Effect of Ligand Field Tuning on the SMM Behavior for Three Related

Dec 11, 2015 - Magnetic studies show no SMM behavior for one compound, whereas ... ligands with a central tertiary amine bearing picolyl and alkoxy ar...
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Effect of Ligand Field Tuning on the SMM Behavior for Three Related Alkoxide-Bridged Dysprosium Dimers Yan Peng,†,‡ Valeriu Mereacre,† Amer Baniodeh,†,‡ Yanhua Lan,‡ Martin Schlageter,‡ George E. Kostakis,‡ and Annie K. Powell*,†,‡ †

Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstrasse 15, 76131 Karlsruhe, Germany Institute of Nanotechnology, Karlsruhe Institute of Technology, Postfach 3640, 76021 Karlsruhe, Germany

‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of three Dy2 compounds, [Dy2(HL1)2(NO3)4] (1), [Dy2(L2)2(NO3)4] (2), and [Dy2(HL3)2(NO3)4] (3), formed using related tripodal ligands with a central tertiary amine bearing picolyl and alkoxy arms, 2-[(2-hydroxy-ethyl)-pyridin-2-ylmethylamino]-ethanol (H2L1), 2-(bis-pyridin-2-ylmethylamino)-ethanol (HL2), and 2-(bis-pyridin-2-ylmethylamino)-propane-1,3-diol (H2L3), are reported. The compounds are rare examples of alkoxidebridged {Dy2} complexes and display capped square antiprism coordination geometry around each DyIII ion. Changes in the ligand field environment around the DyIII ions brought about through variations in the ligand donors can be gauged from the magnetic properties, with compounds 1 and 2 showing antiparallel coupling between the DyIII ions and 3 showing parallel coupling. Furthermore, slow relaxation of the magnetization typical of SMM behavior could be observed for compounds 2 and 3, suggesting that small variations in the ligand field can have a significant influence on the slow relaxation processes responsible for SMM behavior of DyIII-based systems.

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structures of the individual LnIII ions are strongly influenced by these specific details. Therefore, it should be helpful to investigate a robust molecular motif where the coordination geometries of the Ln centers are preserved but where the electronic nature of the encapsulating ligands can be varied through targeted substitutions on the arms of a tripodal and bridging ligand. For these reasons, we chose to explore the structural and magnetic properties of a series of [Dy2] dimer CCs with three related tripodal ligands, all having a central tertiary amine and bearing varying numbers of picolyl and alkoxy arms, namely, 2-[(2hydroxy-ethyl)-pyridin-2-ylmethylamino]-ethanol (H2L1), 2(bis-pyridin-2-ylmethylamino)-ethanol (HL2), and 2-(bis-pyridin-2-ylmethylamino)-propane-1,3-diol (H2L3). In all cases, the ligands provide an alkoxy function that acts to bridge between the two DyIII ions in the form of a {Dy2(OR)2} motif, which is surprisingly uncommon in Dy coordination chemistry.6 Thus, this study provides a means to identify how SMM properties can be tuned by the ligand field for the symmetric dimer compounds [Dy2(HL1)2(NO3)4] (1), [Dy2(L2)2(NO3)4] (2), and [Dy2(HL3)2(NO3)4] (3).

INTRODUCTION

Incorporation of DyIII into coordination clusters (CCs) has proven to be a useful means of introducing magnetic anisotropy into single-molecule magnets (SMMs), and to date, a large number of polynuclear lanthanide complexes has been synthesized and extensively reviewed.1 Some remarkable results in 4f-only systems have been reported, such as the experimental realization of the toroidal arrangement of local magnetization vectors in a Dy3 triangle leading to spin chirality2 and a record energy barrier of 692 K in a {Dy4K2} system.3 Among the many 4f metal ion-based SMMs, dinuclear [Dy2] SMMs show significant variations in their magnetic properties. These [Dy2] SMM clusters can be classified into two categories according to the coordination environments around the two metal centers as either symmetric Dy2 dimeric systems, where the coordination environments of the two Dy centers are identical,4 or asymmetric Dy2 dinuclear systems, where the coordination spheres of the two Dy centers are different.5 Furthermore, it has been established that many factors, including the local ligand field, the coordination geometry, and the strength of the magnetic interaction between lanthanide sites, can influence the SMM behavior of lanthanide-based systems.4b However, it still remains a significant challenge to establish useful guidelines for constructing polynuclear LnIII clusters exhibiting enhanced or unusual SMM properties, not least because the electronic © XXXX American Chemical Society

