Heterospin complex showing spin transition at room temperature

Polyhedron 100 (2015) 132–138

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Heterospin complex showing spin transition at room temperature S.E. Tolstikov a, N.A. Artiukhova a, G.V. Romanenko a, A.S. Bogomyakov a, E.M. Zueva b, I.Yu. Barskaya a, M.V. Fedin a, K.Yu. Maryunina c, E.V. Tretyakov a, R.Z. Sagdeev d, V.I. Ovcharenko a,⇑ a

International Tomography Center, SB RAS, Institutskaya Str. 3A, 630090 Novosibirsk, Russian Federation Kazan National Research Technological University, 68 K. Marx Str., 420015 Kazan, Russian Federation c Department of Chemistry, Graduate School of Science, Hiroshima University, 739-8526 Higashi Hiroshima, Japan d Kazan Federal University, 18 Kremlyovskaya St., 420008 Kazan, Russian Federation b

a r t i c l e

i n f o

Article history: Received 1 June 2015 Accepted 10 July 2015 Available online 20 July 2015 Keywords: Spin crossover Cu(II) complexes Nitroxides Phase transitions Magnetostructural correlations

a b s t r a c t New nitronyl nitroxides LMe and LMe-CP containing a 4-methylpyridin-3-yl substituent were synthesized. It was found that the interaction of Cu(hfac)2 with LMe and LMe-CP gave binuclear [Cu(hfac)2LMe]2 and [Cu(hfac)2LMe-CP]2Solv (Solv = n-C6H14, n-C10H22, n-C16H34) and chain polymer {[[Cu(hfac)2]2LMe 2 ] [Cu(hfac)2]}1 heterospin complexes. An important structural peculiarity of LMe and LMe-CP is a large dihedral angle between the planes of the O–NC@N ? O paramagnetic fragment and the pyridine ring: 55.2 and 56.1°, respectively. The presence of a methyl group in the pyridine ring of the nitroxide molecule in {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1 proved favorable for spin transition at nearly room temperature. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Breathing crystals based on heterospin complexes of Cu(II) with nitroxides have attracted the attention of researchers as objects in which spin transitions are possible [1–4]. These crystals generally have high mechanical stability and do not decompose during phase transitions. This allows one to perform single crystal to single crystal transformations, i.e., to study the structure of polymorphs using the same single crystal and to compare the structural features of the high- and low-temperature phases with the magnetic characteristics of the compound. The possibility of varying the structure of the starting organic paramagnetic ligand many times is a valuable peculiarity of compounds from this class. This is an effective tool for changing the characteristics of the observed spin transitions, which proved highly sensitive to the molecular structure of nitroxide [5–8]. This necessitated studies to reveal the relationship between the structural features of the starting nitroxide and the magnetic properties of the heterospin complex. The goal of this study was to investigate the magnetostructural correlations for Cu(hfac)2 complexes with nonplanar pyridyl-substituted nitronyl nitroxides, in which the dihedral angle (a) between the O–NC@N ? O fragment of the 2-imidazoline ring and the pyridine ring bonded with it is reasonably large [9]. To obtain nitroxides with large a, we introduced a methyl group ⇑ Corresponding author. E-mail address: [email protected] (V.I. Ovcharenko). http://dx.doi.org/10.1016/j.poly.2015.07.029 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

in the 4-position of the pyridine ring of known nitroxides L [10] and LCP [11] and obtained 2-(4-methylpyridin-3-yl)-4,4,5,5-tetra methyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LMe) and 2-(4-methylpyridin-3-yl)-4,5-bis(spirocyclopentane)-4,5-dihydro1H-imidazole-3-oxide-1-oxyl (LMe-CP), respectively.

It was shown that the crystals L are formed by two independent nitroxide molecules that differ in the a angle between the pyridine ring and the O–NC@N ? O fragment of the 2-imidazoline ring: 52.99 and 35.89° (Table 1). Despite the large difference between these values, it is important that the dihedral angle in molecules

S.E. Tolstikov et al. / Polyhedron 100 (2015) 132–138 Table 1 Dihedral angles (a) in nitronyl nitroxides according to the experimental XRD data. Nitronyl nitroxide

a (°) *

LCP

L 10

53.0; 35.9

LMe 11

32.7

LMe-CP *

55.2

56.1*

The results of the present study.

