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Lanthanide-Directed Fabrication of Four Tetranuclear Quadruple Stranded Helicates Showing Magnetic Refrigeration and Slow Magnetic Relaxation Amit Kumar Mondal,⊥ Himanshu Sekhar Jena,⊥ Amita Malviya, and Sanjit Konar* Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal-bypass road, Bahuri, Bhopal-462066, Madhya Pradesh, India S Supporting Information *

ABSTRACT: A rare class of four tetranuclear lanthanide based quadruple stranded helicates namely, [Ln4L4(OH)2](OAc)2·xH2O (Ln = GdIII(1), DyIII(2) and x = 4, 5 respectively), [Er 4 L 4 (OH) 2 ](NO 3 ) 2 ·9H 2 O (3), and [Dy4L4(NO3)](NO3)2·2CH3OH·H2O (4) were synthesized by employing succinohydrazone derived bis-tridentate ligand (H2L) and characterized. Structures of 1−3 are similar to each other except the nature of counterions and number of lattice water molecules. In 4, a distorted nitrate ion was arranged in a hexagonal manner holding four dysprosium centers in a slightly twisted manner. Because of the symmetrical nature of each complex, the C4 axis crosses the center of helicate resulting a pseudo-D4 coordination environment. Each ligand coordinates to lanthanide centers in helical manner forming mixture of left (Λ) and right (Δ) handed discrete units. Complex 1 exhibits antiferromagnetic exchange interaction between nearby GdIII centers and shows magnetic refrigeration (−ΔSm = 24.4 J kg−1 K−1 for ΔH = 7 T at 3 K). AC magnetic susceptibility measurements of 2 and 4 demonstrate slow relaxation behavior, with Ueff (effective energy barrier) of 20.5 and 4.6 K, respectively. As per our knowledge, complexes 1, 2, and 4 represent the first examples of aesthetically pleasing quadruple stranded helicates showing potential magnetocaloric effect and single-molecule-magnet-like behavior.

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helicates are bit explored,12 their magnetic properties are less investigated. Among the recent investigation of molecular magnets, cryogenic magnetic refrigerant13 and single molecule magnet (SMM)14 were noteworthy. Magnetic refrigeration13 depends on the magnetocaloric effect (MCE), which represents the change of isothermal magnetic entropy (ΔSm) and adiabatic temperature (ΔTad) in change of applied magnetic field.13 Therefore, to achieve large ΔSm combination of GdIII with small multidentate ligands with carboxyl, oxo, hydroxo groups were necessary. Such functional groups accumulates large number of metal to ligands and facilitates dense arrangements in the lattice. In general molecular helicates composed of several such functional group, accumulates large number of metal ions in a discrete unit which might facilitate a greater volumetric MCE. Among molecular helicates, double stranded helicates mostly shows SMM-like behavior with few exception to triple stranded one.15 However, the nature of magnetic interaction among metal centers in their higher homologues was barely explored.15 More precisely, slow magnetic relaxation and magnetic refrigeration properties have not been reported for any lanthanide based tetranuclear quadruple stranded

INTRODUCTION Metal-mediated fabrication of supramolecular structures is one among the foremost challenges in coordination chemistry.1,2 Among them, molecular helicates have attracted significant attention owing to their peculiar structural arrangement3 and usages in enantioselective processes, supramolecular devices and molecular magnetism.4−6 Most of the reported selfassembled multistranded helicates comprise of only two or three metal centers whereas higher homologues are quite rare.7 Several attempts have been made to prepare high-nuclearity helicates, however most of them limits to only double and triple stranded except few quadruple stranded and circular helicates.8 Therefore, permutation and combination of several metal ions and ligand might deliver high-nuclearity helicates.9 To our knowledge, transition metal directed formation of circular and quadruple stranded helicates are well understood and many elegant examples, are reported.10 However, lanthanide based such supramolecular moieties are less explored.11 More precisely, only two lanthanide based tetranuclear quadruple stranded helicates have been reported so far.11e,f Therefore, constructing lanthanide based mutliple stranded helicates are leading challenges for supramolecular chemists because of their importance in photofunctional, sensing and in perticular molecular magnetism.4−6 Although the photophysical and sensing properties of lanthanide based © XXXX American Chemical Society

