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From Two-Dimensional Double Decker Architecture to ThreeDimensional pcu Framework with One-Dimensional Tube: Syntheses, Structures, Luminescence, and Magnetic Studies Xun Feng,†,‡ Jiange Wang,† Bin Liu,‡ Liya Wang,*,† Jianshe Zhao,*,‡ and Seikweng Ng§ †

College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, People's Republic of China Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials, Northwest University, Xi’an 710069, People's Republic of China § Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia ‡

S Supporting Information *

ABSTRACT: The hydrothermal reactions of lanthanide salts with substituted imidazole4,5-dicarboxylic acids in the presence of aliphatic carboxylates afforded a new family of lanthanide metal−organic frameworks formulated as {Y3[(Heimda)4(μ2-HCOO)·3H 2 O]·H 2 O} n (1Y), {[Gd 3 (Heim da) 4 (μ 2 -HCOO)·4H 2 O]·2H 2 O } n (2Gd), {[Tb3(Heimda)4(μ2-HCOO)·4H2O]·2H2O}n (3Tb) and {[Nd3(Hpimda)2(μ2-HCOO) (μ2 -C2O4)2·6H2O]·4H2O}n (4Nd), (H3eimda = 2-ethyl-1H-imidazole-4,5-dicarboxylic acid, while H3pimda = 2-propyl-1H-imidazole-4,5-dicarboxylic acid). The structural diversity and photophysical and magnetic properties have been investigated. The polymer 3 triggers intense characteristic lanthanide-centered green luminescence under UV excitation, and it exhibits gradually increasing luminescence intensities when dispersed in water, ethanol, and DMSO as suspensions. After water molecules are liberated from the condensed frameworks of 4, the evacuated product (4a) exhibits nitrogen sorption properties at 77 K with a hysteresis, as well as strong characteristic emissions of the Nd(III) ion in the near-infrared (NIR) region. Polymer 2 displays very weak but significant ferromagnetic couplings between adjacent Gd(III) ions through the carboxylate bridging, whereas the depopulation of the Stark levels or possible antiferromagnetic interactions within both polymers 4 and 4a leads to a continuous decrease of χMT when the samples are cooled from 300 to 2 K.

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INTRODUCTION In recent years, the design and construction of extended frameworks containing rare-earth metals bridged by nitrogenheterocyclic ligands have attracted a great deal of interest in chemistry and materials science fields, owing to their extraordinary molecular architectures and fascinating chemical and physical properties.1 Whereas the series of complexes based on the nitrogen-heterocyclic and aliphatic carboxylate mixedligands have been explored, which exhibited their intriguing architectures as well as diversity of functions,2 the design of metal−organic frameworks (MOFs) with excellent properties that combine dynamic porosity with magnetism or luminescence still poses one of the major challenges in the pursuit of multifunctional materials.3,4 Meanwhile, there exists an escalating interest in the synthesis of MOFs using lanthanides owing to their unique optical preferences, their intense, linelike, and long-lived emissions, which cover a spectral range from the ultraviolet to the visible and even the near-infrared (NIR) region. Especially in the range of 900−1700 nm, luminescence from lanthanide ions such as Nd(III), Er(III), and Yb(III) are of particular interest because of their potential application in various optical and medical devices.5 However, among the many MOFs built from different lanthanide carboxylate cluster SBUs, they usually consist of di-, tri-, or © 2011 American Chemical Society

tetranuclear metal−carboxylate clusters, in which water molecule plays an important role as bridging ligands.6 The luminescence is often quenched by the nonradiative exchange of electronic energy of Ln(III) to the high vibration modes of O−H bond.5 To overcome this difficulty, our strategy employs formate/formic acid as the starting materials to substitute the bridging water molecule, due to its strong coordination ability, to construct stable metal−carboxylate cluster secondary building units (SBUs).7 By contrast with a large number of lanthanide complexes containing only rigid or flexible ligands, the homogeneous constructions involving both nitrogen heterocyclic and the smallest aliphatic carboxylate ligands remain less developed because the products always have low crystallinity. The small aliphatic carboxylate ligands also can provide an opportunity to modulate the functionalities.8,9 In order to further study the coordination behavior that the lanthanide cation exerted on the secondary ligand and better understand the thermal behavior, luminescence, and magnetic properties of these systems, four novel lanthanide polymers based on the semirigid nitrogen heterocyclic carboxylate and Received: October 14, 2011 Revised: December 9, 2011 Published: December 13, 2011 927

dx.doi.org/10.1021/cg2013717 | Cryst. Growth Des. 2012, 12, 927−938

Crystal Growth & Design

Article

Table 1. Crystallographic Refinement Data and Experimental Details for Polymers 1−4 formula formula wt system space group a, Å b, Å c, Å β, deg V, Å3 Z ρcalcd, g/cm3 μ(Mo Kα), mm−1 F(000) crystal size, mm3 θ range, deg index ranges

measured reflns independent reflns obsd reflns (I > σ(I)) GOF for F2 R indexa (I > 2σ(I)) R indexa (all data) residual electron density (max and min) (e Å−3) a

1

2

3

4

C29H34N8O22Y3 1113.37 monoclinic C2/c 33.595(3) 9.1560(9) 13.0325(12) 92.0960(10) 4006.1(7) 4 1.846 4.406 2228 0.37 × 0.24 × 0.19 2.31−25.50 −39 ≤ h ≤ 40 −11 ≤ k ≤ 11 −15 ≤ l ≤ 15 14831 3728 3329 1.086 R1 = 0.0381 wR2 = 0.0981 R1 = 0.0436 wR2 =0.1007 0.955 and −1.260

C29H37N8O24Gd3 1353.42 monoclinic C2/c 33.716(3) 9.2437(8) 13.1971(11) 92.1400(10) 4110.0(6) 4 2.187 4.885 2604 0.23 × 0.18 × 0.15 2.28−25.50 −40 ≤ h ≤ 40 −11 ≤ k ≤ 11 −15 ≤ l ≤ 15 15171 3825 3663 1.007 R1 = 0.0240 wR2 = 0.0576 R1 = 0.0255 wR2 =0.0585 1.701 and −1.664

