Trigonal Prismatic Tris-pyridineoximate Transition ... - ACS Publications

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Trigonal Prismatic Tris-pyridineoximate Transition Metal Complexes: A Cobalt(II) Compound with High Magnetic Anisotropy Alexander A. Pavlov,† Svetlana A. Savkina,† Alexander S. Belov,† Yulia V. Nelyubina,†,‡ Nikolay N. Efimov,‡ Yan Z. Voloshin,†,‡ and Valentin V. Novikov*,† †

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova str. 28, 119991, Moscow, Russia Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii prosp., 31, 117901, Moscow, Russia

‡

S Supporting Information *

ABSTRACT: High magnetic anisotropy is a key property of paramagnetic shift tags, which are mostly studied by NMR spectroscopy, and of single molecule magnets, for which magnetometry is usually used. We successfully employed both these methods in analyzing magnetic properties of a series of transition metal complexes, the so-called clathrochelates. A cobalt complex was found to be both a promising paramagnetic shift tag and a single molecule magnet because of it having large axial magnetic susceptibility tensor anisotropy at room temperature (22.5 × 10−32 m3 mol−1) and a high effective barrier to magnetization reversal (up to 70.5 cm−1). The origin of this large magnetic anisotropy is a negative value of zero-field splitting energy that reaches −86 cm−1 according to magnetometry and NMR measurements.

1. INTRODUCTION Single molecule magnets (SMMs), the compounds that feature magnetic bistability on a molecular scale,1−8 are sought for their potential applications in information storage,9 quantum computing,10 and spintronics;11 many of them have already been found among the complexes of lanthanides, 12,13 actinides,14 and of 3d-transition metals (mostly of cobalt15−37 and iron38−40). The origin of molecular magnetism in SMMs is the energy barrier U to magnetization reversal between states ‘spin up’ (MS = +S) and ‘spin down’ (MS = −S). When it is large (and side relaxation mechanisms are effectively avoided, such as quantum tunneling of magnetization,41 one-quantum direct and two-phonon Raman mechanisms42), the magnetization can be retained at higher temperatures and for a longer time.43 This barrier is proportional to the zero-field splitting energy U ∝ |D|,44 which arises from orbital contribution to the magnetic moment caused by an orbitally degenerate ground state with the first-order orbital momentum45 or by spin−orbit coupling (SOC).46 The value of zero-field splitting is related, through the VanVleck equation,47 to the magnetic susceptibility tensor anisotropy Δχ, the key characteristic of a paramagnetic shift tag.48 Although the largest Δχ values are typically observed for lanthanide compounds,49,50 there are some 3d-transition metal complexes that can do nearly as well; such are cobalt(II) clathrochelates.51 Trigonal prismatic geometry of these complexes with a d7 electronic configuration results in the degeneracy of the highest double-occupied and the lowest single-occupied d-orbitals, leading to the unquenched orbital contribution and to the large negative value of the zero-field splitting (−110 cm−1).52 © 2017 American Chemical Society

In this paper, we clearly show that a combination of the rigid trigonal prismatic geometry and the d7 electronic configuration are necessary for the observation of the SMM behavior in the transition metal complexes of this type: among the series of new clathrochelates, the cobalt(II) complex has a high barrier to magnetization reversal, whereas the complexes of other metals either show a very small magnetic anisotropy (manganese and nickel(II)) or are diamagnetic (zinc and iron(II)).

2. EXPERIMENTAL SECTION Materials and Methods. Reagents used, FeCl2·4H2O, Ni(ClO4)2· 6H2O, NaHCO3, NaClO4·H2O, hydroxylamine hydrochloride, 2acetylpyridine, phenylboronic acid, and organic solvents, were obtained commercially (Sigma-Aldrich ). Co(ClO4)2·6H2O and Zn(ClO4)2·4H2O were synthesized as described in ref 53; Mn(ClO4)2·6H2O was obtained using a similar synthetic procedure. 2Acetylpyridineoxime (AcPyOx) was obtained as described in ref 54. Analytical data (C, H, N contents) were obtained with a Carlo Erba model 1106 microanalyzer. The iron content was determined spectrophotometrically. The cobalt, nickel, manganese, and zinc contents were determined by the X-ray fluorescence method. Matrix assisted laser desorption/ionization time-of-flight (MALDITOF) mass spectra were recorded with and without the matrix using a MALDI-TOF-MS Bruker Autoflex II (Bruker Daltonics) mass spectrometer in a reflecto-mol mode. Ionization was induced by an UV-laser with the wavelength 337 nm. The samples were applied to a nickel plate, and 2,5-dihydroxybenzoic acid was used as the matrix. The accuracy of the measurements was 0.1%. Received: February 18, 2017 Published: May 25, 2017 6943

