Heterometallic CuII–DyIII Clusters of Different Nuclearities with Slow

Dec 24, 2015 - Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, ... ties of two heterometallic CuII−DyIII clusters are r...
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Heterometallic CuII−DyIII Clusters of Different Nuclearities with Slow Magnetic Relaxation Ritwik Modak,† Yeasin Sikdar,† Goulven Cosquer,‡,§ Sudipta Chatterjee,∥ Masahiro Yamashita,*,‡,§ and Sanchita Goswami*,† †

Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata 700009, India Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aramaki-Aza-Aoba, Aoba-ku, 980-8578 Sendai, Japan § Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology (JST), 4-1-8 Kawaguchi, Saitama 332-0012, Japan ∥ Department of Chemistry, Serampore College, Serampore, Hoogly 712 201, India ‡

S Supporting Information *

ABSTRACT: The synthesis, structures, and magnetic properties of two heterometallic CuII−DyIII clusters are reported. The first structural motif displays a pentanuclear CuII4DyIII core, while the second one reveals a nonanuclear CuII6DyIII3 core. We employed o-vanillin-based Schiff base ligands combining ovanillin with 3-amino-1-propanol, H2vap, (2-[(3-hydroxypropylimino)-methyl]-6-methoxy-phenol), and 2-aminoethanol, H2vae, (2-[(3-hydroxy-ethylimino)-methyl]-6-methoxyphenol). The differing nuclearities of the two clusters stem from the choice of imino alcohol arm in the Schiff bases, H2vap and H2vae. This work is aimed at broadening the diversity of CuII−DyIII clusters and to perceive the consequence of changing the length of the alcohol arm on the nuclearity of the cluster, providing valuable insight into promising future synthetic directions. The underlying topological entity of the pentanuclear Cu4Dy cluster is reported for the first time. The investigation of magnetic behaviors of 1 and 2 below 2 K reveals slow magnetic relaxation with a significant influence coming from the variation of the alcohol arm affecting the nature of magnetic interactions. that DyIII, with a high ground state spin value and strong spin− orbit coupling imparting pronounced Ising-type magnetic anisotropy, is responsible for remarkable magnetic behavior.5,6 Moreover, the combination of 3d and 4f transition-metal ions may increase the ground spin state through d−f magnetic interactions. Again, a subarea of 3d−4f cluster chemistry using CuII has attracted special interest.7 Therefore, given the prerequisite of SMMs, a sensible starting point is the synthesis of CuII−DyIII clusters. Until now, many types of heterometallic CuII−DyIII clusters have been structurally and magnetically characterized, most of which possess fascinating structural motifs and intriguing magnetic properties.8−30 In the literature there are several examples of CuII−DyIII heterometallic complexes that display slow relaxation of magnetization,8e,12c,16b,21,23a,30 which is usually regarded as an indication of possible SMM behavior along with a couple of complexes behaving as SMM.8b,9a,b,10,11b,12a,b,d,e,13,15,17b,19−21,23b,24−26,29 Again, gaining control over the nuclearity of 3d−4f clusters is always challenging. Considering the CuII−DyIII subarea only,

1. INTRODUCTION The design and synthesis of high-nuclearity 3d−4f clusters have recently seen a sweeping growth in the arena of coordination chemistry and magnetochemistry, as evident by a constantly increasing number of publications in this area.1 The quest for molecular magnetic materials possessing potential applications in information storage, spin-dependent electronics, and quantum computing is one of the most challenging topics in chemistry, stimulating new ideas for the development of novel materials.2 The catalytic development in this direction in the last two decades has been led by single-molecule magnets (SMM).3 SMMs are capable of showing slow magnetic relaxation of molecular origin as a result of an energy barrier to spin reversal of a magnitude sufficient to observe hysteresis at a workable temperaturea crucial prerequisite for the use of SMMs in molecular devices. Key ingredients to build SMM are large spin multiplicity and maximizing the anisotropy of the system, which in turn is extremely sensitive to small changes in the structure of complexes.4 To satisfy these conditions, one efficient strategy involves the use of lanthanide ions, because of their large anisotropy resulting from the unquenched orbital momentum and crystal field effects. Recent works have shown © XXXX American Chemical Society

