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Article

Modulating the Slow Relaxation Dynamics of Binuclear Dysprosium(III) Complexes through Coordination Geometry

by
Amit Kumar Mondal
,
Vijay Singh Parmar
and
Sanjit Konar
*
Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal 462066, MP, India
*
Author to whom correspondence should be addressed.
Magnetochemistry 2016, 2(3), 35; https://doi.org/10.3390/magnetochemistry2030035
Submission received: 30 August 2016 / Revised: 12 September 2016 / Accepted: 13 September 2016 / Published: 21 September 2016
(This article belongs to the Special Issue Molecules in Quantum Information)
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Abstract

:
A class of two dinuclear dysprosium based complexes 1 and 2 were synthesized by employing salicyloylhydrazone derived pentadentate ligand (L). Structural analysis reveals that in complex 1, two DyIII centers are in muffin (Cs) coordination geometry while in 2, one DyIII center is in bicapped square antiprism (D4d) and other one is in triangular dodecahedron (D2d) coordination geometry. AC magnetic susceptibility measurements disclose that complexes 1 and 2 exhibit single-molecule magnet (SMM) behavior, with effective energy barrier of 36.4 and 9.7 K, respectively. The overall studies reveal that small differences in the coordination environment around the DyIII centers played a significant role in the difference in relaxation dynamics of the complexes. In order to elucidate the role of intermolecular interactions between nearby DyIII centers in the magnetic relaxation behavior, a diamagnetic isostructural YIII analog (3) was synthesized and magnetic behavior was examined.

Graphical Abstract

1. Introduction

In recent years, single molecule magnets (SMMs), a unique class of nano-dimensional magnetic materials, have attracted significant research interest. SMMs have potential applications in a variety of fields, including high-density data storage devices, quantum computing, and molecular spintronics [1]. The importance of SMMs is that, even in the absence of an external magnetic field, they can preserve the magnetization for long period of time at low temperatures. After an extensive research on 3d metal complexes in the last decade [2,3,4], a rapid development of Ln based SMMs has been observed in recent years [5,6,7,8,9]. Compared to 3d metals, Ln based SMMs are mostly investigated owing to the fact that the LnIII ions such as DyIII, TbIII, ErIII, and HoIII have huge and unquenched orbital angular momentum [10,11,12] which causes substantial magnetic anisotropy. The major approach for the construction of such systems is to choose a ligand field (LF) which could offer an axial crystal field acting on the LnIII ion and stabilize the MJ states with a large absolute value of the total angular momentum projection |MJ|, therefore realizing a magnetization easy axis. Outstanding Ln based SMMs include TbIII/DyIII-phthalocyanin (Pc) double-decker complexes [13] and sandwiched ErIII complexes with polyoxometallate based ligands [14]. Dinuclear Ln SMMs represent the simplest molecular units which permit the study of magnetic interactions between two spin carriers. If a dinuclear SMM can be designed in a controllable manner, it can be possible to construct larger molecules via a bottom up molecular approach and make SMMs with higher blocking temperatures. Herein two dinuclear complexes [Dy2(L)2(MeOH)2(NO3)2]·(MeOH)·(CH2Cl2) (1) and [Dy2(L)3(H2O)·(MeOH)]·(MeOH)4·(H2O) (2) are reported, which are obtained from the reaction of a pentadentate organic ligand L with Dy(NO3)3 and DyCl3, respectively (L = 2,6-bis(1-salicyloylhydrazonoethyl) pyridine; Figure S1).
As the overall electronic structure of a DyIII ion is very sensitive to its coordination environment, minor changes in the ligand systems can significantly affect the magnetic properties of the complexes [15,16,17]. Most of the reports on Ln-based SMMs concentrated mainly on either changing the Ln ion keeping the ligand system constant [18,19,20] or changing the ligand but keeping the coordination environment around the Ln ion unchanged [8]. Only few studies are known where modification in the ligand system was done to tune the relaxation behavior in the complexes [21,22,23,24,25,26]. In this paper, differences in slow relaxation of the magnetization behavior were explored in two DyIII dinuclear complexes, where minor changes in the coordination environment around the DyIII ions disturbed the local symmetry.

