Polymorphic Lanthanide Phosphonates Showing Distinct Magnetic

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Polymorphic Lanthanide Phosphonates Showing Distinct Magnetic Behavior Dai Zeng, Min Ren, Song-Song Bao, Zhong-Sheng Cai, Chang Xu, and Li-Min Zheng* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: A series of layered lanthanide phosphonates αLn(2-qpH)(SO4)(H2O)2 (α-Ln; Ln = Gd, Tb, Ho, Er) and βLn(2-qpH)(SO4)(H2O)2 (β-Ln; Ln = Gd, Tb, Ho, Er, Yb) (2qpH2 = 2-quinolinephosphonic acid) have been synthesized and characterized. Compounds α-Ln crystallize in monoclinic space group P21/c, while compounds β-Ln crystallize in triclinic space group P1.̅ Magnetic studies reveal that dominant ferromagnetic interactions are propagated between the magnetic centers in all cases. Field-induced magnetic relaxation is observed in compounds β-Er and β-Yb.

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INTRODUCTION

ions and different magnetization relaxation behavior at low temperature.14 We have been interested in multifunctional lanthanide phosphonates.15 On the basis of 2-pyridylphosphonate16 or 1,4,7-triazacyclononane-1,4,7-triyltris(methylenephosphonic acid) (notpH6),17 a series of 4f or 3d/4d-4f complexes were obtained, some of which show emissive SMM behavior.18,19 More recently, we reported the two layered polymorphic phosphonates α-,β-Dy(2-qpH)(SO4)(H2O)2 (2-qpH2 = 2quinolinephosphonic acid).20 Both show field-induced slow magnetic relaxation at low temperature. The energy barrier of the β phase is nearly 3 times that of the α phase, although the coordination geometries around the dysprosium ions are identical in the two phases. In order to investigate whether polymorphism could have an influence on the magnetic properties of other lanthanide systems, herein we describe a series of heavy lanthanide compounds based on 2-qpH2: namely, α-,β-Ln(2-qpH)(SO4)(H2O)2 (Ln = Gd, Tb, Ho, Er) and β-Yb(2-qpH)(SO4)(H2O)2. The magnetic behaviors of αEr and β-Er are found to be remarkably different, with fieldinduced slow magnetization relaxation observed only in the latter case. Furthermore, field-induced SMM behavior is also observed in compound β-Yb, thus providing a new member to the small family of SMMs based on ytterbium(III).21−25

Lanthanide-based compounds are of intense current interest because of their characteristic luminescence1 as well as their large single-ion magnetic anisotropy due to the unquenched orbital angular momentum and strong spin−orbit coupling. The discovery of single-molecule-magnet (SMM) behavior in bis-phthalocyanine compounds (Bu4N)Pc2Ln (Ln = Tb, Dy)2 has stimulated a tremendous effort in searching for new LnSMMs with high energy barriers.3,4 It is clear now that the anisotropy of lanthanide ions, which is closely related to the energy barrier of the system, is very sensitive to the coordination environment around the 4f ions. Therefore, the manipulation of magnetic behavior of the lanthanide compounds is possible through structural modulation3−6 or external stimulus.7,8 Polymorphism is a well-known phenomenon that concerns compounds with more than one crystalline phase. Controlling polymorphism has been of great interest in the fields of materials science and pharmacology because the various polymorphs of the same substance could show significantly different physical or chemical properties.9 By subtle variation of reaction conditions, a number of polymorphic coordination compounds have been obtained.10 However, polymorphic lanthanide compounds are still limited in number, and most work has focused on the isolation of different polymorphs and their structures.11−14 Little attention has been paid to the structure−property relationships. As far as we are aware, polymorphic lanthanide complexes showing distinct magnetic behavior are extremely rare. The only example is polymorphic mononuclear species of [Dy(NTA)3L], which show significantly different coordination geometries around the dysprosium © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials and Measurements. 2-Quinolinephosphonic acid (2qpH2) was prepared according to the literature method.26 All other starting materials were obtained from commercial sources without further purification. Elemental analysis for C, H, N were performed on Received: February 2, 2016

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DOI: 10.1021/acs.inorgchem.6b00280 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