Received: September 11, 2015

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DOI: 10.1021/acs.inorgchem.5b02107 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Route of 1 and 2

there are many examples of CuII−DyIII clusters reported in the literature with large diversity of structural types such as CuDy,8 CuDy2,9 CuDy4,10 Cu2Dy,11 Cu2Dy2,12 Cu2Dy7,13 Cu3Dy2,14 Cu 3 Dy 3 , 15 Cu 4 Dy, 16 Cu 4 Dy 4 , 17 Cu 4 Dy 12 , 13 Cu 5 Dy 2 , 18 Cu 5 Dy 4 , 19 Cu 6 Dy 2 , 20 Cu 6 Dy 3 , 21 Cu 6 Dy 6 , 22 Cu 8 Dy 2 , 23 Cu 8 Dy 3 , 24 Cu 8 Dy 4 , 25 Cu 8 Dy 9 , 26 Cu 12 Dy 8 , 27 Cu 22 Dy 6 , 28 Cu24Dy8,29 and Cu36Dy24.30 However, the ligand design rationale to fine tune the cluster nuclearity is yet to get a general shape, and therefore, it is still an object of intensive research. In this regard, CuII−DyIII clusters derived from ligand systems that differ only in a spacer, represent considerable potential to reveal the underlying design rationale. Bearing these points in mind, this paper is devoted essentially to the investigation of outlining the influence of the alcohol part (ethanol/propanol) of a Schiff base ligand on the nuclearity of the CuII−DyIII cluster and the detailed magnetic properties of these compounds. Hence, we have chosen two Schiff bases, 2-[(3-hydroxy-propylimino)-methyl]-6-methoxyphenol (H2vap) and 2-[(3-hydroxy-ethylimino)-methyl]-6methoxy-phenol (H2vae) bearing propanol and ethanol arms, respectively, to generate the clusters [Cu II 4 Dy II (vap) 2 (Hvap) 2 (μ 3 -OH) 2 Cl 2 ]Cl·4H 2 O (1) and [CuII6DyIII3(vae)6(μ3-OH)6(H2O)5(MeOH)]NO3·Br2·4H2O· MeOH (2) (Scheme 1). The present study reports the syntheses and characterization of 1 and 2 as well as their structural features and magnetic properties. 1 and 2 will enrich the CuII−DyIII database and thus improve the current knowledge of the structure−property relationship in this field. The novelty of the current work with respect to the previous studies lies in the fact that the work shows how a slight modification of the length of the alcohol arm plays a key role in defining cluster nuclearity.

2. EXPERIMENTAL SECTION 2.1. Chemicals and General Procedure. Solvents and other general reagents used in this work were purified according to standard procedures.31 The starting materials for the synthesis of the ligands, viz., o-vanillin, 3-amino-1-propanol, and 2-aminoethanol (SigmaAldrich, U.S.A), were of reagent grade and used as received. Triethylamine (TEA) and CuCl2·2H2O were obtained from E. Merck (India) Ltd., Mumbai, and were used as such. CuBr2 and Dy(NO3)3·xH2O were obtained from Aldrich Chemical Co. and used as such. All manipulations were performed under aerobic conditions using materials as received unless otherwise indicated. The tetradentate ligands 2-[(3-Hydroxy-propylimino)-methyl]-6-methoxyphenol (H2vap) and 2-[(3-hydroxy-ethylimino)-methyl]-6-methoxyphenol (H2vae) were synthesized using simple 1:1 Schiff base condensation of o-vanillin, 3-amino-1-propanol, and 2-aminoethanol as previously described32 and directly used for the preparation of the complexes. Powder X-ray diffraction (PXRD) patterns were acquired using a PANalytical, XPERT-PRO diffractometer (Netherlands) operated at 40 kV, 30 mA, with graphite-monochromatized Mo Kα radiation of wavelength = 0.71073 Å and a nickel filter. An energydispersive X-ray (EDX) analysis was done by field emission scanning electron microscopy (FESEM-JEOL JSM 7600F). Elemental analysis for carbon, hydrogen, and nitrogen was carried out on a Perkin-Elmer 2400 II analyzer. Fourier transform infrared (FTIR) spectra were recorded with a Perkin-Elmer RXI FTIR spectrophotometer (4000− 400 cm−1). Samples were prepared as KBr disks. ESI-MS experiments were carried out in Waters Xevo G2-SQTof instruments in LC-MS grade solvent. Magnetic measurements were performed in the temperature range of 2−300 K using Quantum Design MPMS-7XL SQUID. The diamagnetic corrections for the compounds were estimated using Pascal’s constants, and magnetic data were corrected for diamagnetic contributions of the sample holder. 2.2. Synthesis of [Cu4Dy(vap)2(Hvap)2(OH)2Cl2]Cl·4H2O (1). H2vap (0.339 g, 1.628 mmol) was dissolved in methanol (20 mL). Dy(NO3)3·xH2O (0.814 mmol) and triethylamine (0.7 mL, 5 mmol) were added to this solution. The reaction mixture stirred for 1 h. At this stage, CuCl2·2H2O (0.259 g, 1.623 mmol) was added, and the B

