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Dec 19, 2017 - ABSTRACT: Functionalized nitronyl nitroxide biradical ligands incorporating pyridine groups hold CoII and LnIII ions together, creating...
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Functionalized Nitronyl Nitroxide Biradicals for the Construction of 3d−4f Heterometallic Compounds Hongdao Li,† Juan Sun,† Meng Yang,† Zan Sun,† Jinkui Tang,*,‡ Yue Ma,† and Licun Li*,† †

Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry and Tianjin Key Laboratory of Metal and Molecule-based Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *

ABSTRACT: Functionalized nitronyl nitroxide biradical ligands incorporating pyridine groups hold CoII and LnIII ions together, creating biradical-based 3d−4f tetranuclear complexes [Ln2Co2(hfac)10(NITPhPybis)2] [LnIII = Gd (1), Tb (2), Dy (3), and Ho (4); NITPhPybis = 5-(4-pyridyl)-1,3-bis(1′-oxyl-3′-oxido4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene; hfac = hexafluoroacetylacetonate]. These complexes have a centrosymmetric cyclic molecular structure in which two biradicals perform as tetradentate ligands to bind two CoII and two LnIII ions, resulting in a rare octaspin system. Direct-current (dc) magnetic susceptibility studies reveal that the strong antiferromagnetic CoII-NO magnetic exchange dominates the present magnetic system, while magnetic coupling of Gd-ON is ferromagnetic. Analysis of the magnetic data of the Gd complex allows us to determine the magnetic parameters through the appropriate magnetic model. Alternating-current (ac) magnetic susceptibility investigations indicate that 2 displays frequency-dependent out-of-phase signals under a zero dc field, while ac magnetic susceptibilities of 3 show field-induced frequency dependence, which is a typical feature of slow relaxation of the magnetization. Complexes 1−4 represent the first nitronyl nitroxide biradical-based 3d−4f compounds.

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INTRODUCTION In the design of molecular magnetic materials, the strategy involving nitroxide free radicals, especially for the nitronyl nitroxides coupled with paramagnetic metal ions, has been proven to be a very successful method.1 This approach has the following merits: (1) nitronyl nitroxides are remarkably stable and easy to chemically modify, thus allowing one to introduce various functional groups within the radical ligands; (2) nitronyl nitroxide radicals as spin carriers can directly coordinate to metal ions, thus leading to the strong possibility of magnetic coupling;1a,b (3) functionalized nitronyl nitroxide radicals may link two or more metal ions, thus producing magnetic molecules with various spin topological structures and dimensionalities. Following this strategy, various magnetic systems have been obtained such as long-range magnetically ordered systems,2 chiral magnets,3 spin-transition-like species,4 single-chain magnets (SCMs),5 and single-molecule magnets (SMMs).6 Recently, special attention of this approach has been placed on molecule nanomagnets (SCMs and SMMs) owing to their potential applications in high-density data storage,7 quantum computing,8 and molecular spintronics,9 and some appealing results have been achieved through this kind of route. For example, the first SCM is the CoII chain connected by nitronyl nitroxide radicals.5a A family of nitronyl nitroxide © XXXX American Chemical Society

radical−lanthanide one-dimensional (1D) chains [Ln(hfac)3(NITPhOPh)] (LnIII = Dy, Tb, Ho, Tm) exhibits SCM behavior.5b,c The SMM behavior has been observed in mononuclear Ln−nitronyl nitroxide complexes10 and Ln dimers bridged by nitronyl nitroxides.6a,11 It should be noted that, following the metal−radical approach, some remarkable SMMs have also been obtained using other organic radicals such as N2̇3−,12 TTF•+,13 thiadiazoyl,14 and HAN3−̇ 15 radical ligands. For example, Long et al. successfully obtained a N2̇3−radical-bridged binuclear TbIII SMM with a high blocking temperature of up to 13.9 K.12 On the other hand, 3d−4f compounds represent another efficient avenue to design molecular nanomagnets.16 In these 3d−4f complexes, the favorable combination of magnetic exchange between the 3d and 4f ions and strong magnetic anisotropy carried out by the Ln ions could promote magnetic relaxation in this system.17 However, it is still a challenge to improve the magnetic behavior of the 3d−4f system. Recently, an emerged strategy is Special Issue: Applications of Metal Complexes with LigandCentered Radicals Received: December 19, 2017