DOI: 10.1021/acs.inorgchem.7b00447 Inorg. Chem. 2017, 56, 6943−6951

Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Parameters for Fe, Ni, Co, Mn, and Zn Fe

Ni

empirical formula

C27H26BClFeN6O7

C27H26BClN6NiO7

formula weight T, K crystal system space group Z a, Å b, Å c, Å α, ° β, ° γ, ° V, Å3 Dcalc (g cm−1) linear absorption, μ (cm−1) F(000) 2θmax, ° reflections measured independent reflections observed reflections [I > 2σ(I)] parameters R1 wR2 GOF Δρmax/Δρmin (e Å−3)

648.65 120 monoclinic P21/c 4 18.552(5) 10.066(3) 16.395(5) 90 112.821(6) 90 2822.0(13) 1.527 6.87 1336 56 21034 6805 3684 391 0.0699 0.1396 1.023 0.823/−0.671

Co

Mn

Zn

651.51 120 monoclinic P21/c 4 19.5866(13) 9.2348(6) 16.7152(11) 90 114.3190(10) 90 2755.1(3) 1.571 8.6 1344 58 44702 7329 5812

C27H26BClCoN6O7· 2(CH2Cl2) 821.58 120 triclinic P1̅ 2 11.1104(4) 11.5375(4) 13.8903(5) 89.3400(10) 81.5970(10) 77.8450(10) 1721.60(11) 1.585 9.41 838 58 21513 9153 7372

C27H26BClMnN6O7· CH2Cl2 732.66 120 triclinic P1̅ 4 12.266(3) 15.054(4) 17.794(4) 109.482(5) 91.477(6) 90.119(6) 3096.4(12) 1.572 7.42 1500 58 64637 16475 8105

CH2Cl2 1401.26 120 monoclinic P21 2 14.496(2) 12.9745(18) 17.424(3) 90 112.979(4) 90 3017.1(7) 1.542 10.47 1436 54 31333 13131 7982

391 0.0355 0.0881 1.006 0.476/−0.315

455 0.0466 0.1299 1.028 1.775/−0.923

836 0.0669 0.1790 1.008 0.889/−0.668

803 0.0605 0.1043 1.001 0.924/−0.662

1

H and 13C NMR spectra were recorded from CD2Cl2 solutions with a Bruker Avance 600 spectrometer. The measurements were done using the residual signals of CD2Cl2 (1H 5.32 ppm, 13C 54.00 ppm). NMR spectra of the reported paramagnetic samples were acquired using the following parameters: for 1H spectra, sweep width 1000 ppm, acquisition time 0.1 s, relaxation delay 0.1 s, pulse duration 6.5 μs, pulse program “zg” within Bruker notation, number of scans 1024, line-broadening factor 3 Hz; for 13C spectra, sweep width 2000 ppm, acquisition time 0.1 s, relaxation delay 0.1 s, pulse duration 9 μs, pulse program “zgpg30” within Bruker notation, number of scans >32k, linebroadening factor 3−30 Hz. UV−vis spectra of the solutions in dichloromethane were recorded in the range 230−1100 nm with a Varian Cary 50 spectrophotometer. The individual Gaussian components of these spectra were calculated using the Fityk program.55 Magnetic measurements were performed using Quantum Design PPMS-9 device; the oscillating AC fields of 1 Oe with the frequencies in the range from 10 to 10000 Hz were employed. Finely ground microcrystalline powders were immobilized in a mineral oil matrix inside a polyethylene capsule. The magnetic data were corrected for the sample holder, the mineral oil, and the diamagnetic contribution. Synthesis. [Fe(AcPyOx)3(BC6H5)]ClO4 or Fe. AcPyOx (1.5 g, 10 mmol) and phenylboronic acid (0.45 g, 3.65 mmol) were dissolved in ethanol (15 mL) under intensive stirring in argon, and FeCl2·4H2O (0.67 g, 3.35 mmol) and NaHCO3 (0.28 g, 3.35 mmol) were added. The reaction mixture was refluxed for 1 h and cooled to r.t., filtered, and rotary evaporated to dryness. The solid product was washed with diethyl ether (15 mL, in three portions) and hexane (2 × 5 mL), dissolved in methanol (20 mL), and precipitated with a NaClO4 saturated aqueous solution (25 mL). The red precipitate was filtered off, dried in air, washed with diethyl ether (20 mL), and extracted with dichloromethane (25 mL). The extract was filtered and rotary evaporated to dryness. The solid residual was washed with diethyl ether (20 mL, in two portions), hexane (20 mL, in two portions), and dried in vacuo. Yield: 1.43 g (65%). Anal. Calcd for C27H26N6O7BClFe (%): C, 49.96; H, 4.01; N, 12.95; Fe, 8.64. Found (%): C, 50.04; H,

