Chair-Shaped MnII2LnIII4 (Ln = Gd, Tb, Dy, Ho) Heterometallic

Publication Date (Web): January 6, 2015. Copyright © 2015 American ... Escuer , and George E. Kostakis. Crystal Growth & Design 2017 17 (4), 1524-153...
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Chair-Shaped MnII2LnIII4 (Ln = Gd, Tb, Dy, Ho) Heterometallic Complexes Assembled from a Tricompartmental Aminobenzohydrazide Ligand Amit Chakraborty,† Prasenjit Bag,† Joydeb Goura,† Arun Kumar Bar,‡,§ Jean-Pascal Sutter,*,‡,§ and Vadapalli Chandrasekhar*,†,∥ †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, F-31077 Toulouse, France § Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France ∥ National Institute of Science Education and Research, Institute of Physics Campus, Sachivalaya Marg, Sainik School Road, Bhubaneshwar 751005, Odisha, India ‡

S Supporting Information *

ABSTRACT: The reaction of a new compartmental ligand, H3L [H3L = N′-(2-hydroxy-3-methoxybenzylidene)-2-(-2hydroxy-3-methoxybenzylideneamino)benzohydrazide], with lanthanide salts followed by reaction of MnII salts in presence of triethylamine along with pivalic acid afforded heterometallic hexanuclear complexes, [{LMn(Cl)(μ3-OH)Gd2(μ3-OH)(μPiv)2}2·3MeCN·7MeOH·H2O] (1), [{LMn(Cl)(μ3-OH)Tb2(μ3-OH)(μ-Piv)2}2·8MeCN] (2), [{LMn(Cl)(μ3-OH)Dy 2 (μ 3 -OH)(μ-Piv) 2 } 2 ·2Et 2 O·4MeCN·3H 2 O] (3), and [{LMn(Cl)(μ3-OH)Ho2(μ3-OH)(μ-Piv)2}2·8MeCN] (4). All of the complexes are isostructural, in which two trianionic ligands [L3−] hold the six metal ions, affording a chair-shaped hexametallic core. The asymmetric unit contains half of the molecule, where Mn(II) ions occupy the inner coordination pocket of the ligand in a distorted square pyramidal geometry (NO3Cl) and the two lanthanide ions occupy the two outer-coordination pockets. Magnetic measurements reveal that antiferromagnetic interactions take place in complexes 1, 3, and 4. On the other hand, complex 2 shows the presence of ferromagnetic interactions.

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INTRODUCTION

based ligands. We were also intrigued by the sparse use of hydrazide ligands for the preparation of 3d−4f complexes.43,44 Our strategy was to design a tricompartmental ligand where the central compartment would preferentially bind to a transition metal ion while the two peripheral compartments would bind to lanthanide ions. We reasoned that appropriate bridging ligands will allow two such trimeric subunits to be attached to each other, forming a hexanuclear ensemble. Accordingly, a new compartmental ligand, H3L, was designed and synthesized. This ligand, apart from possessing three different coordination pockets, also has additional features, such as being flexible and possessing nonsymmetrical arms (Schemes 1−3). Thus, H3L offers a versatile coordination platform to bind with different metal ions to afford polynuclear heterometallic complexes. Apart from the synthesis and characterization of H3L, in this article we report synthesis, structural characterization, and

Polynuclear metal complexes, particularly 3d−4f heterometallic assemblies, are gaining in importance not only from the points of view of their synthesis and structure but also because of their potential applications as molecular materials, such as molecular magnets.1−16 The synthetic challenges in the assembly of these complexes lie in the ability to modulate their nuclearity and the topology.17−20 Generally, compartmental ligands possessing specific coordination sites with the capability of selective binding pockets are the preferred choice for the preparation of heterometallic complexes.1−10,21−34 A variety of ligands have been used in the literature for this purpose. In this regard, our group has reported phosphorus-supported ligands such as SP[N(Me)NCH-C6H3-2-OH-3-OMe]335−39 and [{N2P2(O2C12H8)2}{NP{N(CH3)NCH-C6H3-(2-OH)(3OCH3)}2}]40 as well as ferrocene-based ligands to synthesize heterometallic 3d−4f complexes,41,42 some of which exhibit slow relaxation of magnetization (SMM) behavior at low temperatures. In our exploration of new ligands, we embarked on a strategy to combine the features of Salen- and hydrazide© 2015 American Chemical Society