L can be rather small (35.89°). In LCP crystals, all the molecules are identical, and the a angle is 32.73°. As was anticipated, in solid LMe and LMe-CP the angle proved appreciably larger (Table 1). Rey et al. synthesized and studied the tetranuclear complex of Cu(hfac)2 with nitronyl nitroxide L, which exhibited a spin transition at 110 K [12]. The synthesized complexes with nitronyl nitroxide LCP included the binuclear [Cu(hfac)2LCP]2, tetranuclear [[Cu(hfac)2]4(LCP)2], and chain polymer {[[Cu(hfac)2]2LCP 2 ] [Cu(hfac)2]}1 complexes [11]. For the former two compounds, the temperature variation induced spin transitions at 125 and 100 K, respectively. The leff value of the chain polymer complex changed but slightly when the complex was heated from 5 to 350 K. The possibility of a spin crossover in this compound was not clarified; for this purpose, the compound had to be heated to higher temperatures, but decomposed on heating.

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Ka radiation). The structures were solved by direct methods and refined by the full-matrix least-squares procedure anisotropically for non-hydrogen atoms. The H atoms were calculated geometrically and included in the refinement as riding groups. All calculations were fulfilled with the SHELXTL 6.14 program package. The crystallographic data and details of experiments are presented in Tables S1-S4 in Supplementary Information. 2.5. Quantum-chemical calculations Intra/intermolecular (in molecular complexes) and intrachain (in chain polymeric complex) isotropic exchange parameters were computed using the broken symmetry methodology. The scheme proposed by Yamaguchi and co-workers [16] was employed. All calculations were performed using the crystallographically determined geometries within the framework of the UB3LYP/TZVP computational procedure as implemented in the Gaussian09 package. The intermolecular exchange parameter was estimated using the [[Cu(hfac)2]2L2]. . .[[Cu(hfac)2]2L2] structures, whereas the intrachain exchange parameters were estimated using the {L[Cu(hfac)2][[Cu(hfac)2]2L2][Cu(hfac)2]L} (JCu–O–N, JCu–N. . .N–O, JCu. . .Cu, JN–O. . .O–N) and {[[Cu(hfac)2]2L2][Cu(hfac)2][[Cu(hfac)2]2L2]} (J’Cu–O–N, J’N–O. . .O–N) chain fragments.

2. Experimental sections 2.6. Synthesis 2.1. Materials The reactions were monitored by TLC using Merck «Silica Gel 60 F254, aluminium sheets» and Macherey–Nagel «0.2 mm alumina N/UV254; plastic sheets». Chromatography was carried out with the use of Merck 0.063–0.100 mm silica gel for column chromatography and Al2O3 of chromatographic grade purchased from the Donetsk Plant of Chemical Reagents. The reagents used without additional purification included 3-bromo-4-methylpyridine (96%, Aldrich), n-butyllithium (2.5 M in n-hexane), 2-methyl2-nitrosopropane, and MnO2 (85%). Cu(hfac)2 [13], 1,10 -bis(hydrox yamino)-bicyclopentane sulfate monohydrate [11], 2,3-bis(hydrox yamino)-2,3-dimethylbutane sulfate monohydrate [14], and 4-methylnicotine aldehyde [15] were synthesized by known procedures. 2.2. Characterization The melting points were determined on a «Stuart» microheating table. C, H, and N elemental analyses were carried out on an EA-3000 CHN analyzer at the Chemical Analytical Center of the Novosibirsk Institute of Organic Chemistry. Infrared spectra (4000–400 cm1) were recorded with a Bruker VECTOR 22 instrument in KBr pellets. 2.3. Magnetic measurements The magnetic susceptibility of the polycrystalline samples was measured with an MPMSXL (Quantum Design) SQUID magnetometer at temperatures of 2–360 K in a magnetic field of 5 kOe. The paramagnetic components of magnetic susceptibility v were determined with allowance for the diamagnetic contribution evaluated from Pascal’s constants. 2.4. X-ray crystallography The intensity data for the single crystals of the compounds were collected on a SMART APEX II (Bruker AXS) automated diffractometer with a Helix (Oxford Cryosystems) open flow helium cooler or APEX DUO (Bruker AXS) using the standard procedure (Mo or Cu

