Oxalato-Bridged Neutral Octanuclear Heterometallic Complexes

Article pubs.acs.org/crystal

Oxalato-Bridged Neutral Octanuclear Heterometallic Complexes [Ln4K4(L)4(μ‑H2O)4(NO3)2(μ-Ox)] (Ln = Dy(III), Gd(III), Tb(III), Ho(III); LH3 = N[CH2CH2NCH‑C6H3‑2-OH-3-OMe]3; Ox = (C2O4)2−): Synthesis, Structure, Magnetic and Luminescent Properties Prasenjit Bag,† Amit Chakraborty,† Mathieu Rouzières,‡,§ Rodolphe Clérac,‡,§ Raymond J. Butcher,∥ and Vadapalli Chandrasekhar*,†,⊥ †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India CNRS, CRPP, UPR 8641, F-33600 Pessac, France § Univ. Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France ∥ Department of Chemistry, Howard University, Washington, D.C. 20059, United States ⊥ National Institute of Science Education and Research, Institute of Physics Campus, Sachivalaya Marg, Sainik School Road, Bhubaneswar-751 005, Odisha India ‡

S Supporting Information *

ABSTRACT: Stepwise reaction of LH3 (LH3 = N[CH2CH2NCH-C6H3-2-OH-3-OMe]3) with Ln(NO3)3·nH2O and potassium oxalate [K2Ox] afforded discrete heterometallic neutral octanuclear complexes [Ln4K4(L)4(μ-H2O)4(NO3)2(μ-Ox)] (Ln = Dy(III), Gd(III), Tb(III), Ho(III); LH3 = N[CH2CH2NCH-C6H3-2-OH-3-OMe]3; Ox = (C2O4)2−). The molecular structures of 1−4 were confirmed by single-crystal X-ray crystallography. The asymmetric unit of these compounds contains two lanthanide(III) ions and two potassium cations present in the opposite corners of a distorted rectangle. Such a motif is further bridged by an oxalate anion to generate the octanuclear heterometallic complex. The lanthanides are eight-coordinate in a distorted trigonal dodecahedron geometry. On the other hand, the potassium ions adopt a nine-coordinate coordination sphere in a distorted tricapped trigonal prism geometry. Magnetic studies on 1−4 reveal the absence of significant magnetic interactions between the magnetic centers present in these complexes.

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INTRODUCTION In recent years, the chemistry of lanthanide ions is fastdeveloping due to their potential applications in catalysis,1 sorption,2 optical materials,3 novel magnetic materials,4 MRI contrasting agents,5 and cancer therapy.6 Complexes of lanthanide ions, such as DyIII, TbIII, HoIII, or ErIII, which possess an intrinsic magnetic anisotropy7 (arising from an unquenched orbital angular momentum) and high-spin ground state, are being increasingly investigated as potential single-molecule magnets (SMMs).8 For example, a mononuclear double-decker lanthanide complex is the first example of a lanthanide-ion-based SMM.9 The introduction of magnetic coupling between lanthanide centers in polynuclear systems has also attracted a lot of attention, not only in the case of ferromagnetically coupled © 2014 American Chemical Society

systems but also when the magnetic centers are antiferromagnetically coupled, for their potential use in quantum computation.10 One of the main challenges in this area remains the ability to control the synthesis of various types of heterometallic complexes. Various polynuclear lanthanide aggregates have been prepared based on the so-called serendipitous approach, using, for example, some flexible open-arm Schiff base ligands,11 substituted calixarenes with multiple donating sites,12 or other multisite coordinating ligands.13 However, in all of these syntheses, the nuclearity of the resulting complexes is impossible Received: May 8, 2014 Revised: July 11, 2014 Published: July 21, 2014 4583