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DOI: 10.1021/acs.inorgchem.7b03186 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Bruker TENOR 27 spectrometer using KBr pellets in the range from 400 to 4000 cm−1. Preparation of [Ln2Co2(hfac)10(NITPhPybis)2] [Ln = Gd (1), Tb (2), Dy (3), and Ho (4)]. A mixture of Co(hfac)2·2H2O (0.01 mmol, 0.0050 g) and Ln(hfac)3·2H2O (0.01 mmol) was added to 15 mL of nhexane, and then the solution was heated to reflux for 5 h. After that, a CHCl3 solution (5 mL) containing the NITPhPybis ligand (0.01 mmol, 0.0047 g) was added to the above solution with stirring for another 30 min. Then the mixture solution was cooled to room temperature, filtered, and allowed to slowly evaporate without being disturbed. Square brown crystals, suitable for X-ray diffraction analysis, were obtained after 5 days. The characterization data for these complexes are given below. [Gd 2 Co 2 (hfac) 10 (NITPhPybis) 2 ] (1). Yield: 39%. Calcd for C100H72Co2F60Gd2N10O28 (3432.04 g mol−1): C, 34.97; H, 2.11; N, 4.07. Found: C, 34.71; H, 2.26; N, 3.85. FT-IR (KBr): 3405 (s), 1652 (m), 1505 (m), 1354 (s), 1257 (s), 1210 (s), 1149 (s), 1131 (s), 949 (s), 860 (s), 661 (m), 587 (m), 547 (m) cm−1. [Tb 2 Co 2 (hfac) 10 (NITPhPybis) 2 ] (2). Yield: 37%. Calcd for C100H72Co2F60Tb2N10O28 (3437.40 g mol−1): C, 34.94; H, 2.11; N, 4.07. Found: C, 34.79; H, 2.21; N, 4.09. FT-IR (KBr): 3405 (s), 1651 (m), 1506 (m), 1355 (s), 1257 (s), 1209 (s), 1148 (s), 1131 (s), 949 (s), 861 (s), 663 (m), 587 (m), 546 (m) cm−1. [Dy 2 Co 2 (hfac) 10 (NITPhPybis) 2 ] (3). Yield: 35%. Calcd for C100H72Co2F60Dy2N10O28 (3444.54 g mol−1): C, 34.86; H, 2.10; N, 4.06. Found: C, 34.79; H, 2.12; N, 3.95. FT-IR (KBr): 3405 (s), 1650 (s), 1504 (m), 1357 (s), 1256 (s), 1210 (s), 1147 (s), 1130 (s), 948 (s), 860 (s), 662 (m), 586 (m), 547 (m) cm−1. [Ho 2 Co 2 (hfac) 10 (NITPhPybis) 2 ] (4). Yield: 36%. Calcd for C100H72Co2F60Ho2N10O28 (3449.40 g mol−1): C, 34.82; H, 2.10; N, 4.06. Found: C, 34.89; H, 2.21; N, 4.03. FT-IR (KBr): 3405 (s), 1651 (s), 1505 (m), 1355 (s), 1257 (s), 1209 (s), 1152 (s), 1130 (s), 948 (s), 858 (s), 662 (m), 587 (m), 546 (m) cm−1. Crystallography. Diffraction intensity data of single crystals of 1− 4 were collected at 113 K on a Rigaku Saturn CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The collected data were reduced using the program CrystalClear, and an empirical absorption correction (multiscan) was applied. The structures of compounds 1−4 were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXS-2014 and SHELXL-2014 programs.25 All non-H atoms were refined anisotropically, and H atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms. The restraints of DFIX, ISOR, and DELU were applied in four complexes to keep the disordered atoms reasonable. The final crystallographic data and refinement parameters of the four compounds are listed in Table 1. The important bond lengths and angles are listed in Table 2. CCDC 1574846−1574849 contain the supplementary crystallographic data for this paper. Magnetic Measurements. Magnetic data were recorded on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. Direct-current (dc) magnetic susceptibility measurements were performed on polycrystalline samples of 1−4 in the temperature range 2−300 K and in an applied field of 1000 Oe. The dynamics of the magnetization were investigated from the ac susceptibility measurements in the zero static fields and applied dc fields with a 3.0 Oe ac oscillating field. The experimental magnetic susceptibility data were corrected for the diamagnetism estimated from Pascal’s tables and sample holder calibration.