2(C27H26BClN6ZnO7)·

4.08; N, 12.71; Fe, 8.55. MS(MALDI-TOF): m/z: 549 [ M ]+•. 1H NMR (CD2Cl2, δ, ppm): 2.66 (s, 9H, CH3), 7.06 (d, 3H, 6-Py, 3JHH = 5.6 Hz), 7.36 (m, 3H, m-Ph, p-Ph), 7.51 (t, 3H, 5-Py, 3JHH = 5.6 Hz), 7.76 (m, 2H, o-Ph), 7.94 (d, 3H, 3-Py, 3JHH = 8.1 Hz), 8.06 (t, 3H, 4Py, 3JHH = 8.1 Hz). 13C{1H} NMR (CD2Cl2, δ, ppm): 13.23 (s, CH3), 124.52 (s, 3-Py), 125.77 (s, 5-Py), 127.43 (s, m-Ph), 127.95 (s, p-Ph), 131.71 (s, o-Ph), 137.92 (s, 4-Py), 152.96 (s, 6-Py), 158.02 (s, 2-Py), 160.56 (s, CN). UV−vis (CH2Cl2): λmax/nm (ε·10−3 mol−1·L· cm−1): 242 (21), 259 (8.3), 279 (3.4), 291 (22), 357 (3.9), 436 (1.7), 487 (8.6), 520 (3.6), 527 (11). [Ni(AcPyOx)3(BC6H5)]ClO4 or Ni. AcPyOx (0.48 g, 3.5 mmol) and phenylboronic acid (0.15 g, 1.2 mmol) were dissolved in ethanol (10 mL) under intensive stirring in argon, and a solution of Ni(ClO4)2· 6H2O (0.37 g, 1 mmol) in ethanol (5 mL) and NaHCO3 (0.08 g, 1.0 mmol) were added. The reaction mixture was stirred at 75 °C for 1.5 h and cooled to r.t. The light-brown precipitate was filtered off and washed with ethanol (6 mL, in two portions) and diethyl ether (12 mL, in two portions). The product was extracted from this solid with dichloromethane (100 mL). The extract was filtered and washed with water (25 mL), dried with Na2SO4, filtered, and rotary evaporated to dryness. The solid residue was washed with diethyl ether (20 mL, in four portions), hexane (5 mL), and dried in vacuo. Yield: 0.50 g (77%). Anal. Calcd for C27H26N6O7BClNi (%): C, 49.73; H, 3.99; N, 12.89; Ni, 9.06. Found (%): C, 49.54; H, 3.97; N, 12.79; Ni, 8.90. MS(MALDI-TOF): m/z: 552 [ M ]+•. 1H NMR (CD2Cl2, δ, ppm): −24.04 (s, 9H, CH3), 6.59 (s, 2H, m-Ph), 6.88 (s, 1H, p-Ph), 7.40 (s, 2H, o-Ph), 15.49 (s, 3H, 4-Py), 44.05 (s, 3H, 5-Py), 59.34 (s, 3H, 3Py), 137.34 (br. s, 3H, 6-Py). 13C{1H} NMR (CD2Cl2, δ, ppm): −116.86 (s, 2-Py), −9.60 (s, 6-Py), 54.0 (s, CN), 78.40 (s, i-Ph), 116.26 (s, 4-Py), 128.03 (s, p-Ph), 130.04 (s, m-Ph), 147.65 (s, o-Ph), 322.01 (s, CH3), 393.09 (s, 3-Py), 544.22 (s, 5-Py). UV−vis (CH2Cl2): λmax/nm (ε·10−3 mol−1·L·cm−1): 245 (26), 259 (6.0), 290 (13), 313 (14), 507 (0.035), 791 (0.014), 828 (0.004), 900 (0.025). [Co(AcPyOx)3BC6H5]ClO4 or Co. This complex was obtained like the previous one except that Co(ClO4)2·6H2O (0.37 g, 1.0 mmol) was 6944


