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Spin Transition in the Cu(hfac)2 Complex with (4-Ethylpyridin-3-yl)-Substituted Nitronyl Nitroxide Caused by the “Asymmetric” Structural Rearrangement of Exchange Clusters in the Heterospin Molecule

1
International Tomography Center, SB RAS, Institutskaya Str., 3A, Novosibirsk 630090, Russia
2
Novosibirsk State University, Pirogova St., 1, Novosibirsk 630090, Russia
*
Author to whom correspondence should be addressed.
Crystals 2019, 9(6), 285; https://doi.org/10.3390/cryst9060285
Received: 29 April 2019 / Revised: 23 May 2019 / Accepted: 24 May 2019 / Published: 1 June 2019
(This article belongs to the Special Issue Synthesis and Applications of New Spin Crossover Compounds)
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Abstract

Methods for the synthesis of binuclear [Cu(hfac)2LEt]2 and tetranuclear [[Cu(hfac)2]4(LEt)2] heterospin compounds based on copper hexafluoroacetylacetonate [Cu(hfac)2] and 2-(4-ethylpyridin-3-yl)-4,5-bis(spirocyclopentyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt), were developed. The crystals of the complexes are elastic and do not crash during repeated cooling–heating cycles. It was found that a singlet–triplet conversion occurred in all of the {Cu(II)–O•–N<} exchange clusters in the molecules of the binuclear [Cu(hfac)2LEt]2 which led to spin coupling with cooling. The transition occurred in a wide temperature range with a maximum gradient ΔχT at ≈180 K. The structural transformation of the crystals takes place at T < 200 K and is accompanied by the lowering of symmetry from monoclinic to triclinic, twinning, and a considerable shortening of the Cu–ONO distance (2.19 and 1.97 Å at 295 and 50 K, respectively). For the tetranuclear [[Cu(hfac)2]4(LEt)2], two structural transitions were recorded (at ≈154 K and ≈118 K), which led to a considerable change in the spatial position of the Et substituent in the nitronyl nitroxyl fragment. The low-temperature process was accompanied by a spin transition recorded as a hysteresis loop on the χT(T) curve during the repeated cooling–heating cycles (T½↑ = 122 K, T½↓ = 115 K). This transition is unusual because it causes spin coupling in half of all of the {>N–•O–Cu2+} terminal exchange clusters, leading to spin compensation for only two paramagnetic centers of the six centers in the molecule. View Full-Text
Keywords: spin crossover; Cu(II) complexes; nitroxides; phase transitions; magnetostructural correlations spin crossover; Cu(II) complexes; nitroxides; phase transitions; magnetostructural correlations
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Artiukhova, N.; Romanenko, G.; Letyagin, G.; Bogomyakov, A.; Veber, S.; Minakova, O.; Petrova, M.; Morozov, V.; Ovcharenko, V. Spin Transition in the Cu(hfac)2 Complex with (4-Ethylpyridin-3-yl)-Substituted Nitronyl Nitroxide Caused by the “Asymmetric” Structural Rearrangement of Exchange Clusters in the Heterospin Molecule. Crystals 2019, 9, 285.

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