Field-Induced Slow Magnetic Relaxation in a Mononuclear

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

Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(II) Complex C. Rajnaḱ ,† J. Titiš,*,† J. Moncol,‡ R. Micǒ va,́ † and R. Bocǎ † †

Department of Chemistry, Faculty of Natural Sciences, University of SS Cyril and Methodius, 91701 Trnava, Slovakia Institute of Inorganic Chemistry, FCHPT, Slovak University of Technology, 81237 Bratislava, Slovakia

‡

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S Supporting Information *

The compound under study is the [Mn(4-bzpy)4Cl2] (4-bzpy = 4-benzylpyridine) complex, hereafter 1. Its molecular structure can be viewed in Figure 1. For details about chemical synthesis, analysis, and basic molecular spectra, see the Supporting Information (SI).

ABSTRACT: A mononuclear manganese(II) complex, [Mn(4-bzpy)4Cl2] (4-bzpy = 4-benzylpyridine), exhibits a slow magnetic relaxation with three relaxation modes that is supported by the external magnetic field. The lowfrequency mode shows an exceptionally slow relaxation time: τ = 798 ms at T = 1.9 K and BDC = 0.35 T. The high-frequency domain exhibits unusual acceleration of the relaxation time upon cooling below 3.5 K.

N

ew magnetic materials with tailored magnets, such as single-molecule (-ion) magnets [SMMs (SIMs)], represent the basic design elements for future spintronic devices.1 These compounds (mostly 3d or 4f metal complexes) possess long magnetization relaxation times and spin coherence. Generally, it is accepted that the key property affecting the relaxation mechanisms in SMMs is magnetic anisotropy. Within the spin-Hamiltonian formalism, it is governed by the zero-fieldsplitting (zfs) parameter D, causing a splitting of the groundstate multiplet into sublevels that for S > 1/2 are separated by a barrier to spin reversal.2 Information about slow magnetic relaxation (SMR) and single-ion magnetism over the first transition-metal row is growing rapidly.3 The most widely studied is the group of mononuclear cobalt(II) complexes with coordination modes between three- and eight-coordination.4 Rather scarce is information about vanadium(IV), low-spin manganese(IV), nickel(II), and copper(II) systems.5−9 Also, the Mn(II) SIM is represented only by a single case so far.10 Notice that the former hypothesis that a necessary condition for the SIM behavior is large and negative single-ion anisotropy zfs parameter D has been overcome by finding systems possessing positive D or systems where the D parameter is undefined owing to the orbital degeneracy. In the early view, the six-coordinate, quasi-octahedral manganese(II) complexes are bad candidates for the SIM behavior because of the negligible D parameter and then only a small barrier to spin reversal. In general, the D values of manganese(II) are usually very small, not exceeding 1 cm−1.11 The exception is a polyoxometalate complex in which the manganese(II) is five-coordinate and D ∼ 1.5 cm−1.12 Nevertheless, the present investigation reveals the SMR with manifold relaxation channels in a six-coordinate manganese(II) complex. Moreover, the low-frequency (LF) mode possesses an extraordinarily long relaxation time. © XXXX American Chemical Society

Figure 1. Molecular structure of complex 1 at T = 100 K.

Complex 1 crystallizes in the monoclinic system with space group P21/c. Its crystal structure is formed of neutral molecules [Mn(4-bzpy)4Cl2], with the chromophore {MnN4Cl2} adopting the shape of a tetragonal bipyramid. [A single crystal of 1 was mounted on a Stoe StadiVari diffractometer possessing a Pilatus 3R 300 K detector and a Xenocs Genix3D Cu HF (λ = 1.54186 Å) microfocus source at 100 K. The structure of 1 was solved by Sir2011, refined by SHELXL (version 2018/3), and drawn using the OLEX2 program.15 Crystal data for 1: C48H44N4MnCl2, monoclinic P21/c, a = 11.7040(2) Å, b = 17.7345(2) Å, c = 10.5768(2) Å, β = 109.296(1)°, V = 2072.04(6) Å3, Z = 2, Dc = 1.287 g cm−3, μ = 4.065 mm−1, F(000) = 838, T = 100(1) K, 2θmax = 142.564° (−14 ≤ h ≤ 9, −17 ≤ k ≤ 21, and −11 ≤ l ≤ 12). Final results (251 parameters): R1 = 0.0234 and wR2 = 0.0661 [I > 2σ(I)] and R1 = 0.0250, wR2 = 0.0666, and S = 1.105 for all 60746 reflections. CCDC 1849019.] The distances Mn−N vary in the range 2.30−2.32 Å, whereas Mn−Cl are typically longer at 2.51 Å (for details, see the SI). Individual molecules are interconnected through C−H···Cl−Mn interactions (C···Cl 3.63 Å; Figure S4). The magnetic data in the direct-current (dc) mode were acquired using the SQUID magnetometer (Quantum Design, Received: September 20, 2018

