Ferromagnetic Coupling in the Heterospin Bis-Catecholato

Feb 17, 2017 - Variable-temperature magnetic susceptibility measurements detected intramolecular ferromagnetic coupling between the Mn(IV) S = 3/2 spi...
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Ferromagnetic Coupling in the Heterospin Bis-Catecholato− Manganese(IV) Complex with Pyridine Substituted by Nitronylnitroxide Michael P. Bubnov,*,† Irina A. Teplova,† Eugenia A. Kopylova,† Konstantin A. Kozhanov,† Artem S. Bogomyakov,‡ Marina V. Petrova,‡ Vitaly A. Morozov,‡ Victor I. Ovcharenko,‡ and Vladimir K. Cherkasov† †

G. A. Razuvaev Institute of organometallic chemistry of RAS, 49 Tropinina str., 603950 Nizhniy, Novgorod, Russia International Tomography Center of Siberian Branch of RAS, 3a Institutskaya str., Novosibirsk 630090, Russia

‡

S Supporting Information *

ABSTRACT: A new bis(3,6-di-tert-butyl-catecholato)manganese complex with two 4-NIT-Py ligands was synthesized and characterized [4-NIT-Py = pyridine substituted at position 4 with nitronyl-nitroxide radical, 2-(pyridin-4-yl)-4,4,5,5tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl]. X-ray diffraction indicated an octahedral environment of the manganese atom with a trans arrangement of ligands. Bonds lengths in the inner coordination core of the metal and in the chelate cycles that are representative of the charge distribution between the metal and ligands displayed a Mn(IV)(Cat2−)2 charge distribution. Variable-temperature magnetic susceptibility measurements detected intramolecular ferromagnetic coupling between the Mn(IV) S = 3/2 spins and spins of nitronyl-nitroxyls and intermolecular ferromagnetic interactions of spins of adjacent nitronyl-nitroxide fragments in a chain of molecules at low temperatures. The last phenomenon is revealed by short contacts between nitronyl-nitroxide radicals of adjacent complex molecules.

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INTRODUCTION

General Considerations. The infrared (IR) spectra were recorded on an FSM 1201 Fourier-transform infrared spectrometer. Magnetic susceptibility measurements were taken on a Quantum Design MPMSXL SQUID magnetometer in the temperature range of 2− 350 K at a magnetic field of 5000 Oe. Diamagnetic corrections were made using the Pascal constants. The effective magnetic moment was calculated as μeff(T) = [(3k/NAμB2 )χT] 1/2 ≈ (8χT)1/2. The pseudopotential PW-SCF code of the Quantum Espresso 5.0 package7 was used for the band structure calculations. We utilized ultrasoft pseudopotentials with nonlinear core correction and the PBE version of the exchange-correlation potential. The kinetic energy cutoffs for wave functions and charge density equal 35 and 280 Ry, respectively. The integration in k space in the course of the self-consistency was performed over a 2 × 2 × 2 mesh in the first Brillouin zone as in the Monkhorst−Pack scheme8 with displacement of the k grid at the center of the Brillouin zone and using 0.136 eV Gaussian smearing. The Hubbard correlations on Mn sites were taken into account within the frameworks of the GGA+U approximation in the rotationally invariant version of Dudarev9 with Ud(Mn) = 6.0 eV and Up(O) = 2.7 eV. Initial chemicals and solvents were received from commercial sources (Aldrich and Fluka). Solvents were carefully distilled, dried, and stored under vacuum. All synthetic procedures were performed in evacuated ampules in the absence of traces of oxygen and moisture. The 4-NIT-Py ligand was synthesized according to the method

A study of metal complexes containing both radical and redoxactive ligands is one of the interesting branches among actively developed directions in the design of magnetoactive compounds. Creation of high-spin molecular systems based on such complexes seems to be very promising because of the presence of numerous different magnetic centers and exchange channels. The most impressive compounds of this row are transition metal complexes with nitronyl-nitroxide ligands1 and osemiquinonato−metal complexes.2 Mixed-ligand complexes containing dissimilar radical ligands in the coordination sphere of the metal have been poorly studied thus far. A few examples are known: mixed-ligand bis-o-semiquinonato−cobalt compound with pyridine substituted by nitronyl-nitroxide radical,3 bis-o-semiquinonato−copper complex with 2-(pyridin-3-yl)4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl,4 and the series of redox-isomeric cobalt compounds with 4,4,5,5tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and its imine analogue.5 Caneschi et al.6 showed that in trans-Mn(hfac)2(4-NIT-Py)2 the manganese(II) ion and the 4-NIT-Py ligands are ferromagnetically coupled. Taking into account these results, we have attempted to synthesize the mixed-ligand compound derivative of bis-osemiquinonato manganese with pyridine-substituted nitronylnitroxide. © 2017 American Chemical Society

