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Fine-Tuning of the Coordination Environment To Regulate the Magnetic Behavior in Solvent/Anion-Dependent DyIII Compounds: Synthesis, Structure, Magnetism, and Ab Initio Calculations Lin Sun,†,§ Sheng Zhang,†,‡,§ Chengfang Qiao,†,⊥ Sanping Chen,*,† Bing Yin,*,† Wenyuan Wang,† Qing Wei,† Gang Xie,† and Shengli Gao† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi 710127, China ‡ College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, China ⊥ Shaanxi Key Laboratory of Comprehensive Utilization of Tailings Resources, College of Chemical Engineering and Modern Materials, Shangluo University, Shangluo 726000, P. R. China S Supporting Information *

ABSTRACT: It is crucial to understand and elucidate the selfassembly mechanism in solution systems for the construction of DyIII-based single-molecule magnets (SMMs). Herein, through fine-tuning of the anion and solvent, we prepared three nine-coordinate mononuclear dysprosium compounds, [Dy(2,3′-p cad )(NO 3 ) 2 (CH 3 OH ) 2 ] (1 ), [Dy(2,3 ′Hpcad)2(H2O)3]·3Cl·5H2O (2), and [Dy(2,3′-pcad)(NO3)(H2O)4]·NO3·H2O (3) [2,3′-Hpcad = N3-(2-pyridoyl)-3pyridinecarboxamidrazone]. The reactions of formation for 1−3 are in situ thermodynamically monitored by isothermal titration calorimetry. Magnetic data analysis reveals that 2 shows SMM behavior under a zero direct-current (dc) field, whereas 1 and 3 exhibit distinct slow magnetic relaxation processes upon a 1200 Oe dc field. To deeply understand the different magnetic behaviors, the magnetic anisotropy of 1−3 has been systematically studied by ab initio calculations, which is consistent with the experimental observations. Moreover, the semiconductor behaviors of 1−3 have been investigated by experimental measurements of UV−vis spectroscopy.

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INTRODUCTION Recently, lanthanide compounds have become one of the hottest candidates for the synthesis of single-molecule magnets (SMMs), mainly because of their large single-ion magnetic anisotropy arising from the strong intrinsic spin−orbit coupling (SOC) as well as the crystal-field effect.1 As is well-known, each SMM is capable of retaining magnetization in the scale of an individual molecule at low temperature as a result of its Isingtype anisotropy, thus offering the opportunity of moleculebased information storage and quantum computing and opening a new area of molecular spintronics.2 Among lanthanide SMMs, polynuclear structures are difficult to control the relative orientations of magnetic easy axes of individual centers and usually have a complicated magnetic coupling between spin centers.3 As a consequence, mononuclear SMMs, nowadays frequently denoted as single-ion magnets (SIMs), have attracted the interest of more and more researchers.1i,4 In the last 2 months, Tong et al. reported two examples of sevencoordinate DyIII-based SIMs with approximate D5h local symmetry in 2016, one of which has an effective energy barrier (Ueff) of magnetization reversal up to 1025 K and the other a record magnetic hysteresis temperature of up to 20 K.5 The two © XXXX American Chemical Society

works represent the most recent breakthrough among the SMMs. Although much prominent progress has been obtained for lanthanide mononuclear SMMs, there are still open issues that need to be addressed, particularly the fine-control preparation for target compounds as well as understanding the relaxation mechanism. Commonly, the different environments of the magnetic centers generating various crystal fields usually bring about distinct dynamic magnetic relaxation processes; even subtle changes of the coordination environment can drastically influence the overall magnetic properties of SMMs. The coordination environment is influenced by the amounts of solvent,6 anion ligands,7 and lattice solvents,8 the pH values of the solution systems,9 the counterions,10 or the electrostatic environment around the metal centers on the basis of weakening or strengthening the electron density.11 Obviously, much effort should be committed to exploring the self-assembly regularity to modulate the coordination microenvironment and Received: July 28, 2016