Received: November 8, 2014 Revised: December 26, 2014 Published: January 6, 2015 848

DOI: 10.1021/cg501640y Cryst. Growth Des. 2015, 15, 848−857

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Crystal Growth & Design Table 1. Crystal Data and Refinement Parameters for Complexes 1−4 compound CCDC no. formula MW (g) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρc (g cm−3) μ (mm−1) F(000) crystal size (mm3) θ range (deg) limiting indices

reflns collected ind reflns completeness to θ (%) refinement method data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole (e Å−3)

1

2

3

4

1018346 C66H76Cl2Gd4Mn2N6O22 2115.10 triclinic P1̅ 11.687(5) 14.276(5) 14.847(5) 107.624(5)° 94.648(5)° 108.890(5)° 2189.2(14) 1 1.604 3.391 1030.0 0.15 × 0.14 × 0.13 2.53 to 25.50° −14 ≤ h ≤ 14 −16 ≤ k ≤ 17 −14 ≤ l ≤ 17 15 264 8072 [Rint = 0.0456] 99.2% full-matrix least-squares on F2 8072/0/452 0.952 R1 = 0.0461, wR2 = 0.1117 R1 = 0.0777, wR2 = 0.1193 1.81 and −1.26

1018347 C82H100Cl2Mn2N14O22Tb4 2450.21 triclinic P1̅ 13.425(5) 13.637(5) 14.927(5) 73.564(5)° 69.223(5)° 68.784(5)° 2344.1(15) 1 1.736 3.370 1210.0 0.11 × 0.10 × 0.09 1.63 to 25.50° −13 ≤ h ≤ 16 −15 ≤ k ≤ 16 −18 ≤ l ≤ 16 12 877 8564 [Rint = 0.0430] 98.1% full-matrix least-squares on F2 8564/20/490 1.145 R1 = 0.0616, wR2 = 0.1607 R1 = 0.0831, wR2 = 0.2233 3.80 and −2.68

1018348 C66H76Cl2Dy4Mn2N6O22 2136.10 triclinic P1̅ 13.241(5) 14.275(5) 14.890(5) 103.847(5)°. 110.331(5)°. 101.321(5)°. 2438.7(15) 1 1.454 3.388 1038.0 0.10 × 0.09 × 0.08 2.35 to 25.50° −16 ≤ h ≤ 16 −17 ≤ k ≤ 17 −18 ≤ l ≤ 18 26 889 9078 [Rint = 0.0303] 99.9% full-matrix least-squares on F2 9078/0/476 1.025 R1 = 0.0267, wR2 = 0.0698 R1 = 0.0365, wR2 = 0.0719 1.00 and −0.62

1018349 C82H100Cl2Ho4Mn2N14O22 2474.25 triclinic P1̅ 13.327(5) 13.649(5) 14.848(5) 73.807(5)° 69.316(5)° 69.002(5)° 2322.8(14) 1 1.769 3.762 1218.0 0.13 × 0.12 × 0.11 2.39 to 25.50° −16 ≤ h ≤ 16 −16 ≤ k ≤ 16 −17 ≤ l ≤ 17 16 748 8601 [Rint = 0.0325] 99.6% full-matrix least-squares on F2 8601/6/588 1.019 R1 = 0.0292, wR2 = 0.0608 R1 = 0.0457, wR2 = 0.0656 1.48 and −0.700