2.6.1. 2-(4-Methylpyridin-3-yl)-4,5-bis(spirocyclopentane)-4,5dihydro-1H-imidazole-3-oxide-1-oxyl (LMe-CP) 4-Methylnicotine aldehyde (0.060 g, 0.5 mmol) was added to a solution of 1,1’-bis(hydroxyamino)-bicyclopentane sulfate monohydrate (0.150 g, 0.5 mmol) in water (3 mL) at room temperature. The reaction mixture was stirred for 1 h and then neutralized with NaHCO3. The resulting precipitate was filtered off, washed with water, and dried in a vacuum desiccator. This gave 2-(4-methylp yridin-3-yl)-4,5-bis(spirocyclopentane)-4,5-dihydro-1H-imidazole (0.055 g) as white powder, which was subsequently used without additional purification. MnO2 (0.270 g) was added, while cooling on a water bath, to a suspension of the adduct in MeOH (3 mL), and the mixture was stirred for 1 h. The solution was filtered, and the residue washed with MeOH. The combined filtrates were evaporated. The residue was dissolved in EtOAc, and the solution filtered through a SiO2 layer (2  5 cm). The eluate was evaporated on a rotary evaporator. The product was recrystallized form the Et2O/n-hexane mixture and kept at 4 °C. Yield 0.038 g (25%), dark blue prismatic crystals. Mp 82–85 °C (decomp). IR spectrum, m/cm1: 3448, 2964, 2948, 2871, 1590, 1565, 1462, 1440, 1422, 1409, 1384, 1349, 1333, 1314, 1177, 1141, 1118, 1027, 952, 920, 862, 823, 806, 759, 702, 636, 610, 566, 540, 1177, 1141, 1118, 1027, 952, 920, 862, 823, 806, 759, 702, 636, 610, 566, 540. Found (%): C, 67.7; H, 7.1; N, 13.9. C17H22N3O2. Calculated (%): C, 68.0; H, 7.4; N, 14.0. 2.6.2. 3-(1,3-Dihydroxy-4,4,5,5-tetramethyl-imidazolidin-2-yl)-4methylpyridine 2,3-Bis(hydroxyamino)-2,3-dimethylbutane (0.7 g, 5.0 mmol) was added to a solution of 4-methylnicotine aldehyde (0.6 g, 5.0 mmol) in MeOH (15 mL) at room temperature. The resulting mixture was stirred for 48 h and allowed to stay for 12 h at 5 °C. The white precipitate was filtered off, washed with MeOH, and recrystallized from hot MeOH. Yield 0.83 g (66%). Mp 207–208 °C. IR spectrum, m/cm1: 517, 620, 790, 810, 832, 881, 917, 939, 1006, 1031, 1075, 1115, 1164, 1207, 1233, 1300, 1362, 1374, 1418, 1462, 1605, 1618, 1637, 2868, 2976, 3234, 3415, 3551. Found (%): C, 61.9; H, 8.2; N, 16.4. C13H21N3O2. Calculated (%): C, 62.1; H, 8.4; N, 16.7.