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DOI: 10.1021/acs.inorgchem.6b01803 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

literature.13 A suspension of Dy(NO3)3·6H2O (0.2 mmol, 0.090 g) and 2,3′-Hpcad (0.1 mmol, 0.024 g) in CH3OH (15 mL) was treated. The synthetic yellow solution was stirred for 4 h and then filtered. Paleyellow blocky crystals were gathered after 1 week in a yield of 12% (based on DyIII salts). Anal. Calcd for C14H18DyN7O9 (657.61): C, 28.38; H, 3.06; N, 16.56. Found: C, 28.46; H, 3.07; N, 16.59. IR (KBr, cm−1): 3190(s), 3071(s), 2281(s), 1604(s), 1564(w), 1470(w), 1410(m), 1335(m), 1281(w), 1200(m), 1134(w), 1093(w), 1053(w), 987(m), 933(w), 700(s), 646(m), 533(w), 486(m). Synthesis of [Dy(2,3′-Hpcad)2(H2O)3]·3Cl·5H2O (2). A mixture of DyCl3·6H2O (0.2 mmol, 0.076 g) and 2,3′-Hpcad (0.1 mmol, 0.024 g) in CH3OH (15 mL) was stirred at ambient temperature for 4 h. After filtration, the solution was slowly evaporated, the pale-yellow needlelike crystals of 2 were obtained after 5 days in a yield of 14% (based on DyIII salts). Anal. Calcd for C24H38DyN10O10Cl3 (895.49): C, 32.17; H, 4.28; N, 15.64. Found: C, 32.19; H, 4.32; N, 15.67. IR (KBr, cm−1): 3549(s), 3356(s), 3202(m), 3069 (w), 2094(s), 1607(s), 1535(m), 1467(s), 1394(m), 1261(w), 1180(m), 1134(m), 1107(m), 1040(s), 994(s), 927(s), 820(m), 740(m), 707(s), 601(m), 547(w), 480(s). Synthesis of [Dy(2,3′-pcad)(NO3)(H2O)4]·NO3·H2O (3). The method described above for 1 was used but with the addition of 2 drops of pyridine to the solution. The mixture was stirred for 4 h at room temperature. Yellow block crystals suitable for X-ray analysis were obtained in a yield of 10% (based on DyIII salts). Anal. Calcd for C12H20DyN7O12 (616.85): C, 23.30; H, 3.26; N, 15.86. Found: C, 23.37; H, 3.27; N, 15.90. IR (KBr, cm−1): 3449(s), 3256(s), 3101(m), 3054(w), 2091(s), 1615(s), 1529(m), 1459(s), 1387(m), 1259(w), 1178(m), 1127(m), 1102(m), 1035(s), 991(s), 931(s), 815(m), 735(m), 702 (s), 590(m), 540(w), 475(s). Physical Characterization. The solvent contents of 1−3 were determined by elemental analysis and further confirmed by thermogravimetric analysis under a dry N2 atmosphere (Figures S1− S3). The mass loss values are consistent with two CH3OH (1), eight H2O (2), and six H2O (3) molecules per formula unit, corresponding with the formulas. The PXRD patterns of freshly prepared samples of 1−3 are similar but not identical and are in accordance with the calculated ones (Figures S4−S6). X-ray Crystallography. The single-crystal X-ray experiment was performed on a Rigaku SCX mini CCD diffractometer equipped with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) using ω and φ scan modes. Data integration and reduction were processed with SAINT software. Absorption correction based on a multiscan was performed using the SADABS program. The structures were solved by direct methods using SHELXTL and refined by means of full-matrix least-squares procedures on F2 with the SHELXL-97 program.14 All non-hydrogen atoms were refined anisotropically. Experimental details of the crystal data, data collection parameters, and refinement statistics are summaried in Table S1, while selected bond lengths and angles are presented in Table S2. Ab Initio Calculations. Multiconfigurational ab initio calculations including SOC were performed on the experimental structures of 1−3 to explore the magnetic anisotropy. These types of calculations consist of two steps: (1) a set of SOC-free states, i.e., spin eigenstates, were obtained by the complete-active-space self-consistent-field (CASSCF) method;15 (2) the low-lying SOC states, i.e., Kramers doublets (KD) here, were obtained by state interaction, i.e., diagonalization of the SOC matrix in the space spanned by the spin eigenstates from the first step. All of the calculations were carried out with the MOLCAS 8.0 program.16 In the CASSCF step, the active space consisted of nine electrons in seven orbitals, and all of the spin eigenstates of 21 sextets, 224 quartets, and 490 doublets were included. In the subsequent state interaction, because of hardware limitations, only 21 sextets, 128 quartets, and 130 doublets were mixed by the RASSI-SO module.17 The ANO-RCC basis sets18 of triple-ζ quality, ANO-RCC-VTZP, were used. The extraction procedure according to Chibotaru and Ungur was performed to obtain the g tensors and transition magnetic moment of low-lying KDs with the SINGLE_ANISO module.19