Received: January 23, 2016

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DOI: 10.1021/acs.inorgchem.6b00177 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Data and Refinement Parameters for Complexes 1−4 CCDC number molecular formula formula mass (g/M) temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z DC (g−3) μ (MoKα) (cm−1) F(000) reflection unique GOF on F2 R1a [I > 2σI] R2b [I > 2σI] a

1

2

3

4

1437825 C92H112Gd4N16O34 2614.96 106 (2) triclinic P1̅ 17.336(3) 17.945(3) 18.528(3) 108.258(5) 94.285(5) 90.647(5) 5454.8(16) 2 1.587 2.484 2592 21990 18535 1.233 0.0898 0.2054

1437826 C92H114Dy4N16O35 2653.89 106 (2) triclinic P1̅ 17.336(3) 17.945(3) 18.528(3) 108.258(5) 94.285(5) 90.647(5) 5454.8(15) 2 1.603 2.793 2604 21950 18494 1.095 0.0882 0.1964

1437827 C88H116Er4N18O41 2750.98 104 (2) triclinic P1̅ 13.205(3) 17.931(4) 24.533(5) 69.904(5) 81.134(5) 77.908(5) 5312.8(19) 2 1.708 3.221 2702 20445 18742 1.287 0.0690 0.1767

1437828 C90H106Dy4N19O39 2717.88 103 (2) monoclinic C2/c 28.876(2) 21.597(2) 17.0366(13) 90.00 90.226(4) 90.00 10624.5(15) 4 1.777 2.883 5628 12132 12127 1.146 0.0424 0.1119

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. bR2 = |Σw(|Fo|2 − |Fc|2)|/Σ|w(Fo)2|1/2. followed by full matric least-squares refinments against F2.17 The positions of the remanining non-hydrogen atoms were found by using difference Fourier synthesis and leat square refinments. The exact crystal system, cell dimensions and orientation matrix were determined by the reported procedure followed by multiscan absorption correction and Lorentx polarization.15g All H atoms were calculated geometrically and refined using riding model. The non-hydrogen atoms were refined with anisotropic displacement parameters. SHELXL 97,18 PLATON 99,19 and WinGXsystemVer-1.6420 were used for the refinement and calculations. The details of collection of data and their refinement parameters are included in Table 1. In complexes 1−3, out of four-ligand strand, atoms on one ligand strand are highly disorder and became anisotropic displacement parameters (ADPs) upon anisotropic refinement. This results in B and C level alerts. Attempt to resolve those ADP by command DELU and DFIX in Xhell 6.3.1 as well EADP in OLEX 2 remain unsuccessful. This might be due to the various-low-resolution reflection at lower angle observed during data collections or dense arrangement of ligand in a confined space. In complex 4, coordinated nitrate ion remains disordered with each oxygen atom displaced over two positions with 50% occupancy factors.11e In complexes 1−4, lattice methanol and water molecules are found to be ADP upon anisotropic refinement and attempt to resolve them remain unsuccessful. In complexes, 1−4, locating of hydrogen atoms on the lattice solvent and coordinated water molecules were unsuccessful using riding model but are considered in the molecular formulas and their weights. This results in C and G level alerts regarding the calculated and reported sum formulas and their molecular weights. Synthesis of [Gd4L4(OH)2](OAc)2·4H2O (1). Deprotonation of ligand H2L (44.2 mg, 0.1 mmol) by LiOH·H2O (8.0 mg, 0.2 mmol) was performed in methanol by stirring for 5−10 mints. To the clear solution, Gd(OAc)3·H2O (70.5 mg, 0.1 mmol) dissolved in 8 mL methanol was added dropwise and allowed to stirrer at room temperature for another 3 h. After 1 week, light yellow crystals were isolated from the slow evaporation of the solvent. Yield 57%. Elemental analysis: calcd. (%) for C92H112Gd4N16O34: C 42.26, H 4.32, N 8.57; found C 42.41, H 4.39, N 8.48. Selected IR data (KBr pellet; 4000−400 cm−1):11f 3445(b), 1637(s), 1399(s), 1094(w), 567(b).

