Modulation of the Coordination Environment around the Magnetic

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

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Modulation of the Coordination Environment around the Magnetic Easy Axis Leads to Significant Magnetic Relaxations in a Series of 3d-4f Schiff Complexes Jing-Wei Yang,† Yong-Mei Tian,† Jin Tao,§ Peng Chen,† Hong-Feng Li,† Yi-Quan Zhang,*,§ Peng-Fei Yan,† and Wen-Bin Sun*,†,‡

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Key Laboratory of Functional Inorganic Material Chemistry Ministry of Education, School of Chemistry and Material Science, Heilongjiang University, 74 Xuefu Road, Harbin 150080, P. R. China ‡ Key Laboratory of Chemical Engineering Process & Technology for High-Efficiency Conversion, 74 Xuefu Road, Harbin 150080, P. R. China § Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, China S Supporting Information *

ABSTRACT: A series of Salen-type Zn(II)−Dy(III) complexes [L1Zn(II)ClDy(III)(acac)2]·H2O (1), [L1Zn(II)BrDy(III)(acac)2]·H2O (2), [L1Zn(II)(H2O)Dy(III)(acac)2]· CH2Cl2·PF6 (3), [L2Zn(II)(H2O)Dy(III)(acac)2]·PF6 (4), and Co(III)−Dy(III) complexes [L1Co(III)Br2Dy(III)(acac)2]·CH2Cl2 (5), [L2Co(III)Cl2Dy(III)(acac)Cl(MeO)] (6), [L2Co(III)Cl2Dy(III)(acac)Cl(H2O)] (7), and [L2Co(III)Cl2Dy(III)(NO3)2(MeO)] (8) heterobinuclear singlemolecule magnets (SMMs) were synthesized and magnetically characterized. These complexes were constructed by incorporating diamagnetic Zn(II) and Co(III) ions with acetylacetone (acac) and compartmental Schiff-base ligands (H2L1 = N,N′-bis(2-oxy-3-methoxybenzylidene)-1,2-phenylenediamine; H2L2 = N,N′-bis(2-oxy-3-methoxybenzylidene)-1,2-diaminocyclohexane). In the Zn(II)−Dy(III) (1−4) system, the coordination environments of the Dy(III) ions are nearly identical, but the apical coordination atom to the Zn(II) ion is different. Complexes 1, 2, and 4 displayed no magnetic relaxation in the absence of external magnetic field, but complex 3 displayed more pronounced SMM behavior with a relaxation energy barrier Ueff/kB 38 K and magnetic hysteresis at 1.8 K. The SMM performances of 5, 6, and 7 were enhanced significantly by incorporating an octahedral Co(III) instead of squarepyramidal Zn(II) and replacing one of acac− group around Dy(III) ion by a neutral O atom, displaying Ueff of 167, 118, and 75 K as well as magnetic hysteresis up to 3.5 K. These studies indicated that the remote diamagnetic Zn(II) and Co(III) ions played a key role, and the SMM properties were very strongly related to the special coordination atoms configuration around Dy(III) ion. When this coordination configuration around was broken as 8 exhibited, however, it resulted in a dramatically decreased SMM performance. From this work, the key factors that significantly affect the SMM performance of these heteronuclear Zn(II)/Co(III)−Dy(III) SMMs are unambiguously presented.

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INTRODUCTION Mononuclear lanthanide complexes have attracted a growing interest due to their outstanding single-molecule magnetic (SMM) performance with promising applications in highdensity information storage, quantum information processing, and spintronics.1 The significant SMM behavior (display magnetic hysteresis and frequency dependence of out-of-phase (χ″) ac susceptibility peaks) of these simple 4f-based systems is the result of intrinsic large unquenched orbital angular momentum and spin−orbit coupling of lanthanide ions in combination with an appropriate ligand field, which allows for the significant single-ion anisotropies. Incorporating lanthanide ions into SMMs and/or constructing pure lanthanide-based © XXXX American Chemical Society

mononuclear SMMs (also referred as single ion magnets (SIMs)) is a promising strategy to obtain SMMs with a high energy barrier (Ueff) and blocking temperature (TB).2 To date, this strategy has resulted in compounds with a record Ueff of 1837 K3 and blocking temperatures as high as 60 K.4 In general, the electron density distribution of lanthanide ions shows a strong angular dependence and has a preferred orientation under the electrostatic potential generated by the ligand donor atoms. For the lanthanide ions with the oblate electron distribution like Dy(III) ion, an axial coordination Received: January 9, 2018

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DOI: 10.1021/acs.inorgchem.8b00056 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Orientation of the local main magnetic axes of the ground Kramers doublet on the Dy(III) ion by ab initio calculation (left) for complex [Zn2(L1)2DyCl3]·H2O from ref 16, in which the transverse five coordination atoms perpendicular to the magnetic easy axis with low electronic density, and high electronic density distributed around the main magnetic axis; and by the Magellan procedure (right) for complex [Co(III)Dy(III)L(μ-OAc)2(NO3)2] from ref 17 (L = N,N′-ethylenebis(3-methoxysalicylaldimine)). The H atoms are omitted for clarity. Color codes: Dy, teal; Zn, gold; Co, violet; Cl, bright green; N, blue; O, red; C, gray.

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field contributes to minimize charge repulsion between f electrons and donor atoms of ligand and consequently results in the high anisotropy.5 Thus, stabilizing the MJ = 15/2 sublevel by fine-tuning the ligand field using coordination atoms with varying charge distribution is a good strategy for generating superior SMMs. Many lanthanide SMMs implement this strategy by possessing high axial symmetry around the spin center, such as D4d, D5h, D∞h, C5, and C∞v,6,7 but synthesizing lanthanide complexes with these point groups except for the D4d system is difficult because a large coordination number and various coordination modes are usually seen in lanthanide-based complexes. If a rational configuration of coordination atoms with asymmetric electronic density is employed, lower symmetry lanthanide complexes can also display good SMM properties.8 As for mononuclear single-molecule magnets, the magnetic exchange interactions were absent, and their single-ion anisotropies were mostly derived from the contribution of unquenched firstorder orbit moments, the ligand field and local symmetry.2g,9,10 In addition, the magnetic behavior of most lanthanide SMMs is further hampered by fast quantum tunneling of magnetization (QTM) processes that circumvent the thermal barrier. In order to suppress QTM, many strategies have been adopted, such as magnetic dilution, applying an added dc magnetic field, and enhancing the local axiality of the ligand field around the Ln ions.11 A few recent studies explored the effect that a diamagnetic or paramagnetic 3d ion can have on the QTM in a 4f ion by synthesizing a series of M−Ln−M, M−Ln, or M3−Ln type monolanthanide SMMs.12−15 In the majority of these compounds, the used Schiff-base ligands have two types of oxygen atoms, methoxy groups and phenoxides. The methoxy oxygen atoms have a lower negative charge associated with them than the phenoxide oxygen atoms do. Interestingly, in our previous work about Zn−Ln−Zn complexes [Zn2(L1)2DyCl3]·H2O,16 the phenoxide coordinating atoms coordinated to the axial positions, while the methoxy coordinating atoms coordinated to the equatorial positions (Figure 1), resulting in good SMM performance. The exact role that the diamagnetic metal ion plays in these compounds is unclear. Thus, we present here a series of four Zn(II)− Dy(III) and four Co(III)−Dy(III) SMMs, to explore the impact of diamagnetic transition metals and fine-tuning coordination environment of Dy(III) ions on the slow magnetic relaxation behavior.