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2.6.3. 2-(4-Methylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1Himidazole-3-oxide-1-oxyl (LMe) MnO2 (4 g) was added to a suspension of 3-(1,3-dihydrox y-4,4,5,5-tetramethyl-imidazolidin-2-yl)-4-methylpyridine (0.8 g) in MeOH (15 mL). The mixture was stirred for 6 h. The resulting blue violet solution was filtered, the filtrate was evaporated, and the residue purified by column chromatography (Al2O3, EtOAc) followed by recrystallization from a CH2Cl2/n-heptane mixture. Yield 0.79 g (100%). Rf = 0.5 (EtOAc, 0.2 mm alumina N/UV254; plastic sheets). Mp 129–130 °C. IR spectrum, m/cm1: 533, 617, 679, 732, 763, 798, 833, 860, 1039, 1149, 1164, 1181, 1224, 1299, 1367, 1396, 1454, 1563, 1595, 1618, 1637, 2939, 2983, 3415, 3551 Found (%): C, 62.3; H, 7.1; N, 16.6. C13H18N3O2. Calculated (%): C, 62.8; H, 7.3; N, 16.9. 2.6.4. [Cu(hfac)2LMe]2 A solution of LMe (0.035 g, 0.14 mmol) in CH2Cl2 (3 mL) was added to a solution of Cu(hfac)2 (0.067 g, 0.14 mmol) in n-heptane (4 mL) cooled to 35 °C. The mixture was allowed to stay in an open flask at 4 °C. The resulting bulky red prisms were filtered off and dried in air. Yield 0.085 g (84%). Found (%): C, 37.6; H, 2.9; N, 6.2; F, 31.8. C46H40Cu2F24N6O12. Calculated (%): C, 38.0; H, 2.8; N, 5.8; F, 31.4. 2.6.5. [Cu(hfac)2LMe-CP]2n-C6H14 n-Hexane (5 mL) was added to a solution of Cu(hfac)2 (0.032 g, 0.067 mmol) and LMe-CP (0.020 g, 0.067 mmol) in CH2Cl2 (1 mL). The mixture was allowed to stay in an open conical flask at 4 °C for 1 day. The resulting dark brown crystals in the form of elongated prisms were filtered off, washed with cold n-hexane, and dried in air. Yield 0.030 g (54%). Found (%): C, 43.0; H, 3.4; N, 5.2. C60H62Cu2F24N6O12 Calculated (%): C, 43.9; H, 3.8; N, 5.1. 2.6.6. [Cu(hfac)2LMe-CP]2n-C10H22 and [Cu(hfac)2LMe-CP]2n-C16H34 Complexes were synthesized by a procedure similar to the one used for [Cu(hfac)2LMe-CP]2n-C6H14 (decane and hexane, respectively, were used as a solvent), but in the case of [Cu(hfac)2LMe-CP]2n-C16H34 the mixture of Cu(hfac)2 and LMe-CP was allowed to stay in an open conical flask at room temperature for 1 day. [Cu(hfac)2LMe-CP]2n-C10H22, brownish red needle crystals. Yield 75%. Found (%): C, 48.0; H, 4.0; N, 7.9; F, 21.8. C64H69Cu2F24N6O12. Calculated (%): C, 48.0; H, 4.0; N, 8.0; F, 21.7. [Cu(hfac)2LMe-CP]2n-C16H34, brownish red needle crystals. Yield 82%. In the course of the element analysis, the sample lost some part of solvent molecules. Found (%): C, 42.9; H, 3.6; N, 5.6. C54H48Cu2F24N6O120.25C16H34. Calculated (%): C, 43.2; H, 3.5; N, 5.2. 2.6.7. {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1 A solution of LMe (0.050 g, 0.2 mmol) in CH2Cl2 (2 mL) was added to a solution of Cu(hfac)2 (0.192 g, 0.4 mmol) in n-heptane (10 mL) cooled to 35 °C. The mixture was allowed to stay in an open flask at 25 °C. The resulting red needle crystals were filtered off and dried in air. Yield 0.155 g (40%). Found (%): C, 35.3; H, 2.2; N, 4.6; F, 35.9. C56H42Cu3F36N6O16. Calculated (%): C, 34.9; H, 2.2; N, 4.4; F, 35.5. 3. Results and discussion LMe and LMe-CP were synthesized by condensation of 4-methylnicotine aldehyde with 2,3-bis(hydroxyamino)-2,3-dime thylbutane or 1,10 -bis(hydroxyamino)-bicyclopentane and subsequent oxidation of the resulting adduct with MnO2.

The spin-labeled LMe and LMe-CP were isolated as single crystals suitable for an XRD analysis (Fig. 1a and b). Investigation of these complexes showed that the N–O distances are typical for nitronyl nitroxides [9] (1.272(2)–1.285(1) Å). The dihedral angles between the planes of the nitronyl nitroxide CN2O2 fragment and the pyridine ring in LMe and LMe-CP are close (Table 1). The short contacts between the O atoms of the NO groups of adjacent LMe-CP molecules exceed 5 Å, leading to a nearly constant vT value of 0.37 K cm3/mol in the range 30–300 K (Fig. 1c). This value is in good agreement with the theoretical pure spin value 0.375 K cm3/mol for noninteracting paramagnetic centers with spin S = 1/2 and g = 2. The packing of LMe is formed by pairs of spin-labeled molecules. The O. . .O intermolecular distances inside the pairs are 3.646(2) Å, which is appreciably shorter than the corresponding distances between pairs (over 4.5 Å). According to XRD data, the dependence vT(T) for LMe is well approximated by the dimer model with J = 14.6 ± 0.2 cm1 and g = 1.99 ± 0.01. The [Cu(hfac)2LMe]2 dimer complex was obtained by the interaction of Cu(hfac)2 with LMe in a ratio of 1:1. In its molecule (Fig. 2a), the vertices of the square bipyramid are occupied by the nitroxide oxygen atom (dCu–ONO = 2.465(4) Å) and one of the Ohfac atoms (dCu–Ohfac = 2.249(3) Å). The remaining Ohfac atoms (dCu–Ohfac are 1.938(7)–1.990(8) Å) and the N atom of the pyridine ring of LMe (dCu–N = 2.042(9) Å) lie in the equatorial plane. When the temperature was varied, no significant structural rearrangements were recorded. When [Cu(hfac)2LMe]2 was cooled to 85 K, the Cu–ONO and Cu–N bond lengths decreased by no more than 0.05 Å. A consequence of this was the absence of anomalies on the curve of the vT(T) dependence (Fig. 2b). At 343 K, vT was 1.48 K cm3/mol and increased to 2.84 K cm3/mol when the temperature decreased to 5 K, after which it decreased. The high-temperature value of vT agrees well with the theoretical pure spin value 1.50 K cm3/mol for four noninteracting paramagnetic centers with spins S = 1/2 at a g-factor of 2. The increase in the vT value at lowered temperature suggests the ferromagnetic exchange interactions between the spins of the paramagnetic centers. The vT value at 5 K is higher than the theoretical value 2 K cm3/mol for two centers with S = 1 and agrees well with the theoretical value 3.00 K cm3/mol for the center with S = 2, which points to the presence of effective channels of ferromagnetic exchange. The experimental dependence vT(T) is well described by the four spin exchange cluster model (for the Hamiltonian: H = 2J(S1S2 + S3S4)  2J1(S2S3 + S1S4), where S1 = S3 = 1/2 are the spins of Cu(II) ions and S2 = S4 = 1/2 are the spins of nitroxides) with the following optimum values of parameters: g = 1.96 ± 0.01, J = 12 ± 1 cm1, and J1 = 6.7 ± 0.4 cm1. The antiferromagnetic intermolecular exchange interactions are much weaker, |zJ0 | < 0.5 cm1. The obtained exchange parameters agree well with the results of quantum-chemical calculations, which showed that both direct and indirect interactions between the spins of the Cu2+ ion and nitronyl nitroxide via the pyridine ring are comparable and ferromagnetic in character. Obviously, the calculated direct exchange energy (JCu–O–N = 14 cm1) corresponds to the best fit parameter J, and the indirect exchange energy (JCu–N. . .N–O = 11 cm1) corresponds to the best fit parameter J1. The reaction of Cu(hfac)2 with LMe at a reagent ratio of 2:1 formed a heterospin complex {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1,

