Construction and Magnetic Study of a Trigonal-Prismatic Cobalt(II

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Cite This: Inorg. Chem. 2018, 57, 14047−14051

Construction and Magnetic Study of a Trigonal-Prismatic Cobalt(II) Single-Ion Magnet Binling Yao,†,‡ Yi-Fei Deng,†,‡ Tianran Li,† Jin Xiong,§ Bing-Wu Wang,§ Zhiping Zheng,† and Yuan-Zhu Zhang*,† †

Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055, China Beijing National Laboratory of Molecular Science, College of Chemistry and Molecular Engineering, State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, P. R. China

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S Supporting Information *

huge anisotropy for complexes with the same coordination number may be realized by rational modification toward favorable distortion; for example, four-coordinate mononuclear cobalt(II) complexes were found to show a remarkable range of zero-field-splitting (ZFS) parameters from D = −161 to +16 cm−1.8 However, it should be mentioned that coordinatively unsaturated complexes may be air-sensitive, thus hindering possible practical application. Therefore, research on the more common complexes, such as six-coordinate CoII SIMs, is becoming active. Most six-coordinate cobalt(II) complexes in favor of octahedral geometry exhibit large positive D values,11 while several trigonal-prismatic ones have recently been found to exhibit large uniaxial magnetic anisotropy and typical SIM behavior under small or even zero dc fields.10 A so-called hexadentate clathrochelate ligand is often necessary for this special geometry. It has been found that the rigidity of the ligands lowers the overall stabilization of the system and leads to a smaller energy span of the d orbitals in comparison to the octahedral geometry.13 To retain the trigonal symmetry but less rigidity, by utilizing tripodal tridentate ligands, we and others have recently prepared a series of mononuclear trigonalantiprismatic cobalt(II) complexes, which also exhibited huge magnetic anisotropy along with field-induced slow relaxation of magnetization.12 Both geometries of such trigonal cobalt(II) complexes lead to the very small degeneracy of the highest double-occupied and lowest single-occupied d orbitals on x2−y2 and xy, resulting in a considerable contribution of the unquenched orbital to the magnetic moment.7b With continuous interest in these trigonal molecules, here we explored a new ligand of tris[6-(1H-pyrazol-1-yl)pyridin-2-yl]methanol (tppm; Scheme 1) through the reaction of tris[2-(6bromopyridinyl)]methanol and pyrazole, and the trigonalprismatic cobalt(II) complex [Co(tppm)][ClO4]2·2CH3CN· H2O (1) was further constructed with this ligand. Magnetic studies indicate that 1 exhibited a large negative ZFS parameter of −80.7 cm−1 and SMM behavior under zero dc field with an effective energy barrier of 56 K. Complex 1 crystallized in the monoclinic space group P21/c (Figure 1). The central CoII ion is locked by the hexadentate ligand tppm into trigonal-prismatic geometry (Figure 1b) with

ABSTRACT: A new tripodal hexadentate ligand of tris[6-(1H-pyrazol-1-yl)pyridin-2-yl]methanol (tppm) was synthesized and explored for constructing the trigonal-prismatic cobalt(II) complex [Co(tppm)][ClO4]2·2CH3CN·H2O (1). Magnetic study showed that 1 exhibited large uniaxial magnetic anisotropy with a zerofield-splitting parameter of −80.7 cm−1 and typical singlemolecule-magnet behavior.

