Rational Design and Synthesis of a Chiral Lanthanide-Radical Single

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

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Rational Design and Synthesis of a Chiral Lanthanide-Radical SingleChain Magnet Xiaoqing Liu,† Yuan Zhang,† Wei Shi,*,† and Peng Cheng*,†,‡,§ †

Department of Chemistry, College of Chemistry, ‡State Key Laboratory of Elemento-Organic Chemistry, and §Collaborative Innovation Center of Chemistry Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China

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S Supporting Information *

ABSTRACT: By the reaction of an enantiopure nitronyl nitroxide radical, 2-((1R)-(−)-myrtenal)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide), abbreviated as (1R)-(−)-myrtenal-NIT, and Dy(hfac)3·2H2O (hfac = hexafluoroacetylacetonate), we successfully obtained a homochiral DyIII-radical chain compound [Dy(hfac)3(1R)-(−)-myrtenal-NIT]n (1) with full characterization. 1 crystallizes in the chiral P21 space group and exhibits slow magnetization relaxation, holding an effective energy barrier (Δτ/kB) of 64.6 K (44.9 cm−1) and an attempt time τ0 of 1.0 × 10−7 s. The correlation energy (Δξ/kB) of this chain is 10.4 K (7.2 cm−1), and the effective Curie constant is 2.8 cm3·K·mol−1. Additionally, 1 exhibits temperature-dependent hysteresis loops below 4 K, with a maximum coercive field of 0.51 T at 2 K.

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INTRODUCTION The chiral molecular magnet is a significant archetype of multifunctional molecular materials due to its desirable physical characteristics,1 such as magnetochiral dichromism (MChD),2 chiral magnetostructural effects,3 and multiferroicity,4 and chemists have developed several approaches toward the assembly of chiral molecular magnets; however, the rational design and synthesis of enantiopure molecular magnets are still very challenging. One of the powerful approaches is to enantioselectively synthesize chiral ligands and then introduce the chirality to the molecular system through coordination bonds.5 Among numerous types of ligands for molecular magnets, organic radical species have drawn researchers’ great attention due to their ability to substantially transfer magnetic exchange couplings, which can suppress the quantum tunneling process. The last several years have witnessed a flood of significant breakthroughs for radicalcontaining single-molecule magnets (SMMs) for their high blocking temperatures and large energy barriers.6 Nevertheless, the synthetic and characterization processes for these radicalcontaining SMMs are extremely challenging. On the other hand, air-stable nitronyl nitroxide (NIT) radicals feature a strong ability to transfer effective exchange interactions between spin carriers.7 Moreover, the substituent group of NIT radicals could be chemically modified to realize chirality. Introducing chirality to NIT radicals and NIT-based SMMs or single-chain magnets (SCMs) is desired but they have remained unexplored due to the synthesis challenges for both the chiral NIT and their coordination compounds.8 In this contribution, we rationally design and synthesize an enantiopure nitronyl nitroxide radical 2-((1R)-(−)-myrtenal)© XXXX American Chemical Society

4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide) (abbreviated as (1R)-(−)-myrtenal-NIT) for the construction of chiral lanthanide-radical systems. We successfully obtained a homochiral dysprosium-based chain compound, namely, [Dy(hfac)3(1R)-(−)-myrtenal-NIT]n (1, Scheme 1), which has been systematically characterized by structural analysis, optical activity, and magnetic property measurements. The circular dichroism (CD) spectrum on a polycrystalline sample confirms the optical activity of 1. The magnetic property studies demonstrate the SCM behavior of 1 at low temperature. As far as we know, 1 is the first chiral lanthanide-based SCM based on an enantiopure NIT radical ligand.