further control the local magnetic anisotropy in the construction of a mononuclear SMM. In the present work, the compound N3-(2-pyridoyl)-3pyridinecarboxamidrazone (2,3′-Hpcad), an easily formed mononuclear system in chelating coordination mode,12 was selected as the ligand. In an identical reaction system, the employed anions and nitrate and chlorine ions produced different nine-coordinate DyIII-based compounds, namely, [Dy(2,3′-pcad)(NO 3 ) 2 (CH 3 OH) 2 ] (1) and [Dy(2,3′Hpcad)2(H2O)3]·3Cl·5H2O (2) (Scheme 1). To the nitrateScheme 1. Syntheses of Compounds 1−3

existing system above which the solvent pyridine was added, compound [Dy(2,3′-pcad)(NO3)(H2O)4]·NO3·H2O (3) was obtained (Scheme 1). To explore the self-assembly mechanism in solution systems, the reactions of formation for compounds 1−3 were in situ thermodynamically monitored by isothermal titration calorimetry (ITC). Magnetic characterization indicates that subtle changes of the experimental conditions result in great differences in the coordination environment and dramatically alter the relaxation behavior. To further understand the different magnetic behaviors of compounds 1−3, ab initio calculations were also performed to explore the magnetic anisotropies of the central DyIII ions. In addition, the semiconductor behaviors of 1−3 were investigated by experimental measurements of UV−vis spectroscopy. Crystallographic data for structures 1−3 have been deposited in the Cambridge Crystallographic Data Center as CCDC 1458081, 1458089, and 1458090, respectively, and are given in the Supporting Information.

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EXPERIMENTAL SECTION

Materials and Instructions. All of the materials and reagents were obtained commercially without further purification. The Fourier transform infrared (FT-IR) spectra were recorded in the range of 400−4000 cm−1 using KBr pellets on an EQUINOX55 FT-IR spectrophotometer. Elemental analysis (carbon, hydrogen, and nitrogen) was implemented on a PerkinElmer 2400 CHN elemental analyzer. The phase purities of the bulk or polycrystalline samples were confirmed by powder X-ray diffraction (PXRD) measurements executed on a Rigaku RU200 diffractometer at 60 kV, 300 mA, and Cu Kα radiation (λ = 1.5406 Å), with a scan speed of 5° min−1 and a step size of 0.02° in 2θ. Diffuse-reflectance spectra were obtained by a U-41000 spectrophotometer by applying BaSO4 powder as a 100% reflectance reference. The thermodynamic parameters were measured by a TA/Nano isothermal titration calorimeter. Magnetic measurements were performed in the temperature range of 1.8−300 K with an applied field of 1000 Oe, using a Quantum Design MPMS-XL-7 SQUID magnetometer on polycrystalline samples. The diamagnetic corrections for the compounds were estimated using Pascal’s constants. Alternating-current (ac) susceptibility experiments were performed using an oscillating ac field of 2.0 Oe at ac frequencies ranging from 1 to 1000 Hz. The magnetization was measured in the field range of 0−7 T. Synthesis of [Dy(2,3′-pcad)(NO3)2(CH3OH)2] (1). 2,3′-Hpcad was synthesized through advanced methods, as described in the B