S.E. Tolstikov et al. / Polyhedron 100 (2015) 132–138

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Fig. 1. Molecular structure of LMe-CP (a) and LMe (b); the thermal ellipsoids are given at 50% probability. Experimental vT(T) dependences (c) for LMe (N) and LMe-CP (d); solid lines: calculation.

Fig. 2. Molecular structure of [Cu(hfac)2LMe]2 (a). The atomic displacement parameters are given at 50% probability; the H atoms and CF3 groups are omitted. Experimental (N) and theoretical (solid line) vT(T) dependences (b) for [Cu(hfac)2LMe]2.

whose solid phase has centrosymmetric dimer fragments {[[Cu(hfac)2]2LMe 2 ]} bonded by bischelates [Cu(hfac)2] into polymer chains (Fig. 3). Inside the chain, the distances between the exocyclic Cu(2) atoms and the O(10R) atoms of the coordinated NO groups are much longer than the similar Cu(1)O(1R) distances for the endocyclic Cu atoms. At lowered temperatures, the Cu(1)–O(1R) and Cu(1)–O(1) distances are markedly shortened, while the Cu(1)–O(2) and Cu(1)–O(4) distances are elongated (Table 2). The dependence vT(T) for {[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1 is presented in Fig. 4. The vT value is 1.05 K cm3/mol at 343 K and decreases at lower temperatures to a plateau of 0.45 K cm3/mol below 170 K. This behavior of vT(T) points to a strong antiferromagnetic exchange coupling, which is characteristic of the equatorial coordination of > N–O groups in the Cu(II) complexes [17–20]. The vT value in the range 170–2 K corresponds to the contribution of one isolated Cu2+ ion. Thus, at low temperatures, the spins of the endocyclic Cu2+ ions and nitroxides compensate each other, and the contribution to the paramagnetic character of the complex is made only by the spins of the exocyclic Cu2+ ions. The character of the vT(T) dependence agrees with the XRD data and the results of quantum-chemical calculations (Table 3). For {[[Cu(hfac)2]2LMe 2 ] [Cu(hfac)2]}1, the ground state is a doublet because of the strong antiferromagnetic coupling JCu(1)–O–N in the cyclic dimer fragment, which is strengthened when the Cu(1)O(1R) distances are shortened at lower temperatures (Table 3). The ferromagnetic exchange