to employ paramagnetic radical ligands to bind 3d and 4f ions simultaneously.18 For instance, Dunbar et al. have used TCNQF•− and the dinuclear [Cu(valpn)Tb]3+ unit to achieve a quasi-1D spin organization, exhibiting slow magnetic relaxation behavior.19 Vaz et al. have successfully synthesized a Co−Dy dinuclear compound through a dissymmetric compartmental ligand with the nitronyl nitroxide unit.20 Train and co-workers employ a verdazyl radical as the building block to obtain a six-spin-center 2p−3d−4f SMM.21 We have recently also focused on the preparation of 3d−4f compounds using paramagnetic nitronyl nitroxide ligands, and a series of discrete molecules, one- and two-dimensional (2D) d−f compounds, have been obtained.22 It should be noted that the reported radical-bridged 3d−4f clusters mainly depend on the mononitronyl nitroxide radicals so far, while the chemistry of the nitronyl nitroxide biradicals toward 3d−4f heterometallic complexes remains unexplored. In order to extend the radicalbased 3d−4f system, herein we use the functionalized nitronyl nitroxide biradical NITPhPybis [5-(4-pyridyl)-1,3-bis(1′-oxyl3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene; Scheme 1] as a new synthetic route to assemble 3d− Scheme 1. Coordination Mode of the Biradical Ligand NITPhPybis in Complexes 1−4

4f complexes for the following reasons: (1) This radical ligand could generate intriguing spin-topologic architectures because this radical linker has four NO groups. (2) On the basis of the hard−soft acid−base principle, this radical ligand containing Nand O-donor atoms should be an excellent candidate for constructing 3d−4f heterometallic complexes. (3) The magnetic exchange between two monoradicals through the mphenylene ring is expected to be ferromagnetic.23 Accordingly, four novel nitronyl nitroxide biradical-bridged 3d−4f clusters, namely, [Ln2Co2(hfac)10(NITPhPybis)2] [LnIII = Gd (1), Tb (2), Dy (3), Ho (4); hfac = hexafluoroacetylacetonate], have been successfully obtained. Moreover, an alternating-current (ac) magnetic study shows that slow magnetic relaxation behavior exists in complexes 2 and 3.

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RESULTS AND DISCUSSION Structural Descriptions. X-ray diffraction analysis reveals that complexes 1−4 are isomorphic and crystallize in the monoclinic P21/c space group. The crystallographic data and refinement parameters of the four compounds are listed in Table 1. Selected bond lengths and angles are listed in Table 2. For the sake of brevity, only the structure of complex 1 is depicted. As shown in Figure 1, two NITPhPybis ligands are coordinated to two CoII ions through two N atoms of the

EXPERIMENTAL SECTION

General Synthetic Considerations. All reagents and solvents were commercially purchased and used as received without additional purification. The NITPhPybis biradical ligand was synthesized following literature methods.24 Elemental analysis (for C, H, and N) was implemented on a PerkinElmer 240 elemental analyzer. Fourier transform infrared (FT-IR) spectra of 1−4 were obtained with a B

DOI: 10.1021/acs.inorgchem.7b03186 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinement Summary for 1−4 1 empirical formula Mr T/K cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalcd/g cm−3 μ/mm−1 θ/deg F(000) reflns collected unique reflns/Rint GOF (F2) R1, wR2 [I > 2σ(I)] R1, wR2 (all data)

C100H72Co2F60Ln2N10O28 3432.04 113(2) monoclinic P21/c 19.515(4) 17.124(3) 19.765(4) 90 96.231(1) 90 6566(2) 2 1.737 1.402 3.03−27.58 3376 65786 15206/0.1064 1.066 0.0554, 0.1281 0.0827, 0.1349

2

3

4

3437.40 113(2) monoclinic P21/c 19.543(2) 17.128(1) 19.750(2) 90 96.213(2) 90 6572.6(10) 2 1.737 1.467 3.03−27.69 3380 66416 15070/0.0717 1.025 0.0589, 0.1608 0.0765, 0.1689