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Inorganic Chemistry

Figure 1. Coordination environment of 1 (a), 2 (b), and 3 (c). The hydrogen atoms are omitted for clarity. Symmetry code: A, 1 − x, y, 0.5 − z.

Figure 2. Local coordination geometry of the DyIII ion in 1 (a), 2 (b), and 3 (c).

Figure 3. χMT versus T plots for 1 (a), 2 (b), and 3 (c) at 1000 Oe.

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RESULTS AND DISCUSSION

coordination in the system of reaction S-3, indirectly leading to a decrease of the ΔS values. Description of the Structures. Single-crystal X-ray diffraction analyses reveal that compounds 1 and 3 crystallize in the monoclinic P21/c space group, while 2 pertains to the monoclinic system with the C2/c space group. The coordination environment of the DyIII ion is demonstrated in Figure 1. Compounds 1−3 contain a single mononuclear compound in the asymmetric unit. The asymmetric unit of 1 contains one DyIII ion, one 2,3′-pcad ion, two NO3− ions, and two coordinated methanol molecules. Differently, the DyIII ion in 2 is bound by two tridentate 2,3′-Hpcad ligands and three coordinated water molecules, thus completing the DyN4O5 coordination sphere. In 3, the DyIII ion is coordinated by nine donor atoms consisting of three atoms of the 2,3′-pcad− ligand (two nitrogen atoms and one oxygen atom), two oxygen atoms from NO3−, and six oxygen atoms of the water molecules. For

Thermodynamics of the Reaction System. The selfassembly reactions (S-1, S-2, and S-3) toward the synthesis of 1−3 were in situ monitored by ITC. The titration curves of the three solution systems display heat release and entropy increase at 298 K, as shown in Figures S7−S9. As can be seen from Table S3, the negative values of ΔG imply that these processes are spontaneous in the experiment conditions. For reactions S1 and S-2 with diverse anions, a slight difference of the thermodynamic parameters depends on the inherent nature of the NO3− and Cl− anions. Compared with reaction S-1, the solvent pyridine is added in reaction S-3, and there are obvious changes of the thermodynamic parameters. More definitely, the pyridine molecules as a kind of weak organic base prefer to combine with large amounts of methanol molecules and prompt trace amounts of H2O molecules to take part in C


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Inorganic Chemistry

Figure 4. Experimental M versus H/T plots of 1 (a), 2 (b), and 3 (c) at different temperatures.

Figure 5. Temperature dependence of the in-phase (a) and out-of-phase (b) ac susceptibility signals under a 1200 Oe dc field for 1.

(Figure 3). The χMT values at room temperature are 14.17, 14.77, and 14.43 cm3 K mol−1 for 1−3, respectively, approximating to the expected value of 14.18 cm3 K mol−1 for a DyIII unit (S = 5/2, L = 5, g = 4/3, and 6H15/2). These values remain roughly constant from 300 to 100 K, where a slight decrease begins and hastens. Upon cooling to 2 K, the χMT values decrease gradually to 11.46, 13.01, and 10.25 cm3 K mol−1 for 1−3, respectively, indicating thermal depopulation of the excited-state Stark sublevels.5−11 The field dependence of magnetization was determined at 1.8, 3.0, and 5.0 K in the field range of 0−7 T (Figure 4). Isothermal magnetization data exhibit saturation at 1.8 K for 1− 3 with values of 4.54, 5.71, and 5.39 Nβ, respectively, which significantly deviates from the theoretical saturation of 10 Nβ. The magnetization versus H/T plots at different temperatures cannot be superimposed, indicating the magnetic anisotropy and/or low-lying excited states.21 To probe into the possibility of mononuclear SMM behavior of 1−3, the ac magnetic susceptibilities versus temperature measurements were carried out in a zero applied dc field with an oscillating 2.0 Oe ac field in the temperature range of 1.8− 15 K. A weak out-of-phase (χ″) signal can be observed for 1−3 (Figures S10−S12), whereas χ′ and χ″ of 1−3 increase below 8 K upon cooling, suggesting a fast quantum tunneling of magnetization (QTM) that is frequently associated with mononuclear lanthanide units.5,6,8b,10,11a,c−e,g In order to weaken the QTM effect, ac susceptibility studies were carried