put in gelatin capsules. The magnetic susceptibilities were measured in an applied field of 1 kOe. The molar susceptibility (χM) recorded was corrected for the sample holder and grease as well as for the diamagnetic contribution of all atoms using Pascal’s tables. AC susceptibility was measured with an oscillating field of 3 Oe; measurements have been performed with and without an applied external DC field. X-ray Crystallography. The crystal data for 1−4 were collected on a Bruker AXS SMART APEX CCD X-ray diffractometer using a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The program SMART47 was used for collecting frames of data, indexing reflections, and determining lattice parameters, SAINT,48 for integration of the intensity of reflections and scaling, SADABS,49 for absorption correction, and SHELXTL,49 for space group and structure determination and least-squares refinements on F2. All structures were solved by direct methods using the programs SHELXS-9750 and refined by full-matrix least-squares methods against F2 with SHELXL97.51 Hydrogen atoms were fixed at calculated positions, and their positions were refined by a riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters. The crystallographic figures used in this article were generated using Diamond, version 3.1e, software.52 Compounds 1 and 3 contained highly disordered solvent molecules and could not be modeled satisfactorily. These solvent molecules were removed using PLATON/SQUEEZE.53 The total electron count for complexes 1 and 3 is 204 and 200. For 1, solvent molecules were assigned as 3CH3CN, 7CH3OH, and H2O, and for 3, 2CH3CH2OCH2CH3, 4CH3CN, and 3H2O. Crystal data and cell parameters for compounds 1−4 are summarized in Table 1. CCDC reference numbers for complexes 1−4 are 1018346−1018349, respectively. Synthesis of H3L. To a continuously stirred solution of 2aminobenzhydrazide54 (1.00 g, 6.62 mmol) in EtOH (20 mL) was

magnetic studies of the hexanuclear heterometallic complexes [{LMn(Cl)(μ3-OH)Gd2(μ3-OH)(μ-Piv)2}2·3MeCN·7MeOH· H 2 O] (1), [{LMn(Cl)(μ 3 -OH)Tb 2 (μ 3 -OH)(μ-Piv) 2 } 2 · 8MeCN] (2), [{LMn(Cl)(μ3-OH)Dy2(μ3-OH)(μ-Piv)2}2· 2Et2O·4MeCN·3H2O] (3), and [{LMn(Cl)(μ3-OH)Ho2(μ3OH)(μ-Piv)2}2·8MeCN] (4).

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EXPERIMENTAL SECTION

Reagents and General Procedures. Anthranilic acid, pivalic acid, triethylamine (Spectrochem, Mumbai, India), hydrazine hydrate, ovanillin (SD fine chemicals, India), Mn(ClO4)·6H2O, and LnCl3·6H2O (LnIII = GdIII,TbIII,DyIII, HoIII) (Sigma-Aldrich, USA) were used as received. Solvents and other general reagents used in this work were purified using standard procedures.45,46 Instrumentation. Melting points were measured using a JSGW melting point apparatus and are uncorrected. IR spectra were recorded as KBr pellets on a Bruker Vector 22 FT IR spectrophotometer operating between 400 and 4000 cm−1. 1H NMR spectra was recorded on a JEOL-JNM LAMBDA model 400 spectrometer using CDCl3 operating at 400 MHz. Elemental analyses of these compounds were obtained from Thermoquest CE instruments CHNS-O, model EA/ 110. Electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on a Micromass Quattro II triple quadrupole mass spectrometer. For electrospray ionization (positive ion, full scan mode), methanol was used for desolvation. Capillary voltage was maintained at 2 kV, and cone voltage was kept at 31 kV. Magnetic Measurements. Magnetic measurements for all samples were carried out with a Quantum Design MPMS 5S SQUID magnetometer in the temperature range 2−300 K. The measurements were performed on polycrystalline samples. The crystalline powders of the complexes were mixed with grease and 849

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Crystal Growth & Design Scheme 1. Synthesis of H3L

Scheme 2. Strategy for the Design of H3L Based on Salen64 and Benzohydrazide61 Ligands