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ingle-molecule magnets (SMMs),1 which feature magnetic bistability on a molecular scale, have been proposed as perspective components for high-density information storage, quantum computing, and molecular spintronics.2 The initial research was focused on polynuclear transition-metal clusters,3 while in recent years research on complexes with only one spin carrier, referred to as single-ion magnets (SIMs),4 has achieved significant progress. The key focus underpinning the development of SIMs is optimizing large uniaxial magnetic anisotropy with minimal rhombic terms, achieved by regulation of the metal ions and coordination environment. As a result, lanthanide ions first emerged as mononuclear paramagnetic centers for constructing SIMs because the spin−orbit coupling (SOC) is large enough to compensate for any quenching effect, of which breakthroughs have been achieved recently on both the energy barrier (Ueff) and blocking temperature (TB).5 Meanwhile, their third-row transition-metal analogues, sourced from CrII to NiI/ NiII ions, especially for CoII ions, have also displayed interesting SMM properties.4b,c,6 Interest in high-spin CoII d7 complexes originates from the large magnetic anisotropy with orbital contribution, caused either by an orbitally degenerate ground state with the first-order orbital momentum unquenched by the crystal field or by the SOC when the orbital degeneracy is broken in a low-symmetry ligand field. By the application of a weak ligand field, a variety of CoII SIMs with different coordination environments were reported to exhibit slow magnetic relaxation of magnetization.7−12 However, the examples of CoII-based SIMs with slow relaxation in the absence of a direct-current (dc) field are still limited.9,10 A smart choice for constructing such complexes is the linear two-coordinate strategy, leading to a degenerate ground state and a smaller rate of magnetization tunneling, as in the case of a two-coordinate CoII SIM with a record anisotropy barrier of 413 cm−1 among transition-metal complexes.7c In fact, © 2018 American Chemical Society

Received: September 21, 2018 Published: October 31, 2018 14047

DOI: 10.1021/acs.inorgchem.8b02692 Inorg. Chem. 2018, 57, 14047−14051

Communication

Inorganic Chemistry Scheme 1. Synthesis and Chemical Structure of the tppm Ligand

Figure 2. Temperature dependence of χMT obtained at 1000 Oe (data points) for 1. Solid lines represent the fit by the PHI program. Inset shows the 2−5 K field-dependent magnetization and its fit obtained simultaneously with the χMT fit.

−80.7 cm−1, and |E| = 0.6 cm−1 with R = 1.6 × 10−4. It should be mentioned that imposing a positive D parameter results in a much worse and unacceptable fit. In order to further understand the magnetic anisotropy, CASSCF/CASPT2 calculations considering double-shell effects were performed on this structure (Tables S6−S8) using MOLCAS 8.1 packages.16 To improve the result, a double-occupied ligand orbital was added to the active space [CAS(9,11)]. The calculated axial ZFS parameter D was found to be −124 cm−1 upon treatment with a Co2+ ion as S = 3/2, further suggesting uniaxial magnetic anisotropy. Both the experimental result and theoretical calculation are well in agreement with previous reports for this special geometry.10 A small contraction (C3v) on one of the triangular faces of the trigonal prism (D3h), as in this case, would lightly lift up the energy of the z2 orbital; however, the deviation may not be strong enough to lead to its reversal with the x2−y2 and xy orbitals. As such, the resulting D parameter should be negative and huge.7b We assume that the very small twist deviation may result in a negligible difference between the gx and gy components, and a small E parameter is thus expected based on the following equation (eq 2; ζ is the SOC constant):17

Figure 1. (a) Structure of complex 1 (H atoms, solvent molecules, and counteranions were omitted for clarity). (b and c) Trigonal-prismatic coordination model of the central CoII ion.

Co−N bond lengths around 2.114(4)−2.139(4) Å. The Npy− Co−Npy angles are acute in the range of 81.02−82.52°, while the Npz−Co−Npz ones are larger within the range of 89.35−93.67°. The distortion of the angular parameters ϕ (the Bailar twist angle13) for the trigonal prism10d was calculated as 3.80° (Figure 1c). The SHAPE software14 gave a deviation parameter of 0.589 from the ideal D3h symmetry (0). However, a closer look at the structure reveals that the N···N distances within the pyridine units [2.774(6)−2.819(6) Å] are significantly shorter than those within the opposite pyrazole units [2.986(6)−3.096(7) Å], indicating a significant degree of truncation from the trigonalpyramidal geometry with a lower symmetry of C3v. The nearest intermolecular Co···Co separation is 8.891(2) Å (Figure S1). Direct current (dc) susceptibility measurements of 1 were collected at 1000 Oe over the 2−300 K temperature range (Figure 2). The χMT product at room temperature was 3.22 cm3 K mol−1, which is much larger than the spin-only value of free CoII ion (S = 3/2, g = 2, and 1.875 cm3 K mol−1), indicating significant unquenched orbital momentum. Upon cooling, the χMT value remains roughly constant before a drop at about 100 K and then gradually decreases to a minimum value of 2.5 cm3 K mol−1 at 2 K. The magnetization curves of 1 at 2−5 K exhibited continuous increase, reaching 2.38 Nβ at 7 T and 2 K, far away from the saturation, further confirming the significant magnetic anisotropy (inset of Figure 2). Both χMT versus T and M−H data were fitted simultaneously via the PHI program15 based on the following spin Hamiltonian (eq 1; with gx = gy):