1−3, the Dy−O distances fall in the range of 2.258−2.564 Å and the Dy−N distances in the range of 2.339−2.590 Å. The bond lengths (Dy1−N1, Dy1−N3, and Dy1−O7) of the 2,3′Hpcad ligand in 2 are longer than those from compounds 1 and 3. (Table S2), Nevertheless, small but appreciable differences in the Dy−O bond lengths from solvent molecules are observed from binding of the neutral ligands to Dy in 2 and that of the charged ligand to Dy in 1 and 3: The Dy−O bond length ranges from 2.432 to 2.458 Å in 1, from 2.386 to 2.431 Å in 2, and from 2.365 to 2.419 Å in 3. To evaluate the exact geometry around the DyIII ion and, numerically, the extent of distortion from an ideal shape for compounds 1−3, we carried out continuous-shape measure analysis using SHAPE 2.0 software20 for DyIII ions. As shown in Table S4, the calculation indicates that the DyIII center in 2 and 3, except for 1, is best described as exhibiting a nine-coordinate monocapped square-antiprismatic (C4v) geometry but with different distortions from the ideal geometry (Figure 2). For 1, the results demonstrate that Dy1 locates in a slightly distorted tricapped trigonal-prismatic (D3h) configuration in which N1, O2, and O8 form the top plane of the trigonal prism, and the bottom plane is completed by O6, O7, and O9, while O1, O4, and N3 capped the quadrilateral face formed by (O8, O2, O7, and O9), (N1, O6, O9, and O8), and (N1, O6, O7, and O8). Magnetic Properties. Direct-current (dc) magnetic susceptibilities in an applied field of 1000 Oe were measured on compounds 1−3 in the temperature range of 1.8−300 K D


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Inorganic Chemistry

Figure 6. Temperature dependence of the in-phase (a) and out-of-phase (b) ac susceptibility signals under a 1200 Oe dc field for 2.

Figure 7. Frequency dependence of the in-phase (a) and out-of-phase (b) ac susceptibility signals under 0 Oe for 2.


out under an optimized dc field of 1200 Oe. For 1 and 2, a slow relaxation process was observed (Figures 5 and 6), indicating the presence of field-induced slow magnetic relaxation, whereas χ″ of 3 showed a weak peak at 2 K (Figure S13). At the selected frequency, compounds 1 and 2 go through a maximum and the maxima shift to high temperature with increasing frequency. The magnetization relaxation times τ derived from the temperature-dependent measurements are plotted as a function of 1/T in Figure S14. On the basis of the Arrhenius analysis [τ = τ0 exp(Ueff/T)], the linear fitting of ln τ versus T−1 plots for 1

and 2 in the high-temperature regime gives the anisotropy energy barrier Ueff with a preexponential factor value τ0. The Ueff values of 1 and 2 are 52.14 K (τ0 = 1.5 × 10−6 s) and 72.40 K (τ0 = 3.0 × 10−9 s), corresponding to the expectant τ0 of 10−6−10−11 s for a SMM.22 Furthermore, the dynamics of magnetization of 1−3 have been studied by measuring the frequency dependencies of the ac susceptibility. The χ′ and χ″ signals of 2 show frequency dependence in a zero dc field (Figure 7), while the slow relaxation of magnetization for 1 and 3 is observed only using E


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Inorganic Chemistry


Figure 10. Magnetization relaxation time, ln(τ) versus T−1, plots under a 1200 Oe dc field for 1 (a) and 3 (c) but a 0 Oe dc field for 2 (b). The solid line is fitted with the Arrhenius law.