DOI: 10.1021/acs.inorgchem.5b02107 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry reaction mixture was stirred for a further period of 4 h at 25 °C to afford a clear solution. After obtaining the deep green-colored solution, the solvent was stripped in vacuuo. The resulting green solid was washed with diethyl ether and dried, affording a pure sample of 1. Xray diffraction-quality block-shaped dark green crystals were isolated in ∼40−48% yields after ∼4 days by slow diffusion of diethyl ether into a methanolic solution of the complex. FT-IR (KBr), cm−1: 3434 (b), 2928 (m), 2843 (m), 1631 (s), 1605 (m), 1471 (s), 1454 (m), 1303 (m), 1242 (m), 1224 (s), 1084 (m), 746 (m). Ana. Calcd for C44H64N4O18Cl3Cu4Dy (1460.04 g mol−1): C, 36.20; H, 4.42; N, 3.84. Found: C, 35.85; H, 4.12; N, 3.57. 2.3. Synthesis of [Cu6Dy3(vae)6(OH)6(H2O)5(MeOH)]NO3·Br2· 4H2O·MeOH (2). The similar reaction protocol was applied for the synthesis of compound 2, except using H2vae (0.316 g, 1.628 mmol) and CuBr2 (0.362 g, 1.623 mmol) in place of H2vap and CuCl2·2H2O respectively. After stirring for ∼0.5 h from the addition of CuBr2, a bluish green solid appeared and the stirring was further continued for ∼3 h and then filtered. The resulting precipitate was washed with cold methanol and dried under reduced pressure. Crystals suitable for X-ray studies were obtained by slow diffusion of diethyl ether into the MeCN:MeOH (1:1) solution within 1 week in ∼30−35% yields. FTIR (KBr), cm−1: 3391 (b), 2914 (m), 2854 (m), 1643 (s), 1602 (m), 1471 (s), 1445 (s), 1384 (s), 1308 (m), 1242 (m), 1217 (s), 1064 (m), 971 (m), 739 (m). Anal. Calcd for C62H98N7O38Br2Cu6Dy3 (2578.05 g mol−1): C, 28.88; H, 3.83; N, 3.80. Found: C, 28.44; H, 3.51; N, 3.42. 2.4. X-ray Structural Studies. Crystallographic data for 1 and 2 were collected on a Bruker Nonius Apex II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 296 K. The program SMART33a was used for collecting frames of data, indexing reflection, and determining lattice parameters, SAINT33a for integration of the intensity of reflections and scaling, SADABS33b for absorption correction, and SHELXTL33c,d for space group, structure determination, and least-squares refinements on F2. Both structures were solved by direct methods and refined by full-matrix least-squares calculations using SHELXTL software.33e All non-hydrogen atoms were refined in the anisotropic approximation against F2 of all reflections. The location of the Ln atom was easily determined, and O, N, and C atoms were subsequently determined from the difference Fourier maps. Most of the H atoms were located at their calculated position and refined isotropically with fixed geometry with respect to their carrier atoms. All hydrogen atoms of the bridging μ3-OH and coordinated and noncoordinated water (OH2) molecules were included from a difference Fourier map and refined isotropically. The figures have been generated using Diamond 3.0 software.34 Crystal parameters of clusters are shown in Table S1 (Supporting Information); CCDC-1403374 and CCDC-1403375 contains supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 2.5. Magnetic Measurement (Experimental) Details. Magnetic susceptibility measurements were performed on polycrystalline samples on a Quantum Design MPMS-7XL SQUID magnetometer in applied magnetic fields of 1000 Oe in the T range of 2−300 K. The experimental dc susceptibility data were corrected for the diamagnetism of the sample holder, and the intrinsic diamagnetism of the samples was evaluated using Pascal’s tables. Alternating current measurements were performed in a 3 Oe oscillating magnetic field with and without a static dc field.

neamino-1-propanol.35 In 2007, the same group reported ONO donor Schiff base ligands with variation of ethanol and propanol arms to demonstrate its effect on nuclearity.36 In this context, it is worth noting that Chattopadhyay and coworkers utilized the same set of ligands and obtained a dinuclear CuII2 complex from the ligand having a propanol arm, whereas the ligand containing an ethanol arm afforded a tetranuclear open cubane CuII4 moiety.37 They demonstrated how the incorporation a −CH2− spacer in the ligand framework changed the geometry and magnetic properties of the compounds. Very recently, Murray and co-workers demonstrated that variation of metal chloride/nitrate salts and variation of carboxylate coligand resulted in CuII2DyIII7 and CuII4DyIII12.13 Intrigued by the rich coordination chemistry of o-vanillin Schiff bases,38 we undertook studies of the CuII−DyIII clusters with the o-vanillin-derived Schiff bases H2vap and H2vae having a propanol/ethanol arm to resolve the contribution of alcohol arms. We anticipated that the alcohol arm variation of the ligands would be reflected in the resulting cluster assembly and that this could be exploited for tracing structure control criteria. Each diprotic ligand H2vap/vae can be described as possessing two coordination pockets (I and II) as depicted in Scheme 2. Scheme 2. Structure of the Ligand H2vap and H2vae Consisting of Two Distinct Coordination Pockets

