engleski

Magnetic bistability in the salt of organic radical anion - tetrachlorosemiquinone

Tetrachlorosemiquinone anion radical salt of potassium was prepared [1] and crystallised as 2-butanone solvate. Its structural and magnetic characteristics were studied by variable-temperature single crystal X-ray diffraction, DFT (wB97XD) and CAS-MP2 calculations, and magnetic susceptibility measurements in the temperature range from 10-340 K. The X-ray structure analysis detected the two polymorphs at 100 K and 200 K which undergo reversible phase transition in the range 120-170 K. The 100 K-structure reveals pi-stacked dimers of anion radicals with interplanar separation distance of 3.272 (2) A and ring centroid distance of 3.631(3) A with offset of 1.573 A. The infinite stack of dimers repeated by translation reveals interdimer separation distance of 4.353(3) A and offset of 2.504 A. Diamagnetic response of this crystalline phase involves the antiparallel electron-spin coupling between the two contiguous anion radicals. These dimers continuously uncouple their electron-spins with increasing temperature, which leads to an increase of paramagnetic contribution to magnetisation. The continuous phase transition can be monitored by measuring the unit cell parameters in the range 120-170 K, with a step of 5 K ; the volume of monoclinic unit cell (P21/c) of 100 K-polymorph was reduces two times and unit cell reveals pronounced n-glide plane symmetry (P21/n) in the 200 K polymorph. The paramagnetic 200 K-structure is characterised by interplanar separation distance of radical anions of 3.467 A and offset of 2.090 A. According to the literature [2] solvates and solvent-free crystals reveal different magnetic behaviour, so crystal engineering approach was introduced ; to design crystals of desired magnetic properties one can vary the size and polarisability of the cation, and polarity and size of solvent molecule [3]. [1] H.A. Torrey, W.H. Hunter, J. Am. Chem. Soc., 1912, 34, 702-716 [2] G. Zanotti, A. Del Pra, Acta Cryst. 1980, B36, 313-316 and references therein. [3] K. Molčanov, B. Kojić-Prodić, D, Babić, D. Žilić, B. Rakvin, CrystEngComm. 2011, 13, 5170-5178.

spin-coupled biradical anions; magnetic properties; tetrachlorosemiquinone anion radical

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