interactions J’Cu(2)–O–N between the spins of nitroxides and exocyclic Cu2+ ions increase at lower temperatures, but are much weaker than JCu(1)–O–N and do not change the multiplicity of the ground state. The weaker ferromagnetic exchange interaction between the spins of the Cu2+ ion and nitronyl nitroxide via the pyridine ring JCu–N–O does not change at lowered temperatures and is close to the similar exchange in [Cu(hfac)2LMe]2. The Cu. . .Cu and nitroxide–nitroxide exchange energies inside the metallocycles for the endo- and exocyclic fragments are much smaller in magnitude (Table 3). Thus, when the cooling–heating cycles are repeated at temperatures in the range 5–360 K, a reversible spin transition takes place in {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1, which is accompanied by a change in the coloring from red to wine-red on passing from lowto high-temperature phase (SI, Fig. S-1). Since the complex should not be heated above 360 K to avoid its thermal destruction, the transition in Fig. 4 looks unfinished. However, there is no doubt that this is a spin transition because the significant increase in vT in the range 200–360 K cannot be described within the framework of the model with constant exchange parameter values (the solid line in Fig. 4 corresponds to the theoretical vT(T) dependence in the case of JCu(1)–O–N = 750 cm1 and J’Cu(2)–O–N = 25 cm1). In the reaction of Cu(hfac)2 with LMe-CP at a reagent ratio of 1:1, the solvates of binuclear complexes [Cu(hfac)2LMe-CP]2Solv were obtained (Solv = n-C6H14, n-C10H22, n-C16H34). These compounds have similar vT(T) dependences and related structures. Fig. 5

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Fig. 3. Chain structure in {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1. The atomic displacement parameters are given at 50% probability; the H atoms and CF3 groups are omitted.

Table 2 Selected bond lengths (Å) and angles (°) for {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1. T (K)

360

300

250

150

Cu(1)–O(1R) Cu(2)–O(10R) Cu(1)–N(14R0 ) Cu(1)–O(1) Cu(1)–O(2) Cu(1)–O(4) Cu(1)–O(3) Cu(2)–Ohfac

2.138(5) 2.423(6) 1.974(6) 2.048(10) 2.090(5) 2.178(4) 1.918(6) 1.903(5) 1.940(5) 51.14

2.053(2) 2.411(2) 2.000(2) 2.001(3) 2.194(2) 2.293(2) 1.953(2) 1.932(2) 1.939(2) 49.58

2.011(1) 2.393(1) 1.998(2) 1.985(2) 2.230(2) 2.341(1) 1.951(2) 1.931(2) 1.943(1) 48.84

1.990(1) 2.356(1) 1.994(2) 1.986(1) 2.245(1) 2.364(1) 1.953(1) 1.937(1) 1.951(1) 47.83

a (°)

Fig. 4. Experimental vT(T) dependence for {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1 (N). Solid line: calculated data (see text).

Fig. 5. Structure of the dimer fragment in [Cu(hfac)2LMe-CP]2n-C6H14. The atomic displacement parameters are given at 50% probability; the H atoms and CF3 groups are omitted.

Table 3 The exchange coupling parameters in {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1 according to quantum-chemical calculations. T (K)

150

300

360

JCu(1)–O–N, cm1 J’Cu(2)–O–N, cm1 JCu–N. . .N–O, cm1 JCu. . .Cu, cm1 JN–O. . .O–N, cm1 J0 N–O. . .O–N, cm1

951 41 9 4 1 5

751 23 9 3 1 2

365 14 8 3 3 1

shows the molecular structure of the binuclear heterospin hexane solvate as an example. The structure of [Cu(hfac)2LMe-CP]2Solv (Solv = n-C6H14, n-C10H22, n-C16H34) is formed by two crystallographically independent centrosymmetric molecules (Fig. 5), in which the Cu atoms lie in a distorted octahedral environment. In one of these molecules denoted as molecule A, the O atom of the NO group lies in the equatorial plane of the Cu octahedron with Cu-ONO distances equal to 2.040(3), 2.021(2), and 2.035(3) Å at T = 295 K for Solv = n-C6H14, n-C10H22, and n-C16H34, respectively (Table 4); the axial positions are occupied by the Ohfac atoms (dCu–O 2.230(4)–2.303(2) Å). In the second type of molecules (B), the environment of the Cu atom at room temperature is closer to a flattened octahedron with short axial Cu–Ohfac (1.962(2)–1.973(3) Å) and Cu–N (2.006(3)–2.015(3) Å) distances and equatorial Cu–Ohfac (2.086(3)–2.200(3) Å) and Cu– ONO (2.153(3)–2.188(2) Å) distances. When the crystals of these compounds are cooled, all the distances in the coordination unit of molecule A slightly shorten (Fig. 6, Table 4), while in molecule B, the ONO atom comes closer to the Ohfac atom that lies on the same axis. The Cu–O distances shortened from 2.174(3) to 1.983(2) Å and from 2.186(4) to 2.019(2) Å in [Cu(hfac)2LMe-CP]2n-C6H14 (Fig. 6). The same picture is observed for [Cu(hfac)2LMe-CP]2n-C10H22 and [Cu(hfac)2LMe-CP]2 n-C16H34 (Table 4). Thus, at 100 K, the coordination units in molecules A and B become almost identical with respect to the distances to the ONO, N, and Ohfac atoms of the coordination environment of Cu. The experimental vT(T) dependences for [Cu(hfac)2LMe-CP]2Solv (Solv = n-C6H14, n-C10H22, n-C16H34) are presented in Fig. 7. For all these compounds, vT is 0.72–0.78 K cm3/mol at room temperature, which is close to the theoretical pure spin value 0.75 K cm3/mol for two paramagnetic centers with spin S = 1/2 and g = 2. When the complexes were cooled, vT gradually decreased to 0.01– 0.08 K cm3/mol, indicating that the spins completely vanished from the system. The magnetochemical data are in good agreement with the XRD data and the results of quantum-chemical calculations. In molecules A with short Cu–ONO distances (1.98 Å) both

