Slow Spin Relaxation in Dioxocobaltate(II) Anions Embedded in the

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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Slow Spin Relaxation in Dioxocobaltate(II) Anions Embedded in the Lattice of Calcium Hydroxyapatite Mikhail A. Zykin,† Konstantin A. Babeshkin,‡ Oxana V. Magdysyuk,§ Evgeny O. Anokhin,† Walter Schnelle,∥ Claudia Felser,∥ Martin Jansen,∥,⊥ and Pavel E. Kazin*,‡ †

Department of Materials Science and ‡Department of Chemistry, Moscow State University, 119991 Moscow, Russia § Diamond Light Source Ltd., Harwell Science and Innovation Campus, OX11 0DE Didcot, U.K. ∥ Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany ⊥ Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany S Supporting Information *

ABSTRACT: Pure-phase cobalt-doped calcium hydroxyapatite ceramic samples with composition Ca10(PO4)6[(CoO2)x(OH)1−2x]2, where x = 0−0.2, were synthesized by high-temperature solid-state reaction, and their crystal structures, vibrational spectra, and magnetic properties were studied. Co atoms are found to enter into the apatite trigonal channel formally substituting H atoms and forming bent dioxocobaltate(II) anions. The anion exhibits singlemolecule-magnet (SMM) behavior: slow relaxation of magnetization below 8 K under a nonzero magnetic field with an energy barrier of 63 cm−1. The barrier value does not depend on the concentration of Co ions, virtually coincides with the zerofield-splitting energy as determined from direct-current magnetization, and is very close to the value obtained earlier for cobaltdoped strontium hydroxyapatite. Moreover, the vibration frequencies of the dioxocobaltate(II) anion are found to be the same in calcium and strontium apatite matrixes. The very weak dependence of the SMM parameters on the matrix nature in combination with good chemical and thermal stabilities of the compounds provides wide opportunities to exploit the intrinsic properties of such a SMM-like anion.

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INTRODUCTION Single-molecule magnets (SMMs), compounds demonstrating magnetic bistability on the molecular level and showing slow relaxation of magnetization, continue to arise significant interest because of their potential as active elements in spintronic devices, quantum computers, and media for magnetic data storage “on one molecule”.1 The majority of existing SMMs belong to the family of molecular complexes of d and f elements. The magnetic bistability of a single metal ion, or a cluster of exchange-coupled ions, in such complexes is provided by a high magnetic anisotropy caused by significant contribution of orbital angular momentum to the electronic ground state of the paramagnetic molecule.1d Very recently, significant progress in the SMM performance has been achieved; the energy barrier for magnetization reversal Ueff has been raised to 413 cm−12a for a 3d-metal-based SMM (cobalt amido complex) and to above 1000 cm−1 for 4f metal (dysprosium) complexes.2b−e The most striking feature reported this year is a huge jump of the magnetization blocking temperature to 60 K in a dysprosium metallocene,2c,d which gives hope that SMMs will soon operate at liquid-nitrogen temperature. However, until recently, extended solid inorganic compounds hosting SMM-like atomic © XXXX American Chemical Society

groups were virtually out of consideration. A few years ago we observed slow relaxation of magnetization in the apatite-type compounds doped with copper.3,4 This feature has been attributed to an unusual linear dioxocuprate(III) anion located in the trigonal channel of the hydroxyapatite lattice. The magnetization reversal barrier amounted to several tens of reciprocal centimeters. This finding has defined a new type of SMM representing isolated ions of d metals distributed in a diamagnetic dielectric crystalline inorganic matrix. They can also be considered as single-ion magnets (SIMs). Shortly afterward, Jesche et al. reported on another inorganic solid (Li1−xMx)3N, where M = Fe, Mn, Co, and Ni, in which a monovalent 3d metal atom was also found in linear coordination.5,6 The observed hysteresis in the low-temperature magnetization and slow relaxation of magnetization in the compounds with a low content of M suggested probable magnetic bistability of a single d metal ion. The best performance was found for the Fe+-doped compound, which exhibited a Ueff value of 300 cm−1. Received: August 31, 2017

