Yu. G. Shvedenkov

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

Iron(II) Complexes with 4-R-1,2,4-Triazoles (R = Ethyl, Propyl) Exhibiting 1A1 ⇄ 5T2 Spin Transition

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Synthesis and study of Iron(II) perrhenate complexes with 4-propyl-1,2,4-triazole exhibiting 1 A 1 ⇄ 5 T 2 spin transition

, 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.

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

Methods for the synthesis of iron(II) complexes with 1,2,4-triazole (HTrz) and 4-amino-1,2,4-triazole (NH2Trz) containing the decahydro-closo-decaborate ion [B10H10]2-were developed. The empirical formulas of the complexes were Fe(HTrz)3B10H10 · H2O (I) and Fe(NH2Trz)3B10H10 · H2O (II). The complexes were examined by static magnetic susceptibility measurements (2–300 K) and Moessbauer, IR, and electronic spectroscopy. Complex I exhibits the reversible spin transition 1A1 ⇄ 5T2 and pink ⇄ white thermochromism. The temperatures of the forward and reverse transitions in complex I were 246 and 233 K, respectively. Complex II remained in the high-spin state over the whole temperature range. The sharp decrease in its effective magnetic moment at T < 78 K was attributed to antiferromagnetic exchange interactions between Fe2+ ions.

Cu(II) complexes with 4,6-bis(3,5-dimethyl-1H-pyrazole-1-yl)pyrimidine, 4-(3,5-dimethyl-1H-pyrazole-1-yl)-6-(3,5-diphenyl-1H-pyrazole-1-yl)pyrimidine: Synthesis and catalytic activity in ethylene polymerization reaction

Iron(II) complexes with 1,2,4-triazoles of composition FeL3A2 · nH2O were synthesized, where L is 4-ethyl- or 4-propyl-1,2,4-triazole (Ettrz, Prtrz, respectively) and A = NO–3, ClO–4, Br–; n = 0.5, 1, 2. Magnetochemical studies showed that all these compounds exhibit 1A1 ⇄ 5T2 spin transition (ST) which is accompanied by thermochromism (a reversible pink ⇄ white change of color). The ST pattern, i.e., the temperature of a direct (Ts↑) and reverse (Ts↓) transition, and its contrast substantially depend on the nature of both the ligand and anion and on the availability of water molecules in the complex structure. The highest ST temperatures were observed for Fe(Ettrz)3Br2 · 2H2O: Ts↑ = 327 K, Ts↓ = 314 K.

Cu(II) and Co(II) complexes with 3,5-diphenyl-4-amino-1,2,4-triazole: Synthesis and study

Fe(II) complexes with 4-R-1,2,4-triazoles of compositions FeL3A2 · nH2O (where L is 4-propyl-1,2,4-triazol; A is Br- (n = 4), CF3SO3- (n = 5)), and FeL2A2 · nH2O (where L are 4-ethyl-, 4-propyl-, 4-isopropyl-1,2,4-triazoles (Ettrz, Prtrz, Iprtrz, respectively), A = NCS-, NO3- (n = 0-2)) were synthesized. Magnetochemical studies revealed that FeL3A2 · nH2O complexes exhibit reversible spin transition (ST) 1A1 ⇄ 5T2 that is accompanied by thermochromism (a reversible change of color rose ⇄ white). The temperatures of direct (Ts↑) and reverse (Ts↓) spin transitions are, respectively, 252 and 247 K for Fe(Рrtrz)3Br2 · 4H2O and 207 and 202 K for Fe(Рrtrz)3(CF3SO3)2 · 5H2O. Dehydration of the complexes is attended by significant changes in the type and temperatures of ST.

1,3-Dipolar cycloaddition in the synthesis of pyrazolyl-substituted nitronyl nitroxides

Coordination compounds of iron(II) perrhenate with 4-propyl-1,2,4-triazole (L), [Fe3L6(ReO4)4(H2O)2](ReO4)2(I), [Fe3L6(H2O)6](ReO4)6·H2O(II), and FeL3(ReO4)2(III), were synthesized. Compounds I and III were studied by static magnetic susceptibility measurements and by IR and electronic spectroscopy. These complexes exhibit a reversible 1A1 ⇄ 5T2 spin transition and thermochromism. Compound I exhibits a spin transition without hysteresis at 185 K. The temperatures of forward and reverse transition for III are 250 and 244 K, respectively. Complex II isolated by crystallization from an acidified aqueous solution of compound I has a linear trinuclear structure of the cation (X-ray diffraction data). The structure of complex I was assumed to be similar, while compound III has a polymeric structure.

Dimeric Complexes of Manganese(II) and Nickel(II) Hexafluoroacetylacetonates with Pyrazole-Containing Nitronylnitroxyl Radicals

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.