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S.E. Tolstikov et al. / Polyhedron 100 (2015) 132–138 Table 4 Selected bond lengths (Å) and angles (°) in molecules A and B of [Cu(hfac)2LMe-CP]2Solv (Solv = n-C6H14, n-C10H22, n-C16H34). A T (K)

B Cu–OR

Cu–NR

Cu–Ohfac

[Cu(hfac)2LMe-CP]2n-C6H14 295 2.040(3)

1.991(3)

240

2.004(1)

1.998(1)

190

1.987(1)

1.996(2)

150

1.981(1)

1.995(2)

100

1.980(2)

1.991(2)

1.966(3), 2.230(4), 1.957(1), 2.267(2), 1.957(2), 2.276(2), 1.956(2), 2.278(2), 1.958(2), 2.278(2),

2.043(4) 2.273(3) 2.008(2) 2.312(2) 2.005(2) 2.329(2) 2.004(2) 2.331(2) 2.002(2) 2.331(2)

1.949(2), 2.259(2), 1.955(2), 2.289(2), 1.955(2), 2.286(2),

2.022(2) 2.303(2) 2.005(2) 2.331(2) 2.001(2) 2.333(2)

1.960(3), 2.251(4), 1.961(3), 2.265(4), 1.961(6), 2.275(6),

2.016(4) 2.271(4) 2.014(4) 2.303(4) 2.002(6) 2.327(6)

[Cu(hfac)2LMe-CP]2n-C10H22 295 2.021(2)

1.990(2)

210

1.987(1)

1.996(2)

100

1.978(2)

1.988(2)

[Cu(hfac)2LMe-CP]2n-C16H34 296 2.035(3)

1.998(3)

240

2.006(3)

1.994(4)

85

1.975(5)

1.983(7)

a

Cu–OR

Cu–NR

Cu–Ohfac

a

48.6

2.174(3)

2.015(3)

48.4

2.113(1)

2.009(2)

48.4

2.047(2)

2.006(2)

48.4

2.005(2)

2.010(2)

48.5

1.983(2)

2.009(2)

1.965(3), 2.091(3), 1.965(2), 2.144(2), 1.964(2), 2.253(2), 1.962(2), 2.298(2), 1.963(2), 2.320(2),

2.186(4) 2.121(3) 2.173(2) 2.120(3) 2.050(2) 2.207(2) 2.025(2) 2.238(2) 2.019(2) 2.247(2)

48.8

2.188(2)

2.006(3)

48.2

2.092(2)

2.013(2)

48.2

1.986(2)

2.006(2)

1.962(2), 2.109(2), 1.967(2), 2.198(2), 1.964(2), 2.319(2),

2.086(3) 2.200(3) 2.158(2) 2.094(3) 2.249(2) 2.019(2)

48.5

2.153(3)

2.006(3)

48.3

2.069(3

2.008(4)

48.3

1.982(5)

2.010(7)

1.973(3), 2.143(3), 1.970(4), 2.191(4), 1.964(6), 2.270(6),

2.115(4) 2.171(5) 2.074(4) 2.215(4) 2.018(5) 2.336(5)

53.6 53.5 53.0 52.6 52.6

54.2 53.4 52.5

54.0 53.1 51.6

Fig. 6. Bond length dynamics in the coordination units of molecules A and B in [Cu(hfac)2LMe-CP]2n-C6H14.