helicates. Recently it has been proposed that control alignment of anisotropy axes can possibly give higher D values with increased effective energy barriers.15a Notably, in helicates, the twists of helical strands controls the direction of anisotropy axes. Therefore, in multiple stranded helicates, magnetostructural relationships might allow to make SMMs with much higher energy barriers.15a Herein we report four lanthanide-based tetranuclear quadruple stranded helicates namely, [Ln4L4(OH)2](OAc)2· xH2O (Ln = GdIII(1), DyIII(2) and x = 4, 5 respectively), [Er4L4(OH)2](NO3)2·9H2O (3), and [Dy4L4(NO3)](NO3)2· 2CH3OH·H2O (4) constructed using butanedihydrazidebridged bis(3-ethoxysalicylaldehyde) ligand (H2L). Importantly, their structural rearrangements represent supramolecular cooperativity between metal and ligand resulting beautiful coordination complexes. Additionally, complexes 1, 2, and 4 represents the first set of higher homologues helicate showing magnetic refrigeration and SMM-like behavior.

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EXPERIMENTAL SECTION

All the chemicals and solvents are commerciable available. Succinic dihydrazide, o-vanillin, and lanthanide salts (Gd(OAc)3·H2O, Dy(OAc)3·H2O, Dy(NO3)3·6H2O, and Er(NO3)3·6H2O) were procured from Sigma-Aldrich. Ligand H2L, has been synthesized by following earlier synthetic method.11f Elementar Micro vario Cube elemental analyzer was used for the elemental analysis of all the complexes. PerkinElmer Spectrum BX spectrometer was used to collect the FT-IR spectra. PANalytical EMPYREAN instrument was used for collection of powder X-ray diffraction (PXRD) data using Cu Kα radiation. Quantum Design SQUID VSM magnetometer was used for magnetic mesurments of the complexes. Diamgnetic correction to the samples were applied as estimated from Pascal’s tables.16 Crystal Data Collection and Structure Determination. Brüker APEX-II CCD diffractometer was used for mounting suitable single crystals of 1−4 using Mo Kα (λ = 0.71073 Å, 101 K) radiation. φ and ω scans were used for collection of raw data of each crystals. Direct methods were used for the solution of crystals using SHELXTL B


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Inorganic Chemistry Following the similar synthetic methods, complex 2−4 were synthesized using Dy(OAc)3·H2O, Er(NO3)3·6H2O and Dy(NO3)3· 6H2O respectively. [Dy4L4(OH)2](OAc)2·5H2O (2). Yield 61%. Elemental analysis; calcd. (%) for C92H114Dy4N16O35: C 41.64, H 4.33, N 8.44; found C 41.77, H 4.42, N 8.50. Selected IR data (KBr pellet; 4000−400 cm−1): 3186(b), 1638(s), 1397(s), 1087(w), 590(b). [Er4L4(OH)2](NO3)2·9H2O (3). Yield 65%. Elemental analysis; calcd. (%) for C88H116Er4N18O41: C 39.30, H 4.35, N 9.37; found C 39.41, H 4.23, N 9.43. Selected IR data (KBr pellet; 4000−400 cm−1): 3185(b), 1638(s), 1391(s), 1384(s), 1085(w), 592(b). [Dy4(L)4(NO3)](NO3)2·2CH3OH·H2O (4). Yield 70%. Elemental analysis; calcd. (%) for C90H106Dy4N19O39: C 39.63, N 9.75, H 3.92; found C 39.77, N 9.84, H 4.04. Selected IR data (KBr pellet; 4000− 400 cm−1): 3186(b), 1639(s), 1391(s), 1384(s),1085(w), 590(b).

Figure 1. Illustration of (a) tetranuclear quadruple stranded helicate found in 1 and (b and c) different type of bridging modes of ligand strand. Color codes: Gd (cyan), O (red), N (blue).

