E. V. Tretyakov

Unusual spin transitions

New heterospin complexes of Cu(hfac)2 (hfac, hexafluoroacetylacetonate) with pyrazole-substituted nitronyl nitroxides have been found that in the solid state exhibit thermally induced spin transitions analogous to spin crossover. For the first complex, [Cu(hfac)2Li-Pr], at room temperature, the Cu—OL distances, where OL is the oxygen atom of the nitroxyl group, are very short (2.143 Å). This leads to a strong antiferromagnetic exchange (∼-120cm−1) in the > N—·O—Cu2+—O·—N < exchange clusters. The CuO6 coordination units formed by four O atoms of the two hfac anions and by the nitroxyl O atoms of the two bridging nitroxides have a rare form of flattened octahedra, transformed at low temperatures into elongated octahedra with shorter Cu—OL distances (2.143 Å→2.002Å) and two longer Cu—Ohfac distances (2.130 Å→2.293 Å). For the second complex, [Cu(hfac)2LBu·0.5C6H14], unusual low temperature structural dynamics of heterospin systems have been found. It is characterized by the formation of two types of CuO6 unit. The axial Cu—OL distances are lengthened in one unit (2.250 Å→2.347 Å) and shortened in the other (2.250 Å → 2.006 Å). This leads to a sophisticated μeff(T) dependence with μeff drastically decreased at 163 K as a result of full coupling of two spins in half of all >N—·O—Cu2+—O·—N < exchange clusters and to a shift from antiferromagnetic to ferromagnetic exchange in the other half.

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

Synthesis and Properties of Acetylenic Derivatives of Pyrazoles

\sqrt 2

First Example of a Reversible Single-Crystal-to-Single-Crystal Polymerization–Depolymerization Accompanied by a Magnetic Anomaly for a Transition-Metal Complex with an Organic Radical

, 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.

Jumping Crystals: Oxygen-Evolving Metal-Nitroxide Complexes

Abstract Oxidation of the triple bond in 4-alkynylpyrazoles 2a – e and acetylenic derivatives of the crown-ethers 7a , b with PdCl 2 –DMSO has been carried out to give unsymmetrical hetaryl-1,2-diketones 3a – e ; 8a , b . Attempts to oxidize the triple bond in 5-alkynylpyrazole 6 and alkynylpyridines 5a , b and 9 failed.

A new method for the reduction of nitronyl nitroxides

Publisher Summary This chapter discusses the compounds that have two important functionalities in the same molecule: acetylene (69MI2; 78MI3) and pyrazole (67MI1; 84MI2; 96MI1). It demonstrates the versatile and frequently unexpected nature of acetylene, and pyrazole reactions. The chapter discusses the synthesis of acetylenylpyrazoles, chemical and biological properties of acetylenylpyrazoles, structure, and spectra. Acetylenyl pyrazoles are highly valuable intermediates because a triple bond is susceptible to nucleophilic, electrophilic, radical, and cycloaddition reactions, and because the terminal acetylenes display an unusually high CH acidity that can be used for both functionalizing and building up C–C bonds. Acetylenes occupy a central place in the synthetic organic chemistry because of their availability and the great versatility of their transformations. Because of their stability, acid–base equilibria, coordination behavior, pharmaceutical properties, and easy preparation pyrazoles are among the most used heterocycles.

Synthesis of 2-iminonitroxide-substituted phenols and pyridine-3-oles.: Copper(II) complexes with imino nitroxides containing 2-hydroxyphenyl substituents

Abstract Reaction of arylpropargyl aldehydes with 2,3-dihydroxylamino-2,3-dimethylbutane results in the formation of 2-(1-hydroxy-4,4,5,5-tetramethylimidazolidin-2-ylidene)-1-arylethanones in high yields (70–80%). Allowing availability of propargyl aldehydes, this method gives new possibilities for the preparation of 2-(1-hydroxy-4,4,5,5-tetramethylimidazolidin-2-ylidene)-1-arylethanones.

Spirocyclic derivatives of nitronyl nitroxides in the design of heterospin Cu II complexes manifesting spin transitions

The reaction of copper(II) hexafluoroacetylacetonate [Cu(hfac)2] with the stable nitronyl nitroxide 2-(1-ethyl-3-methyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L(a)) resulted in a paired heterospin complex [[Cu(hfac)2]3(μ-O,N-L(a))2][Cu(hfac)2(O-L(a))2]. The crystals of the compound were found to be capable of a reversible single-crystal-to-single-crystal (SC-SC) transformation initiated by the variation of temperature. At room temperature, the molecular structure of [[Cu(hfac)2]3(μ-O,N-L(a))2][Cu(hfac)2(O-L(a))2] is formed by the alternating fragments of the pair complex. Cooling the crystals of the complex below 225 K caused considerable mutual displacements of adjacent molecules, which ended in a transformation of the molecular structure into a polymer chain structure. A reversible topotactic polymerization-depolymerization coordination reaction actually takes place in the solid during repeated cooling-heating cycles: [[Cu(hfac)2]3(μ-O,N-L(a))2][Cu(hfac)2(O-L(a))2] ⇌ Cu(hfac)2(μ-O,N-L(a))]∞. Polymerization during cooling is the result of the anomalously great shortening of intermolecular distances (from 4.403 Å at 295 K to 2.460 Å at 150 K; Δd = 1.943 Å) between the terminal Cu atoms of the trinuclear fragments {[[Cu(hfac)2]3(μ-O,N-L(a))2]} and the noncoordinated N atoms of the pyrazole rings of the mononuclear {[Cu(hfac)2(O-L(a))2]} fragments. When the low-temperature phase was heated above 270 K, the polymer chain structure was destroyed and the compound was again converted to the pair molecular complex. The specifics of the given SC-SC transformation lies in the fact that the process is accompanied by a magnetic anomaly, because the intracrystalline displacements of molecules lead to a considerable change in the mutual orientation of the paramagnetic centers, which, in turn, causes modulation of the exchange interaction between the odd electrons of the Cu(2+) ion and nitroxide. On the temperature curve of χT, this shows itself as a hysteresis loop. The nontrivial character of the recorded spin transition during the cooling of the sample below 225 K lies in the fact that the magnetic moment abruptly increased. In contrast, heating the sample above 270 K led to a drastic decrease in χT. This behavior of χT is caused by a stepwise change in the character of the exchange interaction in the {>N-(•)O-Cu(2+)-O(•)-N<} fragments. The lengthening of distances between the paramagnetic centers on cooling below 225 K led to a transition from antiferromagnetic to ferromagnetic exchange and, vice versa, the shortening of distances between the paramagnetic centers during the heating of the heterospin polymer above 270 K led to a transition from ferromagnetic exchange to antiferromagnetic exchange.