2. Results and Discussion

Single-crystal X-ray analysis showed that both the complexes crystallize in the triclinic P-1 space groups (Table 1). The molecular structures of the complexes are shown in Figure 1. For complex 1, both DyIII centers contain similar DyN3O6 cores, surrounded by the ligand, one coordinated nitrate anion, one methanol molecule and one phenoxide oxygen atom of another ligand, whereas for complex 2, both DyIII centers contain different DyN6O4 and DyN3O5 cores. Two interlocked pentachelating ligands make up the coordination sphere of one DyIII center, while other one is surrounded by the ligand, one coordinated methanol molecule, one water molecule and one phenoxide oxygen atom of another ligand. In both complexes, ligand coordinates via the pyridyl nitrogen, both hydrazone nitrogen and both carbonyl oxygen atoms. The C–O bond lengths in 1 and 2 (Tables S1 and S2) are all in good agreement with their assignment as carbonyls (1.225(5)–1.265(6) Å) rather than alkoxides [27]. The structural differences between 1 and 2 were deeply investigated since the coordination environment around the DyIII ion has a dramatic influence on the magnetic properties of the resulting complexes [28]. Systematic analysis of the coordination geometries around the DyIII centers using SHAPE 2.1 [29] reveals that the nine-coordinated DyIII centers of complex 1 adopt geometries that are best described as muffin (minimum CShM values of 2.565), whereas for complex 2, the ten- and eight-coordinated DyIII centers adopt bicapped square antiprism and triangular dodecahedron coordination geometries, respectively (minimum CShM values of 3.125 and 3.548 were obtained) (Table S3).
In complex 1, all the hydrogen atoms from the coordinated methanol molecules are involved in intermolecular hydrogen bonding (Table S4) with the phenoxy oxygen atoms and these interactions support the formation of a supramolecular two dimensional arrangement (Figures S2 and S3). In complex 2, all the hydrogen atoms of coordinated methanol and water molecules are involved in intermolecular hydrogen bonding (Table S5) with the phenoxy oxygen atoms and lattice methanol molecules resulting in the formation of a supramolecular two dimensional arrangement (Figures S4 and S5). In addition to the H-bonding interactions, strong CH⋯π interactions are also noticed with CH to centroid distances of 3.595(4) Å and 3.508(6) Å for 1 and 2, respectively.
The purity of the as-synthesized products was confirmed by the good agreements of the bulk phase powder X-ray diffraction patterns with the simulated one (Figure S6). Elemental composition of 1 and 2 were confirmed by the elemental analysis, which matches well with the calculated values. The IR spectra of complexes 1 and 2 show bands at ~3434 cm−1, 1640 cm−1, 1583 cm−1 and 1018 cm−1, which can be assigned to ν(phenolic OH), ν(C=N), pyridine ring stretching vibrations and ν(N–N), respectively. The bands at 1435 cm−1, 1304 cm−1 and 1030 cm−1 clearly identify the presence of coordinated nitrate in complex 1. In the IR spectrum of 2, the bands at ca. 3221 and 819 cm−1 are characteristic of coordinated water molecule in complex 2.
The variable-temperature DC magnetic susceptibility measurements were performed under an applied field of 1000 Oe and in the range of 1.8–300 K. The room temperature experimentally obtained χMT values for complexes 1 and 2 are 28.4 and 28.3 cm3·K·mol−1, respectively (Figure 2 and Figure S7), which are consistent with the theoretical value of 28.34 cm3·K·mol−1 for two isolated DyIII ions [15]. On lowering the temperature from 300 K, the χMT value decreases gradually due to the single ion crystal-field effects. This result is further prominent below 70 K, where it reaches value of 10.9 and 10.7 cm3·K·mol−1 for 1 and 2, respectively, at 2 K. The observation reveals the continuous depopulation of the excited Stark sublevels of the DyIII ions [30].