Received: August 10, 2015

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DOI: 10.1021/acs.inorgchem.5b01793 Inorg. Chem. XXXX, XXX, XXX−XXX

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Magnetic Measurements. Magnetic susceptibility data (1.8−300 K) were collected on powdered samples on a SQUID-based magnetometer (Quantum Design model MPMS-XL instrument) under a 1000 Oe applied magnetic field. Magnetization isotherms were collected at 2, 3, and 5 K between 0 and 70 000 Oe. ac susceptibility measurements were carried out under an oscillating ac field of 3 Oe and ac frequencies ranging from 1 to 1500 Hz. Data were corrected for diamagnetism using Pascal constants and a sample holder correction.

EXPERIMENTAL SECTION

General Information. All chemicals were used as received without any further purification, and apart from the synthesis of the organic ligands, all manipulations were performed under aerobic conditions. Elemental analyses (C, H, and N) were performed using an Elemental Vario EL analyzer. IR spectra were measured on a Perkin Elmer Spectrum One spectrometer with samples prepared as KBr disks. Powder X-ray diffraction was carried out on a STOE STADI-P diffractometer, using Cu-Kα radiation. Synthesis of 2-(Bis-pyridin-2-yl-methylamino)-propane-1,3diol (H2L3). A mixture of 2-amino-1,3-propanediol (2.73 g, 30 mmol) and 2-picolyl chloride hydrochloride (9.84 g, 60 mmol) in MeCN (100 mL) was refluxed under an Ar atmosphere for 48 h in the presence of K2CO3 (16.66 g, 120 mmol) and KI (1.66 g, 10 mmol). The resulting orange solution was filtered, and the solvent was removed under reduced pressure. The pale yellow solid obtained was purified by chromatography using MeOH/ethyl acetate (v/v = 1:3) as the eluant. H3L3 was obtained as a pale yellow oil (6.20 g, 76.5%). 1H NMR (300 MHz, CDCl3): δ (ppm) = 3.32−3.46 (m, 1 H, N−CH), 3.56 (d, 4 H, 3J = 7 Hz, CH2OH), 4.02 (s, 4 H, NCH2), 6.94−7.00 (m, 2 H, HAr), 7.19 (m, 2 H, HAr), 7.59 (td, 2 H, 3J = 7.7 Hz, 1.8 Hz, HAr), 8.54−8.62 (m, 2 H, HAr). Anal. Calcd for C16H21N3O3: C, 65.91; H, 7.01; N, 15.37. Found: C, 65.67; H, 7.22; N, 15.55. Synthesis of [Dy2(HL1)2(NO3)4] (1). A solution of Dy(NO3)3· 6H2O (223 g, 0.50 mmol) in MeOH (5 mL) was added slowly to a stirred solution of H2L1 (66 mg, 0.33 mmol) and Et3N (5 drops) in MeCN (40 mL). After 5 min of stirring, the mixture was filtered and the filtrate was left undisturbed overnight, after which colorless crystals suitable for X-ray diffraction analysis were obtained in a yield of 144 mg, 87% (based on the ligand). Anal. Calcd for C20H30Dy2N8O16 (1): C, 24.93; H, 3.14; N, 11.63. Found: C, 24.85; H, 3.27; N, 11.68. Selected IR data (cm−1): 3217.5 (s), 2996.5 (w), 2958.5 (m) 2905.5 (m), 2858.5 (m), 1636.5 (m), 1607.5 (s), 1575 (m), 1516 (vs), 1484.5 (vs), 1466.5 (vs), 1444 (s), 