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DOI: 10.1021/acs.inorgchem.8b02675 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry model MPMS-XL7) within the interval of 1.9 and 300 K at the external field Bdc = 0.1 T. Raw susceptibility data were corrected for the underlying diamagnetism. The temperature dependence of the effective magnetic moment confirms a Curie-like behavior between T = 300 and 10 K with almost a constant value of μeff = 5.84 μB. Upon further cooling, it drops to μeff = 5.62 μB at T = 1.9 K (Figure 2), which is a fingerprint of some zfs. The

Figure 3. ac susceptibility components (SI units) for 1 as a function of the external magnetic field. Lines: a visual guide. Left: in-phase component. Right: out-of-phase component.

4. In all cases, three relaxation domains are well visible. The LF relaxation channel occurs at f < 1 Hz, which implies the

Figure 2. Magnetic data for 1. Left: Temperature dependence of the effective magnetic moment. Inset: Temperature evolution of the molar magnetic susceptibility. Right: Field dependence of the magnetization per formula unit. Circles: experimental points. Lines: results of the fitting procedure.

magnetization per formula unit is Mmol/NA = 4.67 μB at T = 2.0 K and B = 7.0 T. The standard zfs model has been applied in fitting the joint set of magnetic data (susceptibility and magnetization), wielding giso = 2.00, D/hc = −0.63 cm−1, and the correction to the temperature-independent magnetism χTIM = −0.59 × 10−9 m3 mol−1. A somewhat larger negative D value offers density functional theory (DFT) calculations using the ORCA package:13,16,17 D/hc = −1.49 cm−1. It was found that the d−d spin flip (α → β) excitations dominate the SOC part of the D parameter (75% of the total D).14 On the other hand, the spin−spin (SS) contribution represents 15% of the total D (in the SI). In contradiction are the CASSCF/NEVPT2 calculations, which give the following results: D/hc = 0.07 cm−1, |E/D| = 0.05, and gx = gy = gz ∼ 2.0. Contributions from the sextet to quartet excitations to the D tensor are collected in Table S5. Furthermore, the calculated transition energies (sextet → quartet, Table S6) were compared with the experimental data (Figure S1). Note that the SS interaction is not included in this calculation. The X-band electron paramagnetic resonance (EPR) spectrum of 1 shows a set of transitions that fulfill the condition 6D < hν, where ν = 9.429 GHz. The simulation reveals a small value of |D/hc| ∼ 0.15 cm−1 (see the SI). The susceptibility data in the alternating-current (ac) mode were taken at the amplitude Bac = 0.38 mT. The first scan shows the field dependence of the ac susceptibility components for four representative frequencies of the oscillating field (Figure 3). At the zero field, the out-of phase susceptibility component is silent, which reflects a fast magnetic relaxation. Rather small is the response at Bdc = 0.1 T. The χ′′ component increases with increasing external dc field, however, differently for different frequencies; it culminates around Bdc = 0.35 T. The frequency dependence of the ac susceptibility components for a set of temperatures T = 1.9−9.9 K is shown in Figure

Figure 4. Frequency dependence of the ac susceptibility components (SI units) for 1. Lines: fitted.

relaxation time τLF ∼ 0.8 s (T = 1.9 K). At f < 10 Hz, an intermediate-frequency (IF) relaxation mode is recognized as a shoulder; around 500 Hz, the high-frequency (HF) channel is well identified. The last one is not attenuated until T = 10 K. χ′ and χ′′ (46 data points) were fitted by a three-component Debye model (N = 3) containing 10 free parameters: a common adiabatic susceptibility χS, three isothermal susceptibilities χTk, three distribution parameters αk, and three relaxation times τk. The resulting parameter along with their standard deviations and the discrepancy factors of the fits is deposited in the SI. N