EXPERIMENTAL SECTION

General Information. All solvents and chemicals were used as received without further purification. According to the procedure reported,16,18 the N,N′-bis(2-oxy-3-methoxybenzylidene)-1,2-phenylenediamine (H2L1) and N,N′-bis(2-oxy-3-methoxybenzylidene)-1,2diaminocyclohexane (H2L2) ligands were synthesized by the condensation between o-phenylenediamine or 1,2-cyclohexanediamine and o-vanillin in a molar ratio of 1:2, in which 1,2cyclohexanediamine was racemic. Elemental analyses were carried out on an Elementar Vario EL cube analyzer. FT-IR spectra were obtained on a PerkinElmer Spectrum One spectrophotometer. The powder diffraction measurements were recorded at room temperature on a Bruker D8 diffractometer using Cu Kα (λ = 1.5406 Å) radiation. The accelerating voltage and the applied current were 40 kV and 20 mA, respectively. All measurements were taken with fresh crystals. Single-Crystal X-ray Diffraction. Crystal data for 1−8 were collected for X-ray diffraction analysis on a Xcalibur, Eos diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 293 K. The structures of 1−8 were solved by direct methods and refined on F2 by full-matrix least-squares using the SHELXL-2014.19 All nonhydrogen atoms were refined with anisotropic displacement parameters. The structures of both 1 and 2 contain highly disordered solvent water molecules that were unable to model. The electron density of the solvent was removed using the Squeeze function of PLATON. The squeezed solvent contained 22 electrons in a void of 113 A3, corresponding to one molecule of H2O. Thermogravimetric analysis agrees with this solvent content (Figure S58). The experimental details of the crystallographic data for 1−8 are summarized in Table S1 (in Supporting Information). Magnetic Measurements. Magnetic measurements were performed using a Quantum Design VSM Superconducting Quantum Interference Device (SQUID) MPMS-3 magnetometer on samples of 1−8 that were prepared from fresh crystals collected from stock solution and dried in air for 5 min to ensure the solvent molecule retaining in samples. Alternating-current (ac) susceptibility measurements were performed under an oscillating ac field of 2 Oe or applied external direct-current (dc) field, and ac frequencies range from 1 to 1000 Hz. The dc magnetic susceptibilities were performed using the same instrument under a 1000 Oe dc field between 2 and 300 K. Magnetic hysteresis was measured between −1 and 1 T at temperatures ranging from 1.8 to 4 K. The diamagnetic contribution of the polypropylene bag used to hold the sample was subtracted from the raw data, and the core diamagnetic contributions of the sample were accounted for using Pascal’s constants.20 To ensure that the bulk sample used for the magnetic measurements and the single crystals have the same composition (avoid loss of solvent molecules upon drying), powder diffraction patterns of the fresh bulk samples were measured and compared with the calculated pattern of the single crystal data (Figure S59). Synthesis of Complexes 1−8. Synthesis of [L1Zn(II)ClDy(III)(acac)2]·H2O (1). H2L1 (0.076 g, 0.2 mmol) and ZnCl2 (0.025 g, 0.2 B

DOI: 10.1021/acs.inorgchem.8b00056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of Complexes 1−8 from the Previously Reported H2L1 and H2L2 Ligands16

mmol) were dissolved in absolute MeOH (40 mL) and stirred for 1.5 h, resulting in a cloudy orange-yellow mixture. Then Dy(acac)3·H2O (0.096 g, 0.2 mmol) was added, and the mixture was stirred at room temperature for 4.5 h, resulting in a clear orange-yellow solution. The solution was filtered and layered with diethyl ether, affording yellow crystals after 1 week. Yield: 0.130 g (78.0%). Elemental analysis (%) calcd for C32H34ClDyN2O9Zn: C 45.07, N 3.28, H 4.01 wt %; found: C 45.22, N 3.36, H 4.17 wt %. IR (KBr, cm−1): 3455(w), 2931(w), 1617(s), 1585(m), 1519(s), 1467(m), 1386(m), 1304(w), 1235(m), 1193(m), 1102(w), 1019(w), 975(w), 850(w), 742(w), 649(w), 558(w), 512(w). Synthesis of [L1Zn(II)BrDy(III)(acac)2]·H2O (2). Compound 2 was synthesized according to the above procedure for 1, except using ZnBr2 (0.076 g, 0.2 mmol) in place of ZnCl2. Rectangular yellow crystals were formed after 1 week. Yield: 0.150 g (82.0%). Elemental analysis (%) calcd for C32H34BrDyN2O9Zn: C 42.78, N 3.12, H 3.81 wt %; found: C 42.91, N 3.21, H 3.95 wt %. IR (KBr, cm−1): 3467(w), 2925(w), 1611(s), 1586(m), 1518(m), 1467(m), 1386(m), 1303(m), 1235(m), 1193(m), 1102 (w), 1019(w), 971(w), 849(w), 741(m), 649(w), 558(w), 512(w). Synthesis of [L1Zn(II)(H2O)Dy(III)(acac)2]·CH2Cl2·PF6 (3). To a stirred solution of H2L1 (0.076 g, 0.2 mmol) in CH2Cl2 (40 mL), Dy(acac)3·H2O (0.098 g, 0.2 mmol) was added, and the mixture was further stirred at room temperature for 40 min. Then Zn(acac)2· 2H2O (0.054 g, 0.2 mmol) and NH4PF6 (0.032 g, 0.2 mmol) were added, and the resultant mixture was refluxed for 1 h. The reaction was cooled to room temperature and stirred for an additional 8 h. Then, the reaction was filtered and the filtrate was layered with diethyl