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EXPERIMENTAL SECTION

X-ray Crystallography. Single crystal diffraction data of 1 were measured by an Agilent Supernova diffractometer (120 K) with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) using the ω-scan technique. The crystal structure was solved by direct methods and refined using full-matrix least-squares methods based on F2 with SHELXS-97 program package.9 Anisotropic thermal parameters were assigned to all non-hydrogen atoms, while H atoms of the organic ligands were located at the ideal positions. CCDC-1826700 (1) containing the X-ray structural data can be acquired at www.ccdc.cam. ac.uk/data_request/cif. Physical Characterization. C, H, N analyses were completed on a PerkinElmer 240 CHN elemental analyzer. The infrared (IR) spectrum was obtained on a Bruker ALPHA FT-IR spectrometer with ATR. Thermal gravimetric (TG) analysis was implemented on a Netzsch TG-209 instrument. Solid-state circular dichroism (CD) experiments were performed with finely dispersed powders embedded Received: July 16, 2018

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DOI: 10.1021/acs.inorgchem.8b01981 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Route of 1

Figure 1. (a) D2d-symmetry polyhedra of dysprosium centers of 1. (b) The Dy1-chain of 1. in KBr pellet on a Jasco J-715 spectropolarimeter. A solid-state UV/ vis absorption spectrum was measured on an Agilent Technologies Cary 100 UV−vis spectrophotometer. Static magnetic susceptibility measurements were implemented on a Quantum Design SQUID VSM magnetometer. All data were corrected by the sample holder contribution and diamagnetic contributions estimated using Pascal’s constants. Dynamic (alternating-current, ac) magnetic susceptibilities were conducted on the same instrument. Powder X-ray diffraction patterns were collected on a D/Max-2500 X-ray diffractometer. Materials. 2-Nitropropane and (1R)-(−)-myrtenal were purchased from Sigma-Aldrich, while other reagents were obtained from local suppliers and used directly unless specially noted. Dichloromethane, 95.5%, was stirred for at least 6 h over CaH2 before distillation. Normal heptane, 95%, was dried by distillation over Na. Dy(hfac)3·2H2O was synthesized in accordance with the related literature.10 Synthesis of (1R)-(−)-myrtenal-NIT. (1R)-(−)-Myrtenal-NIT was synthesized with (1R)-(−)-myrtenal and N-[3-(hydroxyamino)2,3-dimethylbutan-2-yl]hydroxylamine using Ullman’s method.11 The resulted crude product was recrystallized in a mixed hexane/ dichloromethane solvent. Yield: ∼40% (based on (1R)-(−)-myrtenal). Elemental analysis: calculated (found) for C16H25N2O2: C, 69.28 (68.87); H, 9.08 (9.24); N, 10.10 (9.65) %. Solid-state circular dichroism (CD) spectrum with powder sample finely dispersed in KBr pellet revealed the optical activity of the NIT radical (Figure S1). IR (cm−1): 2985 (s), 2915 (s), 1690 (w), 1620 (w), 1460 (m), 1410 (vs), 1375 (vs), 1340 (s), 1252 (m), 1217 (s), 1165 (s), 955 (w), 867 (m), 815 (m), 780 (w). We also tried to synthesize the enantiomer of (1S)-(+)-myrtenalNIT, however, the raw material (1S)-(+)-myrtenal is not available from any chemical companies. Synthesis of [Dy(hfac)3(1R)-(−)-myrtenal-NIT]n (1). 0.0400 mmol Dy(hfac)3·2H2O (32.8 mg) was placed in 30 mL treated nhexane to make a suspension which was refluxed for 2 h at 90 °C, followed by dropwise addition of 5 mL dichloromethane solution

containing (1R)-(−)-myrtenal-NIT (0.0400 mmol, 11.1 mg) under stirring. After 5 min, the solution was naturally cooled and filtrated. Pale-blue, sticklike crystals were collected in ∼38% yield (on the basis of Dy) after 1 week. We have also tried our best to synthesize other lanthanide derivatives, but we encountered large difficulties in the syntheses of the chiral ligand and crystallization of the materials at current time. Elemental analysis for 1 (C31H28F18N2O8Dy): calculated (found) C, 35.09 (34.95); H, 2.66 (2.75); N, 2.64 (2.87) %. IR: 1647 (s), 1556 (m), 1526 (m), 1497 (s), 1396 (w), 1346 (m), 1308 (m), 1245 (s), 1192 (s), 1129 (vs), 1095 (s), 949 (w), 869 (m), 798 (s), 735 (m), 651 (s).