ζ (g − gyy) (2) 12 xx The alternating-current (ac) susceptibility measurements were performed to investigate the slow relaxation dynamics. Under zero dc field, variable-temperature ac data showed high and broad frequency-dependent peaks in the range of 5−15 K (Figures 3a and S2 and S3). The upturn of the ac signals below 5 K (Figure S2) indicated heavy quantum tunneling of magnetization (QTM).18 When a 1500 Oe dc field exacted from the optimized field measurements is applied (Table S5 and Figures S4−S6), the out-of-phase susceptibility has a maximum in the whole temperature range of 5−15 K as measured, indicating that the QTM has been suppressed effectively (Figures 3b and S7). The temperature dependence of the relaxation time (τ) in both the 0 and 1500 Oe dc fields was obtained from the fit of the Cole−Cole plots using a generalized Debye model19 (Figures S8 and S9); both cases have small α values (Tables S3 and S4). The relaxation times τ obtained were plotted versus T−1, generating the Arrhenius-like diagram. Both Arrhenius diagrams were temperature-dependent and barely linear in the high-temperature region, yielding estimations of the thermal-activated E=−

2 2 2 Ĥ = D[Sẑ − S(S + 1)/3] + E(Sx̂ − Sŷ )

+ μB

∑ i=x ,y,z

Sî giBi

(1)

where E is the rhombic ZFS parameter, μB is the Bohr magneton, g is the Landé factor, and B is the magnetic induction. The best fit was obtained with parameters gx = gy = 2.31, gz = 2.99, D = 14048

DOI: 10.1021/acs.inorgchem.8b02692 Inorg. Chem. 2018, 57, 14047−14051

Communication

Inorganic Chemistry

Figure 4. Magnetic hysteresis measurements of 1 recorded at 1.9 K with various sweep rates. Figure 3. Frequency-dependent out-of-phase magnetic susceptibilities at 0 (a) and 1500 Oe dc field (b). Temperature dependence of the relaxation rates for 1 under 0 (c) and 1500 Oe (d) dc field. The black lines correspond to the high-temperature Arrhenius fitting. The red lines represent the fitting with eq 3.

prismatic CoII SIM. The complex exhibited large uniaxial magnetic anisotropy with a D parameter of −80.7 cm−1 and zero-field slow relaxation of magnetization. More importantly, butterfly-like hysteresis loops were clearly observed below 3 K. However, the thermal-activated energy barrier (56 K) was far away from the expected value (|2D|), and the dominated QTM and Raman processes were still evidenced for the overall temperature. Further modifications of this system toward an enhanced SMM performance, including better isolation of the spins to remove possible dipole−dipole interactions and attempts to incorporate this geometry into clusters as well as an in-depth understanding of the magnetostructural correlation, are underway in our laboratory.

energy barrier at 56 K, τ0 = 1.7 × 10−4 s, and 61 K, τ0 = 7.8 × 10−6 s, respectively (Figure 3). Such values are far smaller than the barrier expected from the dc data (|2D| = 161 cm−1), indicating the dominating direct, QTM, and/or Raman processes involved in the relaxation pathways. Therefore, a model including these three possible relaxation processes was employed to analyze this relaxation (eq 3):20 B1 τ −1 = AH2T + + CT n 1 + B2 H2 (3)