χ (ω) = χS +

∑ k=1

χk − χk − 1 1 + (iωτk)1 − αk

(1)

According to Figure 4, at low temperature, the magnetic relaxation is dominated by the LF regime: the relaxation time at T = 1.9 K is τLF = 798(32) ms. At these conditions, the individual B

DOI: 10.1021/acs.inorgchem.8b02675 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

applies, and this can be recovered by considering the “strange” term (parameters k > 0 and E1) along with the Raman term

mole fractions are xLF = 0.49, xIF = 0.45, and xHF = 0.07. By heating, the LF and IF fractions decrease in favor of the HF mode; above 5.1 K, the IF mode escapes, and above 7.3 K, also the LF mode expires, so that the HF mode is the only relaxation channel. The retrieved parameters of the Debye model have been used in generating the interpolation/extrapolation lines that are drawn in Figure 4. They are also used in constructing the Argand (Cole−Cole) diagram, as shown in Figure 5, left. Two arcs

τ −1 = CT n + E1T −k

(3)

Using this equation, the data fitting gave n = 2.20(10), C = 57(13) K−n s−1, k = 1.98(12), and E1 = 2.01(19) × 104 Kk s−1. Although the origin of the “strange” term remains unclear so far, it could be associated with intermolecular interactions in the solid state, such as the C−H···Cl−Mn contacts (3.6 Å) in the present case. Notice that the system under study possesses a rather small zfs parameter D = −0.6 cm−1 (0.07 and 0.15 cm−1 by CASSCF/ NEVPT2 calculations and EPR spectrum, respectively). Contrary to this handicap, the system possesses a very long relaxation time because of the dominating Raman process. To our best knowledge, this is the second case of single-ion magnetism detected for manganese(II) complexes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02675. Details about the chemicals used, physical measurements, synthesis, crystal structure determination, EPR data, dc and ac magnetic data, and DFT and ab initio calculations (PDF)

Figure 5. Argand (left) and Arrhenius-like (right) plots for 1. Lines: a visual guide.

referring to the LF and HF relaxation modes are well separated; the HF one involves a shoulder owing to the presence of the IF mode. From the Arrhenius-like plot, ln τ versus T−1 can be seen (Figure 5, right), in which the low-temperature tail does not reach a plateau; it passes through a maximum at T = 3.5 K, and then the relaxation time decreases upon further cooling. This is a rather unusual observation, however, recently reported for a mononuclear copper(II) complex and its nickel(II) analogue.8,9 It is generally accepted that the SMR can be recovered by considering the two-phonon Orbach process (parameters U and τ0), Raman process (parameters n and C), single-phonon direct process (parameters m and A), and quantum tunneling process (parameters D1 and D2) via eq 2. τ

−1

= τ0

−1

n

Accession Codes

CCDC 1849019 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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*E-mail: [email protected] (J.T.).

m

ORCID

exp( −U /kBT ) + CT + AB T

+ D1/(D2 + B2 )

AUTHOR INFORMATION

Corresponding Author

J. Titiš: 0000-0003-2530-7722 R. Boča: 0000-0003-0222-9434

(2)

These terms, however, cannot explain the observed acceleration of the relaxation time upon cooling when the relaxation time passes through a maximum, and then it turns down upon cooling (Figure 6). Evidently, a new term with new physical origin

Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Grant agencies of the Slovak Republic are acknowledged for financial support (Projects APVV-14-0073, APVV-16-0039, and VEGA 1/0534/16). Support of the Research and Development Operational Programme for the project “University Science Park of STU Bratislava” (Project ITMS 26240220084) cofunded by the European Regional Development Fund is also acknowledged.

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Figure 6. Different plots of the HF relaxation time for 1. Lines: fitted with the model of the Raman + strange mechanism. Straight lines: limited low- and/or high-temperature regions for data selected by full points.