C29H37N8O24Tb3 1358.43 monoclinic C2/c 33.598(4) 9.1544(10) 13.0326(14) 92.1400(10) 4005.7(8) 4 2.253 5.342 2616 0.23 × 0.18 × 0.14 2.31−25.50 −40 ≤ h ≤ 40 −11 ≤ k ≤ 11 −15 ≤ l ≤ 15 14853 3727 3398 1.037 R1 = 0.0281 wR2 = 0.0632 R1 = 0.0324 wR2 =0.0652 1.579 and −1.695

C21H35N4O28Nd3 1226.25 orthorhombic Pnma 13.0079 22.5014(18) 12.6096(10) 90 3690.8(5) 4 2.203 4.265 2372 0.36 × 0.18 × 0.10 2.25−25.49 −15 ≤ h ≤ 15 −27 ≤ k ≤ 27 −15 ≤ l ≤ 15 26268 3519 3180 1.055 R1= 0.0228 wR2 = 0.0548 R1 =0.0262 wR2 = 0.0568 1.082 and −0.670

R1= [∑||Fo| − |Fc||/∑|Fo|], wR2 = ∑w[|Fo2 − Fc2|2/∑w(|Fw|2)2]1/2. Syntheses of the Polymers 1−4. {[Y3(Heimda)4(μ2-HCOO)·3H2O]·H2O}n} (1). H3eimda (0.039 g, 0.2 mmol) and sodium formate dihydrate (0.015 g, 0.2 mmol) in a solution of water/alcohol (v/v = 1.5, 10 mL) were mixed with an aqueous solution (10 mL) of Y(NO3)3·6H2O (0.076 g, 0.2 mmol). After the mixture was stirred for 30 min in air, the pH value was adjusted to 4.0 with nitric acid, and the mixture was placed into a25 mL Teflon-lined autoclave under autogenous pressure and heated at 155 °C for 72 h, then the autoclave was cooled over a period of 24 h at a rate 5 °C/h. After filtration, the product was washed with distilled water and then dried. Colorless crystals of 1 were obtained. Yield: 0.0342 g (45% based on lanthanide element). Elemental analysis (%) calcd for 1, C29H34N8O22Y3: C, 31.29; H, 3.08; N, 10.06. Found: C, 30.87; H, 3.19; N, 9.94. IR (KBr pellet, cm−1): 3378s, 3215br, 2976m, 1592s, 1547s, 1472s, 1410m, 1333m, 1279s, 1112s, 866m, 793s, 660s, 517s. {[Gd3(Heimda)4(μ2-HCOO)·4H2O]·2H2O}n} (2). Polymer 2 was synthesized by a method similar to that of 1, except that Gd(NO3)3·6H2O was employed instead of yttrium nitrate. Colorless crystals of 2 were obtained. Yield: 0.037 g (39%). Elemental analysis (%) calcd for C29H37N8O24Gd3: C, 25.73; H, 2.75; N, 8.27. Found: C, 25.64; H, 2.91; N, 8.23. IR (KBr pellet, cm−1): 3357vs, 3208s, 2984m, 1581vs, 1539s, 1432s, 1357m, 1174s, 876s, 685m, 456m. {[Tb3(Heimda)4(μ2-HCOO)·4H2O]·2H2O}n} (3). Polymer 3 was isolated by the same procedure as 1, except that Tb(NO3)3·6H2O was employed, instead of yttrium nitrate. Colorless crystals of 3 were obtained. Yield: 0.037 g (39%). Elemental analysis (%) calcd for C29H37N8O24Tb3: C, 25.64; H, 2.74; N, 8.24. Found: C, 25.53; H, 2.91; N, 8.13. IR (KBr pellet, cm−1): 3363vs, 3176s, 2964m, 1583vs, 1517s, 1454s, 1384m, 1253s, 1218m, 876s, 785m, 522s, 429m. {[Nd3(Hpimda)2(μ2-HCOO)(μ2-C2O4)26H2O]·4H2O}n (4). A mixture of H3pimda (0.0341 g, 0.2 mmol), ammonium oxalate (0.0152 g, 0.1 mmol), and Nd(NO3)3·6H2O (0.0452 g, 0.1 mmol) was added to an aqueous solution (20 mL) of HCOOH/H2O (20 mL, v/v = 0.5/19.5). After the mixture was stirred for 20 min at room temperature, it was transferred to a 25 mL Teflon-lined stainless steel vessel and heated to

aliphatic carboxylate ligands have been synthesized and characterized systematically.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. Lanthanide oxides were purchased from J & K Chemical Limited. Lanthanide nitrate was prepared by the reaction of lanthanide oxides and nitric acid (10 mol/ L−1). Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400 element analyzer. IR spectra were recorded in the range 400−4000 cm−1 using a VECTOR-22 spectrometer using KBr discs. Magnetic data were obtained using a Quantum Design MPMS SQUID 7S magnetometer at an applied field of 2000 G using multicrystalline samples of 2, 4, and 4a in the temperature range of 2−300 K. The magnetic susceptibilities were corrected using Pascal’s constant and the diamagnetism of the holder. The thermogravimetric experiment was performed using a TGA/NETZSCH STA449C instrument heated from 25−900 °C (heating rate of 10 °C/min, nitrogen stream). The powder X-ray diffraction (PXRD) patterns were measured using a Bruker D8 advance powder diffractometer at 40 kV and 40 mA for Cu Kα radiation (λ = 1.5418 Å), with a scan speed of 0.2 s/step and a step size of 0.02° (2θ). The data were analyzed for d-spacing measurements using the EVA program from the Bruker Powder Analysis software package. The simulated powder patterns were calculated using Powder Cell 2.4 program. Absorption spectra were obtained using a Cary 50 Bio UV/visible spectrophotometer (Varian, Inc., Palo Alto, CA) in quartz cuvettes. Liquid state luminescence spectra in the visible ranges were measured at room temperature with an Edinburgh instrument FLS920 fluorescence spectrometer. Emission and excitation spectra were measured at room temperature using a JOBIN YVON/HORIBA SPEX. The multicrystal samples were dispersed in the various solvents into the suspension. The spectra in the near-infrared region were performed on PE-983G IR-spectrophotometer (Perkin-Elmer) with a microsecond flash lamp (μ-F900, Edinburgh) as the excitation source (resolution 1.0 nm). 928



