Thermally Induced Magnetic Anomalies in Solvates of the Bis(hexafluoroacetylacetonate)copper(II) Complex with Pyrazolyl-Substituted Nitronyl Nitroxide

Abstract

We succeeded in synthesizing of a whole family of isostructural solvates of the copper(II) hexafluoroacetylacetonate complex with pyrazolyl-substituted nitronyl nitroxide (L): Cu(hfac)2L x 0.Solv. The main feature inherent in nature of Cu(hfac)2L x 0.5 Solv single crystals is their incredible mechanical stability and ability to undergo reversible structural rearrangements with temperature variation, accompanied by anomalies on the mu(eff(T)) dependence. Structural investigation of the complexes over a wide temperature range before and after the structural transition and the ensuing magnetic phase transition showed that the spatial peculiarities of the solvent molecules incorporated into the solid govern the character of the mu(eff(T)) dependence and the temperature region of the magnetic anomaly. Thus, doping of crystals with definite solvent molecules could be used as an efficient method of control over the magnetic anomaly temperature (T(a)). The investigation of this special series of crystals has revealed the relationship between the chemical step and the magnetic properties. It was shown that "mild" modification of T(a) for Cu(hfac)2L x 0.5 Solv required a much smaller structural step than the typical change of one -CH2- fragment in a homologous series in organic chemistry. Quantum-chemical calculations with the use of X-ray diffraction data allowed us to trace the character of changes in the exchange interaction parameters in the range of the phase transition. In the temperature range of the phase transition, the exchange parameter changes substantially. The gradual decrease in the magnetic moment, observed in most experiments during sample cooling to T(a), is the result of the gradual increase in the fraction of the low-temperature phase in the high-temperature phase.

Breathing Crystals from Copper Nitroxyl Complexes

“…It was shown that a mild modification of T c for [Cu(hfac) 2 L n-Bu ]·0.5solv required a considerably smaller structural step than one methylene unit in the homologous series, typical in organic chemistry. 50 Figure 9.3 illustrates the important circumstance that the packing mode of polymer units is the same in solid solvates with hexane, heptane, octane, and octene, which is confirmed by the form of chains in both perpendicular and axial directions as shown in Figure 9 This can readily be seen, for example, if we compare the μ eff (T) dependences for the solvate with hexane ( Fig. 9.2d) and the solvate with octane ( Fig.…”

“…50 For [Cu(hfac) 2 L n-Bu ]·0.5C 5 H 11 I, the structural characteristics at lowered temperatures are quite different, the dominant tendency being the lengthening of the Cu-O L bond after the transition. Therefore, the abrupt change in μ eff and the character of μ eff (T) are different for this compound ( Fig.…”

“…The calculated J(Cu-nitroxide) values at seven different temperatures were used to reconstruct the experimental μ eff (T) dependence for [Cu(hfac) 2 L n-Bu ]·0.5C 8 H 18 . 50 The resulting μ eff (T) curve is shown in Figure 9.4 as gray lines connecting the calculated magnetic moments. Good agreement is observed between the experimental and calculated μ eff (T) curves.…”

Exchange coupling transformations in Cu (II) heterospin complexes of “breathing crystals” under structural phase transitions

“…Inaccuracy of this basis for these systems was previously demonstrated in Ref. 13 on some other compounds of "breathing crystal" family. Table II of effective magnetization versus temperature.…”

“…12 DFT calculations of the magnetic coupling constant in a three-spin cluster of crystal unit cell were previously published by Gorelik et al 13 and Zueva et al 14 In the former work PBE0 hybrid exchange-correlation functional with 6-31G(d) basis set were used, and the results have shown some limitations of the approach to describe the magnetic coupling in exchange clusters of the "breathing crystals" near the transition temperature. In Ref.…”