REFERENCES

(1) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular nanomagnets; Oxford University Press, 2006; pp 1−408. (2) Boča, R. Magnetic functions beyond the spin Hamiltonian. Struct. Bonding (Berlin) 2006, 117, 1−226. C

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Inorganic Chemistry (3) (a) Craig, G. A.; Murrie, M. 3d single ion magnets. Chem. Soc. Rev. 2015, 44, 2135−2147. (b) Gómez-Coca, S.; Aravena, D.; Morales, R.; Ruiz, E. Large magnetic anisotropy in mononuclear metal complexes. Coord. Chem. Rev. 2015, 289−290, 379−392. (c) Frost, J. M.; Harriman, K. L. M.; Murugesu, M. The rise of 3-d single-ion magnets in molecular magnetism: towards materials from molecules. Chem. Sci. 2016, 7, 2470−2491. (4) (a) Valigura, D.; Rajnák, C.; Moncol, J.; Titiš, J.; Boča, R. A mononuclear Co(II) complex formed from pyridinedimethanol with manifold slow relaxation channels. Dalton Trans 2017, 46, 10950− 10956. (b) 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. (c) 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. (d) Zadrozny, J. M.; Telser, J.; Long, J. R. Slow magnetic relaxation in the tetrahedral cobalt(II) complexes [Co(EPh)4]2− (E = O, S, Se). Polyhedron 2013, 64, 209−217. (e) Sottini, S.; Poneti, G.; Ciattini, S.; Levesanos, N.; Ferentinos, E.; Krzystek, J.; Sorace, L.; Kyritsis, P. Magnetic Anisotropy of tetrahedral CoII singleion magnets: Solid-state effects. Inorg. Chem. 2016, 55, 9537−9548. (f) 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. (g) 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 fourcoordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier. Nat. Commun. 2016, 7, 10467. (5) Atzori, M.; Tesi, L.; Morra, E.; Chiesa, M.; Sorace, L.; Sessoli, R. Room-Temperature Quantum Coherence and Rabi Oscillations in Vanadyl Phthalocyanine: Toward Multifunctional Molecular Spin Qubits. J. Am. Chem. Soc. 2016, 138, 2154−2157. (6) (a) Ding, M.; Cutsail, G. E., III; Aravena, D.; Amoza, M.; Rouzières, M.; Dechambenoit, P.; Losovyj, Y.; Pink, M.; Ruiz, E.; Clérac, R.; Smith, J. M. A low spin manganese(IV) nitride single molecule magnet. Chem. Sci. 2016, 7, 6132−6140. (b) Liu, Y.-M.; Liu, Z.-Y.; Yang, E.-C.; Zhao, X.-J. Field-induced slow relaxation of a manganese(III)-based single-ion magnet. Inorg. Chem. Commun. 2017, 77, 27−30. (c) Realista, S.; Fitzpatrick, A. J.; Santos, G.; Ferreira, L. P.; Barroso, S.; Pereira, L. C. J.; Bandeira, N. A. G.; Neugebauer, P.; Hrubý, J.; Morgan, G. G.; Van Slageren, J.; Calhorda, M. J.; Martinho, P. N. A Mn(III) single ion magnet with tridentate Schiff-base ligands. Dalton Trans 2016, 45, 12301−12307. (d) Pascual-Á lvarez, A.; Vallejo, J.; Pardo, E.; Julve, M.; Lloret, F.; Krzystek, J.; Armentano, D.; Wernsdorfer, W.; Cano, J. Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)-Porphyrin Complex. Chem. - Eur. J. 2015, 21, 17299−17307. (e) Sato, R.; Suzuki, K.; Minato, T.; Shinoe, M.; Yamaguchi, K.; Mizuno, N. Field-induced slow magnetic relaxation of octahedrally coordinated mononuclear Fe(iii)-, Co(ii)-, and Mn(iii)containing polyoxometalates. Chem. Commun. 2015, 51, 4081−4084. (7) Miklovič, J.; Valigura, D.; Boča, R.; Titiš, J. A mononuclear Ni(II) complex: a field induced single-molecule magnet showing two slow relaxation processes. Dalton Trans 2015, 44, 12484−12487. (8) Titiš, J.; Rajnák, C.; Valigura, D.; Boča, R. Field influence on the slow magnetic relaxation of nickel-based single ion magnets. Dalton Trans 2018, 47, 7879−7882. (9) Boča, R.; Rajnák, C.; Titiš, J.; Valigura, D. Field Supported Slow Magnetic Relaxation in a Mononuclear Cu(II) Complex. Inorg. Chem. 2017, 56, 1478−1482. (10) Benniston, A. C.; Melnic, S.; Turta, C.; Arauzo, A. B.; Bartolomé, J.; Bartolomé, E.; Harrington, R. W.; Probert, M. R. Preparation and properties of a calcium(II)-based molecular chain decorated with manganese(II) butterfly-like complexes. Dalton Trans 2014, 43, 13349−13357. (11) Duboc, C. Determination and prediction of the magnetic anisotropy of Mn ions. Chem. Soc. Rev. 2016, 45, 5834−5847. (12) Pichon, C.; Mialane, P.; Riviére, E.; Blain, G.; Dolbecq, A.; Marrot, J.; Sécheresse, F.; Duboc, C. The highest D value for a Mn(II)