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165 °C for 3 days, followed by cooling to room temperature over 24 h. Pink block crystals of 4 were obtained. Yield: 0.0236 g (41% based on lanthanide element). Element analysis calcd for C21H35N4Nd3O28: C, 20.61; H, 2.88; N, 4.58. Found: C, 20.68; H, 2.79; N, 4.60. IR (KBr, cm−1): 3443br, 2858s, 1642s, 1596s, 1438m, 1414s, 1381s, 1275m, 1174w, 884s, 807s, 781m, 725m, 526m, 495s. Crystallographic Data Collection and Refinement. Singlecrystal diffraction data for the polymers were collected on a Bruker SMART APEX II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The structures were solved using direct methods and successive Fourier difference synthesis (SHELXS-97) and refined using the full-matrix least-squares method on F2 with anisotropic thermal parameters for all nonhydrogen atoms (SHELXL-97). An empirical absorption correction was applied using the SADABS program.10 Disordered oxygen atoms of the formate and carbon atoms of the ethyl and propyl groups were restrained in order to obtain reasonable thermal parameters. The hydrogen atoms of organic ligands were placed in calculated positions and refined using a riding model on attached atoms with isotropic thermal parameters 1.2 times those of their carrier atoms. The summary crystallographic data and selected bond lengths and angles are listed in Table 1 and Tables S1−S3 in Supporting Information. Low-Pressure Nitrogen Sorption Measurements. The freshly prepared sample of 4 was soaked in methanol for 3 days, and the extraction was decanted subsequently. The sample was collected by decanting and treated with dichloromethane similarly to remove methanol for 4 days to remove the water molecules and then pumped under a dynamic vacuum at 140 °C overnight subsequently to activate the samples. The sample was dried further using the “degas” function of the surface area analyzer for 96 h at 220 °C. After the water was liberated, the desolvated phase of 4a, whose formula was presumed to be [Nd3(Hpimda)2(μ2-HCOO)(μ2-C2O4)2] was obtained, and it possessed extended 1-D channels and had permanent microporosity. The framework structure nearly remains intact after guest waters are liberated, which is confirmed by powder X-ray diffraction, thermogravimetric analysis (see Results and Discussion section for details), and IR spectra, as well as elemental analysis based on the collected date from the activated sample. Anal. Calcd for [Nd3(Hpimda)2(μ2-HCOO)(μ2-C2O4)2]: C, 23.75; H, 1.62; N, 5.28. Found: C, 23. 62; H, 1.69; N, 5.23. IR (KBr, cm−1): 3413w, 1642s, 1593s, 1429m, 1402s, 1358s, 1271m, 1078w, 802s, 781s, 557w. The low-pressure nitrogen adsorption measurements were performed at 77 K and 0−760 Torr on a Beckman Coulter SA 3100 surface area and pore size analyzer. Before the measurement, the sample was dried again by using the “degas” function of the surface area analyzer for 96 h at 180 °C. High purity nitrogen (99.999%) was used for the measurement. The regulator and pipe were flushed with nitrogen before being connected to the analyzer. The internal lines of the instrument were flushed three times by utilizing the “flushing lines” function of the program to ensure the purity of nitrogen gas. Specific surface area was obtained by fitting the Brunauer−Emmett−Teller equation11 to the adsorption isotherm of nitrogen gas, and total pore volume was calculated using the Dubinin−Radushkevich (D−R) equation.12

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first attempt to react yttrium(III) salt with H3eimda directly using the same quantities of sodium hydroxide in an aqueous solution to deprotonate the H3eimda only gave some precipitates or microcrystalline products unsuitable for singlecrystal X-ray diffraction analysis, which may be due to the higher pH value, whereas the lower pH value cannot ensure the carboxylic acid ligand is fully deprotonated for effective Yb(III) binding. Experiments revealed that when the pH value was adjusted to 4 by the addition of 1 mol/L nitric acid, the large crystal of 1 suitable for crystallographic analysis was successfully isolated in appropriate condition. In order to perform a systematic investigation of the lanthanide polymers incorporating N-heterocyclic dicarboxylate ligands, a series of experiments were carried out, during which we changed the H3eimda ligand as H3pimda and added ammonium oxalate as auxiliary ligand to further obtain polymer 4. In the IR spectra of polymers 1, 2, 3, and 4, the presence of the broad and strong characteristic stretches in frequency region of 3250−3550 cm−1 are assigned to the characteristic peaks of OH vibration of free water. The strong vibrations appeared around 1635 and 1455 cm−1 correspond to the asymmetric and symmetric stretching vibrations of the carboxylate groups.13 The peaks at ca. 1590 cm−1 are assigned to the coordinated HCO2− or C2O42− anions.14 The absence of strong bands ranging from 1690 to 1710 cm−1 indicates the complete deprotonation of carboxylic groups of imidazole dicarboxylic acid. In the IR spectrum, desolvated phase of 4a, the broad and strong peaks around 3443 and 2965 cm−1 have disapeared due to removal of the water molecules coordinated to the metal ion.14 Description of the Structures. The X-ray diffraction analysis reveals that polymers 1Y, 2Gd, and 3Tb exhibit very similar molecular structure except the distinction of metal ion and different numbers of water molecules in the crystal lattice. Therefore, structure of 1 is described in detail to represent their frameworks. The asymmetric unit of 1 contains two crystallographically independent Yb(III) cations, two Heimda ligands, one formate group, and three coordinated waters, as well as one lattice water molecule. As illustrated in Figure S1a, Supporting Information, the two Heimda ligands have the same coordination mode. They both display the μ3-kN,O:kO,OÂ:kO′ mode to connect two Y(III) ions. Both kinds of the Y(III) ions are octacoordinated, exhibiting a slightly distorted square antiprism geometry, but in different coordination environments. The Y(1) ion is coordinated with two imidazolyl nitrogen atoms from Heimda ligands and five carboxylate oxygen atoms of the Heimda ligands with O1N6 donor set, and the coordination sphere of Y(1) ion is completed by another oxygen atom from a terminal water molecule. The Y(2) ion is coordinated to eight oxygen atoms, among which four are from carboxylate groups of Heimda ligands, two are from the formate groups, and the remaining two belong to water molecules. Both carboxylic groups adopt bidentate and monodentate modes (see Scheme 1a). Each Heimda ligand uses the 4-carboxylate group oxygen and 5-carboxylate group oxygen to chelate Y(2) ion. Two adjacent Y(1) ions are linked via the 5-carboxylate group oxygen and 3-imidazole nitrogen of the Heimda to generate an Y2 dimeric unit with Y···Y separation of 6.54(2) Å. These dimers are grafted on to the 1-D infinite ribbon chains along the crystallographic c axis (see Figure 1a). The dihedral angles between the neighboring imidazole rings of Heimda ligands sharing the common Y(III) ion are 78.55° and 14.63°, respectively. The completely deprotonated H3eimda acts as a pentadentate ligand, and the next Heimda ligand further acts as