added a solution of o-vanillin (2.020 g, 13.27 mmol) in EtOH (20 mL) dropwise. After complete addition of the aldehyde solution, the mixture was allowed to reflux for 4 h. Then, the solution was evaporated to reduce the volume and then kept in a refrigerator for 1 day. A pale yellow precipitate was obtained, which was collected by filtration, washed with cold ethanol, and then dried in vacuum. Yield: 2.50 g, 90.1%, mp 110−115 °C. 1H NMR (500 MHz, CDCl3): 11.24 (2H, s, phenolic-H), 8.89 (2H, s, CHN), 6.5−7.99 (10H, Ar−H), 3.87 (6H, s, −OMe). IR (KBr, ν/cm−1): 3582.8 (w), 3527.9 (w), 3309.8 (w, br), 2965.9 (w), 1664.4 (m, br), 1610.1 (s), 1506.10 (s), 1249.5 (s), 735.5 (s). Anal. Calcd for C23H21N3O5 (419.15): C, 65.86; H, 5.05; N, 10.02. Found: C, 65.81; H, 4.91; N, 9.97. ESI-MS: m/z 420.15, [H4L]+. General Procedure for the Synthesis of Complexes 1−4. All complexes were synthesized by following a general procedure. H3L was dissolved in 30 mL of a methanol/chloroform mixture (v/v, 1:1). LnCl3·6H2O was added to the mixture, followed by triethylamine (50 mg, 0.495 mmol). At this stage, the reaction mixture turned turbid. After a while, Mn(ClO4)2·6H2O was added, followed by pivalic acid (PivH). The reaction mixture turned into a clear brown solution. After

this, the reaction mixture was stirred for 4 h at room temperature. The resulting mixture was evaporated in vacuum, and a brown solid was obtained that was washed with diethyl ether and dried. Single crystals of 1−4, suitable for X-ray diffraction, were obtained by vapor diffusion of diethyl ether into the methanol/acetonitrile mixture (v/v, 1:1) of the corresponding compound. The quantities involved in each reaction and other data for each complex are given below. [{LMn(Cl)(μ3-OH)Gd2(μ3-OH)(μ-Piv)2}2·3MeCN·7MeOH·H2O] (1). Quantities: Mn(ClO4)2·6H2O (51.7 mg, 0.142 mmol), GdCl3· 6H2O (105.5 mg, 0.284 mmol), H3L (60.0 mg, 0.142 mmol), PivH (29.0 mg, 0.284 mmol). Yield: 75 mg, 42.5% (based on lanthanide). mp > 250 °C (dec). IR (KBr, ν/cm−1): 3431.2 (m, br), 2958.9 (m), 1617.0 (s), 1547.3(s), 1459.2 (s), 1216.5 (s), 740.1(s). Anal. Calcd For, C79H115Cl2Gd4Mn2N9O30 (2480.58): C, 38.25; H, 4.67; N, 5.08. Found: C, 37.45; H, 4.19; N, 4.54. ESI-MS (positive ion mode): m/z 366.1, [LMnGd + MeOH + THF]2+. [{LMn(Cl)(μ3-OH)Tb2(μ3-OH)(μ-Piv)2}2·8MeCN] (2). Quantities: Mn(ClO4)2·6H2O (51.7 mg, 0.142 mmol), TbCl3·6H2O (106.0 mg, 0.284 mmol), H3L (60.0 mg, 0.142 mmol), PivH (29.0 mg, 0.284 mmol). Yield: 68 mg, 39.1% (based on lanthanide). mp > 250 °C 850

DOI: 10.1021/cg501640y Cryst. Growth Des. 2015, 15, 848−857

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Crystal Growth & Design Scheme 3. Keto−Enol Tautomerism of H3L Ligand

Scheme 4. Synthesis of Hexanuclear Complexes 1−4

Figure 1. ESI-MS of 2: (a) simulated spectrum and (b) experimental spectrum. (dec). IR (KBr, ν/cm−1): 3437.0 (m, br), 2958.9 (m), 1618.1 (s), 1549.7 (s), 1459.7 (s), 1217.9 (s), 740.1 (s). Anal. Calcd For C82H100Cl2Mn2N14O22Tb4 (2450.26): C, 40.20; H, 4.11; N, 8.00. Found: C, 40.12; H, 3.98; N, 7.86. ESI-MS (positive ion mode): m/z 1174.3, [LMnTb2(OH)2 + 2THF + 2MeOH + 3H2O + DMF]+. [{LMn(Cl)(μ3-OH)Dy2(μ3-OH)(μ-Piv)2} 2·2Et2O·4MeCN·3H2O] (3). Quantities: Mn(ClO4)2·6H2O (51.7 mg, 0.142 mmol), DyCl3· 6H2O (107.0 mg, 0.284 mmol), H3L (60.0 mg, 0.142 mmol), PivH (29.0 mg, 0.284 mmol). Yield: 65 mg, 36.5% (based on lanthanide). mp > 250 °C (dec). IR (KBr, ν/cm−1): 3431.0 (m, br), 2958.5 (m), 1618.4 (s), 1551.6 (s), 1460.6 (s), 1219.3 (s), 740.0(s). Anal. Calcd for C82H114Cl2Dy4Mn2N10O27 (2506.63): C, 39.35; H, 4.59; N, 5.60.