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where A, B1, B2, C, and n are coefficients, H is the magnetic field, and T is the temperature. In order to avoid overparameterization, the τ versus H plot was modeled first to obtain direct and tunneling parameters (A = 0.19 s−1 K−1 kOe−2, B1 = 310.6 s−1, and B2 = 10 kOe−2), which were further applied to fit the temperature-dependent Arrhenius-like diagram. Under zero dc field, only the QTM and Raman processes were considered because the direct process should be negligible (H = 0), giving C = 1.7(6) × 10−3 K−6 s−1 and n = 6.0(2). As for the data under 1500 Oe dc field, the good temperature dependence of τ suggests that the QTM process has been well suppressed. Moreover, we found that the contribution of the direct process is also very small compared with that of the Raman process. As a result, the relaxation time data can be modeled well by the power law τ−1 = CTn, with C = 9.1(2) × 10−4 K−5.6 s−1 and n = 5.6(1) (Figures 3d and S10). The ac data under both the zero and 1500 Oe dc fields indicated a dominant optical acoustic Raman process21 responsible for the spin relaxation. To further investigate the SMM behavior of 1, magnetization measurements were performed on the powder sample between 1.9 and 3.0 K at scan rates of 20−500 Oe s−1, respectively (Figures 4 and S11). The hysteresis loops were observed as a function of the temperature and field sweep rate, which exhibited increasing hysteresis with increasing field sweep rate at a constant temperature or decreasing temperature at a constant sweep rate, as found for related polynuclear SMMs.19 At zero dc field, the magnetization was suddenly reduced and behaved as butterfly-like hysteresis loops, which is attributed to the fast QTM and is in agreement with the ac magnetic susceptibility measurements. In summary, a new tripodal hexadentate ligand, tppm, was synthesized and explored for the preparation of a novel trigonal-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02692. X-ray crystallographic data, selected bond lengths and angles, crystal structures, and magnetic measurements (PDF) Accession Codes

CCDC 1856353 for 1 contain the supplementary crystallographic data for this paper. The 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuan-Zhu Zhang: 0000-0002-1676-2427 Author Contributions ‡

Authors have contributed equally

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the High-Performance Computing Platform at Peking University, the National Natural Science Foundation of China (Grant 21671095), Thousand Talents Program-Youth, and a start-up fund from SUSTech. 14049