dx.doi.org/10.1021/cg500677t | Cryst. Growth Des. 2014, 14, 4583−4592

Crystal Growth & Design

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(3581.25): C, 42.26; H, 4.95; N, 7.104. Found: C, 42.06; H, 4.82; N, 7.15. [Gd4K4(L)4(μ-H2O)4(NO3)2(μ-Ox)]·8CH3OH·4H2O (2). Yield: 0.14 g, 31.9% (based on Gd). mp > 250 °C. IR (KBr) cm−1: 3440 (b), 3051 (w), 2897(m), 2835 (m), 1619 (s), 1548 (m), 1453(s), 1410 (s), 1335(s), 1220 (s), 1147(w), 1109 (s), 1032 (w), 1074 (w), 966 (s) ESI-MS m/z, ion: 742.13, [LGdK] + . Anal. Calcd for C 130 H 180 Gd 4 K 4 N 18 O 50 (3580.32): C, 43.61; H, 5.07; N, 7.04. Found: C, 43.16; H, 4.96; N, 7.18. [Tb4K4(L)4(μ-H2O)4(NO3)2(μ-Ox)]·4CH3OH·10H2O (3). Yield: 0.14 g, 32.5% (based on Tb). mp > 250 °C. IR (KBr) cm−1: 3440 (b), 3051 (w), 2897(m), 2835 (m), 1619 (s), 1548 (m), 1453(s), 1411 (s), 1335(s), 1220 (s), 1147(w), 1109 (s), 1032 (w), 1074 (w), 966 (s). ESI-MS m/z, ion: 743.13, [LTbK]+. Anal. Calcd for C126H176K4N18O52Tb4 (3566.92): C, 42.43; H, 4.97; N, 7.07. Found: C, 42.17; H, 4.84; N, 7.18. [Ho4K4(L)4(μ-H2O)4(NO3)2(μ-Ox)]·4CH3OH·8H2O (4). Yield: 0.13 g, 36.6% (based on Ho). mp > 250 °C. IR (KBr) cm−1: 3440 (b), 3051 (w), 2897(m), 2835 (m), 1619 (s), 1548 (m), 1453(s), 1410 (s), 1335(s), 1220 (s), 1147(w), 1109 (s), 1032 (w), 1074 (w), 966 (s) ESI-MS m/z, ion: 749.10, [LHoK] + . Anal. Calcd for C 126 H 172 Ho 4 K 4 N 18 O 50 (3554.94): C, 42.57; H, 4.88; N, 7.09. Found: C, 42.31; H, 4.65; N, 7.16. 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 at 400−4000 cm−1. 1H NMR were recorded on a JEOL-JNM LAMBDA model 400 spectrometer using CDCl3 operating at 400 MHz. Elemental analyses of the compounds were obtained from a Thermoquest CE instruments CHNS-O, EA/110 model. Electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on a Micromass Quattro II triple quadrupole mass spectrometer. Electrospray ionization (positive ion, full scan mode) was carried out using methanol as the solvent for desolvation. Capillary voltage was maintained at 2 kV, and cone voltage was kept at 31 kV. Thermogravimetric analysis (TGA; heating rate of 10 °C/min) was carried out on a PerkinElmer Pyris 6 machine under an argon atmosphere. Powder X-ray diffraction (PXRD) patterns were acquired using a Bruker D8 Advanced powder diffractometer with Cu Kα radiation. Absorption and emission spectroscopic measurements were carried out at room temperature, the former using a PerkinElmer Lambda 45 UV− visible spectrophotometer and the latter using a SPEX Fluorolog HORIBA JOBIN VYON fluorescence spectrophotometer equipped with a double-grating 0.22 m Spex1680 monochromator and a 450 W Xe lamp as the excitation source. Fluorescence lifetime measurements was done by the time S4 correlated single-photon counting (TCSPC) method, at the emission maxima of 1−4 in a Fluoro Log 3, model FL3.21, Horiba Jobin Yvon nanosecond time-resolved spectrofluorimeter. a 340 nm nano LED was used as the excitation source. The decay was measured at 470 nm. The solution-state quantum yields were determined using the following equation22

to predict. Therefore, chemists have tried to develop alternative routes to design polynuclear systems with a reasonable amount of control and predictability. In this research area, our group used phosphorus-supported multidentate ligands for the assembly of heteronuclear trimetallic14 and bimetallic systems15 in which the control of the composition and the structure of the complexes is partially dictated by the ligand geometry. This successful strategy led to 3d/4f trimetallic complexes displaying single-molecule magnet properties.14a−c However, using this approach, we were unable to increase the nuclearity of the complexes beyond three. Therefore, to build polynuclear metal assemblies in a controlled manner, we decided to adapt our synthetic strategy by using, first, a multidentate Schiff base ligand to design a primary core building block, which could be in a second step assembled with well-chosen bridging ligands. In the literature, the use of bridging ligands, such as oxalate,16 cyanide,17 or dicyanamide,18 has been widely developed. Knowing the excellent bridging coordination abilities of the oxalate ion, we decided to use it in combination with a tripodal Schiff base ligand as a synthetic approach for highnuclearity lanthanide-based aggregates. Accordingly, we report, herein, the synthesis, crystal structures, and magnetic properties of several oxalate-bridged octanuclear heterometallic complexes [Ln4K4(L)4(μ-H2O)4(NO3)2(μ-Ox)], where Ln = Dy(III), Gd(III), Tb(III), Ho(III); LH3 = N[CH2CH2NCH-C6H3-2OH-3-OMe]3; and Ox = (C2O4)2−. Magnetic measurements of all the complexes (1−4) show a decrease of magnetic susceptibility with a decrease in temperature and also the absence of significant magnetic interactions between the magnetic centers. Photophysical studies reveal that the gadolinium derivative (2) shows the maximum ligand-based fluorescent intensity.