1385 (s), 1304.5 (s), 1158.5 (w), 1149 (m), 1072.5 (s), 1033 (s), 1025 (m), 1009.5 (s), 920.5 (m), 906 (w), 888 (m), 874 (m), 836.5 (w), 812.5 (m), 789.5 (m), 769.5 (m), 739.5 (m), 732 (w), 636.5 (vw), 587 (m) 555.5 (m), 499 (s), 467.5 (w). Synthesis of [Dy2(L2)2(NO3)4] (2) and [Dy2(HL3)2(NO3)4] (3). Complexes 2 and 3 were prepared in a similar way but using the corresponding ligands and starting from a suspension of Dy(NO3)3· 6H2O (0.5 mmol, 223 mg) and H1L2 (0.33 mmol, 81 mg) or H2L3 (0.33, 91 mg) in MeOH and MeCN (40 mL, v/v = 1:8), which was then treated with Et3N (two drops). The resulting colorless solution was stirred for 5 min and subsequently filtered. The filtrate was left undisturbed, and colorless single crystals of [Dy2(L2)2(NO3)4] (2) or [Dy2(HL3)2(NO3)4] (3) formed after 2 days in yields of 85% (151 mg, based on ligand) for 2 and 90% (166 mg, based on ligand) for 3. Anal. Calcd for C28H32Dy2N10O14 (2): C, 31.80; H, 3.05; N, 13.24. Found: C, 32.04; H, 3.07; N, 13.13. Selected IR data (cm−1) (2): 3414.5 (s), 3102 (w), 2977.5 (w), 2957.5 (w), 2921.5 (w), 2898.5 (m), 2872 (m), 1642 (m), 1607 (s), 1574 (m), 1498.5 (vs), 1468 (vs), 1444 (s), 1385 (s), 1294 (vs), 1159 (m), 1105 (m), 1067.5 (s), 1036.5 (s), 1016 (s), 1005.5 (s), 878.5 (m), 818.5 (m), 801.5 (m), 762.5 (m), 741 (m), 661.5 (w), 638 (m), 628.5 (w), 573.5 (m), 511.5 (m), 497.5 (s), 473.5 (m), 458.5 (m), 418 (m). Anal. Calcd for C30H36Dy2N10O16 (3): 32.24; H, 3.25; N, 12.53. Found: C, 32.34; H, 3.41; N, 12.46. Selected IR data (cm−1) (3): 2947.5 (w), 2911 (w), 2852 (w), 2427.5 (w), 1605.5 (m), 1574.5 (w), 1499.5 (m), 1482 (m), 1448 (m), 1385 (vs), 1298.5 (m), 1239 (w), 1156 (w), 1075 (w), 1046.5 (w), 1016 (m), 995.5 (w), 798 (m), 758 (m), 638 (m), 576.5 (m), 554 (w), 499 (m), 438 (m). X-ray Crystallographic Analysis and Data Collection. Data were collected on an IPDS-II diffractometer with Mo-Kα radiation (λ = 0.71073 Å). All structures were solved by direct methods and refined using full-matrix least-squares on F2 using the SHELX or Olex program. H atoms could be located from the difference Fourier synthesis but were placed in idealized positions and not refined. Crystallographic data and structure refinement results are listed in Table S1, Supporting Information.

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RESULTS AND DISCUSSION In order to undertake this study, ligands 2-[(2-hydroxy-ethyl)pyridin-2-ylmethylamino]-ethanol (H2L1) and 2-(bis-pyridin-2ylmethylamino)-ethanol (HL2) were synthesized according to previously reported synthetic methods,7 whereas the synthesis of 2-(bis-pyridin-2-ylmethylamino)-propane-1, 3-diol (H2L3) is reported for the first time (Scheme 1), with yields of over 75%. Scheme 1. Organic Ligands Used in Isolating the Dimeric Dy2 Units in 1 (Left), 2 (Middle), and 3 (Right)