EXPERIMENTAL SECTION

Received: October 11, 2016 Published: February 17, 2017 2426

DOI: 10.1021/acs.inorgchem.6b02411 Inorg. Chem. 2017, 56, 2426−2431

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Inorganic Chemistry

solution of (THF)2Mn(3,6-DBCat)(3,6-DBSQ) (0.080 g, ∼0.125 mmol) in 20 mL of diethyl ether. The crystalline powder formed at room temperature was filtered, washed with cold diethyl ether, and dried under vacuum. The mother solution was evaporated to one-half of the volume and kept in a cold room. An additional portion of powder was isolated analogously. Yield: 0.1 g (∼83%). Elemental Anal. Calcd for C52H72MnN6O8: C, 64.78; H, 7.53; Mn, 5.70; N, 8.72. Found: C, 64.98; H, 7.69; Mn, 5.72; N, 8.07. IR (nujol, cm−1): 1613 (s), 1541 (w), 1516 (w), 1481 (w), 1460 [v s (nujol)], 1408 (s), 1389 (w), 1375 [v s (nujol)], 1312 (m), 1281 (w), 1260 (w), 1229 (m), 1167 (m), 1121 (w), 1069 (m), 1030 (m), 976 (s), 938 (w), 839 (m), 797 (m), 708 (s), 665 (w), 644 (w), 619 (w), 592 (s), 503 (m).

described in ref 10. Manganese complex (THF)2Mn(3,6-DBCat)(3,6DBSQ) (3,6-DBSQ and 3,6-DBCat are mono- and dianions of 3,6-ditert-butyl-o-benzoquinone, respectively) was obtained according to a known procedure.11 The X-ray data for complex 1 were collected on an Agilent Xcalibur EOS diffractometer at 100 K. The structure was determined by a dualspace12 method and refined on F2 using SHELXTL.13 All hydrogen atoms were placed in calculated positions and were refined in the riding model. ABSPACK (CrysAlis Pro)14,15 was used to perform areadetector scaling and absorption corrections. The details of crystallographic, collection, and refinement data are provided in Table 1, and

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Table 1. Crystallographic Data and Structural Refinement Details for 1 empirical formula formula weight temperature (K) crystal system space group unit cell dimensions a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (calculated) (g cm−3) absorption coefficient (mm−1) maximum and minimum transmission crystal size θ range for data collection (deg) no. of reflections collected/unique R(int) data/restraints/parameters final R indices [I > 2σ(I)] R1 wR2 R indices (all data) R1 wR2 goodness of fit on F2 largest difference peak and hole (e/Å3)

RESULTS AND DISCUSSION Manganese complex (THF)2Mn(3,6-DBSQ)(3,6-DBCat) (3,6DBSQ and 3,6-DBCat are mono- and dianions of 3,6-di-tertbutyl-o-benzoquinone, respectively) was obtained by using a known procedure.11 Treatment of the manganese precursor with pyridine substituted at position 4 by nitronyl-nitroxide radical [4-NIT-Py = 2-(pyridin-4-yl)-4,4,5,5-tetramethyl-4,5dihydro-1H-imidazole-3-oxide-1-oxyl] in diethyl ether leads to target compound (4-NIT-Py)2Mn(3,6-DBCat)2 [complex 1 (Scheme 1)]. It should be mentioned that substitution of THF by 4-NIT-Py leads to the reduction of the semiquinonato ligand by Mn(III) ion into the catecholato ligand. It should be mentioned that compounds of the row (N_N)Mn(Q)2 (where N_N is a nitrogen donor ligand and Q a quinonato ligand) can exist in three redox forms: MnII(SQ)2, MnIII(Cat)(SQ), and MnIV(Cat)2. When N_N = 2 4-NIT-Py, it exists in the last redox form. Complex 1 was isolated and characterized by single-crystal X-ray diffractometry, variable-temperature magnetic susceptibility, and UV−vis−NIR−IR spectroscopy. According to an X-ray investigation, the unit cell contains two independent molecules of 1 in which the manganese atoms are located in an inversion center (Figure 1) and one guest molecule of Et2O. Both six-coordinate manganese atoms have a distorted octahedron environment. The C−O distances in fivemembered metallocycles of 1 vary in the range of 1.362(3)− 1.366(3) Å, which is typical for the catecholato dianion coordination mode [1.35(1)−1.37(1) Å].16,17 Mn−O bond lengths [1.8627(14)−1.8695(16) Å] in the inner coordination core support a Mn4+(Cat2−) charge distribution (Table 2). The dihedral angles between planes containing catecholato ligands and coordinated pyridine substituted by nitronyl-nitroxide (4NIT-Py) are close to orthogonal (86.80° and 88.31°, respectively). The geometrical parameters of 4-NIT-Py molecules in 1 are quite analogous to those in related complexes of Mn.18 It is interesting to note that the dihedral angle between mean planes of the nitronyl-nitroxyl and pyridyl fragments in one independent molecule of 1 is 2.8°, whereas in another, the angle is 20.5°.