ether to afford yellow crystals after 3 days. Yield: 0.180 g (86.0%). Elemental analysis (%) calcd for C33H36Cl2DyF6N2O9PZn: C 37.81, N 2.67, H 3.46 wt %; found: C 37.92, N 2.71, H 3.47 wt %. IR (KBr, cm−1): 3376(w), 2945(w), 1613(s), 1586(m), 1520(s), 1456(m), 1384(m), 1302(w), 1270(w), 1235(s), 1195(s), 1098(w), 960(s), 968(w), 840(w), 739(m), 649(w), 557(m), 529(w). Synthesis of (3Dy0.05Y0.95). A similar procedure was used as 3 except for the admixture of Y(acac)3·H2O (0.084 g, 0.19 mmol)/ Dy(acac)3·H2O (0.005 g, 0.01 mmol) in 19/1 ratios as rare-earth salts. Yield: 0.140 g (81.0%). Stoichiometric doping level for C 33 H 36 Cl 2 Dy 0.05 Y 0.95 F 6 N 2 O 9 PZn: Dy(III), 5.00% and Y(III), 95.00%; experimental derived doping value (ICP): Dy(III), 5.18% and Y(III), 94.82%. Elemental analysis (%) calcd for C33H36Cl2Dy0.05Y0.95F6N2O9PZn: C 40.52, N 2.86, H 3.71 wt %; found: C 40.61, N 2.92, H 3.80 wt %. IR (KBr, cm−1): 3285(w), 2901(w), 1614(s), 1585(m), 1539(s), 1444(m), 1387(m), 1338(w), 1236(s), 1192(s), 1106(w), 1074(s), 977(w), 861(w), 737(m). Synthesis of [L2Zn(II)(H2O)Dy(III)(acac)2]·PF6 (4). Compound 4 was synthesized using the above procedure for 3 except using H2L2 (0.193 g, 0.5 mmol) in place of H2L1. Rectangular yellow crystals were formed after 6 days. Yield: 0.386 g (79.7%). Elemental analysis (%) calcd for C32H40DyN2O9PZnF6: C 39.64, N 2.90, H 4.16 wt %; found: C 39.78, N 2.94, H 4.19 wt %. IR (KBr, cm−1): 3352(w), 2939(w), 1655(w), 1633(w), 1586(m), 1519(s), 1453(m), 1387(m), 1283(m), 1222(m), 1076(w), 1021(w), 950(w), 855(s), 742(m), 660(w), 558(m). Synthesis of [L1Co(III)Br2Dy(III)(acac)2]·CH2Cl2 (5). H2L1 (0.151 g, 0.4 mmol) was dissolved in a mixture of MeOH (5 mL) and CH2Cl2 C

DOI: 10.1021/acs.inorgchem.8b00056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Crystal structures of complexes 1, 3, 5, and 6. The H atoms and CH2Cl2 (in 3 and 5) are omitted for clarity. Color codes: Dy, teal; Zn, gold; Co, violet; P, pink; F, turquoise; Br, dark yellow; Cl, bright green; N, blue; O, red; C, gray. Blue dashed lines represent the π−π interactions in 3. To quantify the degree of distortion of the coordination sphere around the Zn(II)/Co(III)ions and Dy(III) ions, the Continuous-ShapeMeasures (CShMs) were carried out by Shape software.24,25 (CShM value equals 0 which corresponds to the perfect polyhedron, and the larger value indicates the more deviated from the ideal geometry, which are listed in Tables S14−S16). Synthesis of [L2Co(III)Cl2Dy(III)(acac)Cl(H2O)] (7). To stirred solution of H2L2 (0.151 g, 0.4 mmol) in CH2Cl2 (20 mL), CoCl2· 6H2O (0.096 g, 0.4 mmol), and Dy(acac)3·H2O (0.192 g, 0.4 mmol) were added, and the resultant mixture was stirred for 2 h. The filtered solution was layered with n-hexane to afford brownish-black crystals after 2 weeks. Yield: 0.158 g (59.5%). Elemental analysis (%) calcd for C27H33Cl3CoDyN2O7: C 39.29, N 3.33, H 4.03 wt %; found: C 39.32, N 3.39, H 4.15 wt %. IR (KBr, cm−1): 3364(b), 2935(w), 1638(m), 1605(m), 1520(m), 1474(s), 1381(m), 1308(m), 1247(w), 1227(m), 1173(w), 1079(w), 1033(w), 971(w), 924(w), 856(w), 784(w), 739(w), 659(w), 567(w). Synthesis of (7Dy0.05Y0.95). Compound 7Dy0.05Y0.95 was synthesized according to the same procedure as 7 except for the addition of Y(acac)3·H2O (0.166 g, 0.38 mmol)/Dy(acac)3·H2O (0.011 g, 0.02 mmol) in 19/1 ratios as rare-earth salts. Yield: 0.247 g (81.7%). Stoichiometric doping level for C27H33Cl3CoDy0.05Y0.95N2O7: Dy(III), 5.00% and Y(III) 95.00%; experimental derived doping value (ICP): Dy(III), 5.11% and Y(III) 94.89%. Elemental analysis (%) calcd for C27H33Cl3CoDy0.05Y0.95N2O7: C 42.73, N 3.71, H 4.45 wt %; found: C 42.89, N 3.80, H 4.51 wt %. IR (KBr, cm−1): 3362(b), 2933(w), 1641(m), 1606(s), 1520(m), 1474(s), 1385(m), 1303(m), 1246(m), 1227(m), 1170(w), 1079(w), 1031(w), 972(w), 924(w), 856(w), 782(w), 736(w), 657(w), 569(w). Synthesis of [L2Co(III)Cl2Dy(III)(NO3)2(MeOH)] (8). Compound 8 was synthesized according to the above procedure for 6 except Dy(NO3)3·6H2O (0.161 g, 0.35 mmol) was substituted for Dy(acac)3·H2O. Rectangular dark crystals were formed after 1 week. Yield: 0.228 g (78.7%). Elemental analysis (%) calcd for C23H28Cl2DyN4O11Co: C 33.33, N 6.76, H 3.40 wt %; found: C 33.38, N 6.78, H 3.57 wt %. IR (KBr, cm−1): 3433(b), 3057(w), 2982(w), 2843(m), 1611(s), 1533(m), 1452(m), 1353(m), 1309(s), 1251(m), 1198(s), 1167(m), 1091(m), 1015(m), 960(m), 846(m), 795(m), 713(m), 581(m), 469(w).