As can be seen from Scheme 1, ligands H2L1, H1L2, and H2L3 potentially provide N2O2, N3O, and N3O2 coordination pockets, but, more importantly, they provide systematic variation in terms of potential donor groups and all have an alcohol function that provides an alkoxide bridge between the two Dy centers. All compounds were synthesized by reacting the ligands and Dy(NO3)3·6H2O in a 2:3 molar ratio in a solvent mixture of MeOH/MeCN (v/v = 1:8). Changes to the ligand/Dy(NO3)3· 6H2O molar ratio and the use of a single solvent resulted in noncrystalline materials. Although the H2L3 ligand has the potential to provide N3 O2 coordination, the resulting compound utilizes only an N 3 O coordination set, as demonstrated by the structure of compound 3 (Figure 1), and is thus directly comparable to the other two compounds in terms of local coordination geometry provided by the organic ligand. Crystal Structure Analysis. Single-crystal X-ray diffraction analysis revealed that complexes 1 and 2 crystallize in the monoclinic space group P21/c, whereas complex 3 crystallizes in the orthorhombic space group Pbca. The crystallographic data of the three complexes 1−3 are given in Table S1 (Supporting Information) and selected bond lengths and angles are given in Table 1. All compounds comprise neutral, centrosymmetric dimers with an asymmetric unit containing one DyIII ion, one ligand, and two coordinated bidentate nitrates. The centrosymmetric dinuclear complexes are composed of two nine-coordinate DyIII ions bridged by alkoxide groups of the ligands. For the nine-coordinate Dy, the geometry around the metal is a distorted monocapped square antiprism (Figure 1) formed by four coordination atoms from two nitrate anions and five coordination atoms from two ligands with only small differences in the bond lengths and angles. Therefore, only the structure of complex 1 is described herein in detail (as shown in Figure 1). In the B

DOI: 10.1021/acs.inorgchem.5b01793 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Molecules [Dy2(HL1)2(NO3)4] 1 (top left), [Dy2(L2)2(NO3)4] 2 (upper middle), and [Dy2(HL3)2(NO3)4] 3 (top right) and coordination environments for the Dy centers in 1 (lower left), 2 (lower middle), and 3 (lower right).

Table 1. Selected Bonds Length and Angles for [Dy2(HL1)2(NO3)4] 1, [Dy2(L2)2(NO3)4] 2, and [Dy2(HL3)2(NO3)4] 3a compounds 1 Dy1−O1i Dy1−O1 Dy1−O2 Dy1−O4 Dy1−O5 Dy1−O7 Dy1−N1 Dy1−N2 Dy1−O8 Dy···Dy1iintra Dy···Dy1iinter Dy1i−O1−Dy1 a

2 2.255 (5) 2.277 (4) 2.474 (5) 2.554 (5) 2.478 (5) 2.580 (6) 2.528 (6) 2.631 (6) 2.403 (5) 3.709 (7) 7.0399 (8) 109.83° (18)

3

Dy1−O1i Dy1−O1 Dy1−O2 Dy1−O4 Dy1−O5 Dy1−O7 Dy1−N1 Dy1−N2 Dy1−N3 Dy···Dy1iintra Dy···Dy1iinter Dy1i−O1−Dy1

2.259 (7) 2.266 (7) 2.481 (9) 2.459 (9) 2.572 (9) 2.468 (8) 2.551 (10) 2.559 (9) 2.608 (10) 3.706 (3) 8.424 (8) 109.71° (15)

Dy1−O1i Dy1−O1 Dy1−O2 Dy1−O4 Dy1−O5 Dy1−O7 Dy1−N1 Dy1−N2 Dy1−N3 Dy···Dy1iintra Dy···Dy1iinter Dy1i−O1−Dy1

2.257 (17) 2.265 (17) 2.471 (18) 2.498 (19) 2.540 (2) 2.472 (2) 2.553 (2) 2.594 (2) 2.597 (2) 3.719 (3) 8.9980 (4) 110.65° (7)

The atoms related by inversion are labeled with superscript i.