ion: Investigation of a manganese(II) polyoxometalate complex by high-field electron paramagnetic resonance. Inorg. Chem. 2007, 46, 7710−7712. (13) (a) Neese, F. The ORCA program system. WIREs Comput. Mol. Sci. 2012, 2, 73−78. Neese, F. ORCAAn Ab Initio, Density Functional and Semi-empirical Program Package, version 4.0.0; 2017. (14) Zein, S.; Duboc, C.; Lubitz, W.; Neese, F. A systematic density functional study of the zero-field splitting in Mn(II) coordination compounds. Inorg. Chem. 2008, 47, 134−142. (15) (a) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giacovazzo, G. L.; Mallamo, M.; Mazzone, A.; Polidori, G.; Spagna, R. SIR2011: a new package for crystal structure determination and refinement. J. Appl. Crystallogr. 2012, 45, 357−361. (b) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, C71, 3−8. (c) Dolomanov, O.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339−341. (16) (a) Neese, F., Solomon, E. I., Miller, J. S., Drillon, M., Eds. Interpretation and Calculation of Spin-Hamiltonian Parameters in Transition Metal Complexes. In Magnetism: Molecules to Materials IV; Wiley-VCH: Weinheim, Germany, 2002. (b) Cirera, J.; Ruiz, E.; Alvarez, S.; Neese, F.; Kortus, J. How to Build Molecules with Large Magnetic Anisotropy. Chem. - Eur. J. 2009, 15, 4078−4087. (c) Duboc, C.; Collomb, M. N.; Neese, F. Understanding the Zero-Field Splitting of Mononuclear Manganese(II) Complexes from Combined EPR Spectroscopy and Quantum Chemistry. Appl. Magn. Reson. 2010, 37, 229−245. (17) (a) Atanasov, M.; Ganyushin, D.; Pantazis, D. A.; Sivalingam, K.; Neese, F. Detailed Ab Initio First-Principles Study of the Magnetic Anisotropy in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes. Inorg. Chem. 2011, 50, 7460−7477. (b) Angeli, C.; Borini, S.; Cestari, M.; Cimiraglia, R. A quasidegenerate formulation of the second order n-electron valence state perturbation theory approach. J. Chem. Phys. 2004, 121, 4043−4049. (c) Angeli, C.; Cimiraglia, R.; Evangelisti, S.; Leininger, T.; Malrieu, J.-P. Introduction of n-electron valence states for multireference perturbation theory. J. Chem. Phys. 2001, 114, 10252−10264. (d) Angeli, C.; Cimiraglia, R.; Malrieu, J.-P. n-electron valence state perturbation theory: A spinless formulation and an efficient implementation of the strongly contracted and of the partially contracted variants. J. Chem. Phys. 2002, 117, 9138−9153. (e) Neese, F. Efficient and accurate approximations to the molecular spin-orbit coupling operator and their use in molecular g-tensor calculations. J. Chem. Phys. 2005, 122, 34107−34113. (f) Ganyushin, D.; Neese, F. First-principles calculations of zero-field splitting parameters. J. Chem. Phys. 2006, 125, 24103−24111. (g) Neese, F. Calculation of the zero-field splitting tensor on the basis of hybrid density functional and Hartree-Fock theory. J. Chem. Phys. 2007, 127, 164112−164119.

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DOI: 10.1021/acs.inorgchem.8b02675 Inorg. Chem. XXXX, XXX, XXX−XXX