Article

Inorganic Chemistry used instead of Ni(ClO4)2·6H2O. Yield of the light-orange finecrystalline product was 0.46 g (71%). Anal. Calcd for C27H26N6O7BClCo (%): C, 49.73; H, 3.99; N, 12.89; Co, 9.06. Found (%): C, 49.88; H, 3.92; N, 12.90; Co, 8.90. MS(MALDI-TOF): m/z: 552 [ M ]+•. 1H NMR (CD2Cl2, δ, ppm): − 2.50 (s, 3H, 3-Py), 2.42 (s, 9H, CH3), 15.12 (s, 3H, 4-Py), 25.71 (s, 1H, p-Ph), 29.93 (s, 2H, m-Ph), 67.86 (s, 2H, o-Ph), 80.12 (s, 3H, 5-Py), 396.17 (br. s, 3H, 6-Py). 13C{1H} NMR (CD2Cl2, δ, ppm): −476.48 (s, 2-Py), 97.70 (s, CN), 146.74 (s, 4-Py), 155.36 (s, p-Ph), 164.71 (s, m-Ph), 189.38 (s, CH3), 208.63 (s, o-Ph), 263.91 (s, 6-Py), 318.33 (s, 3-Py), 554.92 (s, 5-Py). UV−vis (CH2Cl2): λmax/nm (ε·10−3 mol−1·L·cm−1): 245 (32), 290 (5.5), 307 (9.6), 349 (5.7), 472 (0.10), 682 (0.002), 1003 (0.008). [Zn(AcPyOx)3BC6H5]ClO4 or Zn. AcPyOx (0.31 g, 2.3 mmol) and phenylboronic acid (0.1 g, 0.83 mmol) were dissolved in ethanol (5 mL) under intensive stirring in argon, and Zn(ClO4)2·4H2O (0.26 g, 0.76 mmol) and NaHCO3 (0.06 g, 0.76 mmol) were added. The reaction mixture was refluxed for 1 h and cooled to r.t. The white precipitate was filtered off, washed with ethanol (10 mL, in two portions), diethyl ether (10 mL, in two portions), and extracted with dichloromethane (15 mL). The extract was filtered and rotary evaporated to dryness. The solid residue was washed with diethyl ether (20 mL, in four portions), hexane (5 mL) and dried in vacuo. Yield: 0.50 g (77%). Anal. Calcd for C27H26N6O7BClZn (%): C, 49.28; H, 3.95; N, 12.78; Zn, 9.89. Found (%): C, 49.19; H, 3.95; N, 12.77; Zn, 9.77. MS(MALDI-TOF): m/z: 558 [M − ClO4−]+•.1H NMR (CD2Cl2, δ, ppm): 2.47 (s, 9H, CH3), 7.36 (t, 1H, p-Ph, 3JHH = 7.11 Hz), 7.41 (m, 2H, m-Ph), 7.78 (d, 3H, 6-Py, 3JHH = 7.46 Hz), 7.88 (d, 2H, o-Ph, 3JHH = 7.11 Hz), 7.94 (t, 3H, 4-Py, 3JHH = 5.36 Hz), 8.14 (t, 3H, 5-Py, 3JHH = 7.46 Hz), 9.02 (d, 3H, 3-Py, 3JHH = 5.36 Hz). 13C NMR (CD2Cl2, δ, ppm): 13.23 (s, CH3), 122.59 (s, 3-Py), 127.77 (s, 4-Py), 127.46 (s, m-Ph), 127.46 (s, p-Ph), 131.91 (s, o-Ph), 141.41 (s, 5-Py), 149.71 (s, 6-Py), 150.13 (s, 2-Py), 150.37 (s, CN). UV−vis (CH2Cl2): λmax/nm (ε·10−3 mol−1·L·cm−1): 249(31), 290(18), 307(12), 326(6.7). [Mn(AcPyOx)3BC6H5]ClO4 or Mn. This complex was obtained like the previous one except that Mn(ClO4)2·6H2O (0.36 g, 1.0 mmol) was used instead of Zn(ClO4)2·4H2O. Yield of the bright yellow finecrystalline product was 0.56 g (86%). Anal. Calcd for C27H26N6O7BClMn (%):C, 50.04; H, 4.02; N, 12.97; Mn, 8.49. Found (%): C, 50.07; H, 4.13; N, 13.06; Mn, 8.37. MS(MALDITOF): m/z: 548 [M − ClO4−]+•. 1H NMR (CD2Cl2, δ, ppm): −25.74 (br. s, 6-Py), 7.28 (br. s, m-Ph), 7.45 (br. s, p-Ph), 8.47 (br. s, o-Ph), 46.28 (br. s, 5-Py, 7-Py), 103.40 (br. s, 8-Py). 13C NMR (CD2Cl2, δ, ppm): 11.73 (s, CH3), 127.94 (s, p-Ph), 131.30 (s, m-Ph), 145.37 (s, oPh), 192.17 (br. s, 5-Py), 396.23 (br. s, 6-Py), 614.89 (br. s, 7-Py), 806.99 (br. s, CN). UV−vis (CH2Cl2): λmax/nm (ε·10−3 mol−1·L· cm−1): 233(28), 252(4.0), 265(9.6), 294(20), 318(10), 334(3.1), 340(0.4). X-ray Crystallography. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of saturated solutions of the complexes in dichloromethane−hexane (1:1) mixtures. X-ray diffraction experiments were carried out with a SMART APEX2 CCD diffractometer for the complex Zn and with a APEX2 DUO CCD diffractometer for all others, using graphite monochromated Mo−Kα radiation (λ = 0.71073 Å, ω-scans) at 120 K. The structures were solved by direct method and refined by the full-matrix least-squares against F2 in anisotropic approximation for non-hydrogen atoms. The hydrogen atom positions were calculated, and they were refined in isotropic approximation in riding model. Crystal data and structure refinement parameters for Fe, Ni, Co, Mn, and Zn are given in Table 1. All calculations were performed using the SHELXTL software.56 CCDC 1531859−1531863 contain the supplementary crystallographic data for this paper. Quantum Chemical Calculations. Quantum chemical calculations of the cobalt complex Co were performed with the ORCA package v. 3.0.3.57 The geometry was optimized with a nonhybrid PBE functional,58 scalar relativistic zero-order regular approximation (ZORA),59 and scalar relativistically recontracted (SARC)60 version of the def2-TZVP basis set,61 staring from the experimental molecular