[Dy2(HL1)2(NO3)4] molecule, each nitrate anion provides two donor oxygen atoms coordinating to the DyIII ion, the other four coordination sites of DyIII are occupied by two N atoms and two O atoms from H2L1 for 1 and three N atoms and one O atom for 2 and 3, and the last position is occupied by a bridging alkoxide O from the other ligand to complete the ninecoordination environments: DyO7N2 for 1 and DyO6N3 for 2 and 3. In these complexes, the Dy−O (bridged) bond lengths are 2.255 (5) and 2.277 (4) Å for 1, 2.259 (7) and 2.266 (7) Å for 2, 2.257 (17) and 2.265 (17) Å for 3. The Dy···Dyintra distance is 3.709(7), 3.706(3), and 3.719 (3) Å and the closest Dy···Dyinter distance is 7.0399 (8), 8.424 (8), and 8.9980 (4) Å for 1−3, respectively. The Dy−O−Dy angles of 1−3 are slightly different: 109.83° (18), 109.71° (15), and 110.65° (7), respectively (Table 1). Magnetic Properties. Direct current (dc) magnetic susceptibility studies of 1−3 were carried out with an applied magnetic field of 1000 Oe over the temperature range 300−1.8 K. The plot of χMT vs T is shown in Figure 2. At 300 K, the χMT values are 26.09 cm3·K·mol−1 for 1 and 27.18 cm3·K·mol−1 for 2, which are a slightly lower than the expected value of 28.34 cm3·K·mol−1 for two uncoupled DyIII ions (6H15/2, g =

Figure 2. χT versus T plots for [Dy 2 (HL 1 ) 2 (NO 3 ) 4 ] 1, [Dy2(L2)2(NO3)4] 2, and [Dy2(HL3)2(NO3)4] 3.

/3). As the temperature is decreased, the χMT values decrease gradually and then more dramatically below 100 K to minima of 14.16 and 17.57 cm3·K·mol−1 at 1.8 K for 1 and 2, respectively. The decrease in χMT can be ascribed to thermal 4

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DOI: 10.1021/acs.inorgchem.5b01793 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Plots of χ′ (left) and χ″ (right) vs T under zero dc field (upper) and 2000 Oe dc field (lower) for [Dy2(HL3)2(NO3)4] 3 at different frequencies.

Figure 4. Plots of τ vs T−1 for [Dy2(L2)2(NO3)4] 2 under 1000 Oe dc field (upper) and [Dy2(HL3)2(NO3)4] 3 under zero field (lower left) and 2000 Oe dc field (lower right).