(20 mL), resulting in a clear orange-yellow solution. CoBr2·6H2O (0.088 g, 0.4 mmol) and Dy(acac)3·H2O (0.192 g, 0.4 mmol) were added, and the resulting mixture was stirred at room temperature for 2 h. The reaction was filtered and the filtrate was layered with diethyl ether to afford brownish-black crystals of 5 after 8 days. Yield: 0.332 g (80.9%). Elemental analysis (%) calcd for C33H34DyBr2Cl2N2O8Co: C 38.16, N 2.70, H 3.30 wt %; found: C 38.21, N 2.74, H 3.43 wt %. IR (KBr, cm−1): 2935(w), 1628(s), 1553(m), 1519(m), 1468(m), 1384(s), 1317(m), 1244(m), 1197(m), 1103(w), 1017(w), 980(w), 922(w), 858(w), 776(w), 735(m), 656(w), 532(w). Synthesis of [L2Co(III)Cl2Dy(III)(acac)Cl(MeOH)] (6). Compound 6 was synthesized according to the above procedure for 5 except replacing H2L1 and CoBr2·6H2O with H2L2 and CoCl2·6H2O, respectively. Rectangular dark crystals formed after 1 week. Yield: 0. 26 7 g ( 79 .5 % ). El e m en t al an al y sis (%) ca lc d for C28H35Cl3CoDyN2O7: C 40.07, N 3.34, H 4.20 wt %; found: C 40.11, N 3.39, H 4.37 wt %. IR (KBr, cm−1): 3063(b), 2936(w), 1644(m), 1605(m), 1528(m), 1474(s), 1382(m), 1307(m), 1243(w), 1227(m), 1171(w), 1078(w), 1022(w), 973(w), 924(w), 857(w), 792(w), 734(w), 659(w), 569(w). Synthesis of (6Dy0.05Y0.95). Compound 6Dy0.05Y0.95 was synthesized according to the above procedure for 5 except using Y(acac)3·H2O (0.168 g, 0.38 mmol)/Dy(acac)3·H2O (0.010 g, 0.02 mmol) in 19/1 ratios as rare-earth salts and replacing H2L1 with H2L2. Yield: 0.256 g (83.5%). Stoichiometric doping level for C28H35Cl3CoDy0.05Y0.95N2O7: Dy(III), 5.00% and Y(III) 95.00%; experimental derived doping value (ICP): Dy(III), 5.16% and Y(III) 94.84%. Elemental analysis (%) calcd for C28H35Cl3CoDy0.05Y0.95N2O7: C 43.73, N 3.64, H 4.59 wt %; found: C 43.81, N 3.73, H 4.67 wt %. IR (KBr, cm−1): 3085(b), 2937(w), 1642(m), 1606(s), 1526(m), 1474(s), 1383(m), 1307(m), 1244(m), 1227(m), 1172(w), 1079(w), 1023(w), 972(w), 924(w), 856(w), 792(w), 734(w), 659(w), 569(w). D

DOI: 10.1021/acs.inorgchem.8b00056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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RESULTS AND DISCUSSION Synthesis and Structure. Under aerobic conditions, we used Zn(II) and Co(II) starting materials along with the hexadentate Salen-type Schiff-base ligands (H2L1 and H2L2) to construct a series of heterobinuclear Zn(II)−Dy(III) and Co(III)−Dy(III) SMMs (Scheme 1). The cobalt(II) reagents employed spontaneously oxidized to form the cobalt(III) complexes 5−8, as has been seen for many related complexes in the literature.21 The attempt to obtain the whole system constructed by only the same one Schiff base ligand H2L1 or H2L2 failed; it is mostly due to the flexible coordination mode of Schiff base lanthanide complexes and the sensitivity to the reaction condition during their formation, while the present system ensures that the close coordination atoms around Zn(II) and Dy(III) are identical. In addition, the molar ratio of starting materials of transition metal compounds (ML) and rare earth salts play a key role in forming the final heteronuclear complexes, so do the types of rare earth salts. When ML/LnCl3·6H2O or ML/Ln(β-diketone)3·H2O is 1:1, heterodinuclear M-Ln complexes formed and when ML/ LnCl3·6H2O is 2:1, which resulted in heterotrinuclear M−Ln− M complexes (L = Schiff-based ligand, M = transition metal).22 Complexes 1−8 were synthesized as shown in Scheme 1, and their main crystal parameters are listed in Table S1. Complexes 1−4 have a similar Zn(II)−Dy(III) coordination core except for the different coordination groups (Cl− in 1; Br− in 2; H2O in 3 and 4) on the apical position of Zn(II) ion, and the structure of 1 was selected as an example to discuss in detail (Figure 2 and Figure S1). Complex 1 consists of a Zn(II)−Dy(III) core bridged by the two phenoxide oxygen atoms of L1, and the Zn···Dy distance is 3.507(6) Å. The Dy(III) ion occupies the outer O4 cavity of L1, and it possesses an eight-coordinate biaugmented trigonal prismatic geometry (CShM = 2.237, the CshM values are calculated by Shape software vide infra) with two phenoxide oxygen atoms, two methoxy oxygen atoms, as well as four oxygen atoms from the acac− ligands making up the coordination sphere. The Dy−O bond lengths are in the range of 2.265(4)−2.695(4) Å (Table S4). The two phenoxyl oxygen atoms of the ligand coordinate to Dy(III) ion as the bridged atoms with the distance of Dy− O2 = 2.304(3) Å, Dy−O3 = 2.307(3) Å. The Zn(II) ion invariably occupies the inner N2O2 site of the L1, with a fivecoordinate, square pyramidal geometry (CShM = 0.976), with the N2O2 coordination atoms of L1 forming the square basal plane and the Cl− ion occupying the apical position of the coordination geometry. The distances of Zn−O2 = 2.036(4) Å, Zn−O3 = 2.019(3) Å, Zn−N1 = 2.088(6) Å, Zn−N2 = 2.053(4) Å, Zn−Cl1 = 2.234(1) Å (Table S8). The Zn(II) ion deviated by 0.738(9) Å from the basal N2O2 plane toward the Cl− ion. The basal N2O2 plane of the Zn(II) ion and the pentagonal O5 equatorial plane of the Dy(III) ion are relatively coplanar (dihedral angle = 10.693°, Figure S2 and Table S2). Complex 2 has a similar Zn−Dy coordination core to 1 except the apical coordination atom to the Zn(II) ion is Br− ion in place of the Cl− ion. The addition of NH4PF6 led to two distinct results (3 and 4) that PF6− balances the charge instead of X− ions in 1 and 2. The structures of 3 and 4 contain a H2O ligand in the apical position of the Zn(II) ion, rather than a halogen, resulting in a shorter apical coordination distance of Zn−O9 = 2.050(1) Å and Zn−O9 = 2.035(6) Å, respectively. Consequently, the Zn(II) ion deviation from the N2O2 plane (0.602(9) Å and 0.631(1)Å, respectively) is greatly reduced