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EXPERIMENTAL SECTION

Reagents and General Procedures. Solvents and other general reagents used in this work were purified according to standard procedures.19 Tris(2-aminoethyl)amine (Sigma-Aldrich, USA) was used as received. Potassium oxalate dihydrate (K2C2O4·2H2O) was obtained from SD Fine Chemicals, Mumbai, India. Ln(NO3)3·nH2O was obtained from Aldrich Chemical Co. and was used without further purification. The ligand, tris[4-(2-hydroxy-3-methoxyphenyl)-3-aza-3butenyl]amine (LH3), was prepared as previously described.20 Preparation of the Octanuclear Complexes 1−4. A general synthetic protocol was employed for the preparation of all the metal complexes as described in the following. [Ln4K4(L)4(μ-H2O)4(NO3)2(μ-Ox)]: LH3 (0.110 g, 0.20 mmol) was dissolved in methanol (20 mL). Ln(NO3)3·nH2O (0.20 mmol) and triethylamine (0.09 mL, 0.6 mmol) were added to this solution. The reaction mixture was stirred at 60 °C for 3 h to afford a yellow precipitate that was then filtered off and washed twice with 10 mL of methanol and 10 mL of diethyl ether. This yellow solid, which was previously prepared and suggested to have an (L)Ln(OH2) composition,21 was suspended in 60 mL of methanol before Ln(NO3)3·nH2O (0.20 mmol) was added further. The resulting mixture was stirred for 1 h to afford a clear solution. At this stage, potassium oxalate dihydrate (0.37g, 0.2 mmol) dissolved in 1 mL of water was added, and the solution was further heated under reflux at 80 °C for 8 h. The reaction mixture was allowed to cool to room temperature and filtered. The filtrate was stripped off the solvent in vacuo, affording a yellow solid. This material was recrystallized from a 1:1 mixture of dichloromethane/methanol solution, affording crystals that could be studied by X-ray crystallography. The details of the characterization data for these complexes are given below. [Dy4K4(L)4(μ-H2O)4(NO3)2(μ-Ox)]·4CH3OH·10H2O (1). Yield: 0.12 g, 27.5% (based on Dy). mp > 250 °C. IR (KBr) cm−1: 3440 (b), 3051 (w), 2897(m), 2835 (m), 1619 (s), 1548 (m), 1468 (s), 1453(s), 1411 (s), 1335(s), 1220 (s), 1147(w), 1109 (s), 1032 (w), 1074 (w), 966 (s) ESIMS m/z, ion: 748.13, [LDyK]+. Anal. Calcd for C126H176Dy4K4N18O52

Φ = (Ix /Ist)(A st /A x )(ηx /ηst)Φst Φst is the quantum yield of the reference; Ix and Ist are the integrated areas under the emission spectra of the complex and the reference. Ax and Ast are the absorption coefficients of the complex and the reference (perylene). ηx and ηst are the refractive index of the solvent of the complex and reference, respectively. Magnetic Measurements. Magnetic susceptibility measurements were obtained with the use of a Quantum Design SQUID magnetometer MPMS-XL. This magnetometer works between 1.8 and 400 K for dc applied fields ranging from −7 to 7 T. Measurements were performed on polycrystalline samples of 13.82, 13.12, 9.69, and 12.47 mg for 1−4, respectively, introduced in a polyethylene bag (3 × 0.5 × 0.02 cm). ac susceptibility measurements have been measured with an oscillating ac field of 3 Oe and ac frequencies ranging from 1 to 1500 Hz. M vs H measurements have been performed at 100 K to check for the presence of ferromagnetic impurities that were found to be absent. Magnetic data were corrected for the sample holder and for the diamagnetic contribution. X-ray Crystallography. The crystal data and the cell parameters for 1−4 are given in Table 1. Single crystals suitable for X-ray 4584



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Table 1. Details of the Data Collection and Refinement Parameters for Compounds 1−4 formula M/g cryst system space group wavelength (Mo Kα) unit cell dimensions (Å, deg)

V/Å3 Z ρc/g cm−3 μ/mm−1 F(000) cryst 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

C126H176Dy4K4N18O52 3581.25 monoclinic P21/c 0.71069 a = 15.761(3) b = 29.264 (6) c = 16.660(3) α = 90 β = 108.583(3) γ = 90 7284(2) 2 1.633 2.235 3628 0.16 × 0.13 × 0.09 1.95−25.50 −19 ≤ h ≤19, −35 ≤ k ≤31, −20 ≤ l ≤13 38572 13449 [R(int) = 0.0580] 99.2 full-matrix least-squares on F2 13449/40/918 1.080 R1 = 0.0558 wR2 = 0.1576 R1 = 0.0726, wR2 = 0.1875 2.136 and −1.101

C130H180Gd4K4N18O50 3580.32 monoclinic P21/c 0.71069 a = 15.866(3) b = 29.287(6) c = 16.679(3) α = 90 β = 108.797(3) γ = 90 7337(3) 2 1.621 1.990 3636 0.14 × 0.11 × 0.09 1.90−25.50 −16 ≤ h ≤19, −35 ≤ k ≤23, −20 ≤ l ≤19 38904 13568 [R(int) = 0.0594] 99.3 full-matrix least-squares on F2 13568/44/958 1.067 R1 = 0.0574 wR2 = 0.1545 R1 = 0.0811 wR2 = 0.1865 1.874 and −1.118

