to 19 × 10−7 s were obtained.

Structural and magnetic characterizations of the first manganese(III) Schiff base complexes involving hexathiocyanidoplatinate(IV) bridges

The reactions of [MnIII(L4a)Cl] or [MnIII(L4b)Cl] with [Pt(SCN)6]2− resulted in the formation of a 1D polymeric complex [{Mn(L4a)}2{μ-Pt(SCN)6}]n (1) and a trinuclear compound [{Mn(L4b)(H2O)}2{μ-Pt(SCN)6}] (2), where L4a and L4b are tetradentate dianionic Schiff base ligands, H2L4a = N,N′-ethylenebis(salicylimine) and H2L4b = N,N′-3-methylbenzenebis(3-ethoxysalicylimine). In compound 1, the adjacent [Mn(L4a)]+ subunits are bound together through the phenolic oxygen atoms, thus forming [{Mn(L4a)}2]2+ dimers with the shortest Mn⋯Mn distance of 3.2101(5) A, while in compound 2, the intermolecular bifurcated O–H⋯O hydrogen bonds between the neighbouring molecules form [{Mn(L4b)(H2O)}2]2+ dimers, resulting in a supramolecular 1D chain structure. The Mn⋯Mn distance within the supramolecular dimer is 4.7007(9) A, while the shortest Mn⋯Mn distance between the dimers is 9.2347(10) A. The magnetic analyses comprising the zero-field splitting resulted in a ferromagnetic exchange interaction within the phenolato-bridged dimer in 1 (J = +1.73 cm−1, DMn = −2.65 cm−1) and the antiferromagnetic exchange interaction within the supramolecular dimer in 2 (J = −0.88 cm−1, DMn = −3.06 cm−1). The experimental findings regarding the dominant as well as negligible exchange pathways are in good agreement with the results from DFT calculations at the B3LYP/TZVP level of theory.

An octanuclear Schiff-base complex with a Na2Ni6 core: structure, magnetism and DFT calculations

The octanuclear Na2Ni6 complex (Pr3NH)[Na2Ni6(L)4(Bza)5(HBza)(OH)2(ace)]·Et2O (1), where Pr3NH+ = the tripropylamonium cation, H2L = 2-[(E)-(2-hydroxybenzylidene)amino]phenol, HBza = benzoic acid and ace = acetone, was synthesized and characterized by the elemental analysis, FTIR spectroscopy, single crystal X-ray diffraction analysis, magnetic measurements and DFT calculations. All six NiII atoms are hexacoordinate with the {NiO6} or {NiNO5} chromophores forming two defective dicubane cores. The static magnetic data were fitted to the simplified spin Hamiltonian model which resulted in averaged ferromagnetic and antiferromagnetic exchange parameters J = +5.3 cm−1, and J = −9.2 cm−1, respectively, confirming the predominant role of the antiferromagnetic coupling in 1. The broken symmetry DFT method with various functionals (B3LYP, PBE0, TPPSh and CAM-B3LYP) was used to dissect information about magnetic coupling. As a result, the isotropic exchange parameters (Jab) derived by the PBE0 or B3LYP functionals seem to be the best to match the experimental magnetic data.