The first one provides ONO (a phenolic O from the o-vanillin part of the ligand, an imine N, and an alkoxo O), and the second provides two oxygen donors from the phenol O atom and an O from the −OMe group of the o-vanillin part of the ligand. The phenolic O atom can also bridge between metal ions in the two pockets. The use of H2vap/vae ligands has already been encountered in the literature utilizing the coordination modes discussed above, and a brief overview of its complexes of CuII, LnIII, and 3d−4f is provided in Scheme S1.32a,37,39 Following the strategy described above, we were able to synthesize a pentanuclear CuII4DyIII and a nonanuclear CuII6DyIII3 core starting from H2vap and H2vae, respectively. Initially, both reactions were carried out with copper(II) chloride, but good X-ray-quality crystals were not obtained for 2. Consequently, the synthetic condition was adjusted by changing the associated counteranion. Therefore, copper(II) bromide salt was used to get better diffraction-quality crystals for compound 2.40 The change in CH2 spacer in the ligand framework made a difference in the reaction stoichiometry in methanol as shown in eqs 1 and 2.

3. RESULTS AND DISCUSSION 3.1. Design Rationale and Synthesis. In an attempt to address the issue of nuclearity change, we searched the literature regarding the factors that are responsible for variation of nuclearity of the clusters. In this process, we noted that Oshio and co-workers have long been utilizing ONO donor Schiff base ligands with slight modifications in the framework to assemble heterometallic clusters. In 2005, they reported a dinuclear MnIII−CuII complex based on 5-bromo-2-salicylideC

DOI: 10.1021/acs.inorgchem.5b02107 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 4CuCl 2 + Dy(NO3)3 + 4H 2vap + 8NEt3 + H 2O = [Cu4Dy(vap)2 (Hvap)2 (OH)2 Cl 2]Cl + 8Et3NH+ + 5Cl− + 3NO3−

(1)

6CuBr2 + 3Dy(NO3)3 + 6H 2vae + 18NEt3 + 11H 2O + CH3OH = [Cu6Dy3(vae)6 (OH)6 (H 2O)5 (CH3OH)]2Br .NO3 + 18Et 3NH+ + 10Br − + 8NO3−

(2)

Figure 1. Molecular single-crystal X-ray structure of 1. All hydrogen atoms, counteranion, and solvent molecules were omitted for clarity. Symmetry transformation for equivalent atoms (#) 1 − x, 2 − y, z.

The experimental powder XRD patterns of the bulk crystalline material of 1 and 2 are in good agreement with the simulated XRD patterns of single-crystal X-ray diffraction, confirming purity of the bulk samples (Figures S1 and S2,Supporting Information). Furthermore, in order to investigate the purity of the bulk crystalline material, the energydispersive X-ray (EDX) analysis of 1 and 2 confirm the homogeneity of our crystals and also shows the presence of both Cu and Dy ions in complexes with a Cu:Dy ratio 82.14:17.8 (for 1) and 70.81:29.19 (for 2) corroborated with the results obtained from single-crystal X-ray diffraction (shown in the Supporting Information, Figures S3 and S4). The IR spectra of 1 and 2 are very similar and display a characteristic band of the coordinated ligand (Figure S5, Supporting Information). The broad IR absorption in the region 3391− 3430 cm−1 is attributed to the O−H stretching frequency of H2O/MeOH molecules. A diagnostic sharp absorption at ∼1640 cm−1 is due to the CN stretching of the Schiff base ligand in the complexes. In compound 2 a sharp peak at ∼1380 cm−1 provides an indication of noncoordinated nitrate ion. Besides the solid state characterization of compounds 1 and 2, we made an attempt to explore the principal species present in solution (MeOH/acetonitrile) by virtue of ESI-MS(positive mode) study. For complex 2 the prominent molecular ion peak appeared at 1077.933 amu corresponding to [Cu6Dy3(vae)6(OH)6 + H−]2+(C60H79N6O25Cu6Dy3), which inturn proves that molecular integrity is retained in acetonitrile. Again, a fragmented peak at 949.912 amu corresponds to [Cu5Dy3(vae)5(OH)6]2+, which matches well with the simulated pattern (Figure S6). We did not detect any [Cu4Dy(vap)2(Hvap)2(OH)2Cl2]+ species for complex 1 probably due to the instability of the species in the ESI-MS time scale; instead, we got numerous fragmented peaks. The peak at 541.049 amu matches probably with the species [Cu 3 Dy(vap) 2 (Hvap)(OH) 2 Cl 2 + H + ] 2+ (C 33 H 43 Cl 2 Cu 3 DyN 3 O 11 ). The peak appearing at 813.104 amu can be assigned to the species [Cu2Dy(Hvap)(vap)(OH)(H 2 O)Cl 2 ] + (C 22 H 30 N 2 O 8 Cl 2 Cu 2 Dy 1 ), which matches well with the simulated mass pattern also (Figure S7, Supporting Information). 3.2. Structural Studies. Compound 1 crystallizes in tetragonal P4̅21c space group with the asymmetric unit containing one-half of the cluster. The crystal structure of 1 consists of a cationic entity [Cu4 Dy(vap) 2(Hvap) 2(μ3OH)2Cl2]+ (Figure 1), uncoordinated chloride for charge balance, and four water molecules of crystallization. In this pentanuclear aggregate, the metal centers are linked together by means of two μ3-OH groups. A peripheral ligating environment was provided by two η1:η2:η1:η1:μ2 Hvap− ligands, two η1:η1:η2:μ2 vap2− ligands (Figure 2), and two μ2-chloro. One phenoxo and an alkoxo group bridge two CuII centers to the