C126H176K4N18O52Tb4 3566.92 monoclinic P21/c 0.71069 a = 15.797 (3) b = 29.291(6) c = 16.648(4) α = 90 β = 108.484(4) γ = 90 7306(3) 2 1.621 2.119 3620 0.14 × 0.12 × 0.095 1.90−25.50 −19 ≤ h ≤19, −30 ≤ k ≤35, −20 ≤ l ≤14 38775 13501 [R(int) = 0.0610] 99.3 full-matrix least-squares on F2 13501/34/924 1.039 R1 = 0.0601 wR2 = 0.1569 R1 = 0.0839, wR2 = 0.1760 1.846 and −1.362

C126H172N18O50Ho4K4 3554.94 monoclinic P21/c 0.71069 a = 15.874(5) b = 29.111(5) c = 16.677(5) α = 90 β = 107.326(5) γ = 90 7357(3) 2 1.606 2.331 3596 0.16 × 0.13 × 0.11 2.09−25.50 −19 ≤ h ≤16 −28 ≤ k ≤35 −19 ≤ l ≤20 38480 13653 [R(int) = 0.0532] 99.6 full-matrix least-squares on F2 13653/19/927 1.158 R1 = 0.0857 wR2 = 0.2238 R1 = 0.1027 wR2 = 0.2376 2.127 and −3.711

Table 2. Selected Bond Distances (Å) and Angles (deg) of 1 bond lengths around dysprosium(1)

bond lengths around dysprosium(2)

selected bond angles

Dy(1)−O(11A) 2.256(5) Dy(1)−O(21A) 2.297(5) Dy(1)−O(31A) 2.335(5) Dy(1)−O(11) 2.411(5) Dy(1)−N(32A) 2.499(6) Dy(1)−N(12A) 2.508(6) Dy(1)−N(22A) 2.597(6) Dy(1)−N(1A) 2.684(6) Dy(1)−K(1) 3.776(2) Dy(1)−Dy(2) 5.610(8) bond lengths around potassium(1) K(1)−O(11A) 2.706(6) K(1)−O(22A) 2.706(5) K(1)−O(12A) 2.732(6) K(1)−O(32A) 2.784(5) K(1)−O(1N) 2.806(7) K(1)−O(31A) 2.833(5) K(1)−O(21A) 2.849(5) K(1)−O(2N) 2.880(10) K(1)−O(12) 3.237(5)

Dy(2)−O(11B) 2.277(5) Dy(2)−O(21B) 2.288(5) Dy(2)−O(31B) 2.320(5) Dy(2)−O(12) 2.432(5) Dy(2)−N(32B) 2.516(6) Dy(2)−N(12B) 2.538(7) Dy(2)−N(22B) 2.573(6) Dy(2)−N(1B) 2.647(6) Dy(2)−K(2) 3.769(2)

Dy(1)−O(11A)−K(1) 98.71(18) Dy(1)−O(21A)−K(1) 93.77(16) Dy(1)−O(31A)−K(1) 93.39(16) Dy(2)−O(31B)−K(2) 96.98(17) Dy(2)−O(21B)−K(2) 94.31(16) Dy(2)−O(11B)−K(2) 96.40(18) Dy(1)−O(11)−K(2) 171.7(2) Dy(2)−O(12)−K(1) 174.5(2)

bond lengths around potassium(2) K(2)−O(1OX) 2.606(10) K(2)−O(22B) 2.638(6) K(2)−O(31B) 2.701(5) K(2)−O(12B) 2.726(7) K(2)−O(11B) 2.759(6) K(2)−O(32B) 2.780(6) K(2)−O(21B) 2.827(5) K(2)−O(2OX) 3.118(14) K(2)−O(11) 3.281(5)

SMART CCD diffractometer using a Mo Kα sealed tube. The program SMART23a was used for collecting frames of data, indexing reflection, and determining lattice parameters, SAINT23a for integration of the

crystallographic analyses were obtained by a slow evaporation of a 1:1 mixture of dichloromethane/methanol solution of these compounds (1−4). The crystal data for 1−4 have been collected on a Bruker 4585



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Figure 1. Molecular structure of 1 (hydrogen atoms and solvent molecules are omitted for clarity).

Figure 2. View of the central Dy4K4 core.