Found: C, 38.57; H, 4.18; N, 5.12. ESI-MS (positive ion mode): m/z 858.6, [L2Mn2Dy4 (O)4 + MeCN + H2O]2+. [{LMn(Cl)(μ3-OH)Ho2(μ3-OH)(μ-Piv)2}2·8MeCN] (4). Quantities: Mn(ClO4)2·6H2O (51.7 mg, 0.142 mmol), HoCl3·6H2O (107.7 mg, 0.284 mmol), H3L (60.0 mg, 0.142 mmol), PivH (29.0 mg, 0.284 mmol). Yield: 60 mg, 34.1% (based on lanthanide). mp > 250 °C (dec). IR (KBr, ν/cm−1): 3431.6 (m, br), 2959.1 (m), 1618.6 (s), 1552.8(s), 1461.5 (s), 1220.5 (s), 740.1 (s). Anal. Calcd for C82H100Cl2Ho4Mn2N14O22 (2472.25): C, 39.81; H, 4.07; N, 7.93. Found: C, 39.55; H, 3.92; N, 7.65. ESI-MS (positive ion mode): m/z 784.2, [LMn(Cl)Ho + MeCN + THF]+. 851

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Crystal Growth & Design Scheme 5. Some Earlier-Known Examples of MnIII2Ln4 Complexes74,75

Figure 2. Molecular structure of 2. Some hydrogen atoms and solvent molecules are omitted for clarity.

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RESULTS AND DISCUSSION Synthetic Aspects. 2-Aminobenzohydrazide was synthesized using a previously reported procedure.54 Condensation of 2-aminobenzohydrazide with o-vanillin afforded the multisitecoordinating ligand H3L (Scheme 1). ESI-MS of H3L under positive ion mode showed prominent parent ion peaks [420.15, (H3L + H)+] (Supporting Information). On the basis of a literature study, it is seen that Salen-type ligands are effective for assembling heterometallic complexes.1−10,21−30 On the other hand, aryl hydrazone-based Schiff bases have also been shown to be quite effective for preparing homometallic lanthanide complexes.55−63 In the current design, we have incorporated the features of both of these above-mentioned families of ligands (Scheme 2). The multidentate ligand H3L has several interesting features. First, it can exist in two different tautomeric forms (Scheme 3). It has been shown by us and other that ligands possessing tautomeric structures can adapt to suit the demands of a coordination assembly.55−63,65−73 Second, H3L has three distinct coordination pockets (Scheme 1). It was anticipated

that pocket 1 would preferentially bind transition metal ions, whereas pockets 2 and 3 would facilitate binding of lanthanide ions (Scheme 2). In accordance with this expectation, the reaction of H3L with LnCl3·6H2O (Ln = Dy, Tb, Gd, Ho) and Mn(ClO4)2·6H2O was carried out using pivalic acid as an ancillary ligand in the presence of triethyl amine, affording four isostructural hexanuclear (MnII2Ln4) heterometallic complexes: [{LMn(Cl)(μ3-OH)Gd2(μ3-OH)(μ-Piv)2}2·3MeCN·7MeOH·H2O] (1), [{LMn(Cl)(μ 3 -OH)Tb 2 (μ 3 -OH)(μ-Piv) 2 } 2 ·8MeCN] (2), [{LMn(Cl)(μ3-OH)Dy2(μ3-OH)(μ-Piv) 2} 2·2Et2O·4MeCN· 3H2O] (3), and [{LMn(Cl)(μ3-OH)Ho2(μ3-OH)(μ-Piv)2}2· 8MeCN] (4) (Scheme 4). ESI-MS studies reveal fragmentation of the complexes under these conditions. Thus, 2 reveals the presence of a peak at 1174.3, [LMnTb2(OH)2 + 2THF + 2MeOH + 3H2O + DMF]+ (Figure 1). The latter corresponds to one-half of the molecule. Interestingly, two previously known Mn2Ln4 complexes, with a similar core as that of 1−4, have been prepared either by using (2-hydroxymethyl) pyridine74 or a multisite-coordinating 852