3444.54 113(2) monoclinic P21/c 19.579(5) 17.105(4) 19.721(4) 90 96.231(6) 90 6566(3) 2 1.742 1.530 3.03−26.03 3384 55838 12581/0.1390 1.102 0.0788, 0.1429 0.1667, 0.1724

3449.40 113(2) monoclinic P21/c 19.600(8) 17.125(7) 19.801(9) 90 96.371(10) 90 6605(5) 2 1.734 1.587 3.02−27.56 3388 65974 15043/0.0939 1.050 0.0506, 0.1079 0.0885, 0.1145

Table 2. Important Bond Lengths [Å] and Angles [deg] for Complexes 1−4 Ln−O(rad) Ln−O(hfac) O(rad)−Ln−O(rad) Co−O(rad) Co−N Co−O(hfac) Co−O−N

1 (Gd)

2 (Tb)

3 (Dy)

4 (Ho)

2.494(3), 2.364(3) 2.342(4)−2.389(4) 82.89(12) 2.109(3) 2.089(4) 2.023(3)−2.070(3) 122.5(2)

2.498(5), 2.343(4) 2.332(6)−2.393(4) 82.95(16) 2.110(3) 2.092(4) 2.024(3)−2.073(3) 122.3(3)

2.488(6), 2.336(6) 2.305(7)−2.363(7) 83.2(2) 2.112(5) 2.096(6) 2.025(6)−2.088(5) 122.1(4)

2.470(3), 2.473(3) 2.295(4)−2.379(4) 82.63(11) 2.109(3) 2.094(3) 2.025(3)−2.078(3) 122.4(2)

complex 1 is shown in Figure 2. The closest distance of the intermolecular GdIII···GdIII is 9.79 Å, and the shortest distance of the uncoordinated NO groups is 13.75 Å. Magnetic Properties. The variable-temperature magnetic susceptibility measurements of complexes 1−4 have been carried out in the range 2−300 K under an applied magnetic field of 1 kOe (Figures 3 and 4). At 300 K, the values of χMT are 21.76, 27.66, 30.99, and 29.85 cm3 K mol−1 for compounds 1−4, respectively, which are much lower than the theoretical values for two uncoupled LnIII ions (GdIII, 8S7/2, S = 7/2, L = 0, g = 2, C = 7.88 cm3 K mol−1; TbIII, 7F6, S = 3, L = 3, g = 3/2, C = 11.82 cm3 K mol−1; DyIII, 6H15/2, S = 5/2, L = 5, g = 4/3, C = 14.17 cm3 K mol−1; HoIII, 5I8, S = 2, L = 6, g = 5/4, C = 14.07 cm3 K mol−1), two non-interacting CoII ions (for octahedral high-spin CoII, the typical values of χMT are in the range of 3.0−3.4 cm3 K mol−1),28 and four organic radicals (radical: S = 1 /2, g = 2.0, C = 0.375 cm3 K mol−1). These results are indicative of the existence of strong antiferromagnetic coupling between the CoII ion and the directly coordinated NO group. For complex 1, upon cooling, the χMT value fails to reach a value of 19.33 cm3 K mol−1 at 22 K. Below 22 K, χMT increases slightly to 19.50 cm3 K mol−1 at 7 K and then decreases upon further cooling and reaches a minimum value of 18.94 cm3 K mol−1 at 2.0 K (Figure 3). Such magnetic behavior suggests that strong antiferromagnetic interaction is predominant in complex 1. The field dependence of the magnetization for 1 is

pyridine rings and two NO units; meanwhile, each biradical ligand binds one Gd(hfac)3 unit in chelating mode via two adjacent NO groups, yielding a centrosymmetric cyclic heterometallic tetranuclear cluster. The CoII ion is in the octahedral environment consisting of four O atoms from two hfac− ligands, one O atom from one NO group of the biradical, and one N atom from the pyridine ring of the other biradical. The Co−O(rad) bond length of 2.109(3) Å is longer than the Co−N(pyridine) and Co− O(hfac) bond distances [Co−N, 2.089(4) Å; Co−O(hfac), 2.023(3)−2.070(3) Å]. Each GdIII ion is surrounded by eight O atoms from three hfac− ligands and two NO groups of the biradical. The coordination sphere of the GdIII ion could be described as a C2v biaugmented trigonal-prism geometry based on SHAPE analysis26 (Table 3). The Gd−O(hfac) bond lengths are in the range of 2.342(4)−2.389(4) Å, and the Gd− O(rad) distances are 2.494(3) and 2.364(3) Å, respectively. These bond distances are in good agreement with other Gd(hfac)3−nitronyl nitroxide complexes.27 The dihedral angles between the benzene ring and ON−C−NO of two monoradicals are 33.00(1)° and 28.95(2)°, respectively. The Gd−O−N-C torsion angles are 58.2(7)° and 69.3(7)°, respectively. In the tetranuclear cluster, the separations of Co···Co and Gd···Gd are 9.258 and 17.193 Å, respectively, and the distance between the neighboring GdIII and CoII ions through the NIT motif is 7.921 Å. The crystal packing of C