The reduced magnetization data (M/NμB vs. H) of the complexes were collected at 2, 6 and 10 K. For 1 and 2, with increase in field M/NμB values increase sharply and attained the values of 11.5 and 11.2 NμB respectively (Figure 2 and Figure S7), which are in good agreement with the theoretical value for DyIII-based SMMs [8]. As shown in Figure 2 and Figure S7, all isotherm magnetization curves do not merge, which confirms the presence of large magnetic anisotropy in the complexes [10].
To probe spin dynamics in 1 and 2, ac magnetic susceptibility measurements were carried out at 3.5 Oe ac field and varying the temperature from 1.8–10 K under zero dc field. Complexes 1 and 2 show temperature (Figure 3 and Figure S8) and frequency dependency (Figures S9 and S10) of out of phase (χM″) ac susceptibilities. The phenomenon indicates the single-molecule magnet (SMM) like behavior in the complexes [6]. Moreover, the Cole–Cole plots [31,32] (Figure 3 and Figure S11) were generated from the frequency-dependent ac susceptibility data. The fit of the χM″ vs. χM′ data using the generalized Debye model [31,32] produced the values of α within the ranges 0.05–0.27 (1) and 0.08–0.32 (2), signifying the narrow distribution of the relaxation time. Effective energy barrier (Ueff) and relaxation time (τ0) were calculated from the Arrhenius Equation (1) [33,34,35]:
ln(1/τ) = ln(1/τ0) − Ueff/kT
where k = Boltzmann constant, and 1/τ0 = pre-exponential factor. The linear fit to high temperature data gave values of Ueff = 36.4 K and τ0 = 3.3 × 10−6 s for 1 (Figure 3). However, the out-of-phase signals (χM″) for complex 2 do not show the peak maxima in the mentioned temperature range. Therefore, Debye model and Equation (2) were used to calculate energy barrier and relaxation time [36]
ln(χ″/χ′) = ln(ωτ0) + Ueff/kT
From the best fitting, the value of energy barrier and relaxation time were calculated as Ueff = 9.7 K and τ0 = 1.4 × 10−6 s, respectively (Figure S11), and found to be in good agreement with the expected value of 10−6–10−11 for a SMM [37,38,39].
In order to explore the consequence of inter- and intramolecular exchange interactions on the magnetic behavior, the influence of magnetic dilution on relaxation of the magnetization was studied. To gain more in sight we synthesized the diamagnetic dinuclear YIII analog (3) (see ESI for experimental and X-ray details, Figure S12 and Table S6) and then prepared the doped sample in which the DyIII complex (1) was magnetically diluted with the YIII complex in a 5:95 ratio. AC susceptibility measurements were performed on a polycrystalline sample of the diluted complex. No major difference was found in the energy barrier of the diluted sample compared to the undiluted one (Figure S13). Therefore, it can be concluded that the intermolecular forces and dipolar interactions are insignificant in this case.
From comparative point of view, even though 1 and 2 contain two DyIII centers, their relaxation dynamic behaviors are considerably different. The difference was attributed to the slight changes in the coordination environments around the DyIII centers [19,27,40,41,42]. This is due to the nature and symmetry of crystal field which controls anisotropy and effects on overall effective energy barrier [43,44,45,46,47]. As can be seen, in complex 1, two DyIII centers are in muffin (Cs) geometry, while in 2, one DyIII center is in bicapped square antiprism (D4d) and other one is in triangular dodecahedron (D2d) geometry. Hence, the observed difference in magnetic behaviors of 1 and 2 was mostly because of the different coordination environments around the DyIII centers, which affects the nature of easy axes [43].