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RESULTS AND DISCUSSION Crystal Structure Characterization. 1 crystallizes in a chiral space group P21 with a Flack parameter of −0.011(5), which was determined by single-crystal X-ray diffraction (Table S1). Each asymmetric unit consists of four crystallographically independent Dy(hfac)3(1R)-(−)-myrtenal-NIT entities, with coordination sites around metal atoms occupied by three bidentate hfac− and two (1R)-(−)-myrtenal-NIT units (Figure 1a). The Dy−O bond lengths range from 2.303(5) to 2.416(6) Å (Table S2). The alternating arrangements of [Dy(hfac)3] and radical moieties afford infinite zigzag chains (Figure 1b). The minimum interval between dysprosium atoms residing on adjacent chains is 11.039(4) Å. Additionally, the chains are further packed into supramolecular networks via C−H···F hydrogen bonds (Figure S3 and Table S3). Considering the local coordination symmetry of lanthanide center has a significant effect on the magnetic properties, the semiquantitative method of polytopal analysis12 and classical continuous symmetry measurement (CSM) method13 were implemented. For all DyO8 spheres, they B

DOI: 10.1021/acs.inorgchem.8b01981 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry have the D2d dodecahedron (DD) coordination symmetry of the DyIII centers (Tables S4 and S5). Spectroscopic Studies. Solid-state circular dichroism (CD) spectrum for the sample finely dispersed in KBr pellet showed a positive Cotton effect with λmax of 435 nm, as well as a negative signals with λmax of 376 and 558 nm for 1 (Figure 2), which are basically consistent with the CD spectrum

12−4.5 K indicates a high angular momentum via a significant ferrimagnetic nature between DyIII ions and bridging radicals along the chain.14 The field-dependent magnetization plots at different temperatures show a similar tendencythe magnetizations increase precipitously first under very small external fields; then the magnetizations increase gently until around 3.5 T, followed by a quick upturn again up to 7 T to reach 4.9 μB that is drastically smaller than the theoretical saturation value of 11.0 μB (one DyIII ion plus one radical) (Figure 3b and Figure S4). The magnetization’s behavior illustrates the existence of large magnetic anisotropy and/or low-lying excited states, which also coincides with M vs HT−1 plots without superimposing (Figure S5).15 The two inflections in dM/dH curves indicate the existence of two-step field-induced metamagnetic behavior (Figure S6). The field-cooled and zero-field-cooled (ZFC/FC) susceptibilities show clear divergence below 3.4 K under 50 Oe in a sweeping mode (0.5 K/min) (Figure S7).16 On the basis of the large magnetic anisotropy of 1, dynamic magnetic susceptibilities were collected using a 3 Oe oscillating field to probe the magnetization dynamics. Appreciable temperature and frequency-dependent behaviors appear in the out-of-phase (χM″) components of 1 without applying an external dc field (Figure 4). The calculated parameter φ of 0.17 by the equation φ = (ΔTp/Tp)/Δ(logυ) forecloses the possibility of spin-glass behavior.17 Fitting the ac susceptibilities with a generalized Debye model affords α values of 0.05− 0.17 at 4.7−8.0 K for 1 (Figure 5a and Table S6);18 here the relative narrow distribution of α parameters indicates a single relaxation process can be prospected. By further investigation of the plots, the relaxations times (τ) obey the Arrhenius law denoted as τ−1 = τ0−1 exp[−Δτ/(kBT)] in the temperature range of 4.7−8.0 K, to give an effective energy barrier (Δτ/kB) of 64.6 K (44.9 cm−1) with the corresponding relaxation attempt time of τ0 of 1.0 × 10−7 s (Figure 5b). For a chain compound with Ising-like or Heisenberg magnetic anisotropy, the ln(χM′T) vs 1/T plot follows the expression χM′T = Ceff exp((Δξ/(kBT)) at low temperatures, where Ceff represents the effective Curie constant, Δξ is the correlation energy to create a domain wall within the chain,7c,19 and χM′ is zero-field molar variable-temperature alternating-current susceptibilities collected at 1 Hz and 3 Oe oscillating field. The correlation length exponentially increases in the temperature region of 5.7−9.3 K, yielding the energy gap

Figure 2. Solid-state circular dichroism (CD) spectrum of 1 with powder sample finely dispersed in KBr pellet.