geometry. After the optimization, the g-tensor and the tensors of hyperfine interactions A for hydrogen and carbon nuclei were calculated using the hybrid PBE0 functional and def2-TZVP basis sets, with the primitives with a higher order of the exponent added for a better description of the electron density around nuclei. Ab initio CASSCF calculations were performed using the def2TZVPP basis set for the cobalt atom and def2-TZVP61 for all other atoms, together with the corresponding auxiliary basis sets, starting from the natural orbitals from a tightly converged restricted open shell Hartree−Fock calculation. The active space for CASSCF calculations was chosen to consist of five cobalt d-based molecular orbitals occupied by seven electrons (CAS(7, 5)) with 10 quadruplet and 40 doublet electronic states taken into account. To speed up the calculations, the “chain-of-spheres” (RIJCOSX) approximation to exact exchange as implemented in ORCA was applied. To account for dynamic correlations, N-electron valence perturbation theory (NEVPT2) calculations were performed on top of CASSCFconverged wave functions.

3. RESULTS AND DISCUSSION Synthesis. The boron-capped cobalt, nickel, iron, zinc, and manganese(II) tris-pyridinoximates were synthesized by a procedure similar to one previously used for tris-pyrazoloximates.62 The latter were obtained as precipitates after refluxing the mixture of the corresponding metal(II) chloride with pyrazoloxime and phenylboronic acid with addition of NaHCO3 in ethanol media. Doing the same with FeCl2· 4H2O, 2-acetylpyridineoxime, and phenylboronic acid led to a soluble product, which was reprecipitated with NaClO4. In the case of cobalt(II) chloride, an inseparable mixture of cobalt(II) and cobalt(III) complexes was obtained (according to NMR data), even if other bases (CaCO3, triethylamine) or other solvents (such as methanol, acetonitrile, ethyl acetate, or a mixture of methanol and water) were used in the reaction. Under these conditions, nickel(II) chloride also produced a mixture of complexes; our attempts to obtain the target complex from nickel(II) sulfate or acetate were equally unsuccessful. The use of zinc(II) chloride did not result in the pure target product as well. As changing the base or a solvent used in the reaction did not give the expected results, we have changed the starting metal(II) salts, by choosing the corresponding perchlorates.63−65 It turned out that a light beige precipitate obtained after heating the mixture of 2-acetylpyridineoxime and phenylboronic acid with nickel(II) perchlorate and NaHCO3 in ethanol medium is a pure target nickel(II) complex (according to NMR data). The same synthetic approach also gave phenylboron-capped cobalt(II), zinc(II), and manganese(II) tris-acetylpyridineoximates in high yields (Scheme 1). It seems that the perchlorate ion is crucial for this reaction to succeed, probably, owing to it giving less soluble products that do not undergo further changes. In addition, we carefully chose the order in which the reagents should be introduced in the reaction to suppress possible side processes. Phenylboronic acid and 2-acetylpyridineoxime were first dissolved, and then the corresponding metal salt and NaHCO3 were added. Upon heating the reaction mixture, gradual precipitation of the complex occurred. Triethylamine can also be used as a base; however, it complicates the purification of the target product. Structure. The structure of the complexes obtained has been confirmed by single-crystal X-ray diffraction (Figures 1 and 2). In all of them (Table 2), the metal ion is located almost in the center of the N6 “cage” formed by the encapsulating ligand (Fe−N 1.892(3)−2.000(3) Å; Ni−N 2.0214(15)− 6945