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RESULTS AND DISCUSSIONS Synthetic Aspects. Transition-metal-directed formation of helicates mostly contains ligand such as imines, bipyridine, catechol, benimidazole, and hydrazone derivatives and were well explored.21,22 Recently it has been found that the combination of flexible butanedihydrazide-bridged bis(3ethoxysalicylaldehyde) ligand (H2L) and LnIII ion offers higher homologues of helicates such as quadruple stranded (Ln = LnIII, EuIII) and circular helicates (Ln = LnIII).11e,f Therefore, it was anticipated that using the ligand and other lanthanides (GdIII, ErIII, and DyIII), magnetically potent higher homoluges helicates can be obtained. The deprotonation of ligand and its complexion with LnIII ion in 1:2 ratio results complexes 1−4 (Scheme 1).11f The

analysis of 1 has been discussed in details. In complex 1, out of four GdIII ion, Gd1 and Gd2 are nine coordinated whereas Gd3 and Gd4 are eight coordinated (Figure 2). The observed

Scheme 1. Synthetic Scheme of Complexes 1−4

Figure 2. Polyhedral view of eight (left) and nine (right) coordinated geometries of LnIII centers found in complexes 1−3. Color codes same as Figure 1.

complexion were primarily confirmed from IR-spectra analysis where peaks corresponding to the ν(NH) and ν(CO) groups of free ligand H2L were absent. Nevertheless, a new peak located around ∼1640 cm−1 proves the coordination of LnIII to ligand H2L.11g Additionally a strong peak centered at 1225 cm−1 in 1 and 2 and at 1384 cm−1 in 3 and 4 confirms the presence of free acetate and coordinated nitrate ion, respectively. The phase purity of each complex was confirmed by the powder Xray analysis (Figure S1). Structural Description for Complexes 1−4. In complexes 1−3, the central molecular entity ([Ln4L4(OH)2]2+) contains four deprotonated ligand (L), two hydroxy (−OH) bridge each connected to near by LnIII ions resulting a tetranuclear quadruple stranded helicates. Notably, four ligand backbone wrap around four lanthanide centers to form such supramolecular agregates (Figure 1a). Therefore, the structural

discrepancy of coordination environment were due to the flexible bridging nature of H2L. The nine coordination environment around Gd1 and Gd2 has been fulfilled by two imine-N, two (−C(O)), three phenoxy, one ethoxy and one hydroxy group, respectively. However, eight coordination geometry around Gd3 and Gd4 center was due to the lack of ethoxy bridge. SHAPE 2.123 software was used to ensure the exact geometry of the GdIII center which reveals Biaugmented trigonal prism (minimum CShM values of 4.049 was obtained) for eight coordinated and spherical-capped square antiprism (minimum CShM values of 1.474 was obtained) for nine coordinated GdIII centers respectively (Table S1). Importantly, in the both end of the quadruple stranded helicate, two of nearby GdIII ions are bridged by two η2phenoxo bridge and one η2-OH bridge. On the basis of these bridging modes, they can be differentiate to two types. One contains tridentate as well as tetradentate compartment whereas other contains two tridentate compartment for coordination to three LnIII centers (Figure 1b, 1c). The tetradentate bridging mode of ligand was due to the involvement of one of ethoxy group in the coordination. Importantly, the differnece in the arrangement of ethyl group of ligands (in plane or out of plane with the main ligand backbone) might be due to the self-assembly process which enforces the hanging ethyl group of the ethoxy group to be in such oreintation. The dianionic keto form of the ligands were confirmed by the CO (1.22(2)−1.26(1) Å) and (O)C−NH (1.31(2)− 1.39(3) Å) bond distances which matches well with our earlier reported one.11f Moreover, the observed bond distances (Gd− O; 2.21(1)−2.76(1) Å, Gd−N; 2.54(1)−2.61(1) Å, Table S2) C