RESULTS AND DISCUSSION

Hydrothermal Syntheses and IR Spectra. Our initial attempts to synthesize suitable crystals of lanthanide polymer by reflux, stirring, and diffusing methods at room temperature failed, which prompted us to use the hydrothermal synthesis method alternately. This may be ascribed to the features of hydrothermal synthesis, the special environment with high temperature and pressure to minimize the problems associated with ligand solubility and enhancing the reactivity of reactants.13 However, there are many synthetic parameters that influence the formation of MOFs during the hydrothermal reaction, including the structure of the ligand, the pH values, and the metal-to-ligand ratio and reaction medium chosen. Our 929



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layers is 8.605 Å. This array is comparable to those of series of lanthanide coordination polymers containing the 3,5-pyrazole dicarboxylate in the same crystal system.19 The projection diagram of the 3-D packing structure is illustrated in Figure S2, Supporting Information. Such architectures of MOFs have previously been reported only in several cases.19 Comparing the average Ln−N and Ln−O bond lengths among the polymers 1, 2, and 3 reveals that in the case of 2, the average lengths become a little bit longer than those of 3 and 1 arising from the lanthanide contraction effect. It was also found that within the one-dimensional chain, both the separation of Ln(2)···Ln(2) (bridged by formate) and separation of Ln(1)···Ln(2) (bridged by Heimda) along the c axis display a decreasing trend from 6.998 to 6.902 Å, and from 6.615 to 6.542 Å, respectively, which are also consistent with the lanthanide contraction,20 as shown in Table 2 for details. Interestingly, for 2, when the propyl was replaced by ethyl in the main ligand, the structure obtained is very similar to the complex based on the Hpimda,9d except for a different number of water molecules in the crystal lattice. In addition, the separation of Gd(III) ···Gd(III) in this case is slightly smaller than that of the analogous Gd complex. Polymer 4 (Nd) crystallizes in the orthorhombic system, and the symmetric unit contains three Nd(III) cations, two Hpimda ligands, one formate anion, two oxalate ligands, and six terminal water molecules, as well as four lattice water molecules (see Figures S3 and S4, Supporting Information). There are two crystallographically independent Nd(III) ions in the unit and both the Nd(III) ions are nine coordinated, in tricapped trigonal prismatic geometry. The coordination environments around them are different slightly from each other. The Nd(1) is ligated with two oxygen atoms from the oxalate ions, four carboxylate oxygen atoms of three Hpimda ligands, and one nitrogen from the imidazole ring completing the tricapped trigonal prismatic sphere. The coordination sphere around Nd(2) ions is occupied by nine oxygen atoms, which are from two oxalate, two formate ligands, and three terminal aqua molecules. The Hpimda has just the same coordination mode as the Heimda in polymer 1. To the best of our knowledge, this is one of the rare examples documented in which four bridging or terminal ligands (including two categories of shortest aliphatic carboxylates) coordinate to one metal center.21 The adjacent Nd(1) ions are doubly connected by the 5-carboxylic groups from two neighboring Hpimda ligands in the bridge and chelate μ2-(η2:η1)-O coordination modes (Scheme 1d), resulting in Nd(1)···Nd(1) separation of 4.211 Å in the dinuclear unit, as shown in Figure S5, Supporting Information. These dimer units are further grafted into a 1-D polymeric chain by the ligation of carboxylate oxygens from Hpimda ligands. In this structure, six adjacent Nd(III) ions are alternately connected by four oxalate and two Hpimda ligands to form a hexagonal Nd6 ring-like motif as shown in Figure 3a. The oxalate acts as a tetradentate ligand and forms two Nd− C−O−C−C−O five-membered chlating rings, which can propagate magnetic coupling between paramagnetic ions efficiently,1d and six Nd(III) ions occupied the vertices of the macrocycle with Nd···Nd separation of 8.86(5) and 6.46(4) Å, respectively, among which three Nd(1) centers alternately occupied three vertices of a triangle, and the Nd···Nd···Nd angles are within the range from 92.21° to 128.61°. Close inspection reveals that it is an undulated hexanuclear crown-like metal−organic macrocycle with a large 28-membered ring conformation with C2 symmetry. These hexagonal subunits are further linked by the oxalate and propagate infinitely along the

Scheme 1. Various Coordination Modes of Ligands Observed in the Polymers 1−4

a linker using the 4-carboxylate oxygen and its symmetric partners, consequently connecting these 1-D chains into a 2-D grid along the a axis, as displayed in Figure 1b, and the adjacent binuclear units are further linked together by the μ2-O bridge from the 5-carboxylate group to form a parallelogram configuration. The four vertices were occupied by four neighboring Y(1) ions, and the diagonal-to-diagonal distances are 12.42 and 10.35 Å, respectively (see Figure S1b, Supporting Information). The individual nets are undulated, and it is comparable to the lamellae array of a series of lanthanide complexes based on pyridine-2,6-dicarboxylate.15 The formate groups in 1, meantime, connect the neighboring Y(2) ions in an end to end fashion, resulting in the infinite 1-D {[Y(HCOO)]2+}n chains (see Scheme 1b and Figure 1c), and they are further propagated into a doubled chain network linked by the H3eimda ligand along the c crystal direction, which is similar to many reported compounds with formate as the bridging ligand; thus it is interesting to perform magnetic studies evaluating the eventual exchange interactions between the metal centers.13,16 The Y(1)−O and Y(2)−O bond lengths are 2.357 and 2.328 Å, respectively, and the Y(2)···Y(2) separation is 6.902 Å within the chain. The Y−O bond lengths are comparable to those of bis(phosphinic)diamido yttrium complexes.17 The {[Y(HCOO)]2+}n chains are parallel to the {[Y(Heimda)]+}n chains above-mentioned, resulting in the 1-D double chain along the ac plane (see Figure 2a). The formate and Heimda ligands alternately linked the neighboring Y(1) and two Y(2) ions giving rise to a hexametal parallelogram aggregate along the ac plane as shown in Figure S1c, Supporting Information. The adjacent 2-D coordination networks are further interlinked by the Y(2) chains incorporating formate groups via the 5-carboxylate groups from the third Heimda attached the Y(1) ion to fabricate a highly ordered claylike double layer architecture18 consequently, as demonstrated in Figure 2b. These double layers are interconnected and extended through interlayer hydrogen interactions between carboxylic groups and imidazolyl rings from the adjacent 2-D double sheets, N(2)−H(2)···O(4)#8 [O−N = 2.754 Å, O···H−N = 163.0°], to generate a 3-D supramolecular edifice, and the distance between neighboring 930



