MetalâOrganic Frameworks Built from a Linear ... - ACS Publications

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Crystal Growth & Design

Metal Organic Frameworks Built from a Linear Rigid Dicarboxylate and Different Co-linkers: Trap of the Keto form of Ethylacetoacetate, Luminescence and Ferroelectric Studies Tapan K. Pal,a Rajesh Katoch,b Ashish Gargb and Parimal K. Bharadwaj*a a b

Department of Chemistry, Indian Institute of Technology Kanpur, 208016, India Department of Materials Science and Engineering, IIT Kanpur, Kanpur, 208016, India

ABSTRACT: The ligand 2,6,2',6'-tetranitro-biphenyl-4,4'-dicarboxylic acid (H2L) has been used alone or with a co-ligand to construct a number of metal organic framework (MOF) with different metal ions that

are

X-ray

crystallographically

characterized.

The

porous

3D

MOF

{[Gd2(L)3(DMF)4]·(4DMF)·(3H2O)}n (1) (DMF = N,N´-dimethylformamide) is found to be an excellent host to keto form of ethylacetoacetate to produce, {[Gd(L)1.5(DMF)2(H2O)2](S)(H2O)}n (1a) (S = ethyl 3-oxobutanoate) in single crystal to single crystal (SC-SC) transformation. This involves drastic rearrangement of the channels including several carboxylate shifts and concomitant movement of water molecules from cavity to the metal center. Interestingly, the daughter framework 1a reverts back to the mother framework 1 upon keeping in DMF for 3 days at RT suggesting that the framework 1 can be used as a container. The linker H2L also forms the MOFs,

{[(Cd)4(L)3(HlL1)2(DMF)(H2O)2](DMF)3(H2O)2}n

(2),

{[Cd(L)(L2)]}n

(3),

{[(Cd)1.5(L)1.5(L3)]}n (4) and {[Cd(L)(L4)(H2O)]}n (5) in presence of different co-linkers. Solidstate photoluminescence studies have been performed on MOFs 2-5 at RT showed intra-ligand (π–π*) emission. The MOF 2 being a chiral compound has been subjected to ferroelectric measurements. All the compounds (1-5) have been characterized by X-ray crystallography, elemental analysis, powder X-ray diffraction patterns, thermogravimetry and infrared spectroscopy.

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INTRODUCTION The design and synthesis of metal-organic frameworks (MOF)1−6 continue to attract enormous interest due to their novel architectures as well as potential applications.7−19 In this regard, most of the studies are concentrated towards construction of open robust frameworks that can maintain the structural integrity upon removal of solvent molecules from the lattice as well as metal-bound to generate porous frameworks for various purposes. Single-crystal to singlecrystal (SC−SC)20−24 transformation is an interesting phenomenon that allows crystallographic snap-shots25 of the structural changes accompanying the transformation. If during this process single crystalinity is maintained then any structural changes can be easily read out from single crystal X-ray data. But most of the cases, major obstacle associated with the loss of single crystalinity. The SC-SC transformations in MOFs associated with structural changes mainly promote by some stimuli like heat, light, solvent induced26−28 etc. that involves change in the metal coordination number, removal and insertion of guest and rearrangement of bonds29−31. In this context guest molecule induces structural transformation are extremely interesting. Owing to the high and variable coordination numbers adopted by the lanthanide ions, they can be suitable nodes for the fabrication of flexible MOFs. Changes in the carboxylate coordination modes are known as the “carboxylate shift” is a low-energy process that has considerable biological significance32−34. Although the “carboxylate shift” is not so common amongst coordination polymers,35−37 it is known to play crucial roles in the biological functions38−40 of several enzymes such as methane monooxygenase,41−42 R2 subunit of ribonucleotide reductase,43−44 oxygen transport protein hemerythrin45 and so on. Earlier, we used the ligand H2L (Scheme 1) to construct a flexible framework that allowed direct crystallographic observation of catalytic reactions46 inside the pores. Herein, we report the SC-SC encapsulation of the keto form (S =


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ethyl 3-oxobutanoate) of ethylacetoacetate (EAA) inside the pores of the framework 1. Our interest in ligand H2L is mainly based on the fact that the presence of four nitro groups on the benzene ring creates steric hindrance, allowing the formation of non-interpenetrating 3D structures with sufficient pore size to act as a container. This ligand has also been used here along with L-(+)-lactic acid as a co-linker to afford a chiral MOF for ferroelectric studies. Ferroelectric materials exhibiting reversal of spontaneous electric polarization with respect to an applied field are important as they can be used in a variety of emerging technologies such as electric devices, information storage, capacitors47−52 and so on. Synthesis of MOF based ferroelectric materials are quite challenging because it requires that the compound must crystallize in a chiral space group.53−54 While several techniques have been adopted for the construction of chiral MOFs,55−58 we have used L-(+)-lactic acid (Scheme 1) with H2L and Cd(II) to build a chiral MOF (2) under solvothermal condition. We also report the synthesis of three new frameworks, {[Cd(L)(L2)]}n (3), {[(Cd)1.5(L)1.5(L3)]}n (4) and {[Cd(L)(L4)(H2O)]}n (5). These frameworks (2-5) are found to show luminescence properties. Luminescent MOFs are important for various applications59−61 such as non-linear optics, sensor, photocatalysis and so on.



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Scheme 1. Schematic representation of ligands used in the present study.