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DOI: 10.1021/acs.inorgchem.7b02237 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Extending the research in apatite-based SMMs, we studied the crystal structure and magnetic properties of strontium and barium apatites doped with Co.7,8 The strontium apatite samples include two distinct types of paramagnetic centers with different slow magnetization relaxation features. The first type of paramagnetic center, identified as an intrachannel dioxocobaltate (II) anion with an unusual bent geometry, shows field-induced slow magnetic relaxation with Ueff values of 58−59 cm−1. The samples with low cobalt concentrations exhibit an additional contribution to the magnetization assigned to a paramagnetic center of a second type, which is characterized by slow magnetization relaxation even in the absence of a magnetic field with considerably larger Ueff values of 200−250 cm−1. The Co-doped barium apatite samples contain only the second-type paramagnetic centers with an even higher Ueff amounting to 387 cm−1. It is one of the highest values among 3d-metal-based SMMs, to our knowledge, conceding the superior performance of only one linear cobalt amido complex.2a The core atom of the second-type center was found to be an intrachannel Co atom as well. Obviously, the incorporation of Co ions in an apatite lattice is very promising for the construction of paramagnetic centers with SMM behavior. In order to optimize and understand in depth the properties of such slow-relaxing magnetic centers, in the present work, we introduced Co ions in the trigonal channels of calcium hydroxyapatite and investigated the crystal structure and magnetic properties of the compounds prepared.

Figure 2. Crystal lattice parameters a (crosses) and c (circles) versus Co content x in Ca10(PO4)6[(CoO2)x(OH)1−2x]2. The dotted line designates a linear fit of the c values.

case, a and c are expected to decrease with a Co content increase because of the smaller radius of Co2+ in comparison with that of Ca2+.10 The parameter c changes linearly with the Co content in accordance with Vegard’s law, suggesting a continuous series of solid solutions to form. The crystal structure refinement of 5 using powder synchrotron X-ray diffraction data (see Figure 1 and Tables S2 and S3) shows that the Co atom partially occupies a site in the trigonal channel between the two O(4) atoms, being randomly shifted by ∼0.5 Å apart from the channel’s center and thus forming a bent [O−Co−O]2− anion (Figure 3). Formally, the anion replaces two intrachannel hydroxide ions so that the Co atom acquires a coordination number of 2. Such localization of the [O−Co−O] group was observed earlier in both the strontium and barium apatites, and the bent geometry was attributed to a possible Renner−Teller distortion of a linear dioxocobaltate(II) anion with an orbitally degenerate electronic ground state.7,8 The position of the intrachannel O(4) atom, in fact, is an average of the positions of the O atoms belonging to different species −OH−, [OCoO]2−, and possibly O22− and O2−. Hence, the exact geometry of the dioxocobaltate anion cannot be determined. A reasonable estimate may be obtained using the center of gravity of the O(4) split site, which results in an average Co−O distance of 1.79 Å and an O−Co−O angle of 149°, close to the parameters found in the strontium and barium compounds.7,8 It is worth noting that the Co atom has one longer contact of 2.48 Å to the O(3) atom of a phosphate group. However, the distance looks too long for a bonding and the Co atom may still be considered to have a coordination number of 2. The refined Co site occupancy corresponds to x = 0.22(1), which agrees with the nominal x = 0.2, implying that all of the cobalt loaded is situated in the trigonal channel. Accordingly, the compounds contain dioxocobaltate anions that are statistically distributed in the apatite matrix and separated by a distance of at least 6.9 Å from each other. Fourier Transform Infrared (FT-IR) and Raman Spectroscopy. IR and Raman spectra of the samples prepared are shown in Figure 4. The spectra contain bands typical for the internal vibrations of the phosphate ion, a hydroxide ion libration peak near 630 cm−1, and new bands in the range of 600−750 cm−1 arising from the dioxocobaltate anion,

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RESULTS AND DISCUSSION Crystal Structure. Samples with nominal composition Ca10(PO4)6[(CoO2)x(OH)1−2x]2 (x = 0, 0.02, 0.05, 0.1, and 0.2, designated as 1−5, respectively) were studied. Powder X-ray diffraction analysis reveals the presence of a pure hydroxyapatite phase in all of the samples (Figures 1 and S1−S3). The increase of the cobalt doping level is accompanied by the respective growth of the lattice parameter c, whereas the parameter a remains virtually unchanged (Figure 2 and Table S1). This tendency suggests that Co is incorporated in the apatite trigonal channels9 rather than replacing Ca. In the latter

Figure 1. Synchrotron powder X-ray diffraction pattern of 5. Rietveld refinement. Observed (crosses), calculated (solid line), and difference (solid line below) plots. The positions of the Bragg reflections are shown as bars underneath. B


Article

Inorganic Chemistry

Figure 3. Crystal structure fragments of hydroxyapatite (left) and 5 (right). A section of the trigonal channel formed by face-shared Ca6 octahedra running along the c axis is marked by blue dashed lines. Only one of the phosphate anions surrounding the channel is shown.

