V. N. Ikorskii

Spin-crossover and thermochromism in complexes of iron(II) iodide and thiocyanate with 4-amino-1,2,4-triazole

Abstract New complexes of iron(II) iodide and thiocyanate with 4-amino-1,2,4-triazole were synthesized. The complexes were characterized by magnetic measurements, reflection, Mossbauer and IR spectra. The coordination of iron(II) is discussed on the basis of these data. The compounds posses reversible thermochromism (colour change from white to pink) due to the spin transition 5 T 2 ⇌ 1 A 1 .

Nonclassical Spin Transitions

AbstractA family of heterospin polymer chain complexes Cu(hfac)2 with pyrazole-substituted nitronyl nitroxides (LR, where R = Me, Et, Pr) of the composition Cu(hfac)2LR was found. In the solid state the complexes undergo low-temperature structural rearrangements accompanied by magnetic effects analogous to spin-crossover. Polymer chains with a “head-to-head” or “head-to-tail” motif in Cu(hfac)2LR are formed as a result of the bridging coordination of LR through the imine N atom of pyrazole and one of the O atoms of the nitronyl nitroxide fragment. Despite the low-temperature structural phase transition, the single crystals retain the quality needed for an X-ray investigation, due to which the compounds may be studied at different temperatures and structural dynamics studies are possible. It was found that the major structural changes mainly occur in the coordination polyhedra, leading to phenomena analogous to spin-crossover. This rearrangement is possible due to the Jahn–Teller nature of the Cu(II) ion, which is responsible for the pulled octahedron structure of the coordination unit. The transition of the coordinated nitroxyl O atoms from the axial (dCu–O ∼ 2.2-2.4 Å) to equatorial (dCu–O ∼ 2.0 Å) position is accompanied by a transition of the exchange interaction in the Cu(II)–O•–N< exchange clusters from weak ferromagnetic (or weak antiferromagnetic) to strong anti-ferromagnetic, compensating the spins of the Cu(II) ion and the nitroxyl fragment. The motif of the polymer chain (“head-to-head” or “head-to-tail”) proved to be inessential to the occurrence of thermally induced spin transitions. In both cases, the shortening of the Cu–O distance in the Cu(II)–O•–N< exchange cluster decreases the effective magnetic moment of the complex by a factor of

Excision of the {Mo6Se8} Cluster Core from a Chevrel Phase: Synthesis and Properties of the First Molybdenum Octahedral Cluster Selenocyanide Anions [Mo6Se8(CN)6]7− and [Mo6Se8(CN)6]6−

\sqrt 2

Influence of magnetic dilution on the spin transition in the complex of iron(II) nitrate with 4-amino-1,2,4-triazole

, because spin compensation occurs in only half of all coordination units (Cu(hfac)2LPr, Cu(hfac)2LMe, and Cu2(hfac)4LMeLEt). The low-temperature structural rearrangement in Cu(hfac)2LEt is unusual; it leads to longer Cu–O distances in the Cu(II)–O•–N< exchange cluster and to an abrupt transition from antiferromagnetic to ferromagnetic exchange. At reduced temperatures, the unit cell volume decreased by 5-6% in all of the compounds. The cell parameters are large and so is the absolute value of the decrease (up to 400 Å3). The maximal decrease in the cell dimensions was observed in the directions of the chains and in the directions of the maximal shortening of the Cu...Cu distances. Minimal compression or even extension of the cell took place in the direction of lengthening of the Cu–Ohfac distances. The single crystals of Cu(hfac)2LEt and Cu(hfac)2LPr possess high mechanical stability in repeated cooling-heating cycles.

Electronic and spatial structure of spin transition iron(II) tris(4-amino-1,2,4-triazole) nitrate and perchlorate complexes

New complexes of iron(II) chloride and bromide with 1,2,4-triazole (Htrz) and 4-amino-1,2,4-tri-azole (NH2trz) of composition Fe(Htrz)3Cl2 · 1.5H2O, Fe(NH2trz)3Cl2 · 2H2O, Fe(Htrz)3Br2 · 2H2O, and the Fe(NH2trz)3SO4 · H2O complex were synthesized and studied by magnetochemical, electronic, IR and Mössbauer spectroscopy methods. Magnetochemical studies showed that these complexes exhibit 1A1 ⇄ 5T2 spin transition accompanied by thermochromism (a reversible pink ⇄ white change of color).

Synthesis, molecular and crystal structure, magnetic properties, luminescence, and solid-phase thermolysis of binuclear Ln(III) pivalates with 2,2′-dipyridyl and 1,10-phenanthroline molecules

The synthesis of new molybdenum cluster selenocyanide anionic complexes [Mo6Se8(CN)6]7- and [Mo6Se8(CN)6]6- is reported. The [Mo6Se8(CN)6]7- ion was obtained by excision of the cluster core [Mo6Se8] from a Chevrel phase in the reaction of Mo6Se8 with KCN at 650 degrees C; the [Mo6Se8(CN)6]6- ion is formed by oxidation of [Mo6Se8(CN)6]7-. New cluster salts K7[Mo6Se8(CN)6] x 8H2O (1) and (Me4N)4K2[Mo6Se8(CN)6] x 10H2O (2) were isolated and their crystal structures were solved. Compound 1 crystallizes in the cubic space group Fm3m (a=15.552(2) A, Z=4, V=3761.5(8) A3), compound 2 crystallizes in the triclinic space group P1 (a=11.706(2), b=11.749(2), c=12.459(2) A, alpha=72.25(1), beta=77.51(1), gamma=63.04(1), Z=1, V=1448.5(4) A3). Compound 1 is paramagnetic due to an availability of 21 electrons per Mo6 cluster; cyclic voltammetry reveals a quasi-reversible transition [Mo6Se8(CN)6]7- [Mo6Se8(CN)6]6-, E1/2=0.63 V.

Molecular magnets based on M(hfac)2 and spin-labeled nitrile

A series of novel binuclear ferro- and antiferromagnetic Cu(2+) chelates of structurally broadly varied Schiff bases (derived from o-tosylamino(hydroxyl)benzaldehydes and monoalkylated o-phenylenediamine, o-aminophenol, o-aminothiophenol, 1,2-diaminobenzimidazole, 1-aminobenzimidazoline-2-thione) and β-diketimines (derived from 2,6-di-i-Pr-aniline) has been prepared. The tautomerism of the ligands and structureof their copper complexes have been studied with the use of IR, 1H NMR EPR and EXAFS spectroscopy. Molecular and crystal structure of a β-diketimine copper dimer has been determined by X-ray crystallography. The magnetic measurements (2–300 K) performed for all the complexes showed that the ferro- and antiferromagnetic character of the exchange interaction depends both on the structure of the coordination site (origin of the ligating centers) and the structure of the ligands (in particular, on the type of the cycle annelated to the bridging fragment). Whereas S-binding metal chelates 13 (X = NTs, Y = S, R = H) are diamagnetic, the complexes 15 with annelated azole moieties are ferromagnetic.

Copper(II) complexes with imidazol-4-yl derivatives of 2-imidazoline nitroxides

Influence of magnetic dilution with ZnII ions on the spin transition in the iron nitrate complex of 4-amino-1,2,4-triazole (AT) was studied by magnetochemistry, Mössbauer spectroscopy, and IR spectroscopy. In studies of the properties of solid phases of FexZn1−x(AT)3(NO3)2 (0.01≤x≤0.8), it was demonstrated that magnetic dilution results in a lowered spin transition temperature and an increased share of the high-spin form of the iron(II) complex.