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Inorganic Chemistry were well matched with earlier LaIII based quadruple stranded helicates.11e,f Besides, Gd−O(−OH) distances and Gd−O−Gd angles were fall in between 2.50(1)−2.57(1) Å and 91.6(4)− 101.4(1)°, respectively (Table S3).13d Additionally, each ligand possess pseudo-C confirmation because of the flexible −CH2− CH2− moieties. The torsional angle in between two −(O)C− CH2−CH2−C(O)− units and −C(O)−NH− units of H2L were 71.0−74.0° and 69.9−79.25°, respectively. By considering the length of two facial planes formed by terminal ethoxy group, the observed helical strand is of 15.93(6) Å with an average pitch of 17.06(6) Å.11f The separation between two GdIII centers are of 3.648(1)−6.834(1) Å.11f Notably, it retains pseudo-D4 geometry and allow C4 axis to enter exactly center of it (Figure S2).11f In addition out of four GdIII centers, two of them sited along one C2 axis whereas other two are in a triangular fashion (Figure S2).11f Besides, the helical wrapping of ligand strand in complex 1 results in racemic mixture and contains equal amounts of left- (Λ) and right-handed (Δ) ones (Figure S3). In addition, the hydrazide− N (−C(O)NH−) groups of each helicate were H-bonded with lattice water molecules (N−H···O, 2.74(2)−2.82(2) Å) and acetate ion (N−H···O, 2.71(2)−2.78(2) Å) and extended the discrete helicates to three-dimensional array (Figure S4−S5). Apart from the H-bonding, C−H···π, C−H···N, and C−H···O interactions support their arrangements. Structural analysis of complex 4 ([Dy4L4(NO3)](NO3)2· 2CH3OH·H2O) shows similar helicoidal arrangement of tetranuclear quadruple stranded helicate (Figure 3a).11e,f

Figure 4. Polyhedral view of nine- coordinated geometry of DyIII center found in complex 4 (left); A peculiar hexagonal arrangement of distroted nitrate ion hanging four DyIII center in a bend reactangular fashion (right). Color codes same as Figure 3.

ones.11e,f Similar to complex 1−3, in 4, the dihedral angle between two −(O)C−CH2−CH2−C(O)− moieties and −C(O)−NH− groups the angles are in the range of 61.10− 73.59° and 80.02−80.27°, respectively. Similarly, the total length of the helical ligand strand is 16.06(2) Å having an average pitch of ∼17.44(2) Å. Notably, a distorted nitrate ion was arranged in a peculiar hexagonal way in the center of helicate (Figure 4).11e,f The distances between DyIII centers are in the range of 3.908(1)−6.113(2) Å. Similar to complexes 1−3, in 4 four DyIII centers are arranged in a distorted rectangular fashion where two of them lies along one C2 axis whereas other two are positioned in a triangle and allow the C4 axis through the center of it (Figure S6). Similar to 1−3, complex 4 crystallizes in a racemic form (Figure 5). Notably, the lattice nitrate ions were H-bonded

Figure 3. Illustration of (a) tetranuclear quadruple stranded helicate found in complex 4 and (b) different types of bridging modes of ligand strand. Color codes: Dy (violet), O (red), N (blue).

Figure 5. Illustrations of both left (Λ) and right (Δ) handed arrangement of quadruple-stranded helicate found in 4.

(D···A, 2.78(2) − 3.06(2) Å) to water molecule, phenoxo-O, and hydrazide (−NH−) and form a star-shaped threedimensional supramolecular network (Figure S7−S8).11f Magnetic Property Studies. The variable-temperature DC magnetic susceptibility measurements of 1−4 were carried out under an applied field of 1000 Oe and in the range of 1.8− 300 K.26e For complex 1, at room temperature, the χMT value (χM = molar magnetic susceptibility) found to be 31.08 cm3 K mol−1 (31.20 cm3 K mol−1; expected spin only value for four GdIII centers in a discrete unit) (Figure 6).26e,h The χMT value hold on up to 57 K and then slowly drop to 17.74 cm3 K mol−1 at 2 K. This is because of the combined effects of antiferromagnetic interaction and zero-field-splitting (ZFS) of GdIII ions.24,26g On view of the structural arrangements, the most likely exchange interaction between GdIII centers are though oxo-bridge.26g Thus, taking into account of two magnetically isolated Gd2 centers, the modeling was performed