Figure 2. Coordination and bridging modes of ligands in complexes 1 and 2.

central DyIII ion. Additionally, the two CuII centers are joined by means of μ2-Cl bridges. As a result, the overall vacant cubanes are joined by a DyIII corner. The phenoxo-bridged CuDyO2 core is puckered with a dihedral angle of 18.66°. The Cu2OCl core is also distorted with a dihedral angle of 14.47°. This type of vacant cubanes where CuII centers are joined by μ2-Cl bridges are encountered in a portion of the decanuclear DyII2CuII8 cluster reported by Luneau et al.23b One salient feature of 1 is that although each CuII center has a slightly distorted square pyramidal geometry [Addison parameter (τ) = 0.255 and 0.218 for Cu1 and Cu2, respectively],41 they reside in different coordination environments. For Cu1, the basal plane is made up of one μ3-OH group, an μ2-Oalk coming from the propanol arm of the ligand, Nimine, and μ1-Ophen. For Cu2, the basal plane constitutes one μ3-OH group, μ1-Oalk of the propanol arm, Nimine, and μ2-Ophen. Cu−O distances range between 1.924 and 2.038 Å. The apical Cu−Cl distance is slightly longer than the related clusters (Cu1−Cl1 = 2.728 Å, Cu2−Cl1 = 2.509 Å).23b These two distorted square pyramids (SP) share a base-to-apex edge through a common vertex with a dihedral angle of 62.76° between the two basal planes, as depicted in Figure 3a. The DyIII center is eight coordinate

Figure 3. (a) View showing the arrangement and base-to-apex edge sharing of distorted square pyramidal geometry around CuII centers polyhedra in the cluster. (b) Distorted triangular dodecahedron geometry around the Dy center of 1. D

DOI: 10.1021/acs.inorgchem.5b02107 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry composed of Omethoxy, Ophen, μ3-OH, and μ2-Oalk coming from four ligands. Exact geometry analysis by SHAPE 2.1 software shows that the inner coordination sphere of eight-coordinated DyIII ions are residing in distorted triangular dodecahedron geometry with a significant deviation of 2.571 (out of the range 0.1−3, CShM value) from the ideal D2d symmetry (Figure 3b; Table S2, Supporting Information).42 Dy−O bond lengths range between 2.247 and 2.552 Å. Within the vacant cubane, the Cu−Cu distance is 3.203 Å and average Cu−Dy distance is 3.415 Å. Some selected interatomic distances and angles of the [CuII4DyIII(μ2-Cl)(μ2-Ophe)(μ2-Oalk)6(μ3-OH)2]+ core are tabulated in Table S3, Supporting Information. The supramolecular arrangement also reveals that the shortest intermolecular Dy··· Dy distance is 10.641 Å along the crystallographic c axis shown in the Supporting Information, Figure S8. The crystals of 2 belong to a triclinic system with space group P1̅. Inspection of the crystal structure of 2 unveils the cationic [CuII6DyIII3(vae)6(μ3-OH)6(H2O)5(MeOH)]3+ entity (Figure 4), one uncoordinated nitrate, and two bromide ions

Figure 5. Distorted SAP geometry around DyIII centers and mutual base-to-apex edge sharing two CuII centers in 2.