DOI: 10.1021/acs.inorgchem.7b03186 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Plot of χMT versus T for complex 1. Inset: M versus H plot for 1 at 1.9 K. The solid line represents the simulation curve by using PHI.

Figure 1. (Top) Molecular structure of complex 1. The H and F atoms are omitted for clarity. (Bottom) Coordination polyhedron of the GdIII ion in complex 1.

Table 3. SHAPE Analysis for Complexes 1−4a complex 1 2 3 4

(Gd) (Tb) (Dy) (Ho)

Figure 4. Plots of χMT versus T for complexes 2−4.

SAPR-8

TDD-8

BTPR-8

1.536 1.553 1.617 1.505

1.412 1.457 1.437 1.386

0.944 0.924 0.894 0.844

determined at 2 K in the 0−70 kOe range. The field-dependent magnetization value exhibits a rapid increase at low magnetic fields. At higher fields, the value of M increases to 16.30 Nβ at 70 kOe, which is much lower than the expected saturation value because of the strong antiferromagnetic interactions and/or the spin−orbit effect of CoII in the system. On the basis of the crystal structure, magnetic communication mainly arises from three pathways: magnetic interaction between the CoII ion and the directly coordinated NO group (J1); magnetic coupling between the GdIII ion and the coordinated nitroxide group (J2); magnetic exchange between two coordinated NO groups through the GdIII ion and/or the m-phenylene ring (J3). In addition, the magnetic interaction between the CoII ion and the radical unit through the pyridyl and phenyl rings may be neglected.29 Hence, a linear Co−rad− Gd−rad magnetic unit with a spin Hamiltonian of Ĥ = ̂ Ŝrad1) − 2J2(SGd ̂ Ŝrad1 + SGd ̂ Ŝrad2) − 2J3(Ŝrad1Ŝrad2) − −2J1(SCo 2 DSZ is used to analyze the magnetic data (Scheme 2). D is the axial zero-field-splitting parameter of CoII. The simultaneous fitting of the susceptibility and magnetization data with the PHI package30 affords a good agreement with the experimental data. The best fitting yields the magnetic parameters gGd = grad = 2.00 (fixed), gCo = 2.82, J1 = −125.78

a

SAPR-8: square antiprism. TDD-8: triangular dodecahedron. BTPR8: biaugmented trigonal prism.

Scheme 2. Magnetic Exchange Pathways in Complex 1 Figure 2. Packing diagram along the a axis of complex 1. The H and F atoms are omitted for clarity.