1445 (w), 1404 (w), 1384 (w), 1348 (w), 1294 (w), 1199 (s), 1164 (s), 1140 (s), 1105 (s), 1043 (s), 970 (m), 949 (w), 884 (w), 856 (w), 827 (w), 781 (w), 769 (w), 660 (m), 637 (m), 608 (m), 592 (m), 534 (m), 476 (m), 430 (w). Thermal analysis shows a weight loss of 7.1% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.1%). Data for α-Er. Yield: 70%. Anal. Calcd for α-C9H11ErNO9PS: C, 21.30; H, 2.18; N, 2.76. Found: C, 21.73; H, 2.66; N, 2.86. IR (KBr, cm−1): 3485 (m), 3306 (br), 1642 (m), 1599 (m), 1519 (w), 1486 (w), 1445 (w), 1404 (w), 1384 (w), 1347 (w), 1294 (w), 1200 (s), 1173 (s), 1133 (s), 1105 (s), 1045 (s), 967 (m), 949 (w), 885 (w), 855 (w), 822 (w), 781 (w), 769 (w), 660 (m), 636 (m), 608 (m), 592 (m), 533 (m), 475 (m), 430 (w), 415 (w). Thermal analysis shows a weight loss of 7.0% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.1%). Data for β-Er. Yield: 66%. Anal. Calcd for β-C9H11ErNO9PS: C, 21.30; H, 2.18; N, 2.76. Found: C, 21.64; H, 2.44; N, 2.72. IR (KBr, cm−1):3265 (br), 1642 (m), 1599 (m), 1517 (w), 1485 (w), 1444 (w), 1403 (w), 1383 (w), 1347 (w), 1295 (w), 1200 (s), 1173 (s), 1134 (s), 1103 (s), 1044 (s), 966 (s), 884 (w), 854(w), 823 (m), 777 (m), 660 (s), 635 (s), 605 (s), 592 (s), 532 (s), 475 (w), 4185 (w). Thermal analysis shows a weight loss of 7.5% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.1%). Data for β-Yb. Yield 63%. Anal. Calcd for β-C9H11YbNO9PS: C, 21.06; H, 2.16; N, 2.73. Found C, 20.89; H, 2.41; N, 2.58. IR (KBr, cm−1):3319 (br), 1643 (m), 1600 (w), 1518 (w), 1486 (w), 1446 (w), 1403 (w), 1383 (w), 1347 (w), 1295 (w), 1201 (s), 1176 (s), 1136 (s), 1104 (s), 1045 (s), 967 (w), 885 (w), 854(w), 823 (w), 778 (w), 662 (m), 635 (m), 606 (m), 532 (m), 475 (w), 416 (w). Thermal analysis shows a weight loss of 7.4% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.0%). X-ray Crystallographic Analyses. Suitable single crystals were selected and mounted on a glass rod. The crystal data were collected on a Bruker SMART APEX CCD diffractometer (α-Gd), a Bruker SMART APEX II diffractometer (β-Gd, α-Tb, β-Tb, β-Ho), and a Bruker SMART APEX DUO diffractometer (α-Ho, β-Er, β-Yb) using monochromated Mo Kα radiation (λ = 0.71073 Å) at 296 K for α-Gd, β-Gd, α-Tb, β-Tb, α-Ho, and β-Ho and 123 K for β-Er and β-Yb. The data were integrated using the Siemens SAINT program,28 with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Empirical absorption and extinction corrections were applied. The structures were solved by direct methods and refined on F2 by full-matrix least squares using SHELXTL.29 All nonhydrogen atoms were refined anisotropically. All hydrogen atoms were either put in calculated positions or found from the difference Fourier maps and refined isotropically. Selected bond lengths and angles are given in Table S1 in the Supporting Information.