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DOI: 10.1021/acs.inorgchem.5b01793 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry depopulation of the Stark levels of the DyIII ions and/or antiferromagnetic interactions between the spin carriers, as observed in other dysprosium compounds.8 Magnetization plots (M vs H) for 1 and 2 (Figure S2, Supporting Information) at high fields (up to 7 T) show nonsaturation as well as nonsuperposition onto a single curve, indicating the presence of magnetic anisotropy and/or low-lying excited states. In the case of compound 3, the χMT value is equal to 28.80 cm3·K·mol−1 at 300 K, which is in agreement with the expected value of 28.34 cm3·K·mol−1 for two uncoupled DyIII ions. With decreasing temperature, the χMT product displays only a slight decrease to 27.24 cm3·K·mol−1 at 18 K, which may result from the depopulation of the Stark sublevels and/or significant magnetic anisotropy present in DyIII systems. Upon further decreasing the temperature, the value increases to 29.52 cm3·K·mol−1 at 1.8 K, indicative of a weak intramolecular ferromagnetic interaction in 3. The lack of a superposition of the M vs H data onto a single master curve and the low magnetization of 12.21 Nβ at 7.0 T suggest the presence of a significant magnetic anisotropy and/or low-lying excited states (Figure S2, Supporting Information). In order to explore potential SMM behavior, alternating current (ac) magnetic susceptibility studies were carried out on freshly filtered samples of 1−3. For complex 1, no out-of-phase signal (χ″) was observed, even after a static dc field (Figures S3 and S4, Supporting Information) was applied, indicating the absence of SMM behavior within the measurement parameters of the SQUID magnetometer. Failure to observe slow relaxation could result from very fast quantum tunneling of the magnetization (QTM), which is commonly seen in pure lanthanide complexes.9 For complex 2, a signal was observed in the out-of phase (χ″) vs T plot below 12 K (Figure S5, Supporting Information). This suggests slow relaxation of the magnetization, which is generally attributed to SMM behavior. However, the energy barrier and characteristic relaxation time could not be obtained because the maxima of χ″ were not detected in the available window of our magnetometer. The presence of QTM can reduce the expected energy barrier, but it is often possible to shortcut the QTM by applying a static dc field over the range 0−3000 Oe. Therefore, ac susceptibility measurements were performed at various dc fields (Figure S6) to find an optimum field of 1000 Oe to slow the relaxation time by reducing or suppressing quantum tunneling of the magnetization (Figure S7). By fitting the data to an Arrhenius law, the characteristic SMM energy gap was estimated to be 72.48 K and the preexponential factor τ0 = 8.50 × 10−8 s, (Figure 4) with R = 0.98843. This observation confirms the field-induced SMM nature of 2. For complex 3, in zero dc field, the appearance of maxima in the out-of-phase, χ″, ac susceptibility signals clearly demonstrates slow relaxation of the magnetization at temperatures below 17 K (Figure 3). Plots of in-phase (χ′) and out-of-phase (χ″) ac susceptibilities in zero dc field as a function of frequency in the temperature range 2−12.5 K (Figure S8, Supporting Information) and the resulting Cole−Cole plot (Figure S9, Supporting Information) reveals frequency-dependent features typical of SMM behavior. From these data, the temperature dependence of the relaxation time, τ0, can be deduced (Figure 4). Below 6 K, the dynamics of 3 become temperature-independent in a pure quantum regime, with a τ value of 3.30 × 10−4 s. Above 6 K, the relaxation time becomes progressively thermally activated and the energy barrier of the

thermally activated regime, Ueff, is 41.55 K, with a preexponential factor τ0 = 8.50 × 10−7 s and R = 0.99899. In order to probe the feasibility of lowering the relaxation probability via the quantum pathway, the ac susceptibility was measured at 1.8 K at various applied dc fields (0−3000 Oe; Figure S10, Supporting Information). From these data, the characteristic relaxation frequency as a function of applied dc field for the relaxation modes (i.e., thermal and quantum relaxation modes) can be extracted. The probability of magnetization relaxation via quantum tunneling is minimized at 2000 Oe, and in order to probe the thermally activated regime of relaxation, the in-phase (χ′) and out-of-phase (χ″) ac susceptibilities as a function of frequency at various temperatures were measured by applying a dc field of 2000 Oe. This led to the observation of maxima (Figure 3) that could be fitted to an Arrhenius law, giving an energy gap, Ueff, of 72.37 K and pre-exponential factor τ0 = 1.24 × 10−7 s (Figure 4) with R = 0.99882. This large increase in the effective energy barrier shows that the quantum tunneling of the magnetization is very pronounced. Structure−Property Relationship. Each DyIII ion of complexes 1−3, with a nine-coordinate coordination environment, is linked to the other DyIII ion via a μ2-alkoxide bridge from two monodeprotonated ligands. As seen in Figure 2, complexes 1−3 exhibit distinct magnetic behavior in terms of their dc magnetic susceptibilities. Complexes 1 and 2 show antiferromagnetic interactions between DyIII ions, and these have N2O7 and N3O6 coordination environments, respectively. Ferromagnetic coupling is observed in complex 3, and here the metal ions also have an N3O6 coordination environment, as seen for compound 2. It is noted that the Dy−O−Dy angles of these three compounds show slight differences, being 109.83(18)°, 109.71(15)°, and 110.65(7)°, respectively. Results on previously reported Dy2 systems1 show that the geometry of the Dy2(μ2-O)2 and thus the Dy−O−Dy angles determines the nature of the magnetic interaction, even though such interactions are expected to be very weak. In addition, comparison of the bond distances in complexes 1−3 reveals that the ferromagnetically coupled compound 3 displays the shortest Dy−O bonds in the Dy2O2 cores of three compounds, which would lead to the shortest Dy···Dy distance if the Dy− O−Dy angles were identical for all three compounds. However, the more obtuse angle for compound 3 leads to this system having the greatest Dy···Dy distance. The average Dy−O (NO3−) bond lengths of the three compounds are all similar (∼2.50 Å). Although the average bond lengths of these three compounds are very close, the coordination positions are different. In order to analyze the coordination geometry of the Dy ions, the continuous symmetry measurement (CSM) method10 was performed. The results are given in Table 2 and reveal that it is not really possible to distinguish whether it is better to describe the geometry as C4v, D3h, or Cs. Furthermore, we calculated the anisotropy axes of these three compounds using the Magellan program (Figure 5); the results Table 2. Lanthanide Geometry Analysis Using Shape Software