ν1(PO4) shifts from 962 to 947 cm−1 on going from the calcium compound to the strontium compound]. We noticed similar robustness in the frequency of the [O−Cu−O] symmetric stretching vibration observed in the Raman spectra of the Cu-containing calcium, strontium, and barium apatites as well. 3,4 This feature may testify that an intrachannel dioxometallate ion interacts only weakly with the surrounding apatite matrix. The intensity of the hydroxide libration band decreases with increasing Co content. This tendency is in agreement with the assumed Co-for-H substitution in the apatite channels. Alternating-Current (ac) Susceptibility. Two samples, 2 and 5, with the lowest and highest cobalt concentrations respectively, were selected for further characterization. Pronounced peaks on the imaginary part χ′′ and corresponding inflections on the real part χ′ of the ac susceptibility frequency dependencies measured below 8 K under a direct-current (dc) magnetic field (Figures 5 and S4−S7) point to the presence of slow magnetic relaxation in both samples. The real and imaginary parts of the susceptibility were fitted simultaneously using the modified Debye function11 (see the Supporting Information) in order to determine the relaxation time τ, its distribution width parameter α, the isothermal susceptibility χT, the adiabatic susceptibility χS, and the contribution of a slowrelaxing magnetization χR = χ0 − χS. To reliably access all relaxation processes, we analyzed both the temperature dependence of τ under a chosen dc field of 1.5 kOe and the field dependence of τ at a temperature of 2 K. The dependence of the relaxation time on the temperature and magnetic field is determined by several relaxation processes and can be described by eq 1:12 τ −1 = AH 4T + B1/(1 + B2 H2) + CT n Figure 4. FT-IR (a) and Raman (b) spectra for 2−5. Samples’ color designation: 2, red; 3, orange; 4, green; 5, blue.

+ τ0−1 exp( − Ueff /kT )

(1)

The first term corresponds to the direct process with a phonon emission, the second one to the quantum tunneling (QT), the third one to the Raman process, and the last one to the Orbach mechanism. The field dependence of τ and Arrhenius plots for τ are shown in parts a and b of Figure 6, respectively. The field dependence of τ is determined by the first and second terms in eq 1. At a low magnetic field, QT gives the major contribution and τ increases with the field as the field suppresses QT. At a high magnetic field, a direct process

resembling many of the bands observed in the Co-doped strontium apatite.7 The most pronounced peaks at 728 cm−1 in the FT-IR spectrum and at 717 cm−1 in the Raman spectrum can be attributed to the antisymmetric and symmetric stretching of a [O−Co−O] group, respectively. Interestingly, the peak positions virtually coincide with those found in the strontium apatites,7 at 726 and 716 cm−1, respectively, whereas the bands of the phosphate group vibrations differ by ∼15 cm−1 [e.g., C


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Inorganic Chemistry

Figure 5. ac susceptibility versus frequency dependences for 2 (left) and 5 (right) under a dc field of 1.5 kOe at different temperatures. In-phase (top) and out-of-phase (bottom) parts of the susceptibility. Symbols: experimental points. Lines: fitting.

saturates. At intermediate temperatures, the Raman process prevails, and at higher temperatures, it gives way to an Orbach process, which provides a linear behavior of ln τ(1/T). The Orbach process parameter Ueff is of particular interest. However, the linear part of the curve is not well developed, especially not for 5. Therefore, analysis of the complete ln τ(1/ T) dependencies appears to be required, considering all terms in eq 1. In order to escape overparameterization in a quantitative analysis, first we fitted the τ(H) dependencies to estimate A, B1, and B2 for each sample. The obtained values were used as fixed parameters in a further analysis of the ln τ(1/T) curves. The resulting parameters are presented in Table 1. The contribution of the first term in eq 1 at an applied field of 1.5 kOe is very small, and the direct process effect may be neglected. The contribution of QT is substantial for both of the samples, being roughly an order of magnitude higher for 5. Because the concentration of Co ions in 5 is a factor of 10 larger than that in 2, QT may be determined mostly by dipolar interaction between paramagnetic centers: its energy is inversely proportional to their separation distance to the power of 3 and thus the energy is proportional to the volume concentration of Co ions. The Raman exponents n for both samples are smaller than an expected value of 9 for the Kramers ion Co2+.12a This value is often considered as a plausible one for Co-based SMMs.12b However, n in the range 1−6 may be obtained if relaxation is realized by optical and acoustic phonons jointly,13 and Raman exponents close to 2 and 3 were recently registered for different divalent cobalt complexes.14,15 Unlike the QT parameters, the Ueff values practically coincide for 2 and 5, implying that the energy barrier for magnetization relaxation is independent of the Co concentration. For Codoped strontium apatite, we encountered very similar Ueff values.7 Moreover, the value of τ0 is of the same order of 10−10 s for calcium and strontium compounds as well. Therefore, we consider the SIM center observed in the calcium