a PerkinElmer 240C elemental analyzer. The infrared spectra were recorded on a Bruker Tensor 27 spectrometer with pressed KBr pellets in the 400−4000 cm−1 region. Thermogravimetric analyses were performed on a Mettler-Toledo TGA/DSC STARe thermal analyzer in the range of 25−600 °C under a nitrogen flow at a heating rate of 5 °C min−1. Powder X-ray diffraction patterns (PXRD) were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer using Cu Kα radiation over the 2θ range of 5−50° at room temperature. The magnetic susceptibility data were obtained using polycrystalline samples by a Quantum Design MPMS SQUID VSM magnetometer. The diamagnetic contribution of the sample itself was estimated from Pascal’s constant.27 Synthesis of α- and β-Ln(2-qpH)(SO4)(H2O)2 (α-Ln, β-Ln). Compounds α-Ln and β-Ln were synthesized following a similar procedure except for the pH of the reaction mixtures, which is ca. 1.50 for α-Ln and 0.70 for β-Ln. The synthetic details of α-Gd are thus given as being representative. A mixture of Gd(NO3)3·6H2O (0.0226 g, 0.05 mmol), ZnSO4·6H2O (0.0270 g, 0.10 mmol), and 2quinolinephosphonic acid (0.0105 g, 0.05 mmol) in 6 mL of H2O, adjusted to pH 1.50 with 1 mol/L H2SO4, was kept in a Teflon-lined autoclave at 140 °C for 2 days. After the mixture was cooled to room temperature, white polycrystalline materials of α-Gd were obtained as a pure phase, as confirmed by the powder XRD measurements. Yield: 32.4%. Anal. Calcd for α-C9H11GdNO9PS: C, 21.73; H, 2.23; N, 2.82. Found: C, 22.20; H, 2.50; N, 2.87. IR (KBr, cm−1): 3362 (br), 1639 (w), 1600 (w), 1518 (w), 1488 (w), 1403 (w), 1383 (w), 1294 (w), 1144 (s), 1104 (s), 1041 (s), 969 (w), 883 (w), 855 (w), 829 (w), 775 (w), 657 (m), 639(m), 607 (m), 534 (m), 474 (w), 419 (w). Thermal analysis shows a weight loss of 7.1% in the temperature range 25−250 °C, close to the calculated value for the release of two coordinated water molecules (7.2%). Above 400 °C, the weight loss is due to the decomposition of the organic groups. Data for β-Gd. Yield: 35%. Anal. Calcd for β-C9H11GdNO9PS: C, 21.73; H, 2.23; N, 2.82. Found: C, 21.80; H, 2.64; N, 2.76. IR (KBr, cm−1): 3479 (m), 3282 (m), 1640 (m), 1599(w), 1518 (w), 1485 (w), 1446 (w), 1403 (w), 1383 (w), 1347 (w), 1294 (w), 1198 (s), 1170 (s), 1130 (s), 1102 (s), 1042 (s), 964(m), 885 (w), 856 (w), 822 (w), 775 (w), 656 (m), 636 (m), 591 (m), 532 (m), 474 (w), 430 (w). Thermal analysis shows a weight loss of 7.3% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.2%). Data for α-Tb. Yield: 55%. Anal. Calcd for α-C9H11TbNO9PS: C, 21.66; H, 2.22; N, 2.81. Found: C, 22.00; H, 2.61; N, 2.65. IR (KBr, cm−1): 3359 (br), 1639 (m), 600 (w), 1519 (w), 1486 (w), 1446 (w), 1405 (w), 1384 (w), 1349 (w), 1294 (w), 1162 (s), 1138 (s), 1105 (s), 1042 (s), 970 (m), 883 (w), 855(w), 828 (w), 776 (w), 658 (m), 638 (m), 607 (m), 533 (m), 476 (w), 419 (w). Thermal analysis shows a weight loss of 7.3% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.2%). Data for β-Tb. Yield: 60%. Anal. Calcd for β-C9H11TbNO9PS: C, 21.66; H, 2.22; N, 2.81. Found: C, 21.84; H, 2.50; N, 2.81. IR (KBr, cm−1): 3477 (m), 3337 (br), 3096 (s), 3359 (br), 1639 (m), 1599 (w), 1519 (w), 1485 (w), 1445 (w), 1404 (w), 1383 (w), 1346 (w), 1294 (w), 1198 (s), 1164 (s), 1136 (s), 1103 (s), 1041 (s), 968 (m), 884 (w), 854(w), 826 (w), 775 (w), 657 (m), 636 (m), 605 (m), 532 (m), 475 (w), 418 (w). Thermal analysis shows a weight loss of 7.5% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.2%). Data for α-Ho. Yield: 56%. Anal. Calcd for α-C9H11HoNO9PS: C, 21.40; H, 2.19; N, 2.77. Found: C, 21.58; H, 2.64; N, 2.81. IR (KBr, cm−1): 3383 (m), 3097 (w), 1640 (m), 1600 (w), 1518 (w), 1486 (w), 1446 (w), 1404 (w), 1384 (w), 1348 (w), 1294 (w), 1166 (s), 1137 (s), 1105 (s), 1044 (s), 971 (m), 884 (w), 856 (w), 827 (w), 776 (w), 743 (w), 660 (m), 638 (m), 607 (m), 534 (m), 475 (w), 418 (w). Thermal analysis shows a weight loss of 7.4% in the temperature range 25−250 °C, in agreement with the release of two water molecules (calcd 7.1%). Data for β-Ho. Yield: 40%. Anal. Calcd for β-C9H11HoNO9PS: C, 21.40; H, 2.19; N, 2.77. Found: C, 21.66; H, 2.55; N, 2.68. IR (KBr, cm−1): 3481 (m), 3097 (w), 1641 (m), 1599 (w), 1520 (w), 1486 (w),

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RESULTS AND DISCUSSION Synthesis. Hydrothermal reactions of 2-qpH2 with the corresponding lanthanide nitrates and ZnSO4·6H2O in a 1:1:2 molar ratio (pH 1.1) at 140 °C for 2 days resulted in mixtures of rodlike crystals of α-Ln and platelike crystals of β-Ln (Ln = Gd, Tb, Dy, Ho, Er). In order to obtain pure phases of α-Ln and β-Ln, the reactions were carried out by changing the pH of the reaction mixtures. It was found that pH plays a key role in the formation of particular products. A relatively higher pH facilitates the formation of α-Ln, while a lower pH leads to an increase in the amount of β-Ln. Pure phases of α-Ln and β-Ln can be obtained when the pH of the reaction mixtures is 1.50 and 0.70, respectively, as confirmed by the powder XRD patterns (Figures S2 and S3 in the Supporting Information). At pH >1.50 or pH 2σ(I)) R1,a wR2b (all data) goodness of fit Δρmax, Δρmin (e Å−3) CCDC no.

