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

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 Dy III centers are in muffin (C s) coordination geometry while in 2, one Dy III center is in bicapped square antiprism (D 4d) and other one is in triangular dodecahedron (D 2d) 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 Dy III 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 Dy III centers in the magnetic relaxation behavior, a diamagnetic isostructural Y III analog (3) was synthesized and magnetic behavior was examined.


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 Ln III ions such as Dy III , Tb III , Er III , and Ho III 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 Ln III ion and stabilize the M J states with a large absolute value of the total angular momentum projection |M J |, therefore realizing a magnetization easy axis.Outstanding Ln based SMMs include Tb III /Dy III -phthalocyanin (Pc) double-decker complexes [13] and sandwiched Er III 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 [Dy 2 (L) 2 (MeOH) 2 (NO 3 ) 2 ]•(MeOH)•(CH 2 Cl 2 ) (1) and [Dy 2 (L) 3 (H 2 O)•(MeOH)]•(MeOH) 4 •(H 2 O) (2) are reported, which are obtained from the reaction of a pentadentate organic ligand L with Dy(NO 3 ) 3 and DyCl 3 , respectively (L = 2,6-bis(1-salicyloylhydrazonoethyl) pyridine; Figure S1).
As the overall electronic structure of a Dy III 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 Dy III dinuclear complexes, where minor changes in the coordination environment around the Dy III ions disturbed the local symmetry.

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 Dy III centers contain similar DyN 3 O 6 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 Dy III centers contain different DyN 6 O 4 and DyN 3 O 5 cores.Two interlocked pentachelating ligands make up the coordination sphere of one Dy III 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 Dy III ion has a dramatic influence on the magnetic properties of the resulting complexes [28].Systematic analysis of the coordination geometries around the Dy III centers using SHAPE 2.1 [29] reveals that the nine-coordinated Dy III 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 Dy III 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.pentadentate organic ligand L with Dy(NO3)3 and DyCl3, respectively (L = 2,6-bis(1salicyloylhydrazonoethyl) pyridine; Figure S1).
As the overall electronic structure of a Dy III 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 Dy III dinuclear complexes, where minor changes in the coordination environment around the Dy III ions disturbed the local symmetry.

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 Dy III 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 Dy III centers contain different DyN6O4 and DyN3O5 cores.Two interlocked pentachelating ligands make up the coordination sphere of one Dy III 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 Dy III ion has a dramatic influence on the magnetic properties of the resulting complexes [28].Systematic analysis of the coordination geometries around the Dy III centers using SHAPE 2.1 [29] reveals that the nine-coordinated Dy III 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 Dy III 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 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 χ M T values for complexes 1 and 2 are 28.4 and 28.3 cm 3 •K•mol −1 , respectively (Figure 2 and Figure S7), which are consistent with the theoretical value of 28.34 cm 3 •K•mol −1 for two isolated Dy III ions [15].On lowering the temperature from 300 K, the χ M T 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 cm 3 •K•mol −1 for 1 and 2, respectively, at 2 K.The observation reveals the continuous depopulation of the excited Stark sublevels of the Dy III ions [30].
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 cm 3 •K•mol −1 , respectively (Figure 2 and Figure S7), which are consistent with the theoretical value of 28.34 cm 3 •K•mol −1 for two isolated Dy III 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 cm 3 •K•mol −1 for 1 and 2, respectively, at 2 K.The observation reveals the continuous depopulation of the excited Stark sublevels of the Dy III 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 Dy III -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 (1) where k = Boltzmann constant, and 1/τ0 = pre-exponential factor.The linear fit to high temperature data gave values of Ueff = 36.4K 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 (2) 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].The reduced magnetization data (M/Nµ B vs. H) of the complexes were collected at 2, 6 and 10 K.For 1 and 2, with increase in field M/Nµ B values increase sharply and attained the values of 11.5 and 11.2 Nµ B respectively (Figure 2 and Figure S7), which are in good agreement with the theoretical value for Dy III -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 (χ M ") 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 χ M " vs. χ M 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 (U eff ) and relaxation time (τ 0 ) were calculated from the Arrhenius Equation (1) [33][34][35]: where k = Boltzmann constant, and 1/τ 0 = pre-exponential factor.The linear fit to high temperature data gave values of U eff = 36.4K and τ 0 = 3.3 × 10 −6 s for 1 (Figure 3).However, the out-of-phase signals (χ M ") 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 From the best fitting, the value of energy barrier and relaxation time were calculated as U eff = 9.7 K and τ 0 = 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 Y III analog (3) (see ESI for experimental and X-ray details, Figure S12 and Table S6) and then prepared the doped sample in which the Dy III complex (1) was magnetically diluted with the Y III 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.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 Y III analog (3) (see ESI for experimental and X-ray details, Figure S12 and Table S6) and then prepared the doped sample in which the Dy III complex (1) was magnetically diluted with the Y III 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 Dy III centers, their relaxation dynamic behaviors are considerably different.The difference was attributed to the slight changes in the coordination environments around the Dy III 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 Dy III centers are in muffin (Cs) geometry, while in 2, one Dy III 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 Dy III centers, which affects the nature of easy axes [43].

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 2,6diacetylpyridine 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, From comparative point of view, even though 1 and 2 contain two Dy III centers, their relaxation dynamic behaviors are considerably different.The difference was attributed to the slight changes in the coordination environments around the Dy III 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 Dy III centers are in muffin (C s ) geometry, while in 2, one Dy III center is in bicapped square antiprism (D 4d ) and other one is in triangular dodecahedron (D 2d ) geometry.Hence, the observed difference in magnetic behaviors of 1 and 2 was mostly because of the different coordination environments around the Dy III centers, which affects the nature of easy axes [43].

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

Figure 1 .
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 Dy III centers found in complexes 1 and 2.

Figure 1 .
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 Dy III centers found in complexes 1 and 2.

Figure 2 .
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 .
Figure 2. (a) χ M T vs. T plot measured at 0.1 T for complex 1; and M/Nµ B vs. H (b); and M/Nµ B vs. H/T plots (c) in the field range of 0-7 T and temperature range of 2-10 K for complex 1.

Figure 3 .
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 .
Figure 3. Temperature dependence of the out-of-phase (χ M ") 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.

Table 1 .
X-ray Crystallographic Data and Refinement Parameters for complexes 1 and 2.