DOI: 10.1021/acs.inorgchem.8b02692 Inorg. Chem. 2018, 57, 14047−14051

Communication

Inorganic Chemistry

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S.-L.; Shi, L.; Zhang, Y.-Q.; Wang, X.-Y. Probing the Effect of Axial Ligands on Easy-Plane Anisotropy of Pentagonal-Bipyramidal Cobalt(II) Single-Ion Magnets. Inorg. Chem. 2016, 55, 10859−10869. (7) (a) Jurca, T.; Farghal, A.; Lin, P.-H.; Korobkov, I.; Murugesu, M.; Richeson, D. S. Single-molecule magnet behavior with a single metal center enhanced through peripheral ligand modifications. J. Am. Chem. Soc. 2011, 133, 15814−15817. (b) Gómez-Coca, S.; Cremades, E.; Aliaga-Alcalde, N.; Ruiz, E. Mononuclear single-molecule magnets: tailoring the magnetic anisotropy of first-row transition-metal complexes. J. Am. Chem. Soc. 2013, 135, 7010−7018. (c) Yao, X.-N.; Du, J.-Z.; Zhang, Y.-Q.; Leng, X.-B.; Yang, M.-W.; Jiang, S.-D.; Wang, Z.-X.; Ouyang, Z.-W.; Deng, L.; Wang, B.-W.; Gao, S. Two-coordinate Co (II) imido complexes as outstanding Single-Molecule Magnets. J. Am. Chem. Soc. 2017, 139, 373−380. (8) (a) Yang, F.; Zhou, Q.; Zhang, Y.; Zeng, G.; Li, G.; Shi, Z.; Wang, B.; Feng, S. Inspiration from old molecules: field-induced slow magnetic relaxation in three air-stable tetrahedral cobalt(ii) compounds. Chem. Commun. 2013, 49, 5289−5291. (b) Saber, M. R.; Dunbar, K. R. Ligands effects on the magnetic anisotropy of tetrahedral cobalt complexes. Chem. Commun. 2014, 50, 12266−12269. (c) Boča, R.; Miklovič, J.; Titiš, J. Simple mononuclear cobalt(II) complex: A Single-Molecule Magnet showing two slow relaxation processes. Inorg. Chem. 2014, 53, 2367−2369. (d) 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, 10467−10473. (9) (a) Zadrozny, J. M.; Long, J. R. Slow magnetic relaxation at zero field in the tetrahedral complex [Co(SPh)4]2−. J. Am. Chem. Soc. 2011, 133, 20732−20734. (b) Fataftah, M. S.; Zadrozny, J. M.; Rogers, D. M.; Freedman, D. E. A mononuclear transition metal single-molecule magnet in a nuclear spin-free ligand environment. Inorg. Chem. 2014, 53, 10716−10721. (c) Fataftah, M. S.; Coste, S. C.; Vlaisavljevich, B.; Zadrozny, J. M.; Freedman, D. E. Transformation of the coordination complex [Co(C3S5)2]2− from a molecular magnet to a potential qubit. Chem. Sci. 2016, 7, 6160−6166. (d) Tu, D.; Shao, D.; Yan, H.; Lu, C. A carborane-incorporated mononuclear Co(II) complex showing zerofield slow magnetic relaxation. Chem. Commun. 