C9H11GdNO9PS 497.47 monoclinic P21/c 13.300(3) 6.7919(17) 15.127(4) 90 90.688(5) 90 1366.3(6), 4 2.418 5.171 956 0.0560, 0.1089 0.0907, 0.1179 0.996 1.47, −0.99 1451026 α-Ho

C9H11GdNO9PS 497.47 triclinic P1̅ 6.825(2) 7.624(3) 13.370(5) 90.415(7) 93.204(6) 102.113(5) 679.0(4), 2 2.433 5.203 478 0.0615, 0.1242 0.1042, 0.1403 1.006 2.88, −3.85 1451027 β-Ho

C9H11TbNO9PS 499.14 monoclinic P21/c 13.525(14) 6.837(7) 15.300(16) 90 90.422(16) 90 1415(3), 4 2.343 5.305 960 0.0572, 0.1306 0.0793, 0.1392 1.033 2.88, −3.87 1451028 β-Er

C9H11TbNO9PS 499.14 triclinic P1̅ 6.803(2) 7.603(2) 13.373(4) 90.435(4) 93.202(4) 102.140(5) 675.0(3), 2 2.456 5.560 480 0.0224, 0.0534 0.0238, 0.0543 1.001 0.75, −0.99 1451029 β-Yb

formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3), Z Dc (g cm‑3) μ (mm−1) F(000) R1,a wR2b (I > 2σ(I)) R1,a wR2b (all data) goodness of fit Δρmax, Δρmin (e Å−3) CCDC no.

C9H11HoNO9PS 505.15 monoclinic P21/c 13.274(5) 6.745(3) 15.034(6) 90 90.631(6) 90 1346.0(10), 4 2.493 6.201 968 0.0352, 0.0682 0.0580, 0.0763 0.992 0.90, −0.95 1451030

C9H11HoNO9PS 505.15 triclinic P1̅ 6.7720(8) 7.5628(8) 13.3495(15) 90.3839(15) 93.3413(16) 102.3474(17) 666.62(13), 2 2.517 6.260 484 0.0283, 0.0758 0.0303, 0.0774 1.000 3.12, −1.27 1451031

C9H11ErNO9PS 507.48 triclinic P1̅ 6.7591(5) 7.5493(6) 13.3381(11) 90.4052(11) 93.2907(10) 102.4072(11) 663.47(9), 2 2.540 6.651 486 0.0248, 0.0649 0.0251, 0.0652 1.000 1.36, −2.84 1451032

C9H11YbNO9PS 513.26 triclinic P1̅ 6.6954(7) 7.4801(8) 13.2882(14) 90.491(1) 93.324(1) 102.448(1) 648.61(12), 2 2.628 7.544 490 0.0170, 0.0460 0.0174, 0.0465 1.000 0.90, −1.15 1451033

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

units of 2-qpH− into a two-dimensional layer in the bc plane (Figure 1c). The Tb···Tb distances over the O−P−O bridges are 5.394(4), 5.394(4), and 6.837(7) Å. The interlayer space is filled with quinoline groups. The π−π interactions are present between the quinoline rings with an average centroid−centroid distance of 3.645 Å (Figure 1e). Extensive hydrogen bond interactions are found among the coordination water (O1W, O2W), phosphonate oxygen (O1, O2), sulfate oxygen (O6, O7), and quinoline nitrogen (N1) and carbon (C5) atoms. Compound β-Tb shows a similar layer structure made up of {Tb2O2} dimers and phosphonate linkages. In this case, the Tb−O bond lengths (2.272(3)−2.486(3) Å), Tb−O−Tb angle (114.4(1)°), and Tb···Tb distances over the μ3-O (4.162(1) Å) and O−P−O bridges (5.488(1), 5.242(1), and 6.803(2) Å) are close to those in α-Tb. Significant differences are found in the O1W−Tb−O2W bond angle and the Tb−O···O−Tb torsion angles (Table 2). In β-Tb, the O1W−Tb−O2W bond angle is 129.3(1)° (vs 140.0(3)° in α-Tb), and the Tb−O···O−Tb torsion angles are 23.7, 63.1, and 78.5° (vs 30.7, 67.1, and 73.1°