compared to that of 1 and 2. The distances of Zn−N/O in 1− 4 were similar to that (Zn−O/N range of 2.032(3) −2.093(3) Å) of [Zn2(L1)2DyCl3]·H2O (Table S8). In addition, there existed π-stacking interactions derived from the overlap of two aromatic rings of the ligand from the adjacent moieties in 3, with the offset centroid-to-centroid distance of 3.877 Å and slipping angle of 28.14° (Figure 2 and Figure S3). This centroid−centroid distance is in good agreement with the reported values for a moderately strong π−π interaction.23 In addition, CH2Cl2 molecules were stabilized in 3, where the weak C−H···Cl interactions were formed. Further structural details of 1−4 are listed in Tables S4−S7. Complex 5 has a biphenoxyl-bridged Co(III)−Dy(III) coordination core different to the Zn(II)−Dy(III) core of 1−4, with a Co···Dy distance of 3.393(6) Å (Figure 2). The coordination environment of Dy(III) ions of 5, however, is similar to that of 1−4 and has an eight-coordinate biaugmented trigonal prismatic (CShM = 2.796) with two phenoxide oxygen atoms and two methoxy oxygen atoms from L1, as well as four oxygen atoms originating from the acac− ligands. The Dy−O bond lengths range from 2.256(4) to 2.677(4) Å (Table S9). The Co(III) ion occupies the inner N2O2 site of L1 and displays a six-coordinate elongated octahedral geometry rather than the five-coordinate square pyramidal geometry seen on Zn(II) in 1. The N 2 O 2 coordination atoms form the square plane of the elongated octahedron with distances of Co−O2 = 1.900(3) Å, Co−O3 = 1.899(3) Å, Co−N1 = 1.879(4) Å, Co−N2 = 1.878(4) Å, and two Br− ions occupy the axial positions with distances of Co− Br1 = 2.411(8) Å, Co−Br2 = 2.416(8) Å (Table S13). The Co(III) ion is coplanar with the N2O2 plane (deviation = 0.007(9) Å). The N2O2 equatorial plane of the Co(III) ion is more coplanar with the O5 equatorial plane of the Dy(III) ion (dihedral angle = 6.220°) than is seen in 1 (Figure S2 and Table S2). Compounds 6 and 7 are markedly different than that of 5. The Cl− ions of 6 and 7 are located in the apical positions of Co(III) analogous to the Br− ions in 5. However, over the course of the reaction, one Cl− ion replaces an acac− ligand on the Dy(III) ions of 6 and 7. This substitution opens a coordination site on the Dy(III) ion, permitting a solvent molecule (MeOH for 6; H2O for 7) to coordinate to the ion. The distances of Co−Cl are in the range of 2.250(2)− 2.273(2) Å, and the Co···Dy distance is 3.392(9) Å. The Dy(III) ion is eight coordinated to four oxygen atoms of the ligand, two oxygen atoms of acac−, one Cl− ion, and one oxygen atom from MeOH. The Dy−O distances are in the range of 2.235(5)−2.262(5) Å, and the Dy−Cl distance is 2.632(2) Å (Figure 2 and Table S10). The dihedral angle β is 4.960° for 6 and 3.270° for 7. It is noted that the dihedral angle (3.270−6.220°) of 5−7 was relatively smaller than that (5.790−11.800°) of 1−4 (Table S2); this is mostly due to the different coordination geometries of the Co(III) and Zn(II) ion that consequently lead to the subtle coordination distortion around Dy(III) ions. The different coordination molecules attached to the Dy(III) center, which results in distinct packing modes. Structural analysis revealed that various intermolecular interactions are detected among the adjacent moieties of 6 and 7, respectively. The methanol molecule in 6 formed intramolecular H-bonding with Cl− ions, while water ligands formed both intramolecular and intermolecular H-bonds with Cl− ions in 7 (Figure S4). The distinct intramolecular and E

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temperature is lowered from 300 K to 2 K, the χMT gradually decreases, which is likely due to the thermal depopulation of Stark sublevels of Dy(III) ions in 1−8. Such behavior is consistent with the M versus H/T plots of 1−8 at 2, 3, 5, and 8 K, which showed a rapid increase in the magnetization at low fields, and the magnetization reached the maximum values of magnetization with 4.96, 6.34, 6.34, 5.97, 5.43, 6.07, 5.39, and 5.84 Nβ at 7 T (Figures S6−S13). The saturation values were much lower than the theoretical values (10 Nβ), which are likely due to crystal-field effect and the contribution of lowlying excited states. Further confirmation was obtained from the M vs. H/T plots, and the nonsuperposition field dependence magnetization curves for the M vs. H/T plots at different temperature suggested the existence of large magnetic-anisotropy or low-lying excited states in 1−8.26 Alternating Current Magnetic Properties. To probe the magnetic slow relaxation of 1−4, ac magnetic susceptibilities at various frequencies and temperatures were measured, and the results are shown in Figure 4 and Figures S14−S28. Compounds 1, 2, and 4 only show the beginnings of out-ofphase behavior under zero dc field. Despite the similarities of 3 and 4, clear frequency dependent χ″ peaks were observed under the zero dc field for 3. At the low temperature of 2 K, χ″ reaches a maximum at a frequency of v = 100 Hz, and the peak intensity decreases with increasing temperature up to 4.5 K at the same frequency position, in which the temperature independence of the χ″ in this range can be ascribed to the presence of QTM.27 From 4.5 to 11 K, the peak in χ″ shifts to higher frequencies (Figure 4). In addition, a fast magnetic relaxation was observed above 500 Hz between 2 and 6 K in 3 (Figure 4). Diluting the sample in a diamagnetic matrix of Y (3Dy0.05Y0.95) suppressed this second relaxation, suggesting that it is related to dipolar interactions between neighboring Dy(III) ions in 3. The ac susceptibility measurements of 3Dy0.05Y0.95 show frequency-dependent χ″ peaks from 4 to 18 K under zero dc field in the range of 1 Hz−1000 Hz, and the QTM process was reduced to some extent. Although the QTM processes were not suppressed completely, the relaxation times increased, and the magnetic hysteresis temperature increased from 1.8 K in 3 to 3 K in 3Dy0.05Y0.95 (vide infra and Figure S52). Furthermore, when the external dc filed was applied on the diluted samples, the QTM was suppressed greatly (Figures S29−S32). Complex 5 was designed to preserve the same coordination environment of Dy(III) as 1−2, in which only the Zn(II) ion was replaced by Co(III) ion. The ac susceptibility of 5 (Figure 4) with no applied DC field showed out-of-phase behavior between 5 and 22 K. At 5.5 K, the first clear χ″ peak was observed at a frequency of 3.2 Hz, and the χ″ peak intensity

intermolecular interactions would respond to the subtle difference upon the Dy(III) coordination environment, which might play a role in their magnetic behavior. The Dy(III) ion of 8 has a 9-coordinate spherical capped square antiprism geometry, consisting of four oxygen atoms from L1, four oxygen atoms from the two nitrate ions and one oxygen atom from MeOH. The Dy−O distances are in the range of 2.290(6)−2.545(7) Å (Table S12). Similar to 6 and 7, the Co(III) ion of 8 has an elongated octahedral geometry, with the equatorial plane formed by the N2O2 atoms of L1 and the apical positions occupied by Cl− ions. The Co−O/N distances are 1.860(1) Å−1.903(6) Å, while the Co−Cl distances are 2.212(3) and 2.293(3) Å. The octahedral distortion parameter of Co(III) ions for 5−8 are listed in Table S13. The Co(III) ions of 5−8 are in an axially elongated octahedral geometry (cis, 89.81(7)−91.95(8)°; trans, 176.84(8)−178.24(7)°), which is similar to that (cis, 87.81(8)−91.41(9)°; trans, 179.20(7)°) of the former [Co(III)Dy(III)L(μ-OAc)2(NO3)2] (Table S13).17 According to the relative larger CShM values of 2.558− 3.103 for Dy(III) in 5−8 (Table S14), the coordination environments of Dy(III) center were in a low symmetry. In order to explore the magnetic relaxation behavior corresponding to these complexes, the magnetic measurements were performed. Direct Current Magnetic Properties. The dc magnetic measurements of 1−8 were measured in an applied dc field of H = 1000 Oe in the temperature range of 2−300 K. Figure 3

Figure 3. ΧMT data measured on fresh samples under a magnetic field H = 1000 Oe for 1, 3, 5, and 6, red line, best fit.

and Figure S5 show the temperature dependences of magnetic susceptibility χMT for 1−8 with the values of 13.81, 13.90, 13.91, 14.27, 14.56, 14.80, 13.96, and 13.28 cm3 K mol−1 at 300 K, respectively. These χMT values are in agreement with the theoretical value with 14.17 cm3Kmol−1 for one free Dy(III) ion (S = 5/2, L = 5, 6H15/2, g = 4/3). As the

Figure 4. Out-of-phase susceptibility χ″ vs frequency v (logarithmic scale) for 3 and 5 between 1 and 1000 Hz under zero dc field. F

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Figure 5. Out-of-phase susceptibility χ″ vs frequency v (logarithmic scale) for 6 and 7 between 1 and 1000 Hz under zero dc field.