neighboring atoms (din) along with the dihedral angle between the two mean plane (θ), and magic angles (α), defined by the neighboring ligand donor sites (Supporting Information, Figure S9). For an ideal square antiprism geometry (D4d symmetry) the dpp and din almost should be equal and Ø must be close to 45°.45a,b Also, θ should be close to zero so that the two mean planes will be parallel. For a soft-sphere model with a repulsion energy law ≈ 1/r6, the ideal magic angle (α) which is defined as the angle between the S8 axis of the square antiprism and the central atom ligand bond or as α = γ/2 (γ being the angle between opposite bonds within one hemisphere) amounts to 57.16°.45c,d More obtuse and acute angles α correspond to compression and elongation of the polyhedral geometry, respectively.45e,f The data in the Supporting Information, Tables S4 and S5, supports the distorted square antiprism geometry around the DyIII coordination sphere. All six CuII centers are in a NO4 coordination site, connected in a slightly distorted square pyramidal environment [Addison parameter (τ) = 0.081, 0.185, 0. 154, 0.152, 0.145, and 0.122, respectively, for Cu1−6].41 In each cube, two distorted square pyramids (SP) share a base-to-apex edge with a dihedral angle in the range 22−26.22° between the two basal planes. These structural features of 2 are reminiscent of a [CuII6DyIII3] compound reported by Luneau and co-workers.21 The basic difference between these two compounds lies in the fact that they have utilized a different ligand bearing the ONO donor sites containing the ethanol arm. Another structural feature of 2 that deviates from the reported one is that in our case all the six CuII centers are in pentacoordinated distorted square pyramidal geometry whereas in the reported one four CuII centers have a weakly bound water ligand in the sixth position. Additionally, there are intermolecular hydrogen bonding contacts mediated by Br− counteranions with C(28)−H···Br and C(18)−H···Br distances of 3.88 and 3.859 Å along the crystallographic c axis (see the Supporting Information, Figure S10). All bond length and bond angle parameters are comparable to the reported compound; therefore, only selected interatomic ditances and angles of the [Cu6IIDy3III(μ3-OH)6(μ3-Oalk)6]9+ core are mentioned in the Supporting Information, Table S6. It noteworthy that the same

Figure 4. Molecular single-crystal X-ray structure of 2. All hydrogen atoms, counteranions, and solvent molecules were omitted for clarity.

for charge balance and four water molecules and one methanol for crystallization. All the six vae2− ligands display the same η1:η1:η3:μ3 chelating/bridging mode (Figure 2). A cursory glance at the structure reveals that the [CuII6DyIII3] core is composed of three CuII2DyIII2O4 distorted cubanes. Each CuII2DyIII2O4 cubane is formed by means of two μ3-OH and two μ3-Oalk groups. These three cubanes share their DyIII vertices to generate a triangular topology. Alternatively, it can be said that Dy3 triangle is capped by three CuII2DyIII2O4 cubanes. It is interesting to note here that Tang and co-workers reported a couple of Dy3 triangles close to the Dy3(OH)6 triangle described here.43 The reaction of Dy(ClO4)3·6H2O and H2vae type of ligand leads to the formation of a triangular Dy3 compound bridged by Oalk and μ3-OH.44 Figure 5 depicts the three DyIII ions are in an all oxygen environment. Systematic analysis of the geometries using SHAPE 2.1 reveals that the individual DyIII are best described as square antiprism, with a distortion of 0.688 (Dy1), 0.768 (Dy2) and 0.597 (Dy3) from ideal D4d geometry (Figure 5; Table S2 in the Supporting Information).42 For reinforcement of the polyhedral geometrical determination we also calculated the skew angle (Ø), interplanar distance (dpp), average distance between four E

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Inorganic Chemistry authors reported a related structure in 2007,46 containing CuII3YbIII6 and CuII3GdIII6 cores bearing the same structural motif. They chose the ligand 1,1,1-trifluoro-7-hydroxy-4methyl-5- aza-hept-4-en-2-one containing an ethanol arm. 3.3. Topological Analysis. To get more insight regarding the structures of the CuII−DyIII clusters, we carried out its topological analysis following the concept of the high nuclearity coordination clusters developed by Kostakis et al.47 The topological analysis of 1 reveals a 2,4M5-1 motif composed of two triangles fused at a central Dy node, Figure 6. From the topological viewpoint, 2 can be considered as a 3,6M9-1 motif.

This type of structural motif is also encountered in [ NiII6LnIII3(μ3-HL)6(μ3-OH)6(NO3)3] [H3L = 2-(β-naphthalideneamino)-2-hydroxymethyl-1-propanol; Ln = Gd, Dy and Er] clusters as well.49 Scrutiny reveals that the difference between these two structures is the presence of μ3-Oalk, thus reinforcing the utility of the spacer variation. It seems that ONO donor sites along with the ethanol arm are a prerequisite for designing Cu2Dy2 cubanes. Moreover, this change of coordination mode also induces an adjustment of the coordination sphere around DyIII centers, D2d symmetry for complex 1 and D4d for complex 2. Thus, a significant degree of structural control could be achieved by tuning the alcohol arm of the ligand. It is noteworthy that the methoxy oxygen of the Schiff base ligand chelates DyIII in 1 but remains uncoordinated in 2. 3.5. Magnetic Studies. Magnetic properties of 1 and 2 were investigated in static and oscillating magnetic fields. For 1 (Figure 8), by decreasing the temperature, the susceptibility

Figure 6. Topological motifs of (a) 1 and (b) 2.