Crystal Structures. Compounds α-Ln and β-Ln (Ln = Gd, Tb, Ho, Er) are polymorphs possessing the same molecular components but crystallizing in different space groups. Compounds α-Ln crystallize in the monoclinic space group P21/c, while β-Ln crystallize in the triclinic space group P1̅. Interestingly, only one phase is isolated for the Yb derivative (β-Yb), possibly due to the lanthanide contraction effect. Table 1 gives the cell parameters of compounds α-,β-Ln except for αEr, a single crystal of the latter cannot be obtained. To describe the structures of these compounds in detail, terbium compounds are selected as examples. Figure 1a shows the building unit of α-Tb. The Tb atom has a distorted-dodecahedral geometry, surrounded by three phosphonate oxygen atoms (O1, O2A, O3B), three sulfate O atoms (O4, O5, O5C) and two water molecules (O1W, O2W) (Tb−O = 2.259(10)−2.502(10) Å). The equivalent Tb atoms are doubly bridged by sulfate oxygen atoms O5 and O5C, forming a {Tb2O2} dimer (Tb−O−Tb = 115.6(4)°, Tb···Tb = 4.208(4) Å). The dimers are cross-linked through O−P−O C

DOI: 10.1021/acs.inorgchem.6b00280 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

with that of β-Tb, except that the lanthanide ion is different. The density of the β phase is higher than that of the corresponding α phase, suggesting that the β phase could be more stable. Notably, the cell volumes decrease in the sequence β-Gd > β-Tb > β-Ho > β-Er > β-Yb. Although two diffractometers were used in collecting the crystal data of these compounds (e.g., Bruker SMART APEX II for β-Gd, βTb, and β-Ho and a Bruker SMART APEX DUO for β-Er and β-Yb), the sequences β-Gd > β-Tb > β-Ho and β-Er > β-Yb are in accordance with the effect of the lanthanide contraction. A comparison of bond lengths and angles and the Ln···Ln distances and Ln−O···O−Ln torsion angles of these complexes are given in Table 2. Magnetic Properties of α-Gd and β-Gd. The dc magnetic susceptibility measurements for Gd, Tb, Ho, Er, and Yb compounds were carried out in the temperature range 1.8−300 K under an applied field of 1 kOe. Figure 2 shows the

Figure 1. Building units of (a) α-Tb and (b) β-Tb in ORTEP views (30% thermal ellipsoids), inorganic layer structures of (c) α-Tb and (d) β-Tb, and packing diagrams of (e) α-Tb and (f) β-Tb. Color codes: TbO8, green; PO3C, purple; SO4, yellow. The dashed lines represent the π−π stackings between the layers. The symmetry codes are the same as those in Table S1 in the Supporting Information.

Figure 2. Plots of χMT vs T for α-Gd and β-Gd. Inset: M vs HT−1 plot of β-Gd at 1.8 K.

χMT vs T plots for compounds α-Gd and β-Gd. The χMT products at room temperature are 7.28 and 7.84 cm3 K mol−1 for α-Gd and β-Gd, respectively, close to the theoretical value of 7.88 cm3 K mol−1 for a free Gd3+ ion (8S7/2, g = 2). Upon cooling, the χMT values remain almost constant down to 14 K, below which an abrupt increase is observed in both cases approaching 7.51 and 8.26 cm3 K mol−1 for α-Gd and β-Gd, respectively. The upturn of χMT is indicative of weak ferromagnetic interactions between Gd3+ ions. This is confirmed by the positive Weiss constants of +0.27/+0.10 K for α-Gd/β-Gd, determined by the susceptibility data above 50 K (Figure S6 in the Supporting Information). The dominant ferromagnetic exchange couplings can be propagated through

in α-Tb). The difference is related to the different arrangement of the {Tb2O2} dimers within the layer. In α-Tb, the dimers are arranged in a zigzag form along the c axis (Figure 1c), while in β-Tb, the dimers are arranged in parallel throughout the layer (Figure 1d). The interlayer π−π interactions are again observed in β-Tb between the quinoline rings with an average centroid− centroid distance of 3.628 Å (Figure 1f). The intralayer hydrogen bond networks are also slightly different in the two cases (Table S4 in the Supporting Information). The structures of α-Ln(2-qpH)(SO4)(H2O)2 (Ln = Gd, Ho, Er) are identical with that of α-Tb, and the structures of βLn(2-qpH)(SO4)(H2O)2 (Ln = Gd, Ho, Er, Yb) are identical

Table 2. Selected Structural Parameters of Compounds α-Ln and β-Ln α-Gd β-Gd α-Tb β-Tb α-Dyd β-Dyd α-Ho β-Ho β-Er β-Yb a

Ln−O (Å)

Ln−O−Ln (deg)

2.270(8)−2.485(6) 2.296(8)−2.506(7) 2.259(10)−2.502(10) 2.272(3)−2.486(3) 2.292(2)−2.472(2) 2.265(4)−2.482(4) 2.235(6)−2.459(5) 2.257(4)−2.478(3) 2.247(3)−2.475(3) 2.210(2)−2.453(2)