Figure 6. Out-of-phase susceptibility χ″ vs frequency v (logarithmic scale) for 6Dy0.05Y0.95 and 7Dy0.05Y0.95 between 1 and 1000 Hz under zero dc field.

Figure 7. (a) Plots of ln(τ) versus 1/T at zero dc field for 3, 3Dy0.05Y0.95, 5, 6, 6Dy0.05Y0.95, 7, and 7Dy0.05Y0.95; and (b) at applied dc field for 1−7, 3Dy0.05Y0.95, 6Dy0.05Y0.95, and 7Dy0.05Y0.95.

in a dipolar interaction close to the one in the undiluted sample. On the other hand, this is mostly likely because the dipole coupling was not the major factor in the relaxation dynamic of the present system. Even so, the QTM can be suppressed to some extent when the external dc field was applied. While the ac susceptibilities of 6Dy0.05Y0.95 and 7Dy0.05Y0.95 are qualitatively similar to those of 6 and 7, the diluted samples show magnetic hysteresis at higher temperatures (3.5 and 4 K) than their undiluted counterparts. This suggests that the intermolecular dipolar interactions were reduced in the diluted samples, allowing a thermally activated relaxation process like the Orbach process to have a greater impact on the relaxation process. The more pronounced SMM behaviors observed under zero dc field in 3, 5, 6, and 7 are most likely related to their special coordination environment of local M-Dy core, namely, the transverse five coordination atoms perpendicular to the magnetic easy axil with low electronic density, and axially high electronic density configuration. To prove that, 8 was synthesized using nitrates as the counterion to give a different coordination geometry around the Dy(III) ion, which also displayed very low symmetry. As a result of this change in coordination geometry, 8 displayed no observable SMM behavior.

decrease with increasing frequency up to 1000 Hz. To explore the impact of the first coordination sphere surrounding Dy(III) ions on the relaxation behavior, we introduced Cl− ions and H2O/MeOH to replace the acac− ligand of 5. As shown in Figure 5, 6 displays a temperature dependent, out-of-phase signal between 4 and 20 K. The ac susceptibility of 7 displayed a temperature independent, out-of-phase signal from 2 to 6 K, reflecting the presence of QTM and a temperature dependent signal from 6−18 K, indicating a thermal relaxation pathway. In order to elucidate the effect of intermolecular magnetic interactions on the relaxation process in 6 and 7, the magnetic measurement of diluted sample (6Dy0.05Y0.95 and 7Dy0.05Y0.95) was performed. For diluted sample 6Dy0.05Y0.95, a clear frequency-dependent peak in χ″ between 6 and 23 K with the peak intensity decreased with increasing frequencies was observed. For diluted 7Dy0.05Y0.95, a similar temperature independent, out-of-phase signal from 2 to 6 K and a temperature dependent signal from 6−21 K was also observed (Figure 6). The reason why the most magnetic behaviors of diluted samples are similar to undiluted 6 and 7 may be because the Dy(III) ions tend to cluster close to each other, yielding an average Dy···Dy distance similar to that observed in the undiluted samples, which in turn would result G

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Table 1. Energy Barriers Obtained from the Arrhenius Law Fitting and Equation 1 of the Out-of-Phase (χ’’) ac Susceptibility Data under Zero dc Field Orbach processes relaxation processes 3 3Dy0.05Y0.95 5 6 6Dy0.05Y0.95 7 7Dy0.05Y0.95

Ueff/κB (K) 38.42(4.93) 57.21(0.37) 167.66(0.03) 118.72(4.27) 102.65(2.88) 75.28(13.02) 128.27(9.61)

QTM, Raman, and Orbach processes −1

τ0 (s) 1.08(5) 1.68(1) 8.28(5) 4.76(2) 1.55(1) 3.47(1) 4.51(1)

× × × × × × ×

q (s ) −5

10 10−5 10−8 10−7 10−6 10−6 10−7

−3

1.41(2) × 10 0.20(0.02) 0.16(0.01) 0.12(0.01) 0.09(0.02) 5.29(1)×10−3 0.06(0.01)

C (s−1·K−n)

n

Ueff/κB (K)

7.21(1) × 10−3 0.04(1) 2.16(3) × 10−3 1.51(4) × 10−3 5.30(2) × 10−4 0.02(0.01) 3.99(1) × 10−4

5.21(0.34) 3.76(0.10) 4.39(0.06) 4.82(0.11) 5.22(0.18) 4.15(0.18) 5.14(0.13)

204.33(1) 204.33(1) 312.26(1) 285.04(1) 285.04(1) 220.88(1) 220.88(1)

τ0 (s) 2.62(1) 1.78(6) 1.03(5) 1.04(1) 8.32(5) 3.98(8) 9.36(1)

× × × × × × ×

10−9 10−9 10−10 10−10 10−10 10−8 10−9

Table 2. Energy Barriers Obtained from the Arrhenius Law Fitting and eq 2 of the out-of-phase (χ’’) ac susceptibility data under applied dc field Orbach processes relaxation processes

Ueff/κB (K)

1 2 3 3Dy0.05Y0.95 4 5 6 6Dy0.05Y0.95 7 7Dy0.05Y0.95

25.76(1.29) 20.52(0.99) 29.62(0.11) 101.76(7.96) 37.94(0.13) 157.11(7.08) 130.53(2.98) 120.49(1.81) 128.83(8.16) 160.11(0.46)

Raman and Orbach processes −1

τ0 (s) 6.56(2) 3.94(2) 8.59(1) 2.29(1) 3.82(2) 2.07(4) 2.90(2) 6.03(1) 3.20(4) 1.07(1)

× × × × × × × × × ×

10−6 10−5 10−6 10−6 10−6 10−7 10−7 10−7 10−7 10−7

−n

C (s ·K )

n

Ueff/κB (K)

3.09(0.42) 0.80(0.09) 0.14(0.02) 0.21(0.02) 0.03(0.01) 2.35(2) × 10−4 8.00(2) × 10−5 6.61(1) × 10−5 5.58(2) × 10−4 2.43(3) × 10−5