Noticeably, survey of the database reveals that the 2,4M5-1 motif of 1 could be encountered in another Cu−Dy cluster but the composition of the cluster is reverse, i.e., Dy4Cu.10,48 Therefore, the topology of 1 was reported for the first time, and 2 is only the second example of a 3,6M9-1 motif containing a CuII6DyIII3 core.21 3.4. Structural Comparison between 1 and 2. As already analyzed in section 3.1, the ligands play a key role in dictating the resulting structures. Therefore, it will be of interest to compare the structures of 1 and 2 as a function of alcohol arm of the ligands. In 1, the ligands are arranged like blades of a propeller, Figure 7a, restricting the propanol arm to participate

Figure 8. χmT vs T and magnetization at 1.82 K(inset) curves for 1.

slowly decreases from 17.678 cm3 K mol−1 at 300 K to 17.420 cm3 K mol−1 at 70 K. This decrease can be attributed to the depopulation of the mJ level or, more probably, to a thermally independent paramagnetism.50 The susceptibility value at room temperature is slightly lower than the expected value of 18.17 cm3 K mol−1 for this system in the free ions model. Below 70 K, susceptibility increases up to 19.675 cm3 K mol−1 at 6.75 K, which is a clear sign of ferromagnetic interaction, before decreasing to 18.119 cm3 K mol−1 at 2 K. This last decrease can be antiferromagnetic interaction and/or an acceleration of the depopulation of the mJ levels of DyIII ion. Unfortunately, due to the depopulation of dysprosium, we are not able to perform calculation in order to investigate further interaction in this complex. Magnetization of the sample at 1.82 K continuously increases with field, with a pseudo-saturation from 1 T (inset Figure 8). The value of 7.89 Nβ obtained at 5 T is lower than the expected value of 18 Nβ for one dysprosium and four copper noninteracting. No opening of hysteresis was observed. This complex shows a frequency dependence of the susceptibility in the range 1.85−3 K, without external dc field but without maximum of the out-of-phase susceptibility in the frequency range accessible by our equipment (Figure S12). By applying an external magnetic field (Figure S13), the relaxation time can be shifted below 1000 Hz. An additional relaxation process appears below 1 Hz. The optimal field for both

Figure 7. Metallic core of 1 and 2 described as (a) vertex-sharing [CuII2DyIII(μ2-vap) (Hvap)(μ3-OH)] incomplete cubanes and (b) corner-sharing [CuII2DyIII2(μ3-vae)2(μ3-OH)2] cubanes.

in μ3-bridging. In 1, only the OH− group participates in μ3bridging but the propanol arm is able to form only μ2-bridging instead of μ3-bridging. As a result, closed cubane formation never happened. In 2, there are three sets of ligands, each consisting of two ligands. In each set, the two ligands are oriented in a face-to-face manner, allowing the ethanol arm/ Oalk to participate in μ3-bridging. Therefore, each distorted cube consists of two CuII, two DyIII, two μ3-Oalk, and two μ3OH, i.e., an alkoxo bridging mode is necessary for forming corner-shared three cubane structures, Figure 7b. F

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Inorganic Chemistry relaxation processes was estimated to be 2000 Oe. Frequency dependence of the susceptibility in the range 1.85−3.2 K (Figure S14), with 2000 Oe external dc field being measured and fitted using a generalized debye model (see eqs 1 and 2 in the Supporting Information; Table S7).51 The faster relaxation process was fitted using an Orbach process with Δ/kB = 12.876 K and τ0 = 2.268 × 10−7 s. The slowest relaxation process was fitted using a direct process with a slope of 0.99945 (Figure S15; eqs 6 and 7 in the Supporting Information).52 Magnetic properties of previously reported Cu4Dy cluster have not been measured, or the coordination mode and sphere are not comparable to complex 1, making the comparison difficult. In the case of 2 (Figure 9), by decreasing the temperature, the susceptibility slowly decreases from 40.81 cm3 K mol−1 at

Figure 10. Out-of-phase susceptibility at 0 Oe.

Figure 11. Relaxation time at 0 Oe for 2.