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Crystal Growth & Design

Figure 3. (a) Asymmetric unit of complex 2. (b, c) Hexanuclear Mn2Tb4 core. Corresponding bond distances (Å): Tb(2)−Mn(1), 3.566(2); Tb(1)−Mn(1), 3.557(2); Tb(1)−Tb(2), 3.650(1); Tb(1)−Tb(2)*, 3.760(1); Tb(1)−Tb(1)*, 3.985(2); Bond angles (deg): Tb(2)*−O(12)− Tb(1), 106.0(3); Tb(2)*−O(12)−Tb(1)*, 99.1(2); Tb(1)−O(12)−Tb(1)*, 111.7(3); Tb(1)−O(3)−Tb(2)*, 102.6(3); Mn(1)−O(5)−Tb(2), 106.4(3); Mn(1)−O(11)−Tb(2), 105.4(3); Mn(1)−O(11)−Tb(1), 104.0(3); Tb(2)−O(11)−Tb(1), 101.6(3); Mn(1)−O(4)−Tb(1), 104.3(3).

ligand, H4L75 (Scheme 5). Both of these complexes contain azide and/or pivalate ancillary ligands (Scheme 5). In addition to these known families, an earlier example of an MnII2Ln4 (Ln = Gd, Eu) family of complexes is known that involved the use of p-tert-butylsulfinylcalix[4]arene as the ligand.76 X-ray Crystallography. The molecular structures of 1−4 were confirmed by X-ray crystallography. All of the complexes are isostructural and crystallized in the triclinic space group (P1̅). The only difference among complexes 1−4, apart from the lanthanide ion, is that the solvents of crystallization are different. In the asymmetric unit, for all of the complexes, only one-half of the molecules are present. The molecular structure of complex 2 is shown in Figure 2. This complex is used as a representative example for describing the molecular structures of 1−4. Data for the rest of the complexes are given in the Supporting Information. The molecular structure of 2 reveals that two fully deprotonated L3− are involved in the formation of the hexanuclear heterometallic complex. Each of the ligands is involved in holding an MnLn2 subunit (Figure 3). Two such subunits are joined to each other by two μ3-OH and two pivalate ligands. The bridging occurs between the terbium ions, leading to a central planar tetra-terbium assembly (Figure 3). Within each subunit, MnII is bridged on one side with a terbium center (Tb2) through a bridging phenolate [Mn(1)−O(5), 2.088(7) Å; Tb(2)−O(5), 2.361(6) Å; Mn(1)−O(5)−Tb(2), 106.4(3)°] and a μ3-OH [Mn(1)−O(11), 2.139(7); Tb(2)− O(11), 2.340(7) Å; Mn(1)−O(11)−Tb(2), 105.4(3)°]. This results in the formation of an MnTbO2 four-membered ring. On the other side, MnII is bridged with another terbium (Tb1) through a μ3-OH [Tb(1)−O(11), 2.371(7) Å; Mn(1)− O(11)−Tb(1), 104.0(3)°] and a bridging enolate [Mn(1)−