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intensity of reflections and scaling, SADABS23b for absorption correction, and SHELXTL23c,d for space group and structure determination and least-squares refinements on F2. All structures were solved by direct methods using the program SHELXS-9723e and refined by full-matrix least-squares methods against F2 with SHELXL-97.23e All the non-hydrogen atoms were refined with anisotropic displacement parameters. All the hydrogen atoms of 1−4 except the bridging water molecules were fixed at calculated positions, and their positions were refined by a riding model. All the hydrogen atoms of the bridging and noncoordinated water (OH2) molecules were included from a difference Fourier map and refined isotropically. The figures have been generated using Diamond 3.1e software.23f Selected bond distances and angles are given in Table 2 and Tables S1−S3 (Supporting Information) for 1−4, respectively. Geometrical hydrogen bond parameters for compound 2 are given in Table 4. The short C−C bond distance in the oxalate anion (Ox2−) (“A” alert) can be attributed to the presence of this ion on a symmetry element. Therefore, it is very difficult to obtain a good geometry for the bridging Ox(2−) anion. In all the crystals, the presence of short inter D−H···H− X contacts in the “A” alert (CheckCIF Report) can be attributed to the crystal packing and moderately high thermal parameters of some solvent water and MeOH molecules. This compels H atoms to come close to each other. Coordinates do not form a properly connected set (“A” alert). This is because the bridging oxalate molecule in the asymmetric unit is fragmented; i.e., the oxalate anion (Ox2−) (“A” alert) is present on a symmetry element. Then, only after the symmetry operation, full molecules can be generated.

RESULTS AND DISCUSSION Synthetic Aspects. The multisite coordinating ligand, N[NCH2CH2NCH-C6H3-2-OH-3-OMe]3, LH3, was synthesized by the condensation of N[NCH2CH2NH2]3 with o-vanillin (Supporting Information, Scheme S1). This ligand contains 10 potential coordinating donor sites in the form of the three imino nitrogen atoms, three phenolate oxygen atoms, three methoxy oxygen atoms, and one tertiary amino atom. Also, as shown in Scheme 2, LH3 possesses two distinct coordination compartments: an inner pocket PI (with three imino and one tertiary amino nitrogen atoms and three phenolate oxygen atoms) and an outer pocket PII (with three phenolate oxygen atoms and three methoxy oxygen atoms). Because of this particular coordination geometry, the LH3 ligand should facilitate a stepwise capture of two metal ions into its cavities. Indeed, this idea was further encouraged by a previous report21 suggesting that it was possible to prepare heterodinuclear lanthanide complexes based on the LH3 ligand, but unfortunately, these species were not structurally characterized. Therefore, we reacted LH3 with Ln(NO3)3·nH2O in a 1:1 stoichiometric ratio, followed by the addition of Ln(NO3)3· nH2O (a second stoichiometric amount) and K2Ox·2H2O, affording the complexes, 1−4 [L4Ln4K4(Ox)] in excellent yields (see the Experimental Section). We did not attempt to isolate the solid obtained in the first part of the reaction; however, it was previously identified as LLn(OH2).21 Although the full molecular 4586



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Scheme 1. Synthesis of Compounds 1−4

ionic species in 1−4 were not detected by ESI-MS, it was possible to detect the presence of the [LLnK]+ fragment (see the Experimental Section, Supporting Information). The molecular structures of 1−4 were unambiguously determined by singlecrystal X-ray crystallography as described below. X-ray Crystallography. The molecular structures of 1−4 were determined by single-crystal X-ray crystallography. All the four molecules crystallized in the monoclinic space group P21/c (Table 1). All the complexes are isostructural and possess an octanuclear core with the general formula [Ln4K4(L)4(μH2O)4(NO3)2(μ-Ox)] (Ln = Dy(1), Gd(2), Tb(3), Ho(4)). The details of the molecular and crystal structures of 1−4 are given in the Supporting Information (Figures S1−S6, Tables S1−S4). In view of the structural similarity of 1−4, only the molecular structure of 1 is discussed here. The molecular structure of 1 is given in Figure 1. Selected bond parameters of 1 are given in Table 2. The asymmetric unit of 1 consists of one-half of the molecule where two L3− units are involved in holding two lanthanide and two potassium ions. These ions are located at the opposite corners of a distorted parallelogram. Two such heterometallic tetramers are linked to each other by the bridging coordination action of the oxalate ligand, which links two potassium ions to generate the octanuclear complex (Figures 1 and 2; Scheme 1). The diagonal intermetallic distances are Dy1−Dy2: 5.610 (8) Å and K1−K2: 7.587(2) Å. The edge distances are Dy1−K1: 3.776(2) Å and Dy1−K2: 5.678(1) Å (Figure S2, Supporting Information). The entire intermetallic core itself resembles a double-kite structural motif where the individual rectangular kites comprise the Dy2K2

Scheme 2. Schematic View of the Tripodal Schiff Base Ligand (LH3) Emphasizing the Two Coordination Pockets (PI and PII) Potentially Able To Capture Two Metal Ions

units (Figure S2). Within the octametallic framework, the two antipodal potassium ions (K1 and K1*) are arranged in a trans geometry with respect to each other. These ions are displaced above and below the mean plane defined by Dy1, Dy2, K2, K2*, Dy1*, and Dy2* (Figure S1, Supporting Information). As can be seen from Figure 1, the lanthanide ion is bound by the inner coordination pocket of the ligand (one bridge head nitrogen atom, three imino nitrogen atoms, three phenolate oxygen atoms) while the potassium ion is bound by the outer coordination pocket of the ligand (three phenolate oxygen atoms and three methoxy oxygen atoms). Further, within each tetramer, a pair of lanthanide and potassium ions is bridged by a water molecule. Also, while the coordination of one of the potassium 4587



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Figure 3. Distorted trigonal dodecahedron geometry around the dysprosium(III) metal ions in 1.