3. Materials and Methods

All chemicals were of reagent grade and used without further purification. The elemental analyses were carried out on an Elementar Microvario (Mumbai, India) Cube Elemental Analyzer. FT-IR spectra (4000–400 cm−1) were recorded on KBr pellets using a Perkin-Elmer (Mumbai, India) Spectrum BX spectrometer. Powder X-ray diffraction (PXRD) data were collected on a PANalytical EMPYREAN (Mumbai, India) instrument using Cu-Kα radiation. Magnetic measurements were performed using a SQUID VSM magnetometer (Quantum Design, Mumbai, India). The measured values were corrected for the experimentally measured contribution of the sample holder, while the derived susceptibilities were corrected for the diamagnetism of the samples, estimated from Pascal’s tables [48].
Ligand was prepared by a simple hydrazine condensation reaction of one equivalent 2,6-diacetylpyridine with two equivalents of 2-salicyloylhydrazide in methanol according to an earlier reported procedure [49].
Synthesis of [Dy2(L)2(MeOH)2(NO3)2] (MeOH) (CH2Cl2) (1). L (43 mg, 0.1 mmol) was dissolved in CH2Cl2 (5 mL) and the solution was warmed to 45 °C. LiOH·H2O (4.0 mg, 0.1 mmol) was added to the reaction mixture to deprotonate the ligand. Then, Dy(NO3)3·5H2O (43 mg, 0.1 mmol) dissolved in MeOH (5 mL) and added to the above ligand solution. The solution formed an intense yellow mixture and it was stirred for another 2 h. The solution was filtered off and the filtrate was left in open atmosphere for slow evaporation which gives large X-ray quality yellow crystals of [Dy2(L)2(MeOH)2(NO3)2] (MeOH) (CH2Cl2) (1) after 4 days. The crystals were separated and washed with cold water and Et2O; yield (57%). Anal. Calcd for C52H54Cl4Dy2N12O18: C, 38.99; H, 3.40; N, 10.49%. Found: C, 39.09; H, 3.47; N, 10.41%. Selected IR data (KBr pellet, 4000–400 cm−1) ν/cm−1: 3434, 1640, 1580, 1435, 1304, 1030, 1018.
Synthesis of [Dy2(L)3(H2O) (MeOH)] (MeOH)4 (H2O) (2). L (65 mg, 0.15 mmol) was dissolved in CH2Cl2 (5 mL) and the solution was warmed to 45 °C. LiOH·H2O (4.0 mg, 0.1 mmol) was added to the reaction mixture to deprotonate the ligand. Then, DyCl3·6H2O (38 mg, 0.1 mmol) dissolved in MeOH (5 mL) and added to the above ligand solution. The solution formed an intense yellow mixture and it was stirred for another 3 h. The solution was filtered off and the filtrate was left in open atmosphere for slow evaporation which gives large X-ray quality yellow crystals of [Dy2(L)3(H2O) (MeOH)] (MeOH)4 (H2O) (2) after 5 days. The crystals were separated and washed with cold water and Et2O; yield (65%). Anal. Calcd for C74H80Dy2N15O19: C, 49.15; H, 4.46; N, 11.62%. Found: C, 49.24; H, 4.34; N, 11.69%. Selected IR data (KBr pellet, 4000–400 cm−1) ν/cm−1: 3430, 3221, 1642, 1583, 1020, 819.
Intensity data were collected on a Brüker (Mumbai, India) APEX-II CCD diffractometer using a graphite monochromated Mo-Kα radiation (α = 0.71073 Å). Data collection was performed using φ and ω scan. Direct methods were used for the solution of crystals using SHELXTL followed by full matric least square refinements against F2 [50]. The positions of the remaining non-hydrogen atoms were found by using difference Fourier synthesis and least square refinements. The exact crystal system, cell dimensions and orientation matrix were determined by the reported procedure followed by multi-scan absorption correction and Lorentx polarization. All H-atoms were calculated geometrically and refined using riding model. The non-hydrogen atoms were refined with anisotropic displacement parameters. SHELXL 97 [51], PLATON 99 [52] and WinGXsystemVer-1.64 [53] were used for the refinement and calculations. The details of collection of data and their refinement parameters are included in Table 1.

4. Conclusions

Two important dinuclear dysprosium based complexes have been synthesized and characterized. Both the complexes exhibit single-molecule magnet (SMM) like behavior. It has been observed that a minor difference in the coordination surroundings around the DyIII center affected the relaxation dynamics of the complexes. Therefore, the overall studies propose the importance of the coordination environment around DyIII centers in describing and distinguishing their magnetic properties. Further studies along similar lines are in progress.

Supplementary Materials

The following are available online at www.mdpi.com/2312-7481/2/3/35/s1. Coordination polyhedral, SHAPE analysis table, magnetic plots, PXRD, bond length and bond distances tables and hydrogen bonding tables.

Acknowledgments

A.K.M. thanks UGC for SRF fellowship. V.S.P. thanks IISER Bhopal for fellowship. S.K. thanks DAE BRNS, 37(2)/14/09/2015/BRNS, Government of India and IISER Bhopal for generous financial and infrastructural support.

Author Contributions

A.K.M. designed and performed the experiments; A.K.M. solved the crystal structures and analyzed the magnetic data; A.K.M. and V.S.P. wrote the paper; and S.K. supervised the overall work and organized the project.

Conflicts of Interest

The authors declare no conflict of interest.