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Figure 1. (a) Polyhedral diagram of the 1-D alternate chain of Y(1) cations linked by the Heimda ligands viewed along c axis in 1. (b) Diamond illustration of the 2-D (4, 4) corrugated lamellae. (c) Projective view of the 1-D double chain constructed by Heimda and formate ligands. (Y(1) and Y(2) ions are highlighted in green and pink balls, respectively.).

Table 2. Comparison of the Ln···Ln Metal Separations and Bond Lengths (Å)

polymers

separation of Ln(2)···Ln (2)

separation of Ln(1)···Ln(2)

2 (Gd) 3 (Tb) 1 (Y)

6.998 6.908 6.902

6.615 6.544 6.542

Ln(1)−O Ln(2)−O Ln(1)−N 2.358 2.328 2.324

2.380 2.342 2.328

2.578 2.537 2.544

molecules are located inside the ring viewed from section. The grids are further connected by carboxylic groups to fabricate a noninterpenetrated, 3-D, highly ordered porous framework bearing 1-D channels via the formate bridging ligand (see Figure S6, Supporting Information). The nanotubular channels (size of 10.92 Å × 8.65 Å, defined by the separation between opposite lanthanide ions, deduced van der Waals radii) were found to be large enough to accommodate solvent molecules, as illustrated in Figure 3c. It should also be pointed out that the

Figure 2. Projective view of 2-D double-decker motif consisting of two monolayers connected by the Heimda ligands along the ac plane (Y(1) ion is represented by the green ball, and Y(2) is represented by the pink ball).

ab plane sharing the Nd2 dimer apices mentioned above to produce a 2-D sheet structure consisting of crown-like metal− organic rings (see Figure 3b), and the eight guest water 931



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formate ligand plays an important role in coordination chemistry, which may adopt versatile three-atom binding modes such as monodentate, chelating, and bridging in the syn−syn, syn−anti, and anti−anti configurations. Interestingly, all of the bridging modes can be comparable to the corresponding ones in azide or thiocyanate compounds.16,22 Remarkably, in this case, formate anion serves as a bridging ligand in an end to end fashion as that in 1, linking the adjacent Nd(2) ions to form the 1-D polymeric chain along ab plane, also acting as a barrel brace to enhance the robustness of the 1D nanotubular networks. Another outstanding feature is that along edge of the hexameric metal−organic crown, the two adjacent Nd(1) ions and another Nd(2) ion were connected by two Hpimda ligands to form a triangle array, as demonstrated in Figure S7, Supporting Information. In this 2-D sheet, the Nd6 and Nd3 rings share their edges giving rise to a network with 3464 topology, so-called interesting Kagomé lattice arrangement,23 as displayed in Figure 4, but these 2-D motifs

Figure 4. Schematic illustration of the binodal Kagomé lattice 2-D layer in 4 approximately along the bc plane. Nd(1) and Nd(2) ions are highlighted by the purple and green balls, respectively.

are arranged in a herringbone pattern and a little inclined.24 To the best of our knowledge, this is one of the rare examples of a lanthanide−organic Kagomé lattice based on the formate mediator in coordination polymers.1h A better insight into the nature of the intricate framework of 4 by the application of a topological approach25 reveals that in the spectacular 2-D layer along the bc plane, each Nd(1) center acts as a 3connected node to connect three Hpimda and the Hpimda ligand serves as the planar 3-connected node linking three Nd(III) ions while the Nd(2) ion also acts as a 3-connected node connecting one oxalate and two formate ligands, resulting in a 4·82 topology of the layer. Oxalate ions further connect these layers to afford a 3,4-connected binodal 3-D framework with the Schläfli symbol of (4·62·83)(4·6·8) and vertex symbol of (4·6·6·8·8·107)(4·6·8), as depicted in Figure S8, Supporting Information. If the 4-membered rings (highlighted as purple ball) is simplified as one node, the MOF could be alternatively described as a MOF of pcu with the common six-connected network topology.

Figure 3. (a) Perspective view of the crown-like macrocycle as the SBU in 4 (the green ball represents the Nd(1) ion, and the purple one represents the Nd(2) ion); hydrogen atoms and lattice water molecules have been omitted for clarity. (b) Projection view of the 3-D framework along the bc plane, illustrating the eight included water molecules located inside the hexamer grids. (c) Projection view of the cross-section of 3-D zeolite-like framework of 4, showing the hexameric metal−organic channels after the inside waters are removed. 932



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Figure 5. (a) 1-D nanotube enclosed by neodymium−carboxylate secondary building units and formate ligands along the tube wall in 4, viewed from the crystallographic c axis. (b) Perspective view of the 1-D infinite chain formed by the trans pentanuclear water clusters, illustrating the hydrogen bonding scheme along the crystallographic c axis.