Figure 6. Field dependence of τ at a temperature of 2 K (a) and temperature dependence of τ (Arrhenius plots) under a dc field of 1.5 kOe (b) for 2 (red) and 5 (blue). Symbols: experimental points. Lines: fitting.

prevails and τ drops with the field. As is commonly accepted,1b at low temperature, QT limits τ and the ln τ(1/T) curve D


Article

Inorganic Chemistry Table 1. Parameters Obtained by Fitting ln τ(1/T) for 2 and 5 Using eq 1 sample 2 5

A (s−1·kOe−4·K−1) 0.0020(4) 0.0058(9)

B1 (s−1)

B2 (kOe−2)

56(12) 866(130)

C (K−n·s−1)

1.4(6) 1.1(3)

4.8(4) 33(3)

apatite to be the same as the first-type center in the strontium compound, representing an intrachannel dioxocobaltate(II) anion. At the same time, we did not see any hints of the second type of paramagnetic center with a higher Ueff value, which might correspond to that found in the strontium and barium apatites.7,8 dc Magnetization. The temperature dependences of χT for 2 and 5 are shown in Figure 7. χT(T) exhibits a steep rise with temperature in the low-temperature region, which is consistent with considerable magnetic anisotropy.

τ0 (s−1)

n 2.15(6) 3.04(7)

Ueff (cm−1) −10

1.7(2) × 10 1.1(4) × 10−10

63.1(7) 62(2)

sample from the nominal one. Particularly in the apatite structure, there is a possibility to form defects in which Co ions may cluster into antiferromagnetically coupled entities, e.g., forming intrachannel dimers [O−Co−O−Co−O], as was discussed in ref 8. Such a probability grows with the concentration of Co ions. The entities have to exhibit much smaller susceptibility with relatively weak temperature dependence, so that they will mostly contribute to χTIP. This may explain both lower g and enhanced χTIP for 5 in comparison with those for 2. In this sense, the g parameters obtained for 2 are more reliable.

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CONCLUSIONS We synthesized a range of pure-phase calcium apatites containing different quantities of Co ions in the trigonal channels. The compounds exhibit slow relaxation of magnetization characterized by a single type of SMM-like paramagnetic center identified as a bent dioxocobaltate(II) anion. The energy barrier for magnetization reversal is (1) equal to the energy of the first exited Kramers doublet |2D| determined from dc magnetization data, (2) independent of the Co concentration, and (3) very close to that found in analogous Co-doped strontium apatites. Moreover, the dioxocobaltate(II) vibrational frequencies in the calcium apatite virtually coincide with those in the strontium compound. The observed stability of the SMM parameters in different solid matrixes in combination with the compound robustness provides more opportunities to develop relevant functional materials.

Figure 7. χT versus T plots for 2 (red) and 5 (blue). Symbols: experimental points. Lines: fitting (see Table 2). 3

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(see the Using appropriate analytical equations for S = Supporting Information), we fitted χT(T) and estimated a zerofield-splitting parameter D, Lande factors g∥ and g⊥, and a contribution of temperature-independent paramagnetic susceptibility χTIP (Table 2). D values are found to be negative and