2016, 52, 14326− 14329. (10) (a) Zhu, Y.-Y.; Cui, C.; Zhang, Y.-Q.; Jia, J.-H.; Guo, X.; Gao, C.; Qian, K.; Jiang, S.-D.; Wang, B.-W.; Wang, Z.-M.; Gao, S. Zero-field slow magnetic relaxation from single Co (II) ion: a transition metal single-molecule magnet with high anisotropy barrier. Chem. Sci. 2013, 4, 1802−1806. (b) Novikov, V. V.; Pavlov, A. A.; Nelyubina, Y. V.; Boulon, M.-E.; Varzatskii, O. A.; Voloshin, Y. Z.; Winpenny, R. E. P. A trigonal prismatic mononuclear cobalt(II) complex showing SingleMolecule Magnet behavior. J. Am. Chem. Soc. 2015, 137, 9792−9795. (c) Varzatskii, O. A.; Penkova, L. V.; Kats, S. V.; Dolganov, A. V.; Vologzhanina, A. V.; Pavlov, A. A.; Novikov, V. V.; Bogomyakov, A. S.; Nemykin, V. N.; Voloshin, Y. Z. Chloride ion-aided self-assembly of pseudoclathrochelate metal tris-pyrazoloximates. Inorg. Chem. 2014, 53, 3062−3071. (d) Zhu, Y.-Y.; Zhang, Y.-Q.; Yin, T.-T.; Gao, C.; Wang, B.-W.; Gao, S. A family of CoIICoIII3 Single-Ion Magnets with zero-field slow magnetic relaxation: fine tuning of energy barrier by remote substituent and counter cation. Inorg. Chem. 2015, 54, 5475− 5486. (e) Pavlov, A. A.; Nelyubina, Y. V.; Kats, S. V.; Penkova, L. V.; Efimov, N. N.; Dmitrienko, A. O.; Vologzhanina, A. V.; Belov, A. S.; Voloshin, Y. Z.; Novikov, V. V. Polymorphism in a cobalt-based SingleIon Magnet tuning its barrier to magnetization relaxation. J. Phys. Chem. Lett. 2016, 7, 4111−4116. (f) Ozumerzifon, T. J.; Bhowmick, I.; Spaller, W. C.; Rappé, A. K.; Shores, M. P. Toward steric control of guest binding modality: a cationic Co(II) complex exhibiting cation binding and zero-field relaxation. Chem. Commun. 2017, 53, 4211−4214. (g) Villa-Pérez, C.; Oyarzabal, I.; Echeverría, G. A.; Valencia-Uribe, G. C.; Seco, J. M.; Soria, D. B. Single-Ion Magnets Based on Mononuclear Cobalt(II) Complexes with Sulfadiazine. Eur. J. Inorg. Chem. 2016, 2016, 4835−4841. (h) Peng, Y.; Bodenstein, T.; Fink, K.; Mereacre, V.; Anson, C. E.; Powell, A. K. Magnetic anisotropy of a CoII single ion