PXRD Measurements and Thermal Gravimetric Analysis. The purity and homogeneity of the bulk products of 1, 2, 3, and 4 were determined by comparison of the simulated and experimental X-ray powder diffraction patterns, and the X-ray powder diffraction patterns of activated sample 4a and of the as-synthesized powder of 4 were both measured. The peak positions of the experimental patterns for 1, 2, 3, and 4 nearly matched those of the simulated ones generated from singlecrystal X-ray diffraction data, as depicted in the Figures S10− S13, Supporting Information. The differences in intensity may be due to the preferred orientation of the powder samples. For 4a, only some minor Bragg peak positions and widths show some variations in comparison with the as-synthesized framework of 4, revealing that the removal of the water molecules does not lead to a host-framework transition, as illustrated in Figure S14, Supporting Information. Unfortunately, attempts to obtain the precise single-crystal structure of the dehydrated product were unsuccessful, which was attributed to its poor crystal quality after the lattice water molecules were removed. The thermal stability of 1, 2, 3, 4, and 4a was explored by thermogravimetric (TG) analysis, as illustrated in Figures S15− S17, Supporting Information. The polymers 1, 2, and 3 showed similar thermal behavior; here the complex 2 was used as a representative. The TG curves indicate that the framework of 1 exhibits an initial mass loss of 8.72% corresponding to the dissociation of the two lattice and four coordinated water molecules, and the further weight loss of 1 begins above 260 °C, which is attributed to the release of the formate groups (calcd. 3.41%). The polymer 1 began to decompose upon

As illustrated in Figure 5a, the nanotubular channel is accompanied by the both coordinated and solvate water molecules, and the waters inside produced a series of extensive hydrogen bonding interactions (see Figure 5b and Table S3, Supporting Information) with the coordination water molecules. The five adjacent water molecules created a fused pentameric unit, yielding a 1-D chain aggregate via hydrogen bonds along the channel. The water clusters display a square pyramidal configuration, among which one water molecule in the 3-fold axis was located in the axial position, and these chains extended twist along the crystallographic a axis leading to the adjacent axial water molecules being in nearly opposite direction. The adjacent O···O distances are in the range of 2.930(10)−2.991(8) Å (2.956 Å on average). The value observed here is somewhat larger than the standard value of 2.85 Å in regular ice,26 and the extended water structure based on an odd-numbered water cluster with a high degree of asymmetry propagating in the dynamics of the MOF is seldom observed.27 After hypothetical removal of these guest waters (see Figure 3c and Figures S9 and S13, Supporting Information), the incipient void space is found for the 3-D noninterpentrating framework to be 749.6 Å3 per cell volume (accounting for 20.3% of the total unit cell system, volume of 3690.8 Å3), calculated using the Platon programs.28 Comparison of structures of 1, 2, 3, and 4 reveals that in the present work, MOFs based on the different building blocks demonstrated that the intrinsic features of the linkers play an important role for the topology of the frameworks,29 and further introduction of a simple oxalate as ancillary ligand may introduce control over alternative network arrangements.30 933



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further heating and underwent a rapid and significant weight loss of 33.2% in the temperature range of 450−550 °C, which corresponds to the destruction of the Heimda organic ligands (calcd. 34.2%). The TG curves of 4 display mainly three steps of weight loss processes. The first one occurred at ca. 195 °C with a weight loss of 14.3%, which corresponds to the release of four lattice water molecules and six coordinated water molecules together in one step, ten water molecules per formula, with a calculated value of 13.2%. The second process might include a large range of ca. 310 to 440 °C. It is attributed to the departure of the formate moiety per molecule with a weight loss of ca. 4.02%. The third process began from ca. 550 °C, corresponding to decomposition the two Hpimda organic ligands, as well as two oxalates. The residue weight of 44.6% is close to the calculated value, assuming the final product of Nd2O3. For 4a, the weight loss processes were combined into two main steps due to the water molecules being liberated. The first one occurred from ca. 60 to 350 °C with a weight loss of 4.61%, which can be attributed to the release of minor of waters impurity and one molecule of the formate per formula unit, with a calculated value of 4.33% (based on one formate). The second process might cover a large range from ca. 410 to 720 °C, during which the complex underwent a rapid and significant weight loss of 50.40% upon further heating, which corresponds to the destruction of the Hpimda ligand as well as two oxalates in one step. It is nearly in accordance with the calculated value of 51.42%, based on the single-crystal structure analysis. The residue of 45.43% is close to the calculated 46.62% of the total sample mass as shown in Figure S17, Supporting Information. Photoluminescence Properties. As described in Figure S18, Supporting Information, the gadolinium polymer, 2, exhibits a broad emission band in the near-ultraviolet region with the maximum wavelength of 398 nm, which has been tentatively attributed to ligand-centered transitions within the substituted imidazolecarboxylate ligand, being compared with that of emission of free H3eimda ligand (λmax = 430 nm),31 and characteristic shift of this may contribute to the bonding of the Gd(III) cation involving interaction with the carboxylic oxygen of the nitrogen heterocyclic component.31 The absorbance spectrum of Heimda displays a band with apparent maximum centered at ca. 300 nm, and the excitation polymer 3 at this band maximum produced the most intensive emission band maximum at 545 nm. Upon monitoring of the fluorescence at this emission in methanol, the excitation spectrum of polymer 3 displayed a band with maximum at 309 nm, nearly correlated with the absorbance spectrum, which is attributed to the π−π* transition of the Heimda chromophore, as illustrated in Figure S19, Supporting Information. This indicates that the Tb(III) luminescence can be efficiently sensitized by the Heimda ligand via the “antenna effect”.32 As illuminated in Figure 6a, for polymer 3, upon this photoexcitation, the emission spectrum showed intensive luminescence not only in the UV region with maximum wavelength 398 nm but also in the visible region with the bands peaking at λmax = 492, 545, 586, and 638 nm. These originated from the characteristic 5D4 → 7Fn transition of a sensitized terbium emission, where n = 6, 5, 4, and 3, respectively, and the relative intensity of the sharp-line band is 5 D4 → 7F5 transition.33 As depicted in Figure 6a, the H3eimda ligand displays emission at λmax = 434 nm, which is overlapped by the first emission peak of 3 in the UV region, indicating that it would be attributed to the formation of a ligand-to-metal charge transfer (LMCT) transition.34 It is noteworthy that the Tb(III) polymer 3 shows a higher intensity fluorescence

Figure 6. (a) The photoemission spectra of the polymer 3 (excited at 310 nm) and H3eimda ligand in methanol solution. (b) Solution excitation spectra for powder of polymer 3 dispersed in various solvents. Color scheme: H2O, black; DMSO, blue; DMF, magenta; EtOH, green.