Figure 11. Cole−Cole plots at 2.5−6.0 K of 1 (a), 2 (b), and 3 (c).

preexponential factors (τ0) of 2.9 × 10−6 and 5.5 × 10−8 s for 1 and 3 under an applied dc field of 1200 Oe, respectively. It is worth noting that ln(τ) values of 2 become weakly dependent on 1/T with decreasing temperature. This characteristic reveals a crossover from a thermally activated Orbach mechanism that is predominant at high temperature to a Raman process.23 Cole−Cole diagrams of χ″ versus χ′ for 1−3 (Figure 11) have also been obtained, which were fitted by a generalized Debye model24 (Figures S15−S17). The α values are 0.16−

an applied dc field of 1200 Oe (Figures 8 and 9). Obviously, with increasing temperature, the peaks of χ″ shift from low frequency to high frequency step by step, which is the nature of a superparamagnet. For 1−3, the relaxation times τ extracted from the χ″ peaks at selected temperatures are used to construct the Arrhenius plot shown in Figure 10. The effective energy barrier of Ueff is 39.20 K with τ0 = 3.4 × 10−6 s for 2 under a zero dc field. In addition, Arrhenius analysis gives effective energy barriers (Ueff) of 56.11 and 24.95 K and F


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Inorganic Chemistry

larger than twice those of 1 and 3. However, although quite small compared with gZZ, the values of gXX and gYY are still not negligible, and they imply the existence of QTM in the groundstate KD.19c,26 Because of QTM of the ground-state KD, slow magnetic relaxation needs to be field-induced, and the actual Ueff should be significantly lower than the energy gaps between the ground-state and first excited-state KDs. A detailed analysis of the QTM could provide a deep understanding of the different magnetic behaviors of these compounds. As shown in previous results, the rate of QTM between the components of the doublet is approximately proportional to the square of the tunneling splitting matrix element Δtun,19c,26 which removes the doublet degeneracy. Because DyIII is the Kramers ion, the degeneracy of the KD cannot be lifted by any internal interactions.25 Therefore, the tunneling spiltting can only be induced by Zeeman interaction (eq 1) of the transversal magnetic moments (μ X and μ Y ) with the corresponding components of the external magnetic field H.19c,25

0.07 (Table S5) for 1 and 0.35−0.13 (Table S6) and 0.32−0.25 (Table S7) for 2 and 3, respectively. These α values are all in the range of previously reported SMMs,4−11 indicating a narrow distribution of the relaxation process. Theoretical Investigation. The orientation of the magnetic easy axis of the ground KD in 1−3 is shown in Figures S18−S20, which lies close to the shortest D−O bond. It is worth noting that Dy−O of the carbonyl oxygen atom with negative charge is the shortest coordination bond in this molecule, resulting in the stronger coordination affinity. In addition, the quantized axis should point along the negative charge density direction to minimize the potential energy due to the oblate electron cloud of the Ising limit state of 6H15/2.25 As shown in Tables 1 and S8, for the ground KD, the calculated Table 1. Ab Initio Computed Relative Energies (cm−1), Principle Values of the g Tensors, and Averaged Transition Magnetic Moment μQTM (in μB) of the Four Lowest KDs of Compounds 1−3 KD0

