S. F. Vasilevskii

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

The Cu(II) complexes with bis(pyrazole-1-yl)methane and its derivatives: Synthesis, crystal structure, and magnetic properties

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Spin crossover in the coordination compounds of iron(II) with tris(3,5-dimethylpyrazol-1-yl)methane

, 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.

High-temperature spin transition in the iron(II) trifluoromethylsulfonate, perrhenate, and tetraphenylborate complexes with tris(pyrazol-1-yl)methane

Mononuclear iron(II) coordination compounds with tris(pyrazol-1-yl)methane (HC(Pz)3) described as [Fe{HC(Pz)3}2]A2 × nH2O, where A = Cl−, Br−, I−, 1/2 SO42−, n = 0–7, were synthesized. The compounds were studied by static magnetic susceptibility measurements, IR and UV/Vis spectroscopy, and powder X-ray diffraction. The crystal and molecular structures of all compounds were determined by single crystal X-ray diffraction.

Spin crossover in heteroligand iron(II) complexes with iris(3,5-dimethylpyrazol-1-yl)methane

The procedures for the synthesis of the Cu(II) complexes with bis(pyrazole-1-yl)methane (L1), bis(3,5-dimethyl-4-bromopyrazole-1-yl)methane (L2), and bis(3,5-dimethyl-4-iodopyrazole-1-yl)methane (L3) of the composition Cu2(L1)2Br4 (I), Cu2(L2)2Cl4 (II), Cu(L3)(NO3)2 (III), and Cu(L3)(H2O)(NO3)2 · 2H2O (IV) were developed. The organic ligands in the above complexes are coordinated to Cu(II) in a bidentate cyclic type through the N(2), N(2′) atoms of the pyrazole rings. The molecular and crystal structures of L2, L3, II, III, and IV were determined by X-ray diffraction. The study of the μeff(T) function in a temperature interval 2–300 K showed that compound I, which exhibited ferromagnetic exchange interactions in the chains, undergoes transition to antiferromagnetic state with weak ferromagnetism. The exchange antiferromagnetic interactions predominate in compound II.

Magnetic properties of new complexes of 3d metal chlorides with 3-amino-4-ethoxycarbonylpyrazole

Reactions of Ni(II) and Mn(II) hexafluoroacetylacetonates with pyrazole-containing nitronylnitroxyl radicals (L) result in dimeric heterospin complexes [M(hfac)2L]2or [M(hfac)2L]2· C6H6. The paramagnetic ligands in the centrosymmetric binuclear molecules function as a bridge, with the nitroxyl O atom being bound to one metal and the imine N atom of the pyrazole ring to the other metal atom. The metal environment is a cis-octahedron with short M–ONOdistances, which specifies a strong antiferromagnetic interaction between the lone electrons of paramagnetic centers in M–O•–N< exchange clusters.

Copper(II) complexes with bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane: Synthesis, spectroscopic study, and crystal structure

Iron(II) complexes with tris(3,5-dimethylpyrazol-1-yl)methane {HC(3,5-Me2Pz)3} of the composition [Fe{HC(3,5-Me2Pz)3}2]Am · nH2O (A = Cl− (I), ClO4−(II), SO42−(III), CF3SO3−(IV), m = 1, 2, n = 0.1) are synthesized. The compounds are studied by static magnetic susceptibility, IR and diffuse reflectance spectroscopy, and X-ray structure analysis. The crystal structures of two polymorphous modifications of the [Fe{HC(3,5-Me2Pz)3}2](ClO4)2 (IIa and IIb) and [Fe{HC(3,5-Me2Pz)3}2](CF3SO3)2 (IV) complexes are determined. The temperature dependence μeff(T) shows that the spin crossover 1A1 ai 5T2 is observed in the polycrystalline phase of complex I and in one of the single-crystal phases of complex II (IIa) and is accompanied by thermochromism (the change of the dark pink color ai to white).

Cobalt(II), nickel(II), and copper(II) bromide complexes with 3-amino-4-ethoxycarbonylpyrazole: Syntheses, structures, and magnetic properties

New coordination compounds of iron(II) trifluoromethylsulfonate, perrhenate, and tetraphenylborate with tris(pyrazol-1-yl)methane (HC(Pz)3) of the composition [Fe(HC(Pz)3)2]A2 (A = CF3SO3− (I), ReO4− (II), and B(C6H5)4− (III)) were synthesized and studied by the method of static magnetic susceptibility and IR and electronic spectroscopies. The crystal and molecular structures of compounds I and II were determined by X-ray diffraction analysis. The magnetochemical study of complexes I—III in the interval from 275 to 500 K showed that they possessed the high-temperature spin transition 1A1 ⇄ 5T2 accompanied by thermochromism.