Article

Inorganic Chemistry Scheme 1. Synthesis of Tris-pyridineoximate Transition Metal(II) Complexes

Figure 2. General view of the complexes Mn and Zn with atoms shown as thermal ellipsoids at p = 30%. Only one symmetryindependent molecule of the complex is shown; hydrogen atoms, perchlorate anions, and dichloromethane molecules are omitted for clarity.

2.1202(15) Å; Co−N 2.0757(19)−2.1801(19) Å; Mn−N 2.207(4)−2.249(4) Å; Zn−N 2.122(6)−2.212(6) Å), as typical of low-spin Fe(II) and of high-spin Ni(II), Mn(II) and Co(II) ions. The MN6 polyhedron adopts the geometry that is intermediate between a trigonal prism (TP, the distortion angle φ = 0°) and a trigonal antiprism (TAP, φ = 60°). The degree of this TP-to-TAP distortion, however, depends strongly on the metal ion. It is much closer to TP in the complexes of cobalt, manganese and zinc, while those of iron and nickel have a more TAP-like geometry; the corresponding distortion angle φ varies from 5.8° to 39.6°. The latter values are larger than, e.g., in similar complexes of cobalt(II) but with tris-pyrazoloximate as an encapsulating ligand,62 which also showed the SMM behavior with very high barriers to magnetization reversal.52,66 The reason for this may be that in those previously reported clathrochelates, one of the “caps” was a chloride anion attached to the rest of the clathrochelate framework by strong hydrogen bonds with the NH functionality of the pyrazoloximate moieties,62 while in the complexes discussed here, it is the perchlorate anion loosely bound by weak C−H···OCl contacts (O···H distance from 2.4 Å). Magnetic Properties. Spin state of the paramagnetic ions in the studied clathrochelates Ni, Co, and Mn was confirmed by magnetometry. Variable-temperature DC magnetic susceptibility measurements show that χT values for the complexes Mn and Ni in the temperature range 50−300 K are 4.53 and 1.25 cm3 mol−1 K, respectively, which are close to the theoretical spin-only values for high spin ions (4.38 and 1.00 cm3 mol−1 K

for S = 5/2 and 1) (Figure S8). For the complex Co, a gradual decrease of χT was observed upon cooling from 3.12 cm3 mol−1 K at 300 K; the latter being higher than the spin-only value for a high spin ion (1.88 cm3 mol−1 K) indicates the unquenched orbital contribution to the total magnetic moment that results in the significant magnetic anisotropy.39 The decrease in χT upon cooling from 100 K suggests the presence of large magnetic anisotropy; the shortest Co···Co distance in the crystal is too large (7.99 Å) to explain it by intermolecular interactions. Fitting the observed magnetic data (Figures 3−4) to the following spin Hamiltonian: ⎛ 2 S(S + 1) ⎞ Ĥ = D⎜Sẑ − ⎟ + μB (gx Sx̂ Bx + gySŷ By + gz Sẑ Bz ) ⎝ ⎠ 3 (1)

using the program PHI67 gives high Δg and |D| for the complex Co (g⊥ = 2.08, g∥ = 3.09, D = −86 cm−1), as indicative of its significant magnetic anisotropy; note that simulation of the experimental data using a positive D value was unsuccessful. The magnetic parameters thus-obtained for the complex Co are close to those for phenyl52 and n-hexadecyl boron-capped66 tris-pyrazoloximate cobalt(II) clathrochelates.

Figure 1. General view of the complexes Fe, Ni, and Co with atoms shown as thermal ellipsoids at p = 30%. Hydrogen atoms, perchlorate anions, and dichloromethane molecules (in Co) are omitted for clarity. 6946


1.901(3) 1.983(3) 1.896(3) 2.000(3) 1.892(3) 1.982(3) 1.376(4)−1.382(4) 1.298(5)− 1.304(5) 1.452(6)− 1.463(6) 6.3(5)−7.9(5) av. 7.2 39.6 79.65(14) − 79.98(14) 2.24

M−N1 (Å) M−N2 (Å) M−N3 (Å) M−N4 (Å) M−N5 (Å) M−N6 (Å) N−O (Å) CN (Å) C−C (Å) NC−CN (deg) φ (deg)a α (deg)b h (Å)c