REFERENCES

(1) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic bistability in a metal-ion cluster. Nature 1993, 365, 141−143. (2) (a) Mannini, M.; Pineider, F.; Sainctavit, P.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M.-A.; Cornia, A.; Gatteschi, D.; Sessoli, R. Magnetic memory of a single-molecule quantum magnet wired to a gold surface. Nat. Mater. 2009, 8, 194−197. (b) Ganzhorn, M.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W. Strong spin−phonon coupling between a single-molecule magnet and a carbon nanotube nanoelectromechanical system. Nat. Nanotechnol. 2013, 8, 165−169. (c) Hu, Z.; Dong, B.-W.; Liu, Z.; Liu, J.-J.; Su, J.; Yu, C.; Xiong, J.; Shi, D.-E.; Wang, Y.; Wang, B.-W.; Ardavan, A.; Shi, Z.; Jiang, S.-D.; Gao, S. Endohedral Metallofullerene as Molecular High Spin Qubit: Diverse Rabi Cycles in Gd2@C79N. J. Am. Chem. Soc. 2018, 140, 1123−1130. (3) (a) Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. High-spin molecules: [Mn12O12(O2CR)16(H2O)4]. J. Am. Chem. Soc. 1993, 115, 1804−1816. (b) Ako, A. M.; Hewitt, I. J.; Mereacre, V.; Clérac, R.; Wernsdorfer, W.; Anson, C. E.; Powell, A. K. A ferromagnetically coupled Mn19 aggregate with a record S = 83/2 ground spin state. Angew. Chem. 2006, 118, 5048−5051. (c) Milios, C. J.; Vinslava, A.; Wernsdorfer, W.; Moggach, S.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. A Record Anisotropy Barrier for a Single-Molecule Magnet. J. Am. Chem. Soc. 2007, 129, 2754−2755. (4) (a) Woodruff, D. N.; Winpenny, R. E.; Layfield, R. A. Lanthanide single-molecule magnets. Chem. Rev. 2013, 113, 5110−5148. (b) Frost, J. M.; Harriman, K. L.; Murugesu, M. The rise of 3d single-ion magnets in molecular magnetism: towards materials from molecules? Chem. Sci. 2016, 7, 2470−2491. (c) Craig, G. A.; Murrie, M. 3d single-ion magnets. Chem. Soc. Rev. 2015, 44, 2135−2147. (5) (a) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.-y.; Kaizu, Y. Lanthanide double-decker complexes functioning as magnets at the single-molecular level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (b) Liu, J.; Chen, Y.-C.; Liu, J.-L.; Vieru, V.; Ungur, L.; Jia, J.-H.; Chibotaru, L. F.; Lan, Y.; Wernsdorfer, W.; Gao, S.; Chen, X.-M.; Tong, M.-L. A stable pentagonal bipyramidal Dy(III) Single-Ion Magnet with a record magnetization reversal barrier over 1000 K. J. Am. Chem. Soc. 2016, 138, 5441−5450. (c) Chen, Y.-C.; Liu, J.-L.; Ungur, L.; Liu, J.; Li, Q.-W.; Wang, L.-F.; Ni, Z.-P.; Chibotaru, L. F.; Chen, X.-M.; Tong, M.L. Symmetry-supported magnetic blocking at 20 K in pentagonal bipyramidal Dy(III) Single-Ion Magnets. J. Am. Chem. Soc. 2016, 138, 2829−2837. (d) Ding, Y.-S.; Chilton, N. F.; Winpenny, R. E. P.; Zheng, Y.-Z. On Approaching the Limit of Molecular Magnetic Anisotropy: A Near-Perfect Pentagonal Bipyramidal Dysprosium(III) Single-Molecule Magnet. Angew. Chem., Int. Ed. 2016, 55, 16071−16074. (e) 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. (f) Meng, Y.-S.; Xu, L.; Xiong, J.; Yuan, Q.; Liu, T.; Wang, B.-W.; Gao, S. Low-coordinate Single-Ion Magnets by intercalation of lanthanides into a phenol matrix. Angew. Chem. 2018, 130, 4763−4766. (6) (a) Zadrozny, J. M.; Xiao, D. J.; Atanasov, M.; Long, G. J.; Grandjean, F.; Neese, F.; Long, J. R. Magnetic blocking in a linear iron (I) complex. Nat. Chem. 2013, 5, 577−581. (b) Poulten, R. C.; Page, M. J.; Algarra, A. G.; Le Roy, J. J.; López, I.; Carter, E.; Llobet, A.; Macgregor, S. A.; Mahon, M. F.; Murphy, D. M.; Murugesu, M.; Whittlesey, M. K. Synthesis, electronic structure, and magnetism of [Ni(6-Mes)2]+: A two-coordinate nickel(I) complex stabilized by bulky N-heterocyclic carbenes. J. Am. Chem. Soc. 2013, 135, 13640−13643. (c) Huang, X.-C.; Zhou, C.; Shao, D.; Wang, X.-Y. Field-Induced Slow Magnetic Relaxation in Cobalt(II) Compounds with Pentagonal Bipyramid Geometry. Inorg. Chem. 2014, 53, 12671−12673. (d) Marriott, K. E. R.; Bhaskaran, L.; Wilson, C.; Medarde, M.; Ochsenbein, S. T.; Hill, S.; Murrie, M. Pushing the limits of magnetic anisotropy in trigonal bipyramidal Ni(II). Chem. Sci. 2015, 6, 6823− 6828. (e) Deng, Y.-F.; Han, T.; Wang, Z.; Ouyang, Z.; Yin, B.; Zheng, Z.; Krzystek, J.; Zheng, Y.-Z. Uniaxial magnetic anisotropy of squareplanar chromium(ii) complexes revealed by magnetic and HF-EPR studies. Chem. Commun. 2015, 51, 17688−17691. (f) Shao, D.; Zhang, 14050