observed for the chiral radical (Figure S1), suggesting that the CD signals of 1 mainly originate from the chiral ligand. Moreover, the results agree with the UV/vis spectrum recorded on a powder sample of 1 that shows the main absorption band centered at 350 nm (Figure S2). Static and Dynamic Magnetic Characterizations. Variable-temperature magnetic susceptibilities were harvested on a polycrystalline sample of 1 within 2−300 K at 1-kOe dc field. The yielded room temperature χMT product (14.18 cm3· K·mol−1) deviates to a small extent from 14.55 cm3·K·mol−1 as expected for a free DyIII ion (g = 4/3, J = 15/2) and a radical (g = 2, S = 1/2) (Figure 3a). The χMT vs. T data generally exhibit a “down-up-down” pathway upon cooling − χMT drops gently from 300 to 65 K due to DyIII-radical antiferromagnetic coupling, the thermal depopulation of the MJ manifolds of individual DyIII and/or the crystal-field effect; below 65 K, χMT decreases quickly reaching the minimum of 7.30 cm3·K·mol−1 at 12 K; then χMT ascends precipitously to the maximum 16.28 cm3·K·mol−1 at 4.5 K, after which χMT undergoes a sudden drop reaching 10.24 cm3·K·mol−1 at 2 K. The rise in χMT at

Figure 3. (a) Variable-temperature magnetic susceptibilities of 1 under 1000 Oe dc field. Inset: the enlargement of χMT products at a temperature region of 2−12 K. (b) Plots of field-dependent magnetization (M) (black circle) and first field derivative of the magnetization (dM/dH) (purple line) for 1 at 2 K. C

DOI: 10.1021/acs.inorgchem.8b01981 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Temperature-dependent (a) and frequency-dependent (b) in-phase (χM′) and out-of-phase (χM″) components of ac susceptibilities under zero external field of 1. Full lines are to guide the eyes.

Figure 5. (a) Cole−Cole plots in zero applied external field within 4.7−8.0 K for 1 with full lines corresponding to the best fitting results. (b) Arrhenius plot of temperature-dependent relaxation times for 1. The red full line corresponds to a linear fit.

(Δξ/kB) of 10.4 K (7.2 cm−1) and Ceff of 2.8 cm3·K·mol−1 (Figure 6), where the latter falls inside the scope of SCMs.8,20 Finally, for elucidating the magnetic bistability, the magnetic field was cycled within ±7 T for magnetic hysteresis. Under a scan rate of 100 Oe/s, open magnetic hysteresis loops were monitored with a coercive field of 0.51 T for 1 at 2 K. The hysteresis loops retain opening up to 4 K and closed with the increase of temperature, consistent with the divergence temperatures occurring in the ZFC-FC plots (Figures 7 and S8).

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CONCLUSION In conclusion, an enantiopure nitronyl nitroxide (NIT) radical was successfully synthesized and utilized to construct a homochiral DyIII-radical chain compound which associates optical chirality and magnetic anisotropy. The compound exhibits single-chain-magnet behavior evidenced by the

Figure 6. Temperature dependence of χM′T (χM′ is the molar inphase component of ac susceptibilities at zero external field). The solid line represents a linear fit.

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DOI: 10.1021/acs.inorgchem.8b01981 Inorg. Chem. XXXX, XXX, XXX−XXX

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dynamic magnetic susceptibilities and hysteresis loops. This work offers a novel synthetic strategy for homochiral lanthanide SCMs by introducing an enantiopure NIT radical, which was first achieved experimentally. Inspired by the intriguing results, efforts to explore the relationship and interaction between the chirality and magnetism are underway in our lab. It is hoped that photomagnetism coupling could be realized in such chiral magnetic chain systems that have potential applications such as magnetic field or polarized light responding nanosized molecular devices.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01981. Figures and tables associating with structural and magnetic data as well as other experimental details (PDF) Accession Codes

CCDC 1826700 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 [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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Figure 7. Plots of magnetization vs applied field at a temperature region of 2−4 K for 1 at a field sweeping rate of 100 Oe/s.

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AUTHOR INFORMATION

Corresponding Authors

*(W.S.) E-mail: [email protected]. *(P.C.) E-mail: [email protected]. ORCID

Wei Shi: 0000-0001-6130-1227 Peng Cheng: 0000-0003-0396-1846 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (Grant No. 2018YFA0306002) and the National Natural Science Foundation of China (Grant Nos. 21622105 and 21331003), the Fundamental Research Funds for the Central Universities and the Ministry of Education of China (Grant No. B12015). E

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b01981 Inorg. Chem. XXXX, XXX, XXX−XXX