C56H82MnN6O9 1038.21 100(2) triclinic P1̅ 13.9309(3) 14.1843(4) 16.8795(5) 101.436(2) 94.095(2) 118.079(3) 2831.57(15) 2 1.218 0.291 1.00000 and 0.92277 0.70 mm × 0.10 mm × 0.10 mm 2.932−26.000 43687/11105 0.0654 11105/0/674 0.0558 0.1016 0.0785 0.1094 1.059 0.365 and −0.332

the corresponding .cif file is available as Supporting Information. CCDC 1484017 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via ccdc.cam.ac.uk/ products/csd/request/request.php4. (4-NIT-Py)2Mn(3,6-DBCat)2 (complex 1). A solution of 4-NITPy (0.060 g, ∼0.25 mmol) in 20 mL of diethyl ether was added to a

Scheme 1. Synthesis of (4-NIT-Py)2Mn(3,6-DBCat)2

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Figure 3. Temperature dependence of the magnetic moment of complex (4-NIT-Py)2Mn(3,6-DBCat)2 (blue circles). Theoretical curve [best fit parameters gR = 2.0 (fixed), J = 23 cm−1, and C = 1.79 K cm−3 mol−1] (red solid line). Curve calculated using the quantum chemistry calculations [intermolecular nitroxide−nitroxide exchange interaction, 16.8 cm−1; intramolecular exchange interactions between spins of Mn(IV) and nitroxides, 0.2 and 2.2 cm−1] (green solid line).

Figure 1. Molecular structure of one of the independent molecules of complex 1 (thermal ellipsoids at the 30% probability level). Hydrogen atoms and methyl groups of tert-butyl substituents have been omitted for the sake of clarity.

Table 2. Selected Bond Lengths of Complex 1 first molecule Mn1−O1 Mn1−O2 Mn1−N1 C1−O1 C2−O2 C1−C2 C3−C4 C5−C6 O2−Mn1−O1

1.8668(14) Å 1.8644(14) Å 2.0268(19) Å 1.363(3) Å 1.362(3) Å 1.411(3) Å 1.401(3) Å 1.401(3) Å 85.94(6)°

second molecule Mn2−O5 Mn2−O6 Mn2−N4 C27−O5 C28−O6 C27−C28 C29−C30 C31−C32 O5−Mn2−O6

1.8627(14) Å 1.8695(16) Å 2.0382(19) Å 1.366(3) Å 1.366(3) Å 1.409(3) Å 1.408(3) Å 1.395(3) Å 86.00(7)°

1

/2) and the third on the manganese(IV) ion (S = 3/2). Below 100 K, μeff increases and reaches 6.5 μB at 2 K, which points to a domination of ferromagnetic exchange interactions. Because of the presence of the short intermolecular contacts between paramagnetic moieties of nitronyl-nitroxides in the crystal, a simple model was proposed for an analysis of μeff(T) dependence, but the equation, obtained upon summing the contributions to the magnetic susceptibility from exchangecoupled dimers (H = −2J × SR1SR2) of nitronyl-nitroxide radical spins (S = 1/2) and independent spins of manganese(IV) (Mn4+, d3; S = 3/2) according to the Curie law, poorly describes the experimental data. The theoretical curve (red solid line) is shown in Figure 3 (the best fit parameters are gR = 2.0 (fixed), J = 23 cm−1, and C = 1.79 K cm−3 mol−1; temperature range of 25−300 K). The Curie constant value is close to the theoretical magnitude of 1.875 K cm−3 mol−1 for a single paramagnetic center (S = 3/2) with g = 2. Obviously, the spins of Mn(IV) ions are also involved in exchange interactions. This fact is confirmed by the field dependence of the magnetization study (Figure 4). The curve of the dependence of magnetization on the applied external magnetic field at 2 K is S-shaped. The saturation of magnetization at a field of 45 kOe is 4.54 μB, which is slightly lower than the theoretical value of 5 μB for ferromagnetically coupled spins of Mn(IV) and two nitroxide