Figure 9. χmT vs T and magnetization at 1.82 K(inset) curves for 2.

faster relaxation process, this mechanism occurs at too low temperature to be observed. As a consequence, applying an external field does not affect the relaxation time (Figure S17). Although the topological structures of these two complexes are similar, the magnetic properties, with static or oscillating fields, are significantly different. In the case of the slow relaxation process, if we consider that the dysprosium ion is the main origin for this relaxation, the difference can be explained by the change of the coordination mode of ions, which affects the crystal field and modifies the energy splitting of the mJ levels of dysprosium. In the case of the susceptibility data, it is difficult to conclude the reasons of these differences. In order to understand why the magnetic susceptibilities of these two complexes are different and to determine the origin of the slow relaxation, further experiments will be performed. These investigations will include substitution of 3d or 4f ions by nonanisotropic or diamagnetic ions to analyze magnetic interactions and lanthanide’s mJ sublevel depopulation. We should also prepare a doped analog of these complexes to determine the role played by dipole−dipole interaction. Finally, a computational study will be done to get a better understanding of the mechanism involved in the slow relaxation of these compounds. All these complementary studies will be summarized in a forthcoming publication.

300 K to 7.015 cm3 K mol−1 at 2 K, with a faster decrease from around 50 K. This decrease can be attributed to antiferromagnetic interaction between ions and/or depopulation of mJ levels of dysprosium ions. As for the other complex, the susceptibility at room temperature is lower than 48.51 cm3 K mol−1, as expected for this system in the free ions model. The magnetization curve at 1.82 K show a “two-step” magnetization process, with an acceleration at 1 T and a pseudo saturation from 3 T. The value of 16.28 Nβ obtained at 5 T is significantly lower than the expected value of 42 Nβ for three dysprosium and six copper noninteracting. This two-step magnetization process can be assigned to the presence of antiferromagnetic interaction, with a partial spin flop at 1 T. No opening of hysteresis was observed. This complex shows frequency dependence of the susceptibility up to 4 K, without external dc field (Figure 10; Figure S16 in the Supporting Information). Data were fitted (Table S9) using for 2 a generalized debye model with two relaxation times (eqs 1 and 2 in the Supporting Information) below 3.2 K and one relaxation time over 3.2 K (eqs 3 and 4 in the Supporting Information; Figure 11). The slowest relaxation process was fitted using an Orbach process with Δ/kB = 23.657 K and τ0 = 1.084 × 10−7 s. This is in total agreement with previous work with the same 3,6M9-1 topology.21 The faster relaxation process was fitted using a combination of QTM and Orbach process (eq 5 in the Supporting Information) with Δ/ kB = 25.293 K, τ0 = 3.939 × 10−9 s, and QTM = 5.616 × 10−3 s. Although quantum relaxation was needed to correctly fit the

4. CONCLUSION In the current report, we utilized two Schiff bases that differ in the length of alcohol arms for the generation of two new CuII− DyIII clusters, which enables us to extract a few key insights. G

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Inorganic Chemistry

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The X-ray crystal structures of 1 and 2 reveal that 1 is a pentanuclear CuII4DyIII cluster where the ligand bears a propanol arm. The presence of an ethanol arm in the ligand produces a nonanuclear CuII6DyIII3 cluster. Therefore, it is apparent that the coordination environment of the clusters could be significantly tuned by varying the length of the alcohol arm of the ligand, which plays a crucial structure-driven role in defining the nuclearity and topology of the resulting clusters providing a strategy to rationally design a synthetic mode. Moreover, apart from presenting notable structural and topological features, both 1 and 2 exhibit slow relaxation of magnetization. The foregoing results demonstrate that the length of the alcohol arm appears to be able to modify the susceptibility and slow relaxation behavior by changing the coordination mode of ions in the cluster. This change of coordination mode modifies the interaction between ions, with a ferromagnetic coupling for the pentanuclear cluster and an antiferromagnetic interaction for the nonanuclear complex. The change of coordination mode also induces an adjustment of the coordination sphere of DyIII ions, D2d symmetry for complex 1 and D4d for complex 2, manifested by a slower relaxation of the magnetization for 2 compared to 1. Nevertheless, it is worth noting that further exploration with various other spacer arms toward the design and synthesis of novel CuII−DyIII clusters are currently in progress.

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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.5b02107. Ligand structure, simulated and experimental PXRD, EDX spectrum, FT-IR, HR-ESI-MS spectrum, crystal packing diagram, continuous shape measurement results, metric parameter, additional magnetic data including the equation used in fitting parameter and fitting data (PDF) Crystallographic data (CCDC 1403374−1403375)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M. and Y.S. acknowledge UGC, India, for a RFSMS fellowship (Sanction no. UGC/740/RFSMS) and DST India (Sanction no. (SR/FT/CS-107/2011), respectively, for financial support. DST-FIST and DST-Purse are gratefully acknowledged for providing the X-ray diffraction and HRMS facility at the Department of Chemistry, University of Calcutta.

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REFERENCES

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