115.3(2) 114.1(2) 115.6(4) 114.35(10) 115.6(1) 114.4(1) 115.55(18) 114.47(13) 114.45(11) 114.67(8)

Ln−O···O−Ln (deg)a 28.1, 23.5, 30.7, 23.7, 24.8, 30.0, 30.8, 24.6, 25.2, 25.0,

66.2, 62.7, 67.1, 63.1, 63.6, 66.3, 66.3, 62.6, 62.7, 62.5,

70.5 75.9 73.1 78.5 83.0 67.7 68.9 78.0 78.9 82.4

O1W−Ln1−O2W (deg)

Ln···Lnb (Å)

Ln···Lnc (Å)

140.1(3) 129.6(3) 140.0(3) 129.3(1) 140.1(1) 130.1(1) 140.0(2) 129.6(1) 129.8(1) 129.7(1)

4.163(1) 4.177(1) 4.208(4) 4.162(1) 4.144(1) 4.146(1) 4.122(1) 4.129(1) 4.118(1) 4.083(1)

5.345(1)−6.792(2) 5.262(1)−6.825(2) 5.394(4)−6.837(7) 5.488(1)−6.803(2) 5.334(1)−6.770(1) 5.222(2)−6.770(1) 5.315(2)−6.745(3) 5.204(1)−6.772(1) 5.189(1)−6.759(1) 5.128(1)−6.695(1)

Ln−O···O−Ln torsion angles over the O−P−O bridge. bLn···Ln distances over the μ3-O bridge; cLn···Ln distances over the O−P−O bridges. Reference 20.

d

D

DOI: 10.1021/acs.inorgchem.6b00280 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

ature χMT values of compounds α-Ho and β-Ho are 14.80 and 14.57 cm3 K mol−1, respectively, which coincide with the value of 14.06 cm3 K mol−1 for a single HoIII ion (5I8, S = 2, L = 6, J = 8, g = 5/4). On cooling, the χMT values for both compounds decrease gradually in the range of 300−50 K and then further decrease sharply to reach a corresponding minimum of 5.18 and 4.70 cm3 K mol−1 for α-Ho and β-Ho at 1.8 K, respectively. These behaviors are ascribed to the progressive depopulation of MJ sublevels. The magnetization curves from zero dc field to 70 kOe at different temperatures are shown in the inset of Figure S15 in the Supporting Information. The maximum values of magnetization at 1.8 K are 6.29 and 6.31 Nβ for α-Ho and β-Ho, respectively, which are lower than the expected saturation value of 10 Nβ for each HoIII ion, attributed to the crystal-field effects and low-lying excited states. The temperature-dependent ac susceptibility data reveal that both αHo and β-Ho lack any out-of-phase ac signals under a zero dc field (Figure S17 in the Supporting Information) or applied dc fields (Figure S18 in the Supporting Information), excluding the possibility of slow magnetization relaxation. Magnetic Properties of α-Er and β-Er. For compounds α-Er and β-Er, the χMT values at 300 K (11.97 and 12.81 cm3 K mol−1) are slightly higher than the value of 11.48 cm3 K mol−1 for a single ErIII ion (4I15/2, S = 3/2, L = 6, J = 15/2, g = 6/5). A progressive depopulation of MJ sublevels again occurs when lowering the temperature, causing a continuous decrease of χMT values (Figure 4). The nonsaturation of the magnetization

the μ3-O and/or O−P−O pathways. The magnetization curves were measured at 1.8 K. Both increase rapidly at low field and reach values of 6.39 Nβ (for α-Gd) and 6.90 Nβ (for β-Gd) at 70 kOe (Figure 2 and Figure S7 in the Supporting Information), which are close to the expected saturation value of 7 Nβ for an isolated GdIII. Magnetic Properties of α-Tb and β-Tb. For compounds α-Tb and β-Tb, Figure 3 shows the χMT vs T plots. The χMT

Figure 3. Plots of χMT vs T for α-Tb and β-Tb. Inset: M vs HT−1 plots of α-Tb at different temperatures below 10 K.