3.59(0.09) 3.84(0.08) 4.78(0.06) 2.68(0.05) 5.48(0.07) 5.04(0.04) 5.80(0.08) 5.91(0.08) 5.08(0.13) 5.99(0.05)

86.86(1) 97.77(1) 204.33(1) 204.33(1) 203.13(1) 312.26(1) 285.04(1) 285.04(1) 220.88(1) 220.88(1)

ab initio τ0 (s) 6.46(1) 1.65(1) 2.92(1) 7.03(1) 4.58(1) 1.16(1) 1.18(1) 8.22(1) 4.11(1) 1.02(1)

× × × × × × × × × ×

10−9 10−9 10−9 10−9 10−9 10−10 10−10 10−10 10−9 10−8

Ueff/κB (K) 99.50 106.80 263.95 222.90 412.30 400.30 261.10

Figure 8. Field dependence of magnetization of samples 3, 5, 6, and 7 at a sweep rate of 50 Oe s−1 for 5−7 and 200 Oe s−1 for 3.

correspond to the QTM, Raman, and Orbach processes, respectively.

To calculate the thermal energy (Ueff), the out-of-phase (χ’’) ac susceptibility data under zero dc field were fitted to Arrhenius-type linearity (τ = τ0 exp(Ueff/kBT) and the Ueff values of 3, 3Dy0.05Y0.95, 5, 6, 6Dy0.05Y0.95, 7, and 7Dy0.05Y0.95 were obtained (Figure 7 and Table 1). Allowing for the presence of QTM and Raman, the plots of ln(τ) versus 1/T of 3, 3Dy0.05Y0.95, 5, 6, 6Dy0.05Y0.95, 7, and 7Dy0.05Y0.95 were fitted with eq 1, in which the first, second, and third terms

1/τ = 1/τQTM + CTn + τ0−1 exp( −Ueff /KBT )

(1)

In general, n was equal to 9 for Kramers ions, when both the acoustic and optical phonons were considered depending on the structure of the energy levels, n values varied between 1 H

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Figure 9. Magnetization blocking barriers in 1 (left top), 3 (right top), 5 (left bottom), and 6 (right bottom). The thick black lines represent the Kramers doublets as a function of their magnetic moment along the magnetic axis. The green arrows (QTM); the red arrows represent the most probable path for magnetic relaxation; the blue arrows (Orbach). The numbers at each arrow stand for the mean absolute value of the corresponding matrix element of transition magnetic moment.

and 6.28 The results of the fitting of the relaxation of QTM, Raman and Orbach processes are listed in Table 1. The Y dilution studies above demonstrate that dipole− dipole coupling is not the only source of fast relaxation in these systems. Fast relaxation through quantum tunneling of magnetization can be induced by many factors such as lowsymmetry components of the crystal field, any effective transverse magnetic field, hyperfine interactions with nuclear spins as well as the intermolecular dipolar interactions.2i Using an external dc field to lift the degeneracy of the mJ states can serve to suppress such QTM processes.29 As shown in Figures S14−S52, the ac susceptibilities of 1−7 all show a single relaxation process when a 1300 or 1500 Oe dc field is applied. Cole−Cole plots of these data were fitted using a generalized Debye model with the parameter α < 0.3 (Figures S53−54 and Tables S17−S26), in order to obtain characteristic time constants τ for the relaxations.30 The Arrhenius plots of ln(τ) versus 1/T of 1−7 still exhibit obvious curvature (Figure 7b), indicating another relaxation pathway is likely operative as has been previously observed in many SIMs.31 The out-of-phase (χ′′) ac susceptibility data of 1−7 and Y-doped samples under applied dc field were fit by eq 2,32 and the results of the fitting are listed in Table 2. The obtained Ueff values are similar to those obtained under zero dc field.33 1/τ = CT n + τ0−1exp( −Ueff /KBT )

respectively. Diluting the samples in a diamagnetic matrix slightly improved the temperature at which hysteresis was observable was improved to 3 K for 3Dy0.05Y0.95, 3.5 K for 6Dy0.05Y0.95, and 4 K for 7Dy0.05Y0.95 (Figure S52). The corresponding zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities measurement are shown in Figure S55. Theoretical Calculations. Ab initio calculations34 were performed to gain additional insight into the magnetic properties and analyze the factors that governed the magnetization blocking barrier. The magnetization blocking barrier for complexes 1−8 was calculated using a previously established methodology (Figure 9). The calculated results indicate that the ground state Kramers doublets (KDs) of Dy(III) complexes were well separated from the excited states (Figure 9 and Figure S56). The effective gz values of 1−8 were 19.69, 19.70, 19.77, 19.42, 19.81, 19.70, 19.54, and 19.44, respectively, revealing their magnetically uniaxial anisotropic ground states. The ground state Kramers doublets of 1, 2, 4, and 8 had significant transverse g-tensor components (gx, gy), leading to a large QTM in the ground state. Complexes 3, 5, 6, and 7 were calculated to have almost negligible transverse gtensor components (gxy ≈ 2 × 10−3) (Table S27). For the first excited and second excited Kramers doublets, the transversal components still remain relatively small for 5, 6 and 7 (gxy ≈ 2 × 10−1), while 3 had much larger gxy values (gxy = 1.689, 1.343). The relatively larger transverse components may promote the more pronounced QTM process, which is consistent with its poor magnetic relaxation behavior. As shown in Figure 9 and Figure S56, the smaller transverse magnetic moments (∼5 × 10−4 μB) in the ground state found in 3, 5, and 6 led to reduced QTM and the observed zero-field SMM relaxation behavior. On the basis of the transverse magnetic moments (arrows in Figures 9 and S56), the most probable pathway for magnetic relaxation through the second

(2)

To further probe the dynamic behavior, hysteresis loop measurements were carried out for 1−8. Complex 3 displayed a small open at a sweep rate of 200 Oe s−1 at 1.8 K. The Co(III)−Dy(III) complexes (5, 6, and 7) exhibited wide butterfly shaped hysteresis at 1.8 K at a sweep rate of 50 Oe s−1 (Figure 8). Upon raising the temperature, the openings for complexes 5, 6, and 7 in the hysteresis become narrower and then closed at 3.5 K for complex 5, 3 K for 6, and 3 K for 7, I

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Figure 10. Orientation of the local main magnetic axes of the ground Kramers doublet on Dy(III) of 1, 5, 6, and 8. Color codes: Dy, teal; Zn, gold; Co, violet; Br, dark yellow; Cl, bright green; N, blue; O, red.

