Relationship between the Thermally Induced Reorientations of Aromatic Solvate Molecules in Cu(hfac)₂–Nitroxide Breathing Crystals and the Character of the Magnetic Anomaly

Abstract

A new group of "breathing" crystals has been synthesized. These are aromatic solvates of the copper(II) hexafluoroacetylacetonate complex with spin-labeled pyrazole Cu(hfac)(2)L·0.5Solv, where L is 2-(1-butyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and Solv is benzene, toluene, ethylbenzene, propylbenzene, butylbenzene, styrene, o-xylene, m-xylene, p-xylene, 1,4-bis(trifluoromethyl)benzene, 1-methyl-4-ethylbenzene, 1-methyl-4-vinylbenzene, 1,4-diethylbenzene, 1,2,3-trimethylbenzene, or 1,2,4-trimethylbenzene. The main feature of Cu(hfac)(2)L·0.5Solv single crystals is their remarkable mechanical stability and ability to undergo thermally induced structural rearrangements accompanied by spin-crossover-like phenomena. The structures of Cu(hfac)(2)L·0.5Solv solvates are similar and based on mutually parallel {Cu(hfac)(2)L}(∞) heterospin chains with a "head-to-head" motif. The localization of voids with guest molecules being the same in all crystals, the temperature dependence of the effective magnetic moment (μ(eff)) for Cu(hfac)(2)L·0.5Solv is determined by the structure of the guest molecules, along which the polymer chains are "gliding" when the temperature changes. When the temperature decreased from 300 to 100-50 K, μ(eff) decreased, abruptly or gradually, from 2.7-2.4 to ~1.8 β for the majority of Cu(hfac)(2)L·0.5Solv except the solvates with benzene, toluene, and 1,4-bis(trifluoromethyl)benzene. When Cu(hfac)(2)L·0.5C(6)H(6) and Cu(hfac)(2)L·0.5CH(3)-C(6)H(5) were cooled to 50 K, μ(eff) decreased to ~2.1-2.2 β. When Cu(hfac)(2)L·0.5(1,4-(CF(3))(2)-C(6)H(4)) was cooled to 50 K, μ(eff) initially decreased from ~2.7 to 1.9 β and then abruptly increased to ~2.4 β. A single-crystal X-ray diffraction analysis of each solvate within a temperature range wider than the range of magnetic anomaly temperatures revealed a complex interrelated dynamics of the aromatic solvent guest molecules and heterospin chains. The dynamics largely depended on the orientation of the solvent guest molecules relative to the polymer chains. An analysis of the thermally induced phase transformations revealed a relationship between the structural rearrangement of Cu(hfac)(2)L·0.5Solv and the form of the magnetic anomaly on the μ(eff)(T) curve and between the structural rearrangement of the solvate and the temperature of the magnetic effect.

Breathing Crystals from Copper Nitroxyl Complexes

“…Our study of aromatic solvates showed that the magnetic characteristics of breathing crystals were sensitive not only to slight changes in the structure and composition of incorporated solvent molecules, but also to changes in their orientation relative to the paramagnetic fragments. 42 …”

“…This approach allowed us to summarize all data obtained for solvates with aromatic solvents (ethylbenzene, propylbenzene, butylbenzene, styrene, o-xylene, m-xylene, p-xylene, 1,4-bis(trifluoromethyl)benzene, [42]. Copyright 2011, American Chemical Society.…”

“…9.22, left column) show a different character of magnetic anomalies for each compound. 20,[42][43][44] Typically, at T < 50 K, all spin-triads are already in the strongly coupled state S = 1 / 2 (μ eff ≈ 1.8 β), and the magnetic behavior at lower temperatures is governed by inter-cluster exchange interactions. Two of the compounds, [Cu(hfac) 2 L n-Bu ]·0.5C 7 H 16 and [Cu(hfac) 2 L n-Bu ]·0.5C 8 H 18 ( Fig.…”

“…This unusual effect certainly deserved a special study, but the crystal structure of the compound was not studied below the transition point because the single crystals exploded below 30 K. 40 Later it was found, however, that all the magnetic effects described in, 36,[38][39][40] had the same nature and were associated with a structural phase transition, which was induced by the external treatment and changed the distances and angles between the paramagnetic centers in heterospin exchange clusters and hence the exchange-coupling energy. [19][20][21]38 Based on the accumulated data on the magnetochemical behavior of these heterospin complexes and their properties, it is clear that this is a widespread phenomenon and the spin-transition-like behavior is now recorded even in the case of insignificant structural rearrangements of heterospin solid phases during polymorphic transformations 41,42 .…”

Spatial distribution of phases during gradual magnetostructural transitions in copper(ii)–nitroxide based molecular magnets

“…8,9 Although the spin state of copper(II) cannot be changed, the total spin state of exchange-coupled copper(II)-nitroxide clusters can be changed. 8,9,[21][22][23][24][25][26][27][28][29][30][31][32][33][34] The temperature dependence of the effective magnetic moment (μ eff (T )) strongly varies from compound to compound: abrupt or gradual transitions in one to three steps of different amplitudes may be observed. They are often called "breathing crystals" due to the large and reversible changes in unit cell volume occurring during magnetostructural transitions.…”