Figure 4. Distorted tricapped trigonal prism geometry around the potassium ions in 1.

Table 3. Intramolecular Hydrogen Bond Parameters Found in 1 compound

D−H···A

d(D−H) (Å)

d(H···A) (Å)

d(D···A) (Å)

∠(DHA) (deg)

1

O12−H122···O21A O12−H121···O31A O11−H111···O21B O11−H112···O31B

0.824(17) 0.827(15) 0.825(21) 0.818(25)

2.027(14) 2.158(18) 1.983(13) 2.253(16)

2.743(6) 2.846(5) 2.707(6) 2.942(6)

145.03(33) 140.62(12) 145.98(32) 142.16(33)

Figure 5. View of the supramolecular one-dimensional arrangement through O−H···O interactions in 2 (hydrogen atoms on carbon are omitted for clarity).

ions is completed by the chelating nitrate ligand, that of the other potassium ion is spanned by the chelating oxalate ligand. All the

dysprosium ions are eight-coordinate (4N, 4O) in a distorted trigonal dodecahedron geometry (Figure 3). On the other hand, 4588



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Figure 6. View of the supramolecular two-dimensional wave-like arrangement along the crystallographic “c” axis in 2.

Table 4. Geometrical Hydrogen Bond Parameters for Compound 2 D−H···A

d(D−H) (Å)

d(H···A) (Å)

d(D···A) (Å)

∠(DHA) (deg)

symmetry of A

O1S−H1S···O1N O2S−H2S···O1S O3W−H3WA···O2S C105−H10O···O3W C32B−H32B···C36B

0.821(10) 0.820(11) 0.875(4) 0.959(10) 0.969(7)

1.980(8) 2.012(9) 2.221(12) 2.554(29) 2.854(8)

2.799(13) 2.745(15) 2.774(3) 3.336(31) 3.815 (10)

175.25(26) 148.53(42) 131.61(21) 138.76(46) 171.27 (49)

x, y, z x, y, z x, y, z x, y, z x, y, z

the potassium ions are nine-coordinate (9O) in a distorted tricapped trigonal prismatic geometry (Figure 4). The Dy−N bond length (2.684(6) Å) involving the bridgehead nitrogen atom is longer than the Dy−Nimine average bond length (2.535(6) Å). The Dy−O bond length for the bridging water molecule is 2.411(5) Å. The average K−O bond length involving the phenolate oxygen atoms is 2.741(6) Å while the K−O bond length involving a bridging water molecule is quite large (3.237(5)−3.281(5) Å). All the hydrogen atoms from bridging water molecules are involved in an intramolecular hydrogen bonding with the bridging phenoxy oxygen (Figure 2). The hydrogen bond parameters for these interactions are given in Table 3. In addition, crystal packing among these compounds reveals the presence of C−H···O and C−H···π interactions, which assist in the formation of a supramolecular polymeric association along the crystallographic b axis. (For a representative example, see Figures 5 and 6 and Figure S6, Supporting Information. For the hydrogen bond parameters of these interactions, see Table 4). Thermogravimetric Analysis (TGA) and PXRD of 1−4. We have carried out thermogravimetric analysis (TGA) to check the thermal stability of all four complexes (1−4). From the TGA curve, it is very evident that all four compounds show similar type stability or decomposition pattern toward heat treatment (Figure 7, and Supporting Information, Figure S15). The overall TGA curve consists of a two-step gradual weight loss process. The first step between 50 and 300 °C corresponds to a small weight loss, ∼ 9%, which is probably due to the loss of noncoordinated outer methanol and water solvent molecules present in the crystal

Figure 7. Thermogravimetric analysis curve of 2 (heating rate: 10 °C per min) under an argon atmosphere.

lattice. During the second step, beyond 300 °C, the gradual and rapid decomposition of each compound takes place. Thus, it can be said that all four compounds are thermally quite stable. Efforts were then taken to check the phase purity of all four compounds (1−4) using powder X-ray diffraction analysis (Supporting Information, Figures S16−19), which shows excellent agreement with the simulated pattern. In another major advance to check the response of the four compounds toward heat and moisture, we have taken compound 1 as a representative example. For this purpose, it should be noted that 1 loses all the noncoordinated 4589



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Figure 8. (a) UV−vis and (b) emission spectra of compounds 1−4 and LH3 in CH3OH (concentration: 1 × 10−5 M).