as Daxial) were calculated and listed in Table S29. For an oblate lanthanide like Dy(III), a low Dtransverse and high Daxial lead to slow magnetic relaxation, and the ratio of Daxial/Dtransverse will result in the distortion of the assumed pentagonal ring and then influence the molecular magnetic anisotropy.16 However, the phenoxyl oxygen atoms of 1−8 (O2, O3) have higher Mulliken charges located in the hard-plane than was observed in [Zn2(L1)2DyCl3]·H2O, resulting in a transverse field that is adverse to the oblate Dy(III) ion. Thus, 1−8 show much smaller energy barriers and magnetic blocking temperatures than [Zn2(L1)2DyCl3]·H2O does (Table S29). The diamagnetic Zn(II) and Co(III) ions influence the charge densities on the phenoxyl O atoms bridging the Zn(II)/Co(III) and Dy(III) ions. The large Daxial/Dtransverse of Dy(III) in 3, 5, 6, and 7 promotes their observed SMM behavior in agreement with the [Zn2(L1)2DyCl3]·H2O (Table S31). In addition, as another factor, O5, O7, and O8 donors with a larger Mulliken charge (Table S29) located on two opposite sides of the hardplane were also analyzed in the present Zn(II)/Co(III)− Dy(III) system. The smaller deviation angles between O5, O7, O8 donors and magnetic easy axis (Table S32) revealed that the weaker repulsion contributed to the strong axial ligand field to stabilize the sublevels of Dy(III) ion in complexes 5−7, which resulted in the pronounced SMM performance. So, we could detect significant slow magnetic relaxations in 5, 6, and 7 even at a zero field; however former [Co(III)Dy(III)L(μOAc) 2 (NO 3 ) 2 ] and 8 did not, in which the special coordination configurations were broken by NO3− ligands. In addition to the charge densities of the coordination atoms, the coplanarity of transverse hard-plane composed of five coordination atoms in Zn(II)−Dy(III) and Co(III)− Dy(III) systems also influenced the SMM behavior. For each Dy(III) ion, the charge collocation of the sublevels which possessed maximum Jz quantum number contributed to diffuse the maximum electric density, in which the angle θmax (the angle from the equatorial plane) was used to characterize it.37

KDs for 1−4 and 8 or third KDs for 5 and 7, even fourth KDs for 6 were predicted. However, the experimentally determined Ueff values of 1−7 are all smaller than the ones obtained by theoretical calculations. Likely, this is a consequence of mixing of multiple mJ states in both the ground and first excited states, which can induce quantum tunneling. Notably, even for 6, the magnetic moment of an Orbach relaxation from mJ = −13/2 to mJ = +11/2 was only narrowly smaller than the tunneling between mJ = ± 11/2 states (0.068 μB vs 0.084 μB), and slightly larger than the tunnelling between mJ = ± 13/2 states (0.04 μB), the result suggested that a competition between these three pathways may contribute to the lowering of the experimental energy barrier.35 Additionally, for 1, 2, 4, and 8, a relatively larger transverse magnetic moment of 10−1 μB and 10−2 μB and gxy value in the ground state was observed (vide supra), promoting significant QTM, which contributed to lowering the energy barriers. The deviations observed in particular values may also be due to the exclusion of electron dynamic correlation in the calculations.36 The direction of the calculated anisotropy axis is shown in Figure 10 and Figure S57, which nearly paralleled the Dy−O5 direction of 1−8. The angles between easy axis and the N2O2 plane were in range of 69.23−85.18° for 1−8, respectively, which is much larger than that (22.89°) of the precious Zn(II)−Dy(III)−Zn(II) SMM [Zn 2 (L 1 ) 2 DyCl 3 ]·H 2 O 16 (Table S3). Figure 10 and Figure S57 showed that there is a pentagonal ring consisting of five coordination atoms for 1−8, which is located perpendicularly to the magnetic anisotropy easy axis direction (bright green arrows). This plane was normally denoted as a hard-plane,2h,16 and the angle between anisotropy axis and hard-plane was in the range of 59.88− 85.64° and 71.53−81.97° for the 1−4 system and 5−7 system, respectively. These values are close to the angle of 83.07° in [Zn2(L1)2DyCl3]·H2O16 (Table S3). The average charge densities of these five coordination atoms (denoted as Dtransverse) and the coordination atoms along easy axis (denoted J

DOI: 10.1021/acs.inorgchem.8b00056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Different lanthanide ions had different θmax, and the relatively large θmax suggested a relatively substantial repulsion between 4f electrons and donor atoms, which might result in a reduction of the magnetic anisotropy.38 In view of this, we calculated the degree of distortion of five coordination atoms from the hard-plane to probe the impact of transverse coordination atom position associated with each sublevel (MJ sublevels) surrounding Dy(III) ions on their magnetic anisotropy. The mean plane of five transverse O atoms of Dy(III) was calculated, and the distances that the atoms deviated from the plane were measured (Table S30). Compounds 3, 5, 6, and 7 have much smaller values of deviation, favoring good SMM behavior. Meanwhile, the relative smaller dihedral angles (β) also play a key role in minimizing the repulsion between Dy(III) and donor atoms. Both of these parameters might explain why significant slow magnetic relaxation is observed in 3, 5, 6, and 7 under zero dc field, while 1, 2, and 4 show no SMM behavior.

ORCID

CONCLUSIONS The series of Zn(II)/Co(III)−Dy(III) SMMs presented here examine the effects that transition metal charge and coordination geometry can have on the SMM behavior of a lanthanide ion. Dynamic magnetic measurements reveal that the SMM performance can be significantly enhanced by utilizing Zn(II) and Co(III) ions to perturb the charge distribution on the phenoxyl bridging O atoms. Utilizing an octahedral Co(III) ion rather than a square pyramidal Zn(II) ion results in smaller dihedral angles between the N2O2 plane around the transition metal and the plane of transverse coordination atoms around the Dy(III) ion and improves SMM behavior (higher Ueff and TB). These factors combine to give a coordination geometry containing lower electron density in the transverse plane and higher electron density in along the magnetic easy axis. This configuration forms a strong axial ligand field and stabilizes the high MJ sublevels, favoring SMM behavior.

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Peng Chen: 0000-0002-2689-0297 Hong-Feng Li: 0000-0003-4646-0515 Yi-Quan Zhang: 0000-0003-1818-0612 Peng-Fei Yan: 0000-0002-5124-1707 Wen-Bin Sun: 0000-0003-3358-050X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Nos. 21572048, 21102039, and 51472076), the Educational Commission of Heilongjiang Province (1254G045, 12541639), and Heilongjiang University (JCL201605). Natural Science Foundation of Jiangsu Province of China (BK20151542) and Natural Science Foundation of the Jiangsu Higher Education Institutions of China (16KJB430020) are also acknowledged. W.B.S. thanks Dr. B. Dolinar of Texas A&M University for helpful discussions and writing.

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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.8b00056. Magnetic measurement plots for 1−8, continuousshape-measures (CShMs) of the coordination geometry for Dy(III) ion in Dy-based complexes, ab initio computational details, and orientation of the local main magnetic easy axis on the Dy(III) ion (PDF) Accession Codes

CCDC 1544545−1544552 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.

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*(S.W.-B.) E-mail: [email protected]. *(Z.Y.-Q.) E-mail: [email protected]. K

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DOI: 10.1021/acs.inorgchem.8b00056 Inorg. Chem. XXXX, XXX, XXX−XXX