O(4), 2.140(7); Tb(1)−O(4), 2.363(7) Å; Mn(1)−O(4)− Tb(1), 104.3(3)°]. Tb1 and Tb2 are also further bridged by a pivalate ligand [Tb(1)−O(10), 2.379(8) Å; Tb(2)−O(9), 2.346(8) Å]. Finally, MnII is connected to both Tb1 and Tb2 through a μ3-OH. The manganese ions are present in a distorted square pyramidal (NO3Cl) geometry; the chloride occupies the apical position [Mn(1)−Cl(1), 2.344(3) Å] (Figure 3). The oxidation state of the manganese ions (+2) has been confirmed by BVS calculations (Supporting Information, Table S4).77−80 The bond distances around MnII are unexceptional (Figure 3). Two types of octa-coordinate TbIII ions are present. One of these (Tb1) is surrounded by 7 oxygen atoms and one nitrogen atom (7O, N), whereas another (Tb2) is surrounded by eight oxygen atoms (8O). An evaluation of the actual shape of the Ln coordination spheres was performed by Continuous Shape Measures analysis.81,82 It revealed that the coordination environments around Ln ions deviate from an ideal geometry and consist of either distorted square antiprism or biaugmented trigonal prism geometries (Table S5). The bond distances are summarized in the caption of Figure 4. In the hexanuclear framework, all of the terbium ions are present in the same plane, whereas the two manganese ions are arranged in a trans geometry with respect to each other and are displaced by 1.966(2) Å from the Tb4 plane. Thus, the hexanuclear core adopts the chair conformation of a 1,4-dioxan moiety (Figure 3). Besides this, each MnTb2 unit possesses an open book-like topology, whereas the inner tetra-terbium core possesses a defect-dicubane-like topology. Within the hexanuclear core, the inter lanthanide distances and the Mn−Ln distances (along with the τ value for Mn) in 1−4 are summarized in Table 2. 853

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Article

Crystal Growth & Design

mol−1 at 2 K. Such behavior reveals dominant antiferromagnetic interactions. For 1, exchange interactions possibly take place via Gd−O−Gd and Gd−O−Mn exchange pathways. Considering the number of magnetic centers, the various linkages between them, and the featureless variation of χMT, we did not attempt to model this behavior. However, the fact that a decrease of χMT is observed only below 20 K is indicative of very weak exchange interactions. The field-dependent magnetization recorded at 2 K (Figure 6) reaches 36.9 μB for 5 T, which is slightly lower than the expected value (38 μB) for four isolated GdIII metal ions (7 μB for each GdIII ion, gJ = 2) and two isolated MnII metal ions (5μB for each MnII). Considering the isotropic nature of the ions involved, the gradual increase of M and the fact that magnetization does not reach saturation can be attributed to the contributions of antiferromagnetic interactions. The χMT value at 300 K for isostructural complex 2 is 56.3 cm3 kmol−1 (Figure 6), which is well in agreement with the expected value (56.03 cm3 K mol−1) from four isolated TbIII ions with S = 3, gJ = 3/2, χMT = 11.82 cm3 K mol−1 for each center and two isolated MnII metal ions. Upon cooling, the χMT value gradually decreases to 46.6 cm3 kmol−1 at 10 K. It is welldocumented that TbIII ion has large orbital angular momentum and strong spin−orbit coupling. The low-lying mJ states have lower spin states than the higher states. Hence, magnetic susceptibility is expected to decrease upon lowering the temperature. At the same time, there are Tb−O−Tb and Tb−O−Mn magnetic exchange pathways. Their synchronized exchange interactions could allow the system to attain a largespin ground state that is prominently populated at low temperature in the case of ferromagnetic interactions, thereby increasing the χMT value upon cooling. Presence of such mutually antagonist effects accounts for the minimum (at ∼10 K) in the χMT vs T plot.84,85 Strong spin−orbit coupling is also reflected in the field-dependent magnetization studies (Figure 6). The observed magnetization (27 μB; Figure 6) at 2 K and high field (5 T) did not reach saturation. The magnetic behavior of isostructural complexes 3 and 4 is similar to that of complex 1. The χMT value at 300 K for 3 (65.6 cm3 K mol−1) agrees with the calculated value (65.43 cm3 K mol−1), corresponding to four magnetically exchange-free DyIII ions (with S = 5/2, gJ = 4/3, χMT = 14.17 cm3 K mol−1 for each) and two MnII ions. For 4, the observed χMT value at 300 K is 65.35 cm3 K mol−1 (Figure 6), which agrees well with the calculated value (65.03 cm3 K mol−1), corresponding to four magnetically exchange-free HoIII ions (with S = 2, gJ = 5/4, χMT = 14.07 cm3 K mol−1 for each) and two MnII ions. In the case of complex 3, the χMT value slowly decreases to reach a slight plateau at 59 cm3 K mol−1 between 25 and 13 K followed by a sharp decrease to 42 cm3 K mol−1 at 2 K. For complex 4, χMT slowly decreases until 100 K and thereafter the decrease is more pronounced, leading to a value of to 39.7 cm3 K mol−1 at 2 K. Such behaviors can be ascribed to the crystal field effect in conjunction with the possibility of exchange interactions among the metal centers. However, such behavior precludes any conclusion on the sign of the exchange interactions. It cannot be excluded that some weak ferromagnetic interactions are operative within the spin system.86,87 The field-dependence magnetization studies showed that the observed magnetizations at 2 K and 5 T (30.2 μB for 3 and 32.0μB for 4) did not reach saturation, as expected for these anisotropic Ln ions. AC susceptibility measurements have been performed for all compounds. No contribution for the out-of-phase part of the