Notably, complex 4 is isostructural with the EuIII analogues reported recently.11f Its cationic asymmetric unit is contained four DyIII ions bridged by four deprotonated ligands and a distorted nitrate ion.11f In contrast to 1−3, in complex 4 all the DyIII are nine coordinated and forms a distorted monocapped square antiprisms geometry around each center (Figure 4). SHAPE 2.123 software was undertaken to ensure the exact coordination environment, which reveals a minimum value of 2.146 suggesting monocapped square antiprism geometry. Unlike to 1−3, complex 4 contains only one set of ligand with both tridentate as well as in tetradentate coordination pocket (Figure 3b). Tetradentate coordination pocket was formed due to the involvement of ethoxy-O group and η2 bridging mode of phenoxo-O group.11f The observed bond angles and distances were matched well with complexes 1−3 and also with reported D


Article

Inorganic Chemistry

under an applied field of 7 T, M(H) reaches value of 20.6 NμB, at 2 K. For 2 and 4, with increase in field M/NμB values rise abruptly and attained the values of 20.53 and 19.14 NμB respectively (Figure S10). On comparing with the expected value for isolated DyIII ions (gJ × J = 4/3 × 15/2 = 10 μB per DyIII), the obtained values are comparatively less which might be due to the presence of anisotropy and significant crystal field effect.28 As shown in Figure S11, all isotherm magnetization curves do not merge, which confirms the existence of substantial magnetic anisotropy in the complexes.28 To probe spin dynamics in 2−4, ac magnetic susceptibility measurements were undertaken at 3.5 Oe ac field and varying the temperature from 1.8 to 10 K. Notably, under zero dc field, the in-phase (χM′) and out-of-phase (χM″) ac signals were indepedant, which is due to the quantum tunnelling of the magnetization (QTM),14a,35e Therefore, measurements were carried out in an optimum static dc field of 1000 Oe to conquer QTM.14c Notably complex 2 and 4 shows temperature (Figure S12−S13) and frequency dependency (Figures 8 and S14) of out of phase (χM″) ac susceptibilities whereas complex 3 does not show of it (Figure S15).This phenomenon signifies the field induced SMM-like behavior in 2 and 4.14c Moreover, the Cole− Cole plots29 (Figure 9) were done from the frequencydependent ac susceptibility data. Effective energy barrier (Ueff) and relaxation time (τ0) were calculated from the Arrhenius equation (eq 1)30

Figure 6. Illustration of χMT versus T plot for 1−4 measured at 0.1 T (red line represents the best fit obtained).

for complex 1. The best fit resulted in JGdGd = −0.12 ± 0.06 cm−1 and g = 2.0 ± 0.07 (Figure 6), suggesting a very weak antiferromagnetic interactions between them.13c Additionally, the χMT data were fitted using Curie−Weiss equation and the values of C = 33.33 cm3 mol−1 K and θ = −1.99 K were obtained from the best-fitting (Figure S9).24d At low temperature, the value of field dependence of the magnetization measurements was 27.4 NμB at 7 T and was consistent with the expected hypothetical value of 28 NμB (Figure 7 (left)).26e Using Maxwell equation, ΔSm (T) ΔH = ∫ [δM(T,H)/ δT]HdH,25 the magnetic entropy change (ΔSm) at different field and temperature was calculated (Figure 7 (right)). It was found that, on decreasing the temperature, the value increases slowly attaining a value of −ΔSm = 24.4 J kg−1 K−1 for ΔH = 7 T at 3 K finally. The volumetric entropy change was found to be 38.7 mJ cm−3 K−1 (Table S4).26 The maximum entropy change per mole was intended as nRln(2s + 1) = 8.2R (n = 4GdIII; s = 7/2), which is corresponding to a value of 26.1 J kg−1 K−1. The differences between the theoretical and experiential magnetic entropy change might be due to the expected antiferromagnetic interactions.26e The room temperature χMT values for complexes 2−4 are 55.9, 45.6, and 56.1 cm3 K mol−1 respectively, which are consistent with expected theoretical values (56.7 and 45.9 cm3 K mol−1) for four isolated DyIII ions (4I15/2, S = 3/2, L = 6, g = 6/5) and ErIII ions (6H15/2, S = 5/2, L = 5, J = 15/2, g = 4/3) (Figure 6).26h When the temperature dropped from 300 K, the χMT value decreases slowly because of the single ion crystalfield effects.26g This result is further prominent below 70 K, where it attains value of 39.1 cm3 K mol−1 for 2, 25.6 cm3 K mol−1 for 3, and 38.3 cm3 K mol−1 for 4 at 2 K, which reveals the continuous removal of the excited Stark sublevels of the LnIII ions.26g,27 The reduced magnetization data (M/NμB vs H) of complexes 2−4 were collected at 2, 6, and 10 K. For 3,

ln(1/τ ) = ln(1/τ0) − Ueff /k T

(1)