Thorough analysis of crystal packing (Figure 2) of 1 has shown the presence of the intermolecular C···C interactions [C(49)···C(5), 3.340 Å19]. These interactions lead to rotation of the nitronyl-nitroxide fragment relative to the pyridine plane connected to it. As a result of such rotation, the O(8)···N(3) (3.035 Å) and O(8)···C(20) (2.889 Å) short contacts appear. Taking into account the spin density distribution inside nitronyl-nitroxide radical, we find that O(8)···N(3) and O(8)···C(20) short contacts can result in magnetic exchange between them. The effective magnetic moment (μeff) value in the temperature range of 100−300 K is ∼4.55 μB and remains approximately constant (Figure 3, blue circles). This value is very close to a theoretical spin-only one (4.58 μB), calculated for three noninteracting spins: two on the NIT-Py ligands (S =

Figure 2. Fragment of crystal packing of 1. The tert-butyl groups and H atoms have been omitted for the sake of clarity. 2428

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The O(8)···C(20) short contacts (2.889 Å) promote exchange interaction between atoms with positive and negative spin densities (Figure 5b) and result in ferromagnetic coupling according to the McConnell mechanism.20 The level of intramolecular exchange between spins of Mn(IV) ion and nitroxides is much lower, and the exchange is ferromagnetic in character too. The difference in the dihedral angle between mean planes of the nitronyl-nitroxide and pyridyl fragments in independent molecules of 1 leads to the different values of the exchange interaction between spins of Mn(IV) and two nitroxide radicals (JMn−R). The calculated JMn−R values are 0.2 and 2.2 cm−1, and the first corresponds to the larger dihedral angle between mean planes of the nitronyl-nitroxide and pyridyl fragments. Despite small JMn−R values, the ferromagnetic character of the exchange interaction between spins of Mn(IV) and two nitroxide radicals leads to a significant increase in μeff with a decrease in temperature. The theoretical μeff(T) dependence (Figure 3, green solid line) was calculated for a ring chain of two {R−Mn−R} units, which was limited by computing resources, and describes the experimental one much better than the simple model of exchange-coupled nitronylnitroxide radical spins and independent spins of Mn(IV). Therefore, it seems that molecules of the catecholato− manganese complex play the role of a matrix that arranges nitronyl-nitroxide moieties in such a way that their spins in the ground state are ferromagnetically coupled. The first example of intermolecular ferromagnetic exchange of spins of nitroxide radicals was observed in the lattice of nitroxide biradical 1,2,5,7tetramethyl-2,6-diazaadamantane-N,N′-dioxyl.21 Two known structural analogues of complex 1, (4-NITPy)2Mn(hfac)2 and (NITpPy)2Mn(Phtfac)2,6 both demonstrating ferromagnetic exchange between odd electrons of high-spin Mn(II), d5 and unpaired electrons of nitronyl-nitroxide radical, have another packing feature. Each of their structures is presented by one independent molecule. Short contacts of the −N−O group consist of −N−O···Me-NIT-Py (3.322 Å) for the first complex and −N−O···Ph-ac (3.376 Å) for the second. When the spin density distribution is taken into account, these contacts should not contribute to the magnetic properties of complexes. Ferromagnetic exchange in both complexes (∼1 cm−1 for the first complex and ∼3 cm−1 for the second) is a consequence of interaction of unpaired electrons of nitroxylnitroxide radicals with unpaired electrons of high-spin metal through the π-system of the pyridine bridge. The electronic absorption spectrum of the complex at room temperature is similar to that of the previously investigated

Figure 4. Dependence of magnetization (M) on the applied magnetic field/temperature ratio (H/T) for complex 1: experimental at 2 K (blue squares), experimental at 7 K (green circles), Brillouin functions for S = 5/2 (green line), for three noninteracting spins, two on the 4NIT-Py ligands (S = 1/2) and the third on the manganese(IV) ion (S = 3 / 2) (red line), and for two noninteracting spins, one for ferromagnetically coupled spins of the 4-NIT-Py ligands (S12 = 1) and the second on the manganese(IV) ion (S = 3/2) (violet line).