D

DOI: 10.1021/acs.inorgchem.7b03186 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry cm−1, J2 = 3.10 cm−1, J3 = −4.92 cm−1, and D = 69.07 cm−1. It is worth noting that the poor fitting is obtained with a negative D value. The determined J1 value indicates that there is strong antiferromagnetic interaction between the CoII ion and the coordinated NO group, which is similar to the reported cobalt(II)−nitronyl nitroxide complexes.31 In 1, the Co−O−N angle is about 120°, and the Co−O(rad) distance of 2.108 Å is short, which are beneficial to the effective overlap of the magnetic orbitals of CoII ions and the radical π* orbital. The positive value of J2 demonstrates that the magnetic exchange between the GdIII ion and the coordinated NO group is ferromagnetic. As is well-known, the magnetic exchange (J) between two spins may be the sum of the contributions of ferromagnetic (JF) and antiferromagnetic (JAF) couplings, i.e., J = JAF + JF. The ferromagnetic contribution (JF) comes from charge transfer from the π* orbital of the radical to the empty orbitals of the GdIII 5d/6s/6p.32 The antiferromagnetic component (JAF) mainly arises from the direct overlap between the Gd magnetic orbitals and the π* orbitals of the radical.32c Thus, the sign and magnitude of magnetic coupling between the Gd ion and the coordinated NO group are determined by two competing contributions. Ishida et al. recently proposed an empirical relationship between Gd−O−N−C torsion angle and magnetic coupling which indicates the larger Gd−O−N−C torsion angles are favorable for ferromagnetic coupling.27c,33 More recently, density functional theory calculations have shown that the larger Gd−O−N−C torsion angles will favor charge transfer to increase the ferromagnetic exchange strength.32c In the present Gd complex, the determined ferromagnetic interaction follows these results owing to the larger Gd−O−N−C torsion angles (58.2° and 69.3° in 1). It is worth noting that there are two exchange pathways for two coordinated NO groups (J3): one is through the empty 6s or 5d orbitals of the GdIII ion, in which antiferromagnetic interaction should be observed;34 the other is via the m-phenylene ring, which usually leads to ferromagnetic coupling.23 The observed negative value of J3 may be ascribed to the combination of two kinds of magnetic exchanges, and the former dominates in the system, which is consistent with the reported similar Ln− biradical complex.35 For complexes 2 and 4, as the temperature is lowered, the χMT products remain almost constant from 300 to 150 K. Then, for 2, the χMT value gradually decreases to 23.76 cm3 K mol−1 at 14 K and finally abruptly drops to 21.68 cm3 K mol−1 at 2.0 K. For 4, the χMT value steadily decreases to 17.62 cm3 K mol−1 at 2 K (Figure 4). In the cases of 2 and 4, except for the above-mentioned magnetic couplings (J1, J2, and J3) and the spin−orbit of CoII, there is depopulation of the excited MJ states of the TbIII or HoIII ion. All of these together result in the observed χMT versus T behavior. In general, the Ln−nitronyl nitroxide interaction is ferromagnetic for the LnIII ion with 4f7− 4f10 electronic configurations.36 Therefore, the magnetic coupling between TbIII/HoIII and the coordinated nitroxide group is anticipated to be ferromagnetic. For complex 3, the χMT product is almost constant, with a value of 30.6 cm3 K mol−1 above 140 K, and then steadily falls to 26.21 cm3 K mol−1 at 10 K, which indicates the presence of antiferromagnetic exchange interactions and thermal depopulation of the DyIII Stark sublevels. Below 10 K, the χMT product of 3 starts to increase with decreasing temperature to reach a value of 26.74 cm3 K mol−1 at 4 K, resulting from ferromagnetic interaction between the DyIII ion and the coordinated NO group.31a In the low-temperature region (T < 4 K), the value of

χMT falls upon further cooling and attains a minimum value of 17.63 cm3 K mol−1 at 2.0 K (Figure 4). It is noteworthy that the increase of the χMT product of 3 below 10 K is not observed for complexes 2 and 4. This may be related to weaker ferromagnetic Tb/Ho−nitronyl nitroxide interaction, whose influence on χMT are just overwhelmed by the stronger contributions of the crystal-field effect compared to Dy.36 The magnetization measurements of 2−4 are performed at 1.9, 3, and 5 K in the field range 0−70 kOe (Figures S7, S9, and S11). The M versus H plots of 2−4 show that the M values increase rapidly at low fields and do not reach the saturation value at 70 kOe. Furthermore, the superposition of the M versus H/T data on a single master curve is lacking at high field. This behavior suggests the presence of significant magnetic anisotropy in 2−4. In addition, no hysteresis is observed at 1.9 K for compounds 2−4 (Figures S8, S10, and S12). It is interesting to probe the slow magnetic relaxation behavior of complexes 1−4 due to the presence of anisotropic metal ions. For complex 1, ac magnetic susceptibility investigation indicates that no nonzero out-of-phase (χ″) signals are observed under zero or external dc fields (Figures S13 and S14). The dynamic susceptibility of complex 2 exhibits frequency-dependent tails of the χ″ signals below 3.5 K in the absence of one dc field, revealing magnetic relaxation behavior (Figure 5). Nevertheless, no peaks are observed in the χ″