Co 2.1118(19) 2.1619(19) 2.107(2) 2.1801(19) 2.0757(19) 2.157(2) 1.374(2)−1.382(2) 1.282(3)−1.284(3) 1.479(3)−1.486(3) 1.1(3)−6.3(3) av. 3.2 14.7 73.83(7)−75.28(7) 2.53

Ni 2.0391(15) 2.0954(15) 2.0214(15) 2.1202(15) 2.0382(14) 2.0900(15) 1.3769(18)− 1.3814(18) 1.285(2) − 1.289(2) 1.476(2) − 1.483(2) 1.2(2)−9.8(2) av. 6.6 30.1 76.19(6)−76.82(6) 2.41

Znd 2.122(6) [2.139(7)] 2.177(7) [2.202(7)] 2.141(7) [2.152(7)] 2.143(7) [2.169(7)] 2.212(6) [2.179(6)] 2.178(6) [2.129(6)] 1.372(8)−1.393(7) [1.371(8)−1.391(8)] 1.262(10)−1.286(9) [1.279(10)−1.302(10)] 1.475(11)−1.484(11) [1.461(11)−1.499(11)] 1.3(11)−12.7(10) [0.9(11)−2.9(10)] av. 5.8 [1.6] 17.3 [15.9] 73.5(2)−74.3(2) [73.5(3)−74.5(2)] 2.54 [2.55]

Mnd 2.218(4) [2.218(4)] 2.243(4) [2.233(4)] 2.207(4) [2.215(4)] 2.222(4) [2.243(4)] 2.236(4) [2.209(4)] 2.226(4) [2.249(4)] 1.370(4)−1.379(4) [1.376(4)−1.381(4)] 1.282(5)−1.289(5) [1.279(5)−1.289(5)] 1.479(6)−1.483(6) [1.485(6)−1.487(6)] 0.4(6)−5.4(5) [1.9(6)−5.4(5)] av. 2.9 [3.9] 6.4 [5.8] 71.51(13) − 71.97(13) [71.71(14) − 72.14(13)] 2.58 [2.59]

a Angle of the TP-to-TAP distortion of the coordination polyhedron. bBite (or N−M−N) angles. cHeight of the TP-TAP polyhedron. dValues in the parentheses are for the second symmetry-independent molecule of the complex in the crystal.

Fe

M

Table 2. Main Geometrical Parameters of the Clathrochelate Framework in the Complexes Obtained

Inorganic Chemistry Article

Figure 3. Variable-temperature DC magnetic susceptibility data and its fit (solid red line) for a microcrystalline sample of the complex Co (red solid circles). The black line represents the data from ab initio CASSCF calculation.

Figure 4. Magnetization curves for the complex Co. Solid lines are their best fits by the eq 1. The dashed lines represent the data from ab initio CASSCF calculation.

In the NMR spectroscopy, the presence of paramagnetic species results in the paramagnetic shifts of all the nuclei:

δobs = δdia + δCS + δ PCS

6947

(2)

(δobs  observed chemical shift, ppm; δdia  diamagnetic contribution; δCS  contact shift that arises from spin polarization conveyed through molecular orbitals and that becomes negligible at a distance of 5−6 covalent bonds; δPCS  pseudocontact shift that arises from dipolar coupling between magnetic moments of a nucleus and of an unpaired electron and has a 1/r3 dependence on the distance to the paramagnetic species68−70). In the 1H and 13C spectra of the complexes Ni and Mn, chemical shifts of the nuclei in the pyridineoximate moieties reach hundreds of ppm (Figures S1−S4), while for the phenyl group located at more than five chemical bonds from the metal ion, they are close to those for the diamagnetic complexes Fe and Zn. The latter indicates that the induced paramagnetic shifts are of the contact nature and that the pseudocontact


Article

Inorganic Chemistry contribution is weak. In contrast, the nuclei of the phenyl group in the complex Co feature large and very different paramagnetic shifts (Figures 5 and S5).

Figure 5. 1H NMR spectrum (600 MHz, 20 °C) of the complex Co.

To evaluate the contact contribution to the chemical shifts, quantum chemical calculations were performed for all the above complexes, starting from their experimental geometries as obtained by X-ray diffraction. The contact shifts were estimated using the spin-Hamiltonian (3) based on the calculated g-tensor and isotropic values of hyperfine interaction tensors Aiso: T T Ĥ = μB BTgŜ + hŜ DŜ + hA isoŜ I ̂ + μ N gN BTI ̂

(3)

Fitting the data for the complex Co (Figure 6) with eq 4 produces the value of the axial magnetic susceptibility tensor anisotropy (Δχax = 22.5 × 10−32 m3 mol−1) that is among the largest reported to date for transition metals complexes at room temperature.51 1 δiCal = Δχ (3 cos2 θi − 1) + δiCS + δidia 12πri 3 ax (4)

Figure 6. Correlation plot of experimental (x-axes) vs theoretical (yaxes) 1H and 13C chemical shifts for the complex Co.