radicals. At 7 K, the magnetization curve versus H/T becomes close to linear and coincides with the Brillouin function for S = 5 /2 (Figure 4, green line), which confirms ferromagnetic interactions between spins of Mn(IV) and two nitroxide radicals. Furthermore, the M versus H/T plots at low temperatures are not superposed. It may be supposed that the transition to the ferromagnetically ordered state should be observed at Tc values ≤2 K. For comparison, the Brillouin functions are presented in Figure 4: (a) a red line for three noninteracting spins, two on the NIT-Py ligands (S = 1/2) and the third on the manganese(IV) ion (S = 3/2), and (b) a violet line for two noninteracting spins, one for ferromagnetically coupled spins of the NIT-Py ligands (S12 = 1) and the second on the manganese(IV) ion (S = 3/2). For the detailed analysis of magnetic properties, quantum chemistry calculations for (4-NIT-Py)2Mn(3,6-DBCat)2 were performed. The calculated intermolecular nitronyl-nitroxide exchange interaction parameter value is 16.8 cm−1. The quite strong ferromagnetic exchange interactions arise from a specific arrangement of the (4-NIT-Py)2Mn(3,6-DBCat)2 molecules relative to each other, which provides an orthogonality of magnetic orbitals of the adjacent nitronyl-nitroxides (Figure 5).

Figure 5. (a) Magnetic orbitals of adjacent nitronyl-nitroxyls. (b) Spin density distribution in adjacent nitronyl-nitroxyls. 2429

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Inorganic Chemistry complex (4-t-Bu-py)2(3,6-DBQ)2.22 It is determined by an intense band that belongs to derivatives of Mn(III) as well as Mn(IV) in the 850 nm region. It is known that mixed valence quinonato−manganese complexes display the interligand band at ∼2300 nm.11 No absorption of complex 1 in the NIR region at room temperature was observed (Figure S3). It seems that no valence tautomerism exists in complex 1.

Means of Complexation with Magnetic Metal Ions. A Molecule-Based Magnet with Three-Dimensional Structure and High TC of 46 K. J. Am. Chem. Soc. 1996, 118, 1803−1804. (c) Oshio, H.; Ito, T. Assembly of imino nitroxides with Ag(I) and Cu(I) ions. Coord. Chem. Rev. 2000, 198, 329−346. (d) Iwamura, H.; Inoue, K. In Magnetism: Molecules to Materials II; Miller, J. S., Drillon, M., Eds.; Wiley/VCH: Weinheim, Germany, 2001; p 60. (e) Volodarsky, L. B.; Reznikov, V. A.; Ovcharenko, V. I. Synthetic Chemistry of Stable Nitroxides; CRC Press: Boca Raton, FL, 1994. (f) Ovcharenko, V. I.; Sagdeev, R. Z. Molecular ferromagnets. Russ. Chem. Rev. 1999, 68, 345−363. (g) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Toward molecular magnets: the metal-radical approach. Acc. Chem. Res. 1989, 22 (11), 392−398. (h) Ovcharenko, V. Metal−Nitroxide Complexes: Synthesis and Magnetostructural Correlations. In Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds; Hicks, R. G., Ed.; John Wiley & Sons Ltd.: Hoboken, NJ, 2010; p 461. (2) (a) Pierpont, C. G.; Attia, A. S. Spin Coupling Interactions in Transition Metal Complexes Containing Radical o-Semiquinone Ligands. A Review. Collect. Czech. Chem. Commun. 2001, 66, 33−51. (b) Ovcharenko, V. I.; Bagryanskaya, E. G. Properties and Applications. In Spin-Crossover Materials; Halcrow, M. A., Ed.; Wiley: Hoboken, NJ, 2013; pp 239−280. (3) Yamaguchi, A.; Awaga, K. Inner- and outer-sphere magnetic moments in the cobalt−quinone valence tautomeric system. J. Mater. Chem. 2001, 11, 2142−2145. (4) Ovcharenko, V. I.; Gorelik, E. V.; Fokin, S. V.; Romanenko, G. V.; Ikorskii, V. N.; Krashilina, A. V.; Cherkasov, V. K.; Abakumov, G. A. Ligand Effects on the Ferro- to Antiferromagnetic Exchange Ratio in Bis(o-Semiquinonato)copper(II). J. Am. Chem. Soc. 2007, 129, 10512−10521. (5) Fursova, E. Yu.; Kuznetsova, O. V.; Tretyakov, E. V.; Romanenko, G. V.; Bogomyakov, A. S.; Ovcharenko, V. I.; Sagdeev, R. Z.; Cherkasov, V. K.; Bubnov, M. P.; Abakumov, G. A. Heterospin complexes based on cobalt semiquinolate with nitroxides. Russ. Chem. Bull. 2011, 60 (5), 809−815. (6) (a) Caneschi, A.; Gatteschi, D.; Renard, J. P.; Rey, P.; Sessoli, R. Ferromagnetic phase transitions of two one-dimensional ferrimagnets formed by manganese(II) and nitronyl nitroxides cis octahedrally coordinated. Inorg. Chem. 1989, 28, 3314−3319. (b) Caneschi, A.; Ferraro, F.; Gatteschi, D.; Rey, P.; Sessoli, R. Ferro- and antiferromagnetic coupling between metal ions and pyridinesubstituted nitronyl nitroxides. Inorg. Chem. 1990, 29, 4217−4223. (7) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21, 395502−395521. (8) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188−5192. (9) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 1505−1509. (10) (a) Tretyakov, E.; Tolstikov, S.; Mareev, A.; Medvedeva, A.; Romanenko, G.; Stass, D.; Bogomyakov, A.; Ovcharenko, V. New Cascade Syntheses of Nitronyl Nitroxides and a New Synthetic Approach to Imino Nitroxides. Eur. J. Org. Chem. 2009, 2009, 2548− 2561. (b) Shimono, S.; Tamura, R.; Ikuma, N.; Takimoto, T.; Kawame, N.; Tamada, O.; Sakai, N.; Matsuura, H.; Yamauchi, J. Preparation and Characterization of New Chiral Nitronyl Nitroxides Bearing a Stereogenic Center in the Imidazolyl Framework. J. Org. Chem. 2004, 69, 475−481. (11) Attia, A. S.; Pierpont, C. G. New Semiquinone/Catecholate Complexes That Exhibit Valence Tautomerism. Synthesis and Characterization of MnIII(thf)2(3,6-DBSQ)(3,6-DBCat) and Observations on the MnIV(3,6-DBSQ)2(3,6-DBCat)/MnIII(3,6-DBSQ)3 Equilibrium in the Solid State. Inorg. Chem. 1998, 37, 3051−3056. (12) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