KD1

KD2

KD3

E gZZ gXX gYY μQTM E gZZ gXX gYY μQTM E gZZ gXX gYY μQTM E gZZ gXX gYY μQTM

1

2

3

0.00 19.5190 0.0178 0.0374 9.20 × 10−3 145.12 16.4230 0.1394 0.3196 8.39 × 10−2 230.717 16.8982 1.1379 1.4985 4.64 × 10−1 270.360 11.9314 0.2059 2.7564 7.34 × 10−1

0.00 19.5355 0.0404 0.0617 1.70 × 10−2 338.38 15.8634 0.2506 0.2736 1.02 × 10−1 399.53 18.4928 0.8142 1.2376 3.96 × 10−1 534.96 10.1516 3.7478 5.4546 1.90

0.00 19.3367 0.0393 0.0754 1.91 × 10−2 149.52 11.7016 1.6842 6.6806 1.79 186.57 11.3643 2.8489 3.8336 1.20 274.18 9.9288 2.8257 4.9633 1.39

|Δtum | = |μX HX + μY HZ|

(1)

Therefore, the magnitude of Δtun, which determines the strength of QTM, is the product of the transition magnetic moment within the KD and external field. One important source of the external magnetic field is the existence of other magnetic centers in the environment, e.g., other DyIII ions in the whole crystal for the complexes studied here.19c,26 Ab initio results (Table 1) indicate that nonnegligible transition magnetic moments of close magnitude do exist for the ground-state KDs of the compounds here. The calculated values are 9.20 × 10−3, 1.70 × 10−2, and 1.91 × 10−2 μB for 1−3 respectively. They are apparently larger than those of recently reported DyIII complexes (10−5−10−4 μB)5a,b,27 in which QTM is efficiently suppressed even at zero field. Therefore, theoretical results imply the existence of operative zero-field QTM for the ground-state KDs of 1−3, and this implication is consistent with the fact that the observed slow magnetization relaxations of the complexes here are mainly of the fieldinduced type. Although 2 is the only one showing a partial zero-field signal here, its transition magnetic moment is not the smallest among the three complexes. At first glance, this seems controversial because the QTM of 2 should be weaker than those of 1 and 3. However, as indicated above, the magnitude of Δtun is determined not only by the transition magnetic moment but also by the external field, which is experienced by the central DyIII. The coordination environment of 2 is apparently different from those of 1 and 3. The existence of two large 2,3′-Hpcad ligands in the coordination sphere of 2 should lead to longer distances between the nearest-neighbor DyIII ions. Actually, the nearest-neighbor Dy···Dy distances lie within the range of 10.09−12.20 Å in the crystal structure of 2 (Figure S21). This range is significantly larger than those of 1 and 3, which are 7.63−9.09 Å (Figure S22) and 7.25−8.60 Å (Figure S23), respectively. Thus, it could be deduced that the strength of the external magnetic field experienced by the central DyIII ion of 2 is remarkably weaker than those of 1 and 3; thus, the final Δtun of 2 is the smallest. Although the Ueff value of 2 fitted from zero-field experimental data is only 39.20 K, when utilizing the data under an applied dc field, the Ueff value is fitted to be 72.40 K. This value is apparently larger than those of 1 (52.14 K) and 3 (24.95 K) fitted from the data under an applied dc field.

principal g values along the easy axis, i.e., gZZ, are all approaching the Ising limit of 20, where compound 2 possesses the largest one, 19.5355. These results verify the strong axial magnetic anisotropy of the ground KD of 1−3, which should lead to the observation of SMM behavior.19c According to the crystal-field parameters from ab initio calculation (Table S9), the magnitudes of axial components of rank = 2, i.e., B(2,0), are larger than those of rank = 4 and 6, i.e., B(4,0) and B(6,0), by at least an order of 2. Therefore, the axialities of the compounds here are decided mainly by the contribution of rank = 2 axial components of the crystal field. All of the B(2,0) parameters are negative, and thus they should favor the |mJ| = 15/2 components of the H15/2 multiplet. The magnitude of B(2,0) of 2, −0.32 × 101, is apparently larger than those of 1 (−0.20 × 101) and 3 (−0.16 × 101). Therefore, the crystal field experienced by DyIII of 2 is more axial than those of 1 and 3. The difference in the crystal field experienced by the central DyIII ion should be the reason for the different axialities among compounds 1−3. The energy gaps between ground-state and first excited-state KDs are large, especially for 2, where the value is 338.38 cm−1, G