solvent molecules upon heating to 200 °C, as confirmed by TGA analysis (Figure S15), and we have collected the PXRD data of the same desolvated crystal of 1 (Figure S20, Supporting Information). Later, the same dehydrated crystal of 1 was exposed to the water/methanol (1:1 v/v) vapor for 5 days (see the Supporting Information, Figure S21). The structure determination using PXRD revealed that the native form of 1 was not obtained, but the pattern obtained is similar to the desolvated one, confirming the irreversible nature of the tetrameric structure. Photophysical Properties. Absorption Spectra. The absorption spectra of all four complexes along with the ligand (LH3) were recorded in methanol solution (10−5 M) and are characterized by peaks at both high and low energies. Table S4 (Supporting Information) summarizes the absorption, emission, and quantum yield data of all four complexes along with the ligand. The ligand shows an absorption maxima, localized around 422 nm with a molar absorption coefficient (εmax) of 4.58 × 105 L mol−1 cm−1 along with the two other overlapping bands at 290 and 262 nm with a stronger molar absorption that can be attributed to a ligand-centered spin-allowed singlet π−π* transition (Figure 8a).24 The overall features of the absorption spectra of all the complexes (1−4) follow a similar trend displayed by the free ligand (Figure 8a). However, the absorption maxima of 422 nm observed for the free ligand (LH3) is significantly blue-shifted in all the complexes and is found to be at ∼363 nm with a concomitant presence of another band at ∼275 nm with a stronger molar absorption coefficient. Also, it is to be noted that, in the metal complexes, a higher molar extinction coefficient is observed in comparison to the ligand. Emission Spectra of 1−4. Both the solid and the solutionstate (in methanol) emission spectra were carried out for all four compounds (1−4) along with the ligand (Supporting Information, Figure S22, and Figure 8b). When the four complexes were excited in their corresponding ligand absorption maxima (λex = 325 nm for solid and 363 nm in solution), none of them display a lanthanide-based emission; only ligand-based fluorescence is found (see above) with an emission maxima centered around 470 nm. This proves the inability to sensitize the lanthanide emission of the o-vanillin derivative. Recently, Chandrasekhar and co-workers have shown in a detailed study the inability to sensitize lanthanide(III) present in complexes containing the o-vanillin ligand as the chromophore.25a The singlet energy level (1ππ*) of the ligand chromophore is at 23 529 cm−1, determined from the high wavelength absorption

spectra of the gadolinium derivative. This value is similar to that reported previously for an o-vanillin chromophore.25a The triplet energy level for o-vanillin is around 19 100 cm−1.25a Because of the low singlet−triplet energy gap [ΔE(3ππ* − 1ππ*) = 4400 cm−1] of o-vanillin as well as the low energy difference between the ligand triplet state and that of the emitting level of either Tb3+ (5D4: 20 400 cm−1) or Dy3+ (4F9/2: 20 800 cm−1), energy transfer to the lanthanide ions is forbidden.25 On the other hand, this low energy gap induces back energy transfer from the lanthanide emitting level to the ligand triplet state and, therefore, quenches both the ligand as well lanthanide luminescence, as is evident in the fluorescence spectra of 1, 3, and 4, which show less quantum efficiency in the ligand emission compared to that of the free ligand.26 Among these compounds, the gadolinium derivative (2) shows considerable increase in ligand fluorescence intensity with a quantum yield of ∼15%. The lowest-lying excited energy level (6P7/2) for Gd3+ is located at 32 150 cm−1, which is higher than either the singlet or the triplet state of the ligand and, therefore, prohibits any energy transfer to the gadolinium ion.27 The photophysical data pertaining to all four complexes are given in Table S4. The fluorescence quantum yields (determined relative to that of perylene)22 lie in the range of 0.58−14.8% for compounds 1−4. The fluorescence decay measurements (Supporting Information, Figures S23−25) at the emission maxima of all of these compounds show that the lifetime lies in the nanosecond time scale (Supporting Information, Table S4), suggesting that the emission originates from the vertically excited states of the ligand. Magnetic Properties. Magnetic susceptibility measurements were performed on polycrystalline samples of 1 (Dy4K4), 2 (Gd4K4), 3 (Tb4K4), and 4 (Ho4K4), respectively, in the 1.8−280 K temperature range with an applied dc field of 1000 Oe (Figure 9). The χT products at room temperature for these compounds are 62.7 (1), 28.6 (2), 42.4 (3), and 59.5 cm3·K mol−1 (4). These values are in good agreement with the expected paramagnetic contributions of four isolated Dy3+ (4f9, 6H15/2, C = 14.17 cm3 K mol−1 with g = 4/3), Gd3+ (4f7, 8S7/2, C = 7.875 cm3 K mol−1 with g = 2), Tb3+ (4f8, 7F6, C = 11.81 cm3 K mol−1 with g = 3/2), and Ho3+ (4f10, 5I8, C = 14.06 cm3 K mol−1 with g = 5/4) ions, respectively (Table S5, Supporting Information).28 For the analogous GdIII complex, the χT product is constant over the entire temperature range, demonstrating the simple Curie-type behavior of the susceptibility (χ = C/T with a Curie constant C = 28.6 cm3·K/mol) and thus the absence of significant magnetic interactions between the S = 7/2 GdIII centers (Figure 9). This 4590



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Ln(III)−Ln(III) distance prohibits any further exchange interactions, resulting in lowering of the χT value on lowering the temperature because of the Stark effect, which tends to lower the local contributions of the lanthanide ions. On the other hand, in the case of the heterodinuclear (Ln, Gd) (Ln = Nd, Ce, Yb, Dy, Er) complexes synthesized by Costes et al.,21 presumably, both the inner and the outer pockets of the L3− ligand contain the lanthanide ions that are bridged by three phenoxo oxygen atoms. In these compounds [(Gd, Nd), (Gd, Ce), and (Yb, Gd)], a considerable increase of χT value is seen on lowering the temperature. This may be due to the ferromagnetic interactions between the Gd(III) ion and the other lanthanide(III) ion mediated presumably, through the phenoxide bridges.