values at 300 K are 12.72 and 12.83 cm3 K mol−1, respectively, consistent with the expected value of 11.76 cm3 K mol−1 for a free Tb3+ ion (7F6, g = 3/2, J = 6). Upon cooling, the χMT values decrease slowly until reaching a minimum of 10.84 cm3 K mol−1 at 14 K (for α-Tb) and 12.63 cm3 K mol−1 at 60 K (for β-Tb), attributed to thermal depopulation of the MJ sublevels of Tb3+. Below the minimum temperatures, the χMT value increases again, indicating that ferromagnetic interaction is dominant in the two phases. The magnetization curves show a linear increase at low fields and are not saturated at 70 kOe (5.17 Nβ for α-Tb and 4.80 Nβ for β-Tb) (Figure S8 in the Supporting Information). The nonsuperimposed M vs H/T curves also suggest the presence of magnetic anisotropy and/or low-lying excited states (Figure 3, inset). To probe the magnetic dynamics of the two phases, ac susceptibility measurements were performed at zero dc field. Neither peaks nor frequency dependences of the in-phase (χM′) and out-of-phase (χM″) signals are observed down to 2.0 K (Figure S10 in the Supporting Information), possibly due to the fast quantum tunneling of the magnetization (QTM) and/or exchange couplings that destroy the anisotropy of the terbium ion. The QTM effect can be suppressed by applying an external dc field. Thus, the ac susceptibility measurements were performed at 2.0 K under variable dc fields for both compounds (Figure S11 in the Supporting Information). Attempts to fit these data were unsuccessful due to the absence of frequencydependent peaks. Therefore, a moderate external field of 2 kOe was applied to measure the ac susceptibility of both compounds. In both phases, nonzero χM″ signals are observed below 10 K but without distinct frequency dependence (Figure S13 in the Supporting Information). The χM″ vs ν plots measured at different temperatures are almost superimposed, with only tails appearing at low frequency (Figure S14 in the Supporting Information). Obviously, slow magnetization relaxation is not visible in compounds α-Tb and β-Tb down to 2 K. Magnetic Properties of α-Ho and β-Ho. As shown in Figure S15 in the Supporting Information, the room-temper-

Figure 4. χMT vs T plots for α-Er and β-Er. Inset: M vs HT−1 plots of β-Er at different temperatures below 10 K.

at 70 kOe (3.57 and 5.11 Nβ for α-Er and β-Er) and nonsuperimposed M vs H/T curves suggest the presence of magnetic anisotropy and/or low-lying excited states (Figure 4, inset, and Figure S19 in the Supporting Information). The ac susceptibility data reveal that both α-Er and β-Er show neither peaks nor frequency dependence of the χM′ and χM″ signals at zero dc field (Figure S21 in the Supporting Information). For α-Er, the χM″ signals are silent even on applying a dc field of 3 kOe (Figure 5a). However, for β-Er, the application of an optimum field of 2 kOe leads to the emergence of frequency-dependent ac signals below ca. 6 K, indicating slow relaxation of magnetization (Figure 5b). The peaks in the χM″ component can be found at frequencies higher than 316 Hz. The magnetization relaxation times τ are derived from the χM″ peaks assuming τ = (2πν)−1. The relaxation follows a thermally activated mechanism, and Arrhenius law fitting gives an energy barrier of 18.0 K, with the preexponential factor τ0 of 2.8 × 10−6 s. Frequency-dependent ac susceptibilities were also measured for β-Er under 2 kOe dc E

DOI: 10.1021/acs.inorgchem.6b00280 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Magnetic Properties of β-Yb. For compound β-Yb, the χMT value is 2.47 cm3 K mol−1 at 300 K (Figure 7a), in

Figure 5. χM′ and χM″ vs T plots for α-Er under 3 kOe dc field (a) and for β-Er under 2 kOe dc field (b).

field at various temperatures (2.2−5.0 K). The decline in the χM′ component concomitant with the appearance of the peaks in the χM″ component indicates slow relaxation of magnetization (Figure 6a). The magnetization relaxation time τ can be

Figure 7. (a) χMT vs T plots and magnetization curves at 2 K (inset) for β-Yb. (b) The χM′ and χM″ vs ν plots for β-Yb under 2 kOe dc field. (c) Plot of ln τ vs T−1 from the χM″ vs ν data for β-Yb. The solid lines are guides to the eye (a, b) or best fit (c).

agreement with the expected value of 2.57 cm3 K mol−1 for one isolated YbIII ion (2F7/2, S = 1/2, L = 3, J = 7/2, g = 8/7). The χMT value decreases with temperature and finally drops to a minimum of 1.22 cm3 K mol−1 at 4 K, below which the χMT product increases slightly to 1.23 cm3 K mol−1 at 2 K. Such a behavior is attributed to the depopulation of excited MJ sublevels as well as the presence of very weak ferromagnetic exchange couplings between the magnetic centers, similar to the case for the related Gd complexes. The unsaturated magnetization (1.75 Nβ) together with the nonsuperimposition of M vs H/T curves suggests the presence of magnetic anisotropy and/or low-lying excited states in β-Yb. At zero dc field, the ac susceptibility data of β-Yb show neither peaks nor frequency dependence of the χM′ and χM″ signals (Figure S26 in the Supporting Information). The application of a 2 kOe dc field switches on the χM″ signals, which are frequency dependent (Figure 7b). On the basis of the χM′ and χM″ vs frequency data, semicircular shaped Cole−Cole plots can be obtained in the temperature range of 2.2−6.0 K (Figure S28 in the Supporting Information). These data can be fitted by the generalized Debye model, giving a distribution coefficient value of 0.10−0.37. A linear fitting of the ln τ vs T−1 plot according to the Arrhenius law leads to an energy barrier of 11.2 K (τ0 = 8.6 × 10−6 s) (Figure 7c).