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CONCLUSION The bis-catecholato complex of manganese with two nitronylnitroxide ligands described in this paper is the first example of the structurally characterized Mn(IV) compounds with nitronyl-nitroxide radicals. The relatively large distance between manganese ion (Mn4+, d3, S = 3/2) and nitronyl-nitroxide radical and short contact between nitronyl-nitroxide moieties of adjacent molecules on one hand and the positive sign of the exchange energy of both magnetic exchange channels on the other determine the ferromagnetic behavior of the complex. The bis-catecholato−manganese complex serves as a matrix that arranges the nitroxide radicals in such a way that their spins are coupled ferromagnetically.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02411. Crystallographic data (CIF) Molecular structure of complex 1 with detailed numbering of atoms (Figure S1), field (oersteds) dependence of magnetization (Gauss per cubic centimeter per mole) of complex 1 (Figure S2), detailed field (oersted per kelvin) dependence of magnetization (NμB) of complex 1 (Figure S3), and the NIR−IR spectrum of complex 1 (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +7 831 4627682. Fax: +7 831 4627497. E-mail: bmp@ iomc.ras.ru. ORCID

Michael P. Bubnov: 0000-0002-0536-3711 Artem S. Bogomyakov: 0000-0002-6918-5459 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Russian Science Foundation (Grant 14-13-01296). Eugenia A. Kopylova is thankful for a Russian President Grant supporting scientific schools (NSh7916.2016.3) that supported the spectroscopic studies. Marina V. Petrova thanks RFBR (Grant 16-33-00124) for supporting quantum chemical calculations. X-ray crystallography was performed by Group of Roentgen Diffraction Studies of G.A. Razuvaev Institute of Organometallic Chemistry of RAS.

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REFERENCES

(1) (a) Caneschi, A.; Gatteschi, D.; Rey, P. The chemistry and magnetic properties of metal nitronyl nitroxide complexes. Prog. Inorg. Chem. 1991, 39, 331−429. (b) Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y. Assemblage and Alignment of the Spins of the Organic Trinitroxide Radical with a Quartet Ground State by 2430

DOI: 10.1021/acs.inorgchem.6b02411 Inorg. Chem. 2017, 56, 2426−2431

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DOI: 10.1021/acs.inorgchem.6b02411 Inorg. Chem. 2017, 56, 2426−2431