All of the samples were synthesized by a high-temperature solid-state reaction. CaCO3, (NH4)2HPO4, and CoCO3 were used as starting materials. The reagents in a stoichiometric ratio were mixed and ground in an agate mortar and, subsequently, heat-treated according to the following procedure: heating to 400 °C, dwelling for 1 h, heating to 600 °C, dwelling for 1 h, heating to 800 °C, dwelling for 5 h, and slow cooling to room temperature. The obtained materials were ground, pressed into pellets, sintered at 1200 °C for 4 h, and then quenched in air. This procedure was repeated three times with intermediate regrinding and pressing. Synchrotron powder X-ray diffractograms of samples 2−5 were recorded at the Diamond Light Source, I12-JEEP beamline17 (λ = 0.23369 Å), using a Pixium RF4343 2D-area flat-panel detector. Wavelength calibration and data reduction were performed using the software DAWN.18,19 Powder X-ray diffraction of sample 1 was performed using a Rigaku D/MAX 2500 diffractometer with a rotating anode (Cu Kα radiation). The crystal structure was refined using the JANA 2006 program.20 IR spectra were measured on a PerkinElmer Spectrum One spectrometer and Raman spectra on a Renishaw in Via Reflex spectrometer with laser excitation at a wavelength of 514 nm. Magnetic measurements were performed on a Quantum Design SQUID magnetometer (SVSM). A ceramic sample of ca. 50−100 mg mass was fixed by GE-varnish (ca. 1 mg) at the center of a quartz-glass paddle. Magnetization was measured under fields of 1 and 10 kOe for 2 and 5, respectively, in the temperature range 1.8−300 K. The ac susceptibility was measured at an ac field amplitude of 3 Oe under a dc field of 1.5 kOe in the temperature range 2−8 K and under different fields in the range of 0−20 kOe at a temperature of 2 K. The susceptibility values were corrected for core diamagnetism using Pascal’s constants.

Table 2. Parameters Obtained by Fitting χT versus T for 2 and 5 sample

D (cm−1)a

g||

g⊥

χTIP (emu mol−1)

2 5

−31.9(1) −32.5(3)

2.397(1) 2.145(2)

2.091(1) 1.906(2)

8.2(2) × 10−4 16.3(1) × 10−4

EXPERIMENTAL SECTION

a The energy separation between the |MS| = 3/2 and 1/2 levels (defining Ueff) is equal to 2|D|.

to practically coincide for both samples. Accordingly, the ground Kramers doublet has |MS| = 3/2, which defines easy-axis magnetic anisotropy. The first exited doublet with |MS| = 1/2 is at 2|D| higher energy, and one expects that the Orbach process would include excitation to this energy level. Indeed, the Ueff values obtained from the magnetization relaxation analysis are very close to the estimated 2|D| values in excellent agreement with the theory. The absolute values of g are different for 2 and 5, reflecting the difference in the χT(T) values. It should be noted that the refined g values, in fact, include a scaling factor connected to the possible deviation of the paramagnetic ion content in the E