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Magnetochemistry 02 00035 g001a 1024Magnetochemistry 02 00035 g001b 1024
Figure 1. View of the molecular structures of: complex 1 (a); and complex 2 (b). Polyhedral view of: nine- (c); ten- (d); and eight- (e) coordinated geometries of DyIII centers found in complexes 1 and 2.
Figure 1. View of the molecular structures of: complex 1 (a); and complex 2 (b). Polyhedral view of: nine- (c); ten- (d); and eight- (e) coordinated geometries of DyIII centers found in complexes 1 and 2.
Magnetochemistry 02 00035 g001aMagnetochemistry 02 00035 g001b
Magnetochemistry 02 00035 g002 1024
Figure 2. (a) χMT vs. T plot measured at 0.1 T for complex 1; and M/NμB vs. H (b); and M/NμB vs. H/T plots (c) in the field range of 0–7 T and temperature range of 2–10 K for complex 1.
Figure 2. (a) χMT vs. T plot measured at 0.1 T for complex 1; and M/NμB vs. H (b); and M/NμB vs. H/T plots (c) in the field range of 0–7 T and temperature range of 2–10 K for complex 1.
Magnetochemistry 02 00035 g002
Magnetochemistry 02 00035 g003 1024
Figure 3. Temperature dependence of the out-of-phase (χM″) AC magnetic susceptibility plots for: complex 1 (a); and complex 2 (b); Illustration of (c) ln(1/τ) vs. 1/T plots for 1 (red lines represents the best fit of the Arrhenius relationship). Cole–Cole plots for 1 are shown in the inset.
Figure 3. Temperature dependence of the out-of-phase (χM″) AC magnetic susceptibility plots for: complex 1 (a); and complex 2 (b); Illustration of (c) ln(1/τ) vs. 1/T plots for 1 (red lines represents the best fit of the Arrhenius relationship). Cole–Cole plots for 1 are shown in the inset.
Magnetochemistry 02 00035 g003
Table
Table 1. X-ray Crystallographic Data and Refinement Parameters for complexes 1 and 2.
Table 1. X-ray Crystallographic Data and Refinement Parameters for complexes 1 and 2.
12
FormulaC52H54Cl4Dy2N12O18C74H76Dy2N15O19
Mw (g·mol−1)1601.871804.50
Crystal size (mm)0.45 × 0.18 × 0.160.43 × 0.15 × 0.10
Crystal systemTriclinicTriclinic
Space groupP-1P-1
T (K)296(2)151(2)
a (Å)10.868(3)12.7428(8)
b (Å)10.872(3)16.2939(9)
c (Å)12.955(3)19.1396(11)
α (°)84.323(14)89.6900(18)
β (°)85.574(13)85.6640(18)
γ (°)76.415(13)75.657(2)
V (Å3)1478.3(7)3838.7(4)
Z12
ρcalcd (g·cm−3)1.7991.561
µ (MoKα) (mm−1)2.7712.012
F(000)794.01818.0
Tmax, Tmin0.652, 0.5450.828, 0.723
h, k, l range−14 ≤ h ≤ 13, −14 ≤ k ≤ 14, −16 ≤ l ≤ 16−15 ≤ h ≤ 15, −20 ≤ k ≤ 20, −23 ≤ l ≤ 23
Collected reflections664515689
Independent reflections56679904
Goodness-of-fit (GOF) on F21.0381.456
R1, wR2 (I > 2σI)0.0444, 0.11630.0494, 0.0794
R1, wR2 (all data)0.0518, 0.12290.0887, 0.0842
CCDC Number14824391482440
R1 = Σ||Fo| − |Fc||/Σ|Fo| and wR2 = |Σw(|Fo|2 − |Fc|2)|/Σ|w(Fo)2|1/2.

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Mondal, A.K.; Parmar, V.S.; Konar, S. Modulating the Slow Relaxation Dynamics of Binuclear Dysprosium(III) Complexes through Coordination Geometry. Magnetochemistry 2016, 2, 35. https://doi.org/10.3390/magnetochemistry2030035

AMA Style

Mondal AK, Parmar VS, Konar S. Modulating the Slow Relaxation Dynamics of Binuclear Dysprosium(III) Complexes through Coordination Geometry. Magnetochemistry. 2016; 2(3):35. https://doi.org/10.3390/magnetochemistry2030035

Chicago/Turabian Style

Mondal, Amit Kumar, Vijay Singh Parmar, and Sanjit Konar. 2016. "Modulating the Slow Relaxation Dynamics of Binuclear Dysprosium(III) Complexes through Coordination Geometry" Magnetochemistry 2, no. 3: 35. https://doi.org/10.3390/magnetochemistry2030035

APA Style

Mondal, A. K., Parmar, V. S., & Konar, S. (2016). Modulating the Slow Relaxation Dynamics of Binuclear Dysprosium(III) Complexes through Coordination Geometry. Magnetochemistry, 2(3), 35. https://doi.org/10.3390/magnetochemistry2030035

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