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Figure 9. Temperature dependence of the χT product at 1000 Oe (with χ defined as the molar magnetic susceptibility equal to M/H) for compounds 1−4.

CONCLUSION In conclusion, using the combination of a compartmental multidentate ligand, LH3, along with the bridging oxalate (Ox) ligand, a novel series of octanuclear heterometallic complexes [L4Ln4K4Ox] have been synthesized in high yields. The methodology employed is a sequential synthesis where a multidentate Schiff base ligand is used to generate, in situ, a monolanthanide complex (LLnOH2) as a metalloligand. Utilizing the latter as a primary building block, in a second step, the octanuclear complexes 1−4 have been assembled by involving the oxalate ion as the bridging ligand. In these compounds, the lanthanide ions are eight-coordinate in a distorted trigonal dodecahedron geometry while the potassium cations are nine-coordinate in a distorted trigonal tricapped prismatic geometry. A detailed study of the magnetic properties of these four complexes reveals the absence of any significant interlanthanide magnetic interactions.

result as well as the crystal structure description suggests that intermolecular interactions are also absent for the other compounds. Therefore, the decrease of the χT product upon lowering the temperature observed for the other analogues (Figure 9) is certainly the result of the thermal depopulation of ligand field excited levels. At 1.8 K, the χT product decreases down to 55.1, 20.9, and 24.9 cm3 K/mol for 1, 3, and 4 respectively. As a complementary characterization, the field dependence of magnetization for 1−4 has been measured below 8 K (Figures S7−S10, Supporting Information). As expected in view of the Curie behavior of 2, the M vs H data for this complex obey to a simple sum of four S = 7/2 Brillouin functions (with g = 1.9; the magnetization reaches 26 μB at 7 T and 1.8 K), further confirming the presence of noninteracting isotropic magnetic sites. In the case of the complexes 1, 3, and 4, the intrinsic magnetic anisotropy of the lanthanide ions composing these octanuclear species is clearly seen on the M vs H or M vs H/T data. After a rapid increase at low field without an inflection point (i.e., S-shaped curve, which could be indicative of weak antiferromagnetic interactions), the magnetization increases linearly without a clear saturation, even at 1.8 K under 7 T, highlighting the effect of the magnetic anisotropy of these lanthanide ions. As expected in this situation, the M vs H/T plot (Figures S7, S9, S10) is not superimposed on a single master curve. At 1.8 K, the magnetization reaches 24.6, 18.2, and 25.9 μB at 7 T for 1, 3, and 4 respectively. It is worth mentioning that none of these compounds exhibit an out-of-phase ac signal (above 1.8 K and for a frequency up to 1500 Hz) that could be indicative of slow relaxation of the magnetization, i.e., SMM properties. It is interesting to compare the magnetic properties of 1−4 with that of a series of heterodinuclear lanthanide (III) complexes synthesized by Costes et al.21 As mentioned above, the gradual decrease of the magnetic susceptibility of 1−4, at low temperatures, is probably due to the depopulation of the excitedstate Stark sublevels.28 From the discussion on the crystal structures, it is very clear that there are two tetranuclear subunits in the octanuclear complex, which are further connected to each other by the bridging coordination action of the oxalate ligands. Each of the two tetranuclear subunits itself contains two dinuclear motifs. Within each dinuclear unit, the inner pocket of the L3− ligand contains the Ln(III) ion with a N4O3 coordination environment. The outer pocket contains the diamagnetic K+ ion. No direct contact between the lanthanide ions is present within the tetranuclear subunit. Also, the large

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ASSOCIATED CONTENT

S Supporting Information *

Scheme showing the synthesis of ligand LH3; tables containing selected bond distances and angles, ligand-centered photophysical data, and additional magnetic data; and figures showing molecular structures, field dependence of magnetization, ESIMS, thermogravimetric analysis, PXRD, excitation and emission spectra, and life decay profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Science and Technology, India, and the Council of Scientific and Industrial Research, India, for financial support. V.C is thankful to the Department of Science and Technology, for a J.C. Bose fellowship. P.B. and A.C. thank the Council of Scientific and Industrial Research, India, for a Senior Research Fellowship. R.C. and M.R. are grateful to the CEFIPRA, the University of Bordeaux, the CNRS, the Aquitaine Region, and the ANR for financial support.

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Oxalato-Bridged Neutral Octanuclear Heterometallic Complexes

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