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Inorganic Chemistry

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the Axial Limit. Angew. Chem., Int. Ed. 2017, 56, 11445−11449. (d) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular Magnetic Hysteresis at 60 K in Dysprosocenium. Nature 2017, 548, 439−442. (e) Krylov, D. S.; Liu, F.; Avdoshenko, S. M.; Spree, L.; Weise, B.; Waske, A.; Wolter, A. U. B.; Buechner, B.; Popov, A. A. Record-High Thermal Barrier of the Relaxation of Magnetization in the Nitride Clusterfullerene Dy2ScN@C80-Ih. Chem. Commun. 2017, 53, 7901−7904. (3) Kazin, P. E.; Zykin, M. A.; Zubavichus, Y. V.; Magdysyuk, O. V.; Dinnebier, R. E.; Jansen, M. Identification of the Chromophore in the Apatite Pigment [Sr10(PO4)6(CuxOH1‑x‑y)2]: Linear OCuO− Featuring a Resonance Raman Effect, an Extreme Magnetic Anisotropy, and Slow Spin Relaxation. Chem. - Eur. J. 2014, 20, 165−178. (4) Kazin, P. E.; Zykin, M. A.; Schnelle, W.; Felser, C.; Jansen, M. Rich Diversity of Single-Ion Magnet Features in the Linear OCuIIIO− Ion Confined in the Hexagonal Channels of Alkaline-Earth Phosphate Apatites. Chem. Commun. 2014, 50, 9325−9328. (5) Jesche, A.; McCallum, R. W.; Thimmaiah, S.; Jacobs, J. L.; Taufour, V.; Kreyssig, A.; Houk, R. S.; Bud’ko, S. L.; Canfield, P. C. Giant Magnetic Anisotropy and Tunnelling of the Magnetization in Li2(Li1‑xFex)N. Nat. Commun. 2014, 5, 3333. (6) Jesche, A.; Ke, L.; Jacobs, J. L.; Harmon, B.; Houk, R. S.; Canfield, P. C. Alternating Magnetic Anisotropy of Li2(Li1−xTx)N (T = Mn, Fe, Co, and Ni). Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 180403. (7) Kazin, P. E.; Zykin, M. A.; Schnelle, W.; Zubavichus, Y. V.; Babeshkin, K. A.; Tafeenko, V. A.; Felser, C.; Jansen, M. Cobalt-Based Single-Ion Magnets on an Apatite Lattice: Toward Patterned Arrays for Magnetic Memories. Inorg. Chem. 2017, 56, 1232−1240. (8) Kazin, P. E.; Zykin, M. A.; Trusov, L. A.; Eliseev, A. A.; Magdysyuk, O. V.; Dinnebier, R. E.; Kremer, R. K.; Felser, C.; Jansen, M. A Co-Based Single-Molecule Magnet Confined in a Barium Phosphate Apatite Matrix with a High Energy Barrier for Magnetization Relaxation. Chem. Commun. 2017, 53, 5416−5419. (9) Renaudin, G.; Gomes, S.; Nedelec, J. M. First-Row Transition Metal Doping in Calcium Phosphate Bioceramics: a Detailed Crystallographic Study. Materials 2017, 10, 92−113. (10) Veselinović, L.; Karanović, L.; Stojanović, Z.; Bračko, I.; Marković, S.; Ignjatović, N.; Uskoković, D. Crystal Structure of CobaltSubstituted Calcium Hydroxyapatite Nanopowders Prepared by Hydrothermal Processing. J. Appl. Crystallogr. 2010, 43, 320−327. (11) Aubin, S. M. J.; Sun, Z.; Pardi, L.; Krzystek, J.; Folting, K.; Brunel, L.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Reduced Anionic Mn12 Molecules with Half-Integer Ground States as Single-Molecule Magnets. Inorg. Chem. 1999, 38, 5329−5340. (12) (a) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions; Clarendon Press: Oxford, U.K., 1970; Vol. 1, p 652. (b) Buvaylo, E. A.; Kokozay, V. N.; Vassilyeva, O. Y.; Skelton, B. W.; Ozarowski, A.; Titiš, J.; Vranovičová, B.; Boča, R. Field-Assisted Slow Magnetic Relaxation in a Six-Coordinate Co(II)−Co(III) Complex with Large Negative Anisotropy. Inorg. Chem. 2017, 56, 6999−7009. (13) Shrivastava, K. N. Theory of Spin-Lattice Relaxation. Phys. Status Solidi B 1983, 117, 437−458. (14) Chandrasekhar, V.; Dey, A.; Mota, A. J.; Colacio, E. Slow Magnetic Relaxation in Co(III)-Co(II) Mixed-Valence Dinuclear Complexes with a CoIIO5X (X = Cl, Br, NO3) Distorted-Octahedral Coordination Sphere. Inorg. Chem. 2013, 52, 4554−4561. (15) Rechkemmer, Y.; Breitgoff, F. D.; van der Meer, M.; Atanasov, M.; Hakl, M.; Orlita, M.; Neugebauer, P.; Neese, F.; Sarkar, B.; van Slageren, J. A Four-Coordinate Cobalt(II) Single-Ion Magnet with Coercivity and a Very High Energy Barrier. Nat. Commun. 2016, 7, 10467. (16) Boča, R. Zero-Field Splitting in Metal Complexes. Coord. Chem. Rev. 2004, 248, 757−815. (17) Drakopoulos, M.; Connolley, T.; Reinhard, C.; Atwood, R.; Magdysyuk, O.; Vo, N.; Hart, M.; Connor, L.; Humphreys, B.; Howell, G.; Davies, S.; Hill, T.; Wilkin, G.; Pedersen, U.; Foster, A.; De Maio, N.; Basham, M.; Yuan, F.; Wanelik, K. I12: the Joint Engineering,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02237. Powder synchrotron X-ray diffraction patterns, crystal structure parameters, plots of ac susceptibility under different dc fields, and equations for magnetic data treatment (PDF) Accession Codes

CCDC 1571798 contains 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Martin Jansen: 0000-0003-0762-0985 Pavel E. Kazin: 0000-0002-1415-2190 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation under Grant 16-13-10031. REFERENCES

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