E

coordination geometry

CSAPR (C4v)

TCTPR (D3h)

MFF (Cs)

1 2 3

2.94808 2.25212 2.26792

3.16798 2.04771 2.01812

3.01272 2.40413 2.40337

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indicate that these three compounds have very similar anisotropy axial directions.11

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01793. XRD, M vs H, and χ′ and χ″ vs T plots for 1−3; Cole− Cole plot for 3; and crystallographic data for 1−3 in tablular and CIF formats (PDF, CIF, CIF, CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 5. Ground-state magnetic anisotropy of compounds 1 (left), 2 (middle), and 3 (right). The blue rods represent the orientations of the anisotropy axes for each DyIII ion, as calculated by the electrostatic model.

REFERENCES

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A further influence on the magnetic properties could arise from subtle variations in the coordination spheres of the DyIII ions. For compound 1, each DyIII has an N2O7 coordination environment, whereas for 2, each Dy III has an N 3 O 6 coordination environment through the formal replacement of one alkoxide O by a pyridine N. For 3, there is the same first coordination sphere environment for the Dy centers as that seen for compound 2, but there is the extra, unprotonated alcohol arm of the ligand that does not participate in the coordination of the Dy centers. Put differently, all three compounds have essentially the same nine-coordinate first coordination sphere (Figure 1), but the donor atoms are subject to small, but possibly significant, at least for lanthanide ions, electronic variations that are manifest in these examples by the increasing suppression of QTM effects across the series of compounds 1−3.

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CONCLUSIONS Three new Dy2 compounds (1−3) have been synthesized using ligands H2L1, H1L2, and H2L3 and are examples of alkoxide Obridged Dy2 complexes that all have the same nine-coordinate monocapped square antiprism coordination geometry around each DyIII ion. From magnetic studies, we can conclude that compound 1 does not show SMM behavior within the parameter window of conventional SQUID measurements, and we suggest that this arises from a very fast quantum tunneling process. On the other hand, SMM behavior can be seen for compounds 2 and 3, but there is a more dominant QTM regime for compound 2. The apparent suppression of QTM for compound 3 could be the result of the fine details of the electronic structure of the coordination sphere around the Dy centers arising from small changes in the details of the tripodal ligand. For both 2 and 3, applying dc fields in order to suppress the QTM behavior leads to very similar Ueff values (around 72 K) and similar pre-exponential factors from the Arrhenius treatment of the out-of-phase ac data. Future studies will be aimed at untangling the contributions of the coordination environment, the ligand field (including that beyond the first coordination sphere), and Dy−Dy coupling interactions in such symmetric [Dy2] dimer systems. F

DOI: 10.1021/acs.inorgchem.5b01793 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (11) Chilton, N. F.; Collison, D.; McInnes, E. J. L.; Winpenny, R. E. P.; Soncini, A. Nat. Commun. 2013, 4, 2551.

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DOI: 10.1021/acs.inorgchem.5b01793 Inorg. Chem. XXXX, XXX, XXX−XXX