Article pubs.acs.org/IC
Cite This: Inorg. Chem. 2019, 58, 9187−9194
Pressure-Controlled Migration of Paramagnetic Centers in a Heterospin Crystal Victor Ovcharenko,*,† Galina Romanenko,† Alexey Polushkin,† Gleb Letyagin,†,△ Artem Bogomyakov,† Matvey Fedin,† Kseniya Maryunina,‡ Sadafumi Nishihara,‡,§ Katsuya Inoue,‡,§ Marina Petrova,† Vitaly Morozov,†,△ and Ekaterina Zueva∥,⊥,# †
International Tomography Center SB RAS, 3A Institutskaya Street, Novosibirsk 630090, Russia Department of Chemistry, Graduate School of Science and Chirality Research Center (CResCent), and §Institute for Advanced Materials Research, Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-1, Kagamiyama, Higashi Hiroshima, Hiroshima 739-8526, Japan ∥ Department of Inorganic Chemistry, Kazan National Research Technological University, 68 K. Marx Street, Kazan420015, Russia ⊥ A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov Street 8, Kazan 420008, Russian # Kazan Federal University, Kremlyovskaya Street 18, Kazan 420008, Russia △ Novosibirsk State University, Pirogova Street 1, Novosibirsk 630090, Russia
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‡
S Supporting Information *
ABSTRACT: A study of the single-crystal-to-single-crystal transformation induced by temperature variation for the chain polymer Cu(II) complex with nitronyl nitroxide showed that an increase in the hydrostatic pressure of up to ∼0.07 GPa completely changes the intracrystalline displacements of molecules relative to one another. This, in turn, significantly affects the interaction energy of the unpaired electrons of the paramagnetic centers and hence the form of the temperature dependence of the magnetic susceptibility χT. The cooling of crystals under normal conditions causes a rearrangement of the intrachain exchange clusters {>N−•O−Cu(II)−O•−NN−•O−Cu(II)−O•−N< }) spin heterospin exchange clusters occurs during the temperatureinduced structural phase transition, leading to a change in the energy of exchange interaction between the unpaired electrons of the paramagnetic centers. This results in anomalies on the temperature dependence of the χT similar to spin transitions and is often accompanied by a significant change in the color Received: March 21, 2019 Published: June 26, 2019 9187
DOI: 10.1021/acs.inorgchem.9b00815 Inorg. Chem. 2019, 58, 9187−9194
Article
Inorganic Chemistry
Figure 1. Chains in the structure of [Cu(hfac)2L]-I (a) and [Cu(hfac)2L]-II (b) and dependences χT(T) at P = 1 atm (red) and 0.07 GPa (purple) for [Cu(hfac)2L]-I61 (c) and [Cu(hfac)2L]-II (d). The H atoms and the geminal CH3 and CF3 groups are omitted.
of the compound.36 It should be noted that despite the significant structure rearrangements, the phase transitions can occur without decomposition of single crystals, which is especially important for correlating the structural changes with magnetic properties for this group of compounds.14,15,37−39 In contrast to the actively studied pressure effect on the structure and magnetic properties of iron, cobalt, and manganese complexes,1−6,8,17−26,45−56 exhibiting spin transitions and valence tautomerism, the pressure effect on the magnetic properties of 3d metal complexes with nitroxides was studied only for a few bulk magnetic-ordered 1D complexes of Mn57,58 and Cu(II) coordination compounds exhibiting effects similar to SCO.59−62 An increase in pressure completely suppressed the magnetic anomaly for the Cu(hfac)2 complex with dinitroxide,59 and provoked an increase in the transition temperature for Cu(II) complexes with pyrazolyl-substituted nitronyl nitroxides.61,62 The study of changes in the structure of the compound when the pressure changes was not carried out in any of the above cases.
formation under these conditions. As the compound can be obtained in the form of different polymorphs depending on the synthesis conditions,34 for further study we chose the modification [Cu(hfac)2L]-II with a head-to-head chain motif (Figure 1). For [Cu(hfac)2L]-II, the temperature dependence of χT is much smoother. Therefore, it was assumed that the modification had high mechanical elasticity, which favored a detailed study of structure transformations induced by a decrease in temperature at different pressures. Experiments revealed unique changes in the magnetic properties of [Cu(hfac)2L]-II at increased hydrostatic pressure. For [Cu(hfac)2L]-I, the spin transition temperature increased (Figure 1c) and the general character of χT(T) remained the same, whereas for [Cu(hfac)2L]-II, even a slight increase in pressure completely altered the form of χT(T) (Figure 1d). This response of magnetic properties to a small increase in external pressure had never been encountered for transition metal compounds with organic ligands exhibiting spin transition effects and predetermined interest in studies of the [Cu(hfac)2L]-II structure at increased external pressure. However, before considering the effect of pressure in more detail, it is instructive to address structural behavior of this compound at variable temperatures. The most important characteristics of the structure of [Cu(hfac)2L]-II under the normal conditions are the relatively short Cu−ONO distances (2.336(1) Å) in the {CuO6} units and the long Cu−N distances (2.557(2) Å) in the CuO4N2 units. When the temperature is lowered to 30 K, the Cu−ONO distances are shortened stepwise by 0.336 Å (Table S1). The Cu−O bonds on one of the Ohfac−Cu−Ohfac axes are
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RESULTS AND DISCUSSION We performed these experiments for crystals of the Cu(II) chain-polymer complex with 4,4,5,5-tetramethyl-2-(1-methyl1H-pyrazol-4-yl)-imidazoline-3-oxide-1-oxyl (L) with a headto-tail chain motif [Cu(hfac)2L]-I (Figure 1), for which the temperature induced structural transformation14,15,34,35 and the thermocompression dependence of magnetic properties61 had been studied before. However, the [Cu(hfac)2L]-I crystals were destroyed even at a slightly increased pressure (P ∼0.07 GPa), which hindered the study of the structure trans9188
DOI: 10.1021/acs.inorgchem.9b00815 Inorg. Chem. 2019, 58, 9187−9194
Article
Inorganic Chemistry lengthened by a comparable value (0.337 Å). Thus, the long axis of the Cu bipyramid in the CuO6 square bipyramid actually changes: the coordinated O atoms of the nitroxyl group pass to the equatorial position, displacing two Ohfac atoms to the axial positions. In the {CuO4N2} units, the Cu−N axial distances are shortened to a lesser extent (by 0.100 Å), while the N−Cu−N axis remains elongated. The O···O interchain contacts between the noncoordinated NO groups of the paramagnetic ligands decrease from 3.901 Å at 295 K to 3.571 Å at 30 K. These data completely correlate with the data of Q- and W-band CW EPR measurements, which unambiguously confirm that the Jahn−Teller axis in the {CuO6} units (>N−•O−Cu−O•−N N− •O−Cu(II)−O•−N< exchange clusters observed during the cooling of the crystal is the cause of the increase in the fraction of the low-temperature phase with strong antiferromagnetic exchange interactions in the {>N−•O−Cu(II)−O•−NN−•O−Cu(II)−O•−NN− •O−Cu(II)−O•−NN−•O−CuII−O•−NN-•O−Cu(II)−O•−N