emission than that of the N-heteroatomic free ligand.35 The luminescence measurements of 1 mg of polymer 3 dispersed in 5 mL of solvents after 24 h clearly displayed that the 5D4 → 7F5 emissions at 545 nm were the strongest intensities in DMSO (dimethyl sulfoxide) and ethanol suspensions, but the intensities were significantly reduced in DMF and H2O suspensions (Figure 6b). Presumably, this is because the small molecules of H2O could enter the coordination spheres of the Tb(III) ions within the framework, which quenches the luminescence intensities effectively.36 The EtOH and DMSO molecules, which possess sterically bulky alkyl groups, might protect Tb(III) ions from quenching by O−H oscillators. Interestingly, the different polarities of solvates do not alter the luminescence peak positions, except that the emission peak maximum at ca. 350 nm originating from the ligand-to-metal charge transfer transition showed a slight shift. The absorption spectrum of the H3pimda ligand (see Figure S20, Supporting Information) displays a band with apparent maximum centered at 245 nm, which is assigned to the π → π* 934



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transition of the conjugation in the free H3pimda ligand. The excitation spectrum recorded upon monitoring the emission band of Nd(III) ion (see Figure S21, Supporting Information) also contains the band centered at ca. 245 nm that adopts the same profile as the absorption of H3pimda ligand, which means the absorption band of ligand is overlapped partly with the excited spectrum of polymer 4, also indicating that H3pimda can sensitize Nd(III) via the antenna effect.32 As displayed in Figure 7, upon excitation at 298 nm, the polymer 4 exhibits the

Figure 8. Nitrogen gas sorption−desorption isotherms at 77 K for polymer 4a. The filled (■) and open (○) symbols represent adsorption and desorption data, respectively.

high-point with the maximum adsorption of 312 cm3 (STP)/g of N2 at 77 K and 760 Torr, displaying much nitrogen absorbed and some mesopore formed by powder interspaces of the dehydrated material, or may be attributed to N2 condensation on crystallite surface. Fitting the Brunauer−Emmett−Teller (BET) equation to the adsorption isotherm of N2 for 4a gives the estimated BET surface area of 296 m2/g. By use of the Dubinin−Radushkevich equation, the specific pore volume of 4a is estimated to be 0.145 cm3/g (void = 20.2%) calculated with N2 adsorption data in region of 0.1−0.95P0, which is nearly in aggrement with the VP value calculated from the crystal structure with PLATON.28 This value is significantly smaller than the corresponding values of the coordination framework based on 8-connected {Co3(OH)} clusters.39b But it is higher than that of binary lanthanide polymer [Tb2(bdc)3]·4H2O.39b,c The Horvath−Kawazoe (HK) model indicates a pore diameter of ca. 8.7 Å, which is nearly consistent with the crystal X-ray analysis. The adsorption isotherm is not saturated, and a higher nitrogen uptake is expected at higher pressure.40 The profile of adsorption isotherm may be compared with the chromium naphthalene dicarboxylate open framework with giant pore.41 Magnetic Properties. The temperature dependence of the χMT product and χM for 2 is illustrated in Figure 9. At room temperature, the value of and χMT is ca. 8.77 cm3 mol−1 K, which is somewhat larger than the theoretical value of 7.88 cm3 mol−1 K, the expected value for a free Gd(III) ion with S = 7/2, g = 2.0.1c,h,8d Upon cooling, this value gradually increases up to the maximum of 9.04 cm3 mol−1 K until 76 K, whereas below that the χMT quickly decreases continuously to a value of 7.13 cm3 mol−1 K at 2 K. This behavior is indicative of the existence of weak ferromagnetic interactions between adjacent Gd(III) ions. According to the Gd(III) ion having a 8S7/2 (L = 0) ground-state configuration, which is spherically symmetric and is therefore not susceptible to crystal field effect,42 and the large Gd···Gd separation through the carboxyl bridge allows us to consider that the crystal structure of 2 would behave as a uniform chain of Gd(III) ions, bridged by Heimda ligands, from a magnetic point of view (see Figure 1a). Having this in mind and taking into account the large value of the local interacting spin [S = 7/2], the classical spin expression derived by Fisher to describe the magnetic behavior of a uniform chain

Figure 7. Normalized emission spectra of polymer 4 (blue) and 4a (red) (λexc = 298 nm) in the NIR region upon excitation of the antenna chromospheres.

characteristic emission bands maxima at 1062 and 1343 nm in the NIR region, which are attributed to the 4F3/2 → 4I11/2 and 4 F3/2 → 4I13/2 transitions of Nd(III) ion, respectively.32 The band at 1062 nm is dominant and most important because it is potentially applicable to laser emission, fluoroimmuno assays and in vivo detection.37 The photoluminescent properties of 4 are different from those of Nd−Cd−organic frameworks based on imidazole-4,5-dicarboxylate. This is probably attributed to introduction the electron-donating species (propyl groups) and gauche coligand of oxalate, thus facilitating the electron transfer in this case.38 The luminescence spectra recorded of polymer 4a is similar to the solvated phase of 4; however the intensities of the emission bands for the dehydrated species are significantly enhanced. It is because the coordinated and lattice water molecules were removed from the inner coordination sphere of Nd(III).7 Nitrogen Gas Adsorption Studies. After the guest and coordinated waters are liberated, a microporous polymer 4a is obtained, which has extended 1-D tunnels and possesses permanent microporosity, and the permanent microporosity is further confirmed by the nitrogen gas adsorption isotherm. The nitrogen adsorption isotherms for the activated sample 4a were measured at 77 K. As illustrated in Figure 8, type III sorption behavior, which is characteristic of solids with micropores, is observed in the relative pressure range of P/P0 = 0−1.0. The adsorption isotherm shows a smooth rise at low pressure and a plateau after saturation, then a very slow stepping up at 0.82 P/ P0 region, which is characteristics of microporous adsorbents, and what occurred before 0.85 P/P0 can be attributed to the process of the nitrogen filling into the micropores.39a The isotherm goes up steeply above P/P0 = 0.85 and reaches the 935



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Figure 9. Plots of χMT (□) and χM (○) versus T for polymer 2. The solid line represents the best fit obtained from the Hamiltonian given in the text.

with large spins eq 1 applies for 2.43 In this expression, u is the Langevin function defined as