(where k = Boltzmann constant, 1/τ0 = pre-exponential factor). The linear fit to high temperature data gave values of Ueff = 20.5 K and τ0 = 3.76 × 10−6 s for 2 (Figure 9). Nevertheless, the out-of-phase signals (χM″) for complex 4 do not display the peak maxima in the mentioned temperature range. Thus, Debye model and equation (eq 2), was used to evaluate energy barrier and relaxation time31 ln(χ ″ /χ ′) = ln(ωτ0) + Ueff /k T

(2)

The value of energy barrier and relaxation time were found to be Ueff = 4.6 K and τ0 = 1.9 × 10−6 s respectively upon the best fit (Figure 9), and found to be in good agreement with the estimated value of 10−6−10−11 for a SMM.32 On comparative point of view, although 2 and 4 comprise four DyIII centers; their relaxation dynamic behaviors are different. The observed difference was due to the minor changes in the coordination surroundings around the DyIII centers.33 This is because of the nature and symmetry of crystal field which regulates anisotropy and affects overall anisotropy energy barrier.34,35 As it is noted that, in complex 2, two DyIII centers

Figure 7. Plots of M/NμB vs H at the indicated temperatures (left). Temperature dependencies (3−10 K) of −ΔSm as obtained from magnetization data (right) for 1. E


Article

Inorganic Chemistry

Figure 8. Frequency dependency of the out-of-phase (χM″) AC magnetic susceptibility plots of 2 (left) and 4 (right) at 1000 Oe.

Figure 9. Illustration of (a) ln (1/τ) vs 1/T plots for 2 (red lines represents the best fit of the Arrhenius relationship) and (b) natural logarithm of the ratio of χ″ over χ′ vs 1/T for 4 (solid lines represent best fit obtained from eqn 2). Cole−Cole plots for complexes 2 (c) and 4 (d).

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are in Biaugmented trigonal prism and other two are in Spherical capped square antiprism geometry whereas in 4, all of them are in monocapped square antiprism. Thus, the observed difference in magnetic behaviors of 2 and 4 was mainly owing to the different coordination surroundings around the DyIII centers, which effects nature of easy axes.15b,34

Corresponding Author

*E-mail: [email protected]. Fax: +91-755-6692392. Tel: +91755-6692336. Author Contributions ⊥

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These authors contributed equally.

Notes

CONCLUSION In conclusion, four aesthetically pleasing tetranuclear lanthanide based quadruple stranded helicates have been synthesized by employing a flexible succinohydrazone derived bis-tridentate ligand and well characterized. Structural analyses reveals that nature of binding sites, length and flexibility of the ligand strand and coordination adaptability of lanthanide ion plays a major role for such supramolecular self-assembly. Complex 1 appears to be the first report of a cryogenic magnetic refrigerant in helicate systems. Whereas complex 2 and 4 represents the first examples of lanthanide based tetranuclear quadruple stranded helicates showing single-molecule magnet (SMM) like behavior. Thus, the overall studies suggest the importance of the coordination environment around DyIII ions in defining and distinguishing their magnetic properties. Further studies on the along the similar line are under way.

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AUTHOR INFORMATION

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.M. thanks UGC for SRF fellowship, and H.S.J. thanks IISER Bhopal for postdoctoral fellowship. A.M. thanks DST for Inspire fellowship. S.K. thanks DAE BRNS, 37(2)/14/09/ 2015/BRNS, Government of India, and IISER Bhopal for generous financial and infrastructural support.

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REFERENCES

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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.6b00177. CCDC (1437825−1437828) contains the supplementary crystallographic data. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Additional data and plots, PXRD, and magnetic characterizations (PDF) Crystallographic information file for compounds 1−4 (CIF) F


Article

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