Figure 5. (Top) Temperature dependence of χ″ for 2 in a 0 kOe dc field with an oscillation of 3 Oe. (Bottom) Natural logarithm of χ″/χ′ versus 1/T of 2. The solid lines represent the fitting results.

signals above 2 K, which is evidence that quantum tunneling of the magnetization (QTM) exists in the system. To check for the dc field effect on QTM, the field-dependent χ″ signals are determined at 997 Hz and 1.9 K (Figure S16), in which the χ″ signal under 700 Oe is the strongest; thus, an optimized dc field of 700 Oe is applied for suppressing QTM. Unfortunately, under a 700 Oe dc field, there is still no peak of the χ″ signal for 2 (Figure S17). For complexes 3 and 4, there are no χ″ signals under a zero dc field because of the presence of QTM (Figures S18 and S21). To shut down the tunneling pathway, the ac susceptibility E

DOI: 10.1021/acs.inorgchem.7b03186 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of 3 is measured under an optimized applied dc field of 1000 Oe. As shown in Figure 6, the χ″ signals display frequency dependence below 7.5 K, but no peaks are observed above 2.0 K for the χ″ signals.

3d−4f compounds. Our work not only provides intriguing examples of nitronyl nitroxide 3d−4f complexes but also promises a new strategy for constructing radical 3d−4f complexes by means of functionalized nitronyl nitroxide biradicals. This work illustrates the versatility of functionalized nitronyl nitroxide biradicals in the design of magnetic molecules. They are particularly appealing spin carriers in molecular magnetic engineering. Various isolated species or extended 2D or 3D systems exhibiting different magnetic behavior could be achieved. This new approach is anticipated to open wider space in the radical−metal magnetic material field. Further investigations for the design and synthesis of new nitronyl nitroxide−biradical 3d−4f compounds are currently underway.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03186. Table of bond lengths and angles, coordination polyhedron of Ln ions, crystal structure and packing diagrams, and dc and ac magnetic data (PDF) Accession Codes

CCDC 1574846−1574849 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 6. (Top) Temperature dependence of χ″ for 3 in a 1 kOe dc field with an oscillation of 3 Oe. (Bottom) Natural logarithm of χ″/χ′ versus 1/T of 3. The solid lines represent the fitting results.

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Corresponding Authors

Because there are no peaks of the χ″ signals for Tb and Dy derivatives in zero and external dc fields, the energy barrier and preexponential factor cannot be obtained by means of the common Arrhenius fitting. In this case, the energy barrier and preexponential factor can be crudely estimated via the following relationship ln(χ″/χ′) = ln(ωτ0) + Δeff/kBT.37 The best linear fitting on the plots of ln(χ″/χ′) versus 1/T give Δeff/kB ≈ 7.98 K and τ0 ≈ 5.4 × 10−6 s for 2 under zero dc field (Figure 5, bottom) and Δeff/kB ≈ 6.03 K and τ0 ≈ 5.2 × 10−5 s for 3 under 1000 Oe dc field (Figure 6, bottom). As shown in 1, no slow magnetic relaxation is observed. Consequently, the spin dynamics of 2 and 3 may arise from the Tb/Dy−biradical motif. In light of the large D value (69.07 cm−1) of CoII in 1, this implies that there exists important magnetic anisotropy. Accordingly, the magnetic relaxation behavior is anticipated; however, no such behavior is observed for 1. This might be tentatively ascribed to the weak Co−Gd magnetic exchange, which will induce a considerable QTM effect.38 Similar to that of 1, the weak Co−Ln magnetic couplings in 2−4 could produce random transversal fields that facilitate QTM,38,39 which results in poor magnetic performance for 2−4.

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jinkui Tang: 0000-0002-8600-7718 Licun Li: 0000-0001-8380-2946 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21471083 and 21773122) and the 111 Project (B12015).

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REFERENCES

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CONCLUSION In summary, a family of unprecedented nitronyl nitroxide biradical-based 3d−4f clusters has been successfully achieved, and the dynamic magnetic properties of 2 and 3 reveal slow magnetic relaxation behavior. To the best of our knowledge, they are the first examples of nitronyl nitroxide biradical-based F

DOI: 10.1021/acs.inorgchem.7b03186 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03186 Inorg. Chem. XXXX, XXX, XXX−XXX