Another way to estimate the magnetic anisotropy from the experimental data is based on the temperature-dependence of the paramagnetic shifts in the NMR spectra.71 For the complex Co, they have been collected at different temperatures from 195 to 270 K. The values of Δχax at each of these points (Figure 7) were fitted by the spin-Hamiltonian (1), with χ-tensor elements expressed as χii =

NAkT ∂ 2 ln Zi 10 ∂ 2Bi

(5) i=1

(Zi  a partition function equal to ∑N e−Ei / kT , i = x,y,z). The temperature dependence of Δχax from the NMR data was fitted as follows: χyy + χxx Δχax = χzz − (6) 2 The data can be described by the following magnetic parameters: g⊥ = 2.22, g∥ = 2.86, D = −95 cm−1, which are close to the values obtained by magnetometry; the difference may be due to the different phase states of the sample (microcrystalline in the case of magnetometry and liquid for NMR). The values

Figure 7. Temperature dependence of the values Δχax of the complex Co as obtained from NMR; black solid line is the fit.

from ab initio CASSCF calculations (gx = 2.03, gy = 2.05, gz = 2.92, D = −78 cm−1, E/D = 0.019) are also quite similar. Such a large negative zero-field splitting energy D may provide slow magnetization relaxation. For this reason, AC 6948


Article

Inorganic Chemistry

4. CONCLUSION New mononuclear boron-capped zinc, iron, nickel, manganese, and cobalt(II) clathrochelates reported here tend to adopt a trigonal prismatic geometry, which in the case of the cobalt(II) complex leads to a very large value of axial magnetic susceptibility tensor anisotropy at room temperature (Δχax = 22.5 × 10−32 m3 mol−1), making it potentially useful as a paramagnetic shift tag for biomedical applications. The large negative value of the zero-field splitting energy (D = −78 cm−1 from magnetometry) also leads to a slow magnetic relaxation with a high effective barrier to magnetization reversal (70.5 cm−1 from magnetometry). These findings were supported by NMR spectroscopy, which thus emerges as a convenient tool for prescreening new magnetic compounds (shift tags and SMMs) before turning to much more time-consuming and less accessible magnetometry.

magnetic susceptibility measurements were performed for a microcrystalline sample of the complex Co (Figures S9−S10). Fitting the obtained Cole−Cole plots within the generalized Debye model (Figure S11) gives the dependences of the relaxation time τ vs the temperature (Figure 8).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00447. Supplementary Figures S1−S14 and Tables S1−S4 (PDF)

Figure 8. Arrhenius plot of the relaxation time τ vs the inverse temperature for the complex Co obtained in the absence of a DC field and under the DC field of 1000 Oe. The solid lines show fits to the data using eq 7; for parameters of the fits, see Supplementary Table S4. The dot lines show the fits to the data by the Arrhenius law.

Accession Codes

CCDC 1531859−1531863 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

In the absence of an external DC magnetic field, the relaxation time is independent of the temperature below 8 K, indicating the prevalence of the quantum tunneling of magnetization. Although QTM is formally forbidden for complexes with odd electron count and axial magnetic anisotropy, hyperfine21 or dipole−dipole41 coupling may still result in a nonzero tunneling contribution. In contrast, the data under the DC magnetic field of 1000 Oe show the decrease in its contribution. The field dependence of the relaxation time reaches a plateau at fields more than 1000 Oe (Figures S12− S14), so the measurements were done at this field. Simultaneously fitting the temperature- and the field-dependence of the relaxation time by the eq 7, which takes into account all the possible relaxation pathways (Orbach, quantum tunneling, direct and Raman processes),39 results in the Orbach barrier to magnetization reversal of 194.6 cm−1. The latter value is close to 2|D|. τ −1 = AH2T +

B1 1 + B2 H

2

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +7-499-135-50-85. ORCID

Yulia V. Nelyubina: 0000-0002-9121-0040 Valentin V. Novikov: 0000-0002-0225-0594 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The study was supported by Russian Science Foundation (Project 14-13-00724). NMR measurements were funded by the Russian Foundation for Basic Research (Project 16-3300233) and by the Council of the President of the Russian Federation (Project MK-2179.2017.3).

■

+ CT n + τ0−1 exp( −U /kT )

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