Figure 4. (a) Distorted square pyramidal geometry around the manganese(II) metal ions in 2. (b, c) Distorted square antiprism (left) or biaugmented trigonal prism (right) geometry around the terbium(III) metal ions in 2. Corresponding bond distances (Å): Mn(1)− O(5), 2.088(7); Mn(1)−O(11), 2.139(7); Mn(1)−O(4), 2.140(7); Mn(1)−N(3), 2.207(8); Mn(1)−Cl(1), 2.344(3); Tb(1)−O(3), 2.348(7); Tb(1)−O(12), 2.363(7); Tb(1)−O(4), 2.363(4); Tb(1)− O(11), 2.371(7); Tb(1)−O(10), 2.379(8); Tb(1)−O(8)*, 2.396(7); Tb(1)−O(12)*, 2.452(7); Tb(1)−N(2), 2.451(8); Tb(2)−O(7), 2.286(8); Tb(2)−O(12)*, 2.344(7); Tb(2)−O(9), 2.346(8); Tb(2)−O(11), 2.341(7); Tb(2)−O(5), 2.361(6); Tb(2)−O(3)*, 2.468(7); Tb(2)−O(2)*, 2.491(7); Tb(2)−O(6), 2.569(8); Tb(2)− Tb(1)*, 3.760(1).

Table 2. Calculation of the Degree of Trigonality of Complexes 1−4

a

complex

Mn−Ln distancea (Å)

Ln−Ln distancea (Å)

τb (MnII)

Mn2Gd4 (1) Mn2Tb4 (2) Mn2Dy4 (3) Mn2Ho4 (4)

3.563 3.562 3.537 3.540

3.806 3.798 3.737 3.750

0.43 0.52 0.51 0.51

Average bond parameters. bτ = the trigonality indices.83

The supramolecular architecture of 2 and 4 is similar and reveals a 1D polymeric arrangement formed through C−H···Cl and C−H···π interactions (Supporting Information, Figure S6). On the other hand, C−H···Cl hydrogen bonds alone lead to formation of the 1D supramolecular polymeric structure for 3 (Supporting Information, Figure S5). Besides this, C−H···Cl hydrogen bonds along with π···π stacking results in the formation of a 2D layer supramolecular architecture for 1 (Figure 5). Magnetic Properties. Magnetic susceptibility of the complexes 1−4 was investigated over the temperature range 2−300 K. The temperature dependence of the molar susceptibilities in the form of χMT vs T (where χM = molar magnetic susceptibility and T = temperature) is shown in Figure 6. The χMT value for complex 1 at 300 K is 40.4 cm3 K mol−1, which is close to the calculated value (40.27 cm3 K mol−1) for six magnetically exchange-free metal ions (four GdIII ions with S = 7/2, gJ = 2.00, χMT = 7.88 cm3 K mol−1 for each center and two MnII metal ions with S = 5/2, g = 2.00, χMT = 4.375 cm3 K mol−1 for each center). Upon cooling, the χMT value remained practically unaltered from the temperature 300 to ∼100 K and thereafter a sharp decrease occurs to 21.1 cm3 K 854

DOI: 10.1021/cg501640y Cryst. Growth Des. 2015, 15, 848−857

Article

Crystal Growth & Design

Figure 5. View of the supramolecular two-dimensional honeycomb-like arrangement in 1. Some hydrogen atoms have been omitted for clarity. The metric parameters involved are C12−H12···Cl1, 2.774 (2) Å;