at room temperature and at 100 K, the strong antiferromagnetic coupling JCu–O–N (Table 5) is dominant. Due to this, the spins of the Cu(II) ions and coordinated nitroxides compensate each other,

Fig. 7. Experimental vT(T) dependences for [Cu(hfac)2LMe-CP]2n-C6H14 (j), [Cu(hfac)2LMe-CP]2n-C10H22 (s), and [Cu(hfac)2LMe-CP]2n-C16H34 (4).

the singlet being the ground state. The paramagnetism of [Cu(hfac)2LMe-CP]2Solv at room temperature is determined only by the spins of molecules of type B, in which the ONO atom lies in the equatorial position with Cu–ONO distances 2.153(3)– 2.188(2) Å, and the antiferromagnetic exchange coupling JCu–O–N is much weaker than in the molecules of type A (Table 5). When the temperature is lowered to 100 K, the Cu-ONO distances shorten, which leads to a strengthening of the antiferromagnetic coupling JCu-O–N and, as a consequence, to a drastic decrease in vT. The length of the hydrocarbon chain of the solvent molecule in [Cu(hfac)2LMe-CP]2Solv does not affect the structure of the dimers nor the spin transitions. This is a significant distinction of [Cu(hfac)2LMe-CP]2Solv that differentiates it from the solvates of chain polymer Cu(hfac)2 heterospin complexes with spin-labeled pyrazoles studied earlier, whose magnetic properties are highly sensitive not only to the nature and geometry of the solvent molecule, but also to its orientation in the interchain space [8,21]. The spin transitions in [Cu(hfac)2LMe-CP]2Solv occurring in the range 200–250 K are accompanied by a change in the color of the sample from orange to brown on heating (SI, Fig. S-2) and vice versa on cooling.

138

S.E. Tolstikov et al. / Polyhedron 100 (2015) 132–138

Table 5 The exchange coupling parameters (cm1) for [Cu(hfac)2LMe-CP]2Solv according to the quantum-chemical calculations. T (K)

100

Molecule

A

[Cu(hfac)2LMe-CP]2n-C6H14 JCu–O–N 898 JCu–N. . .N–O 8 JCu. . .Cu 5 JNO. . .O–N 1 JN–O. . .O–N(inter) T (K)

T (K)

295 B

A

B

Appendix A. Supplementary data

858 7 5 1

709 10 4 1

143 11 1 0

CCDC 1401279-1401297 contains the supplementary crystallographic data for LMe, LMe-CP, [Cu(hfac)2LMe]2, {[[Cu(hfac)2]2LMe 2 ] [Cu(hfac)2]}1 and [Cu(hfac)2LMe-CP]2Solv. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.poly.2015.07.029.

32

19

100

Molecule A [Cu(hfac)2LMe-CP]2n-C10H22 JCu–O–N 903 JCu–N. . .N–O 8 JCu. . .Cu 5 JNO. . .O–N 1 JN–O. . .O–N(inter)

295 B

A

B

863 8 5 1

784 9 3 2

92 11 11 11

33

19

85

Molecule A [Cu(hfac)2LMe-CP]2n-C16H34 JCu–O–N 902 JCu–N. . .N–O 8 JCu. . .Cu 5 JNO. . .O–N 1 JN–O. . .O–N(inter)

RFBR (14-03-00517 and 15-53-10009 Grants), A.S.B. and M.V.F. thank the RF President’s Grants (MK–247.2014.3 and MD–276.2014.3).

296

32

B

A

B

865 6 5 1

768 7 4 1

226 10 1 1 20

4. Conclusions We synthesized sterically nonrigid heterospin Cu(hfac)2 complexes with nitronyl nitroxide containing a 4-methylpyridin-3-yl substituent and possessing a large dihedral angle between the planes of the O–NC@N ? O paramagnetic fragment and the pyridine ring. The presence of a methyl group in the pyridine ring of the nitroxide molecule in {[[Cu(hfac)2]2LMe 2 ][Cu(hfac)2]}1 favored a spin transition at approximately room temperature. It was found that [Cu(hfac)2LMe-CP]2Solv show reversible spin transitions in the range 200–250 K accompanied by a change in the color of the sample. An important peculiarity of these compounds was that the hydrocarbons included in the solid phase do not affect the magnetic behavior of the compound. Acknowledgments This study was financially supported by the Russian Science Foundation (Grant 15–13-30012). G.V.R. and N.A.A. thank the

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