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Inorganic Chemistry

of DyIII-based SMMs through variations in the solvent or anion, mainly the third chemicals (pyridine) arousing the coordination competition among solvents in self-assembly reactions and realizing the oriented synthesis on the basis of thermodynamic interpretation. Furthermore, the experiment of semiconductor behaviors demonstrates that 1−3 could be wide-band-gap semiconductors.

For a comparison between 1 and 3, the transition magnetic moment of 1 is less than half that of 3 and the nearest-neighbor Dy···Dy distances in the crystal of 1 are also weakly longer than that of 3. Therefore, the QTM in the ground-state KD of 1 should be weaker than that of 3. Although the calculated energy gaps between the ground-state and first excited-state KDs for 1 and 3 are quite close to each other, 145.12 and 149.52 cm−1, respectively, the Ueff value of 1 should be apparently higher than that of 3 because of its weaker QTM. This theoretical prediction is verified according to experimental results, in which the Ueff value of 1, 56.11 K, is larger than twice that of 3, 24.95 K. Semiconductor Behaviors of Compounds 1−3. For the purpose of exploring the semiconductivity, the diffusereflectance data are transformed into a Kubelka−Munk function to obtain the band gap (Eg). The semiconducting properties of the ligand and compounds 1−3 have been studied by diffuse-reflectance spectroscopy in Tauc plots (Figure 12).

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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.6b01803. Magnetization data and PXRD patterns (PDF) X-ray crystallographic file for C14H18DyN7O9 (CIF) X-ray crystallographic file C24H38DyN10O10Cl3 (CIF) X-ray crystallographic file C12H20DyN7O12 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.C.). *E-mail: [email protected] (B.Y.). Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 21673180, 21373162, 21473135, 21173168, and 21103137) and the Natural Science Foundation of Shanxi Province (Grants 2015JQ2053, 2016JQ2038, and 2016JM2026).

Figure 12. Tauc plots of the ligand (2,3′-Hpcad) and compounds 1− 3.

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The results demonstrate that the ligand and compounds 1−3 are wide-band-gap semiconductors with Eg values of 3.20, 2.48, 2.56, and 2.67 eV, respectively. The band gap of compound 3 is wider than the band gaps of compounds 1 and 2.

REFERENCES

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CONCLUSIONS We have successfully synthesized and characterized three DyIII compounds through fine-tuning of the anion and solvent. To understand the self-assembly regularity of DyIII-based SMMs in a solution system, the reactions of formation for 1−3 are in situ thermodynamically monitored by ITC. For 1−3, the DyIII ions are nine-coordinate with different coordination geometries by crystal structure analysis. What is more, magnetic characterization indicates that compounds 1−3 exhibit distinct magnetic behaviors. Among 1−3, 2 shows SMM behavior under a zero dc field, whereas 1 and 3 exhibit a slow magnetic relaxation process merely upon a 1200 Oe dc field. When 1 and 3 are compared, it is seen that they exhibit distinct slow magnetic relaxation (Ueff = 56.11 K of 1 and 24.95 K of 3) because of their different coordination geometries. The magnetic behaviors above can be attributed to the different local environments surrounding crystallographically independent DyIII ions, associated with the subtle structural changes due to fine-tuning of the solvent and anion. Notably, the results of ab initio calculations are in accordance with the dynamics of magnetic relaxation obtained by the experiment. This work might offer a promising entry into the fine-tuning for the magnetic properties H


Article

Inorganic Chemistry

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