Figure 6. (a) χM′ and χM″ vs ν plots and (b) ln τ vs T−1 plot from the χM″ vs ν data for β-Er under a 2 kOe dc field. The solid lines are guides to the eye (a) or best fit (b).

extracted by fitting the Cole−Cole plots using the generalized Debye model for a single relaxation process (Figure S24b in the Supporting Information).30 The α values are in the range of 0.03−0.12, indicating a narrow distribution of the relaxation. The Δ/kB and τ0 values deduced from the Arrhenius laws using the data of the high-temperature regime are 17.8 K and 1.9 × 10−6 s, respectively (Figure 6b), in accordance with those based on the χM″(T) data. F

DOI: 10.1021/acs.inorgchem.6b00280 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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It is very interesting that polymorphic compounds α-Er and β-Er show remarkably different magnetic behavior. Fieldinduced slow magnetic relaxation is observed below 6 K for βEr but not for α-Er down to 2 K. The difference can be explained by the structural difference in the two polymorphs. As described above, both phases display layer structures made up of {Er2O2} dimers and phosphonate linkages. The arrangements of the dimers within the layer, however, are different in the two cases. In α-Er, the dimers are arranged in a zigzag form along the c axis, which reduces the overall anisotropy due to the nonparallel arrangement of the easy axes. However, in β-Er, they are arranged in parallel. Considering that ferromagnetic interactions are dominant between the ErIII ions, a parallel arrangement of easy axes is essential to enhance the overall magnetic anisotropy and hence the observation of slow magnetization relaxation. The same phenomenon was also observed in the dysprosium analogues.20 To the best of our knowledge, layered ErIII or YbIII compounds showing SMM-like behavior have never been documented except for [Er(notpH4)(H2O)]ClO4·3H2O.19a Another interesting point is related to the magnetic behavior of β-Ln. Field-induced magnetization relaxation is observed in β-Dy, β-Er, and β-Yb but not in the β-Tb and β-Ho derivatives, although their structures are identical. The absence of slow relaxation in compounds β-Tb and β-Ho with integer J values is reminiscent of the spin parity effect on the tunneling efficiency, which was previously observed in the mononuclear complexes [N(C2H5)4]3[Ln(dipic)3]·nH2O (dipic = pyridine-2,6-dicarboxylate),21 Na[LnDOTA(H2O)]·4H2O,31 and (Et3NH)[Ln(3NO2-salen)2].24c According to Rinehart and Long,32 large magnetic anisotropy may be achieved when the negative charges of the ligands are concentrated in an axial position for oblate 4f ions and in an equatorial position for the prolate 4f ions. Thus, the observation of SMM behavior in isostructural DyIII, ErIII, and YbIII compounds is unusual, because the dysprosium(III) ion has an oblate electron density while the erbium(III) and ytterbium(III) ions have a prolate electron density. The result can be explained by the distorted-dodecahedral geometry of the lanthanide ions in β-Ln, which provides an intermediate environment that could stabilize a high |MJ| value for both oblate and prolate electronic shapes.25 The energy barriers follow the sequence β-Dy (83.0 K) > β-Er (17.8 K) > β-Yb (11.2 K), as a result of interaction between the single 4f ion electron density and the crystal field environment.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00280. Structural, IR, PXRD, and additional magnetic data (PDF) Crystallographic data (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for L.-M.Z.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2013CB922102). REFERENCES

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CONCLUSIONS

We have synthesized and characterized nine lanthanide phosphonate compounds with the formulas α-Ln(2-qpH)(SO4)(H2O)2 (α-Ln; Ln = Gd, Tb, Ho, Er) and β-Ln(2qpH)(SO4)(H2O)2 (β-Ln; Ln = Gd, Tb, Ho, Er, Yb). The α and β phases of the same lanthanide are polymorphic, showing similar layer structures except the arrangement of the {Ln2O2} dimers within the layer: e.g., zigzag mode in the α phase and parallel mode in the β phase. The slight structural difference leads to significant changes in their magnetic behavior. Therefore, field-induced slow magnetic relaxation is observed in β-Er and β-Yb but not in α-Er. This work provides the third examples of polymorphic lanthanide compounds that show distinct magnetic behavior. G

DOI: 10.1021/acs.inorgchem.6b00280 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b00280 Inorg. Chem. XXXX, XXX, XXX−XXX