χ=

Ng 2β2 ⎡1 + u ⎤ S(S + 1)⎢ ⎣ 1 − u ⎥⎦ 3KT

(1)

u = coth[JS(S + 1)/(KT)] − KT/[JS(S + 1)] where N, β, k, and g have their usual meanings and J is the exchange coupling parameter between adjacent spins. We neglect all exchange interactions between nearest-neighbor Gd(III) ions, including that of being connected by formate. The best least-squares fitting parameters are J = +0.038 cm−1, g = 2.02, and R = 1.3 × 10−5 (R is the agreement factor defined as R = ∑[(χM)obs − (χM)calc]2/∑[(χM)obs]2). Beyond 50 K, the temperature dependence of the reciprocal susceptibilities (1/χM) obeys the Curie−Weiss law [χM = C/(T − θ)] above 40 K with θ = 0.11 K, C = 8.71 cm3 K mol−1, and R = 6.14 × 10−5. (See Figure S22, Supporting Information). The θ value also supports the presence of weak ferromagnetic interaction within the polymer 2. The magnetic behavior in this case is similar to the reported compounds with formate as the bridging ligand.1h It is found that the reported ferromagnetic gadolinium compounds with bridging carboxylic motifs have J values between 0.024 and 0.080 cm−1, and the Gd−O−Gd angle values are in the region of 114° to 119°, while Gd···Gd distances are between 3.937 and 6.516 Å.1c,e,f,h,8d The plots of χMT/χM versus T of 4 and 4a are reported in Figure 10a,b, respectively. The χMT product of 4 at 300 K is 1.38 cm3 K mol−1, which is smaller than the expected value of 1.64 cm3 K mol−1 for a noninteracting Nd(III) ion in the 4I9/2 ground state (g = 8/11).44a This value and the reduction of χMT at lower T are clearly related to the thermal depopulation of the crystal-field energy levels of the multiplet.43,44 As the temperature is lowered, the χMT product decreases monotonously and slowly within the entire temperature range and reaches 0.61 cm3 K·mol−1 at 2 K. This is comparable to results reported previously,43,44 implying that the magnetic susceptibility of 4 deviates from the Curie law because of the thermal depopulation of Stark level or indicates the presence of possible antiferromagnetic interactions between the adjacent Nd(III) cations bridged by the oxalate ligand or the depopulation of

Figure 10. Temperature dependence of χMT (□) and χM (○) for 4 (a) and 4a (b), respectively. The solid line represents the best fit obtained from the Hamiltonian given in the text.

Stark sublevels together with crystal affection on the consideration of Ln(III) strong spin−orbital coupling.44b From the magnetic point of view, the basic unit of 4 can also be considered as the mononuclear motif, in which the Nd(III) ions are doubly linked through a pair of carboxylate bridges since the coupling through Hpimda and formate are almost negligible due to the long distance. To obtain a rough quantitative estimation of the magnetic interaction between Ln(III) ions, the Nd(III) ion may be assumed to exhibit a splitting of the mj energy levels (Ĥ = ΔJẑ 2) in an axial crystal field.44a,45 Thus, beyond 50 K, the susceptibility data were fitted according to approximately χNd =

Ng 2β2 [81e−81Δ /(4kT ) + 49e−49Δ /(4kT ) 4kT + 25e−25Δ /(4kT ) + 9e−9Δ /(4kT ) + e−Δ /(4kT )/(e−81Δ /(4kT ) + e−49Δ /(4kT ) + e−25Δ /(4kT ) + e−9Δ /(4kT ) + e−Δ /(4kT ))]

(2)

described as eq 2. In the expressions, Δ is the zero-field splitting parameter, and the Zeeman splitting was treated isotropically for the sake of simplicity. The zJ′ parameter based on the molecular field approximation in eq 3 is introduced to take into account the molecular field approximation introduced to simulate the magnetic interaction between the Nd(III) 45

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ions.46 2 2

χ = χNd /[1 − (2zJ ′ /Ng β )χNd]

(3)

χNd = [(NgJ 2β2)J(J + 1)]/(1 + 3KT ) where J = 9/2, gJ = 8/11.47 The fitting analyses of the magnetic data give the results Δ = −1.90 cm−1, g = 0.76, zJ′ = −0.37 cm−1, and R = 3.45 × 10−6. Comparing 4 with 4a, we can find that the χMT value of the evacuated phase at 300 K is somewhat higher for 4a than 4, and the Δ value is also larger than that of assynthesized polymer of 4. This is probably due to the intramolecular hydrogen bond interactions or the coordination evironment and ligand field around the paraparamagnetic cation within the framework having been altered.48

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CONCLUSIONS In summary, the present investigation demonstrated the structures and properties of four new lanthanide polymers based on both nitrogen heterocyclic imidazole dicarboxylates and simple aliphatic carboxylate auxiliary ligands via solvothermal reactions. The additional introduction of oxalate leads to the structure changing from the 2-D double-decker configuration to 3-D lanthanide−organic framework with pcu topology. The evacuated phase 4a was obtained through the “from the condensed lanthanide coordination solids to microporous frameworks” approach, and it exhibited nitrogen gas adsorption properties as well as strong emission in the nearinfrared region. The studies of the detailed magnetic and gas adsorption behaviors and the use of substituted imidazole dicarboxylate ligand in conjunction with other lanthanide metals are currently under investigation. ASSOCIATED CONTENT

* Supporting Information S

The additional structures and characterizations of polymers (Figures S1−S22) and selected bonds lengths and angles for polymers (Tables S1−S3). This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic files in CIF format have been deposited with the Cambridge Crystallographic Data Center. Copies of this information may be obtained free of charge, by quoting the publication citation and deposition numbers CCDC: 816027 (1), 833020 (2), 830510 (3), and 756340 (4).

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REFERENCES

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The least-squares analysis of the magnetic data gives Δ = −1.79 cm−1, g = 0.69, zJ′ = −0.45 cm−1, and R = 2.73 × 10−4. For 4a, the value χMT at 300 K is 1.53 cm3 K mol−1, which is somewhat smaller than the value (1.64 cm3 K mol−1) calculated according to the equation

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AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected]; [email protected]. Tel: +86037965523593; +86 02988302604.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21071074 and 21171139), Henan tackle key problem of science and technology (No 102102210450), and the Foundation of Education Committee of Henan province, China (No 2010A150016). 937



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