DOI: 10.1021/acs.inorgchem.8b02692 Inorg. Chem. 2018, 57, 14047−14051

Communication

Inorganic Chemistry magnet with distorted trigonal prismatic coordination: theory and experiment. Phys. Chem. Chem. Phys. 2016, 18, 30135−30143. (11) (a) Vallejo, J.; Castro, I.; Ruiz-García, R.; Cano, J.; Julve, M.; Lloret, F.; De Munno, G.; Wernsdorfer, W.; Pardo, E. Field-induced slow magnetic relaxation in a six-coordinate mononuclear cobalt(II) complex with a positive anisotropy. J. Am. Chem. Soc. 2012, 134, 15704−15707. (b) Herchel, R.; Váhovská, L.; Potočňaḱ , I.; Trávníček, Z. Slow magnetic relaxation in octahedral cobalt(II) field-induced Single-Ion Magnet with positive axial and large rhombic anisotropy. Inorg. Chem. 2014, 53, 5896−5898. (c) Gómez-Coca, S.; Urtizberea, A.; Cremades, E.; Alonso, P. J.; Camón, A.; Ruiz, E.; Luis, F. Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy. Nat. Commun. 2014, 5, 4300. (12) (a) Zhang, Y.-Z.; Gómez-Coca, S.; Brown, A. J.; Saber, M. R.; Zhang, X.; Dunbar, K. R. Trigonal antiprismatic Co(ii) single molecule magnets with large uniaxial anisotropies: importance of Raman and tunneling mechanisms. Chem. Sci. 2016, 7, 6519−6527. (b) Zhang, J.; Li, J.; Yang, L.; Yuan, C.; Zhang, Y.-Q.; Song, Y. Magnetic anisotropy from trigonal prismatic to trigonal antiprismatic Co(II) complexes: experimental observation and theoretical prediction. Inorg. Chem. 2018, 57, 3903−3912. (13) Knight, J. C.; Alvarez, S.; Amoroso, A. J.; Edwards, P. G.; Singh, N. A novel bipyridine-based hexadentate tripodal framework with a strong preference for trigonal prismatic coordination geometries. Dalton Trans 2010, 39, 3870−3883. (14) SHAPE: continuous shape measures calculation, version 2.0; Electronic Structure Group, Universiat de Barcelona: Barcelona, Spain, 2010. (15) Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S. PHI: A powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and fblock complexes. J. Comput. Chem. 2013, 34, 1164−1175. (16) (a) Roos, B. O.; Taylor, P. R.; Siegbahn, P. E. M. A complete active space SCF method (CASSCF) using a density matrix formulated super-CI approach. Chem. Phys. 1980, 48, 157−173. (b) Andersson, K.; Malmqvist, P. A.; Roos, B. O.; Sadlej, A. J.; Wolinski, K. Second-order perturbation theory with a CASSCF reference function. J. Phys. Chem. 1990, 94, 5483−5488. (c) Andersson, K.; Roos, B. O. Excitation energies in the nickel atom studied with the complete active space SCF method and second-order perturbation theory. Chem. Phys. Lett. 1992, 191, 507−514. (d) Aquilante, F.; Autschbach, J.; Carlson, R. K.; Chibotaru, L. F.; Delcey, M. G.; De Vico, L.; Fdez Galván, I.; Ferré, N.; Frutos, L. M.; Gagliardi, L.; Garavelli, M.; Giussani, A.; Hoyer, C. E.; Li Manni, G.; Lischka, H.; Ma, D.; Malmqvist, P. Å.; Müller, T.; Nenov, A.; Olivucci, M.; Pedersen, T. B.; Peng, D.; Plasser, F.; Pritchard, B.; Reiher, M.; Rivalta, I.; Schapiro, I.; Segarra-Martí, J.; Stenrup, M.; Truhlar, D. G.; Ungur, L.; Valentini, A.; Vancoillie, S.; Veryazov, V.; Vysotskiy, V. P.; Weingart, O.; Zapata, F.; Lindh, R. Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table. J. Comput. Chem. 2016, 37, 506−541. (e) Chibotaru, L. F.; Ungur, L.; Aronica, C.; Elmoll, H.; Pilet, G.; Luneau, D. Structure, magnetism, and theoretical study of a mixedvalence Co(II)3Co(III)4 heptanuclear wheel: lack of SMM behavior despite negative magnetic anisotropy. J. Am. Chem. Soc. 2008, 130, 12445−12455. (17) Neese, F.; Solomon, E. I. In Magnetism: Molecules to Materials IV; Miller, J. S., Drillon, D., Eds.; Wiley-VCH Verlag GmbH, 2002. (18) Carlin, R. L.; Van Duyneveldt, A. J. Magnetic Properties of Transition Metal Compounds; Springer: New York, 1976. (19) Gatteschi, D.; Sessoli, R.; Villain, R. Molecular Nanomagnets; Oxford University Press, 2006. (20) Lucaccini, E.; Sorace, L.; Perfetti, M.; Costes, J.-P.; Sessoli, R. Beyond the anisotropy barrier: slow relaxation of the magnetization in both easy-axis and easy-plane Ln(trensal) complexes. Chem. Commun. 2014, 50, 1648−1651. (21) Shrivastava, K. N. Theory of Spin−Lattice Relaxation. Phys. Status Solidi B 1983, 117, 437−458.

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DOI: 10.1021/acs.inorgchem.8b02692 Inorg. Chem. 2018, 57, 14047−14051