EXPERIMENTAL SECTION Materials. Reagent grade 4-chloro-3,5-dinitrobenzoic acid, L-(+)-lactic acid (H2L1), 1,3di(pyridin-4-yl)propane

(L2)

(E)-1,2-di(pyridin-4-yl)ethane

(L3),

phenanthroline

(L4),

ethylacetoacetate, Gd(NO3)3.6H2O and Cd(NO3)2·6H2O salts were procured from Aldrich and used as received. N,N′ -dimethylformamide (DMF), ethanol, and copper dust were acquired from S. D. Fine Chemicals, India. All solvents were purified prior to use following standard procedures. Physical measurements. Infrared (IR) spectra were performed (KBr disk, 400−4000 cm−1) on a Perkin-Elmer model 1320 spectrometer. Thermogravimetric analyses (TGA) were acquired on a Mettler Toledo Star system (heating rate of 5 °C/min). Microanalyses of all the compounds were carried out by using a CE-440 elemental analyzer (Exeter Analytical Inc.). Powder X-ray diffraction spectra (Cu Kα radiation, scan rate 3°/min, 293 K) were obtained on a Bruker D8 Advance series 2 powder X-ray diffractometer. 1H NMR and 13C NMR spectra were recorded on a JEOL –ECX 500 FT (500 and 125 MHz respectively) instrument in CDCl3 or in DMSO-d6 with Me4Si as the internal standard. ESI mass spectra were recorded on a WATERS Q-TOF Premier mass spectrometer. The ferroelectric measurements were performed by making a pellet of the powdered sample (diameter 3.4 mm and thickness 0.6 mm, sandwiched between silver electrodes) with a ferroelectric tester (Precision Premier II, Radiant Technologies) at room temperature. The frequency dependence of dielectric constant was measured on an Agilent 4294A impedance analyzer at room temperature. Solid-state photo-excitation and emission


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spectra were recorded using an UV−vis-NIR spectrophotometer (Varian Model Cary 5000) and Jobin Yvon Horiba Fluorolog-3 spectrofluorimeter at room temperature. Single-Crystal X-ray Studies. Single-crystal X-ray data of compound 1-5, were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å) as reported earlier.10 The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from the International Tables for X-ray Crystallography.62 The data integration and reduction were carried out with SAINT63 software. Empirical absorption correction was applied to the collected reflections with SADABS64 and the space group was determined using XPREP.65 The structure was solved by the direct methods using SHELXTL-9766 and refined on F2 by full-matrix least-squares using the SHELXL-9767 program package. In compound 2, the disordered solvent molecules could not be located in the successive difference Fourier maps and hence PLATON68 squeeze refinement was performed. The hydrogens of the coordinated water molecules were located in the difference Fourier maps, whereas H atoms of the lattice water molecules could not be located in 1. The H atoms attached to C atoms were positioned geometrically and treated as riding atoms using SHELXL default parameters. Several DFIX and DANG commands were used to fix the bond distances in 2 and 5. In 1a nitro groups displayed rotational disorder and they were refined by fixing their perfect occupancy. Selected bond distances and bond angles are given in Table S1 (Supporting Information) while crystal parameters of all the complexes, data collection and refinement parameters are summarized in Table S2. Synthesis of the Ligand. The ligand H2L (Scheme 1) was synthesized according to a literature procedure.69 Synthesis of the Complexes.



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{[Gd2(L)3(DMF)4]·4DMF·3H2O}n (1). The compound 1 has been synthesized according to our reported procedure.46 A mixture of Gd(NO3)3·6H2O (0.5 mmol), H2L (0.5 mmol) were taken in DMF (10 mL) and ethanol (5 mL) and heated to 100 °C under autogenous pressure in a Teflonlined steel bomb for 2 days, followed by slow cooling (5° C h-1) to room temperature. The yellowish block-shaped crystals formed were collected, washed with DMF and air-dried Yield: 64%; IR (KBr cm-1): 3397(br), 3085 (m), 2933 (m), 2880 (w), 1675 (s), 1631 (s), 1543 (s), 1461 (m), 1387 (s), 1341 (s), 1253 (w), 1187(w), 1093 (m), 1062 (w); Anal. calcd. for C66H74N20O47Gd2 : C, 35.80, H, 3.36, N, 12.65%; found: C, 35.71, H, 3.52, N, 12.58%. {[Gd(L)1.5(DMF)2(H2O)2](S)(H2O)}n (1a). When a single crystal of 1 (mother crystal) is dipped in ethylacetoacetate at RT for 5 h, compound 1a, {[Gd(L)1.5(DMF)2(H2O)2)]·(S).(H2O)}n, (S = ethyl 3-oxobutanoate) is formed without losing crystallinity. Anal. calcd. for C33H34N8O26Gd: C, 35.52; H, 3.07; N, 10.04%. Found: C, 36.70; H, 3.05; N, 10.10%. IR (KBr, cm-1): 3422(br), 2875 (w), 1674(s), 1629(s), 1546(s), 1460(m), 1386(s), 1342(s), 1253(w), 1203(w), 1103(m).

{[(Cd)4(L)3(HlL1)2(DMF)(H2O)2](DMF)3(H2O)2}n. (2) A mixture containing H2L (0.05 mmol), L-(+)-lactic acid (0.05 mmol), Cd(NO3)2·4H2O ( 0.1 mmol) taken in DMF (2 mL) and ethanol (1 mL) and 3 mL water, was sealed in a Teflon-lined autoclave and heated under autogenous pressure to 100 °C for 2 d and then allowed to cool to RT at the rate of 1 °C per minute. Blockshaped colorless crystals of 2 were collected in ~20% yield. IR (KBr, cm-1): 3421(br), 2930(br), 1675(s), 1598(s), 1378(w), 1217(s), 1096(w), 858(s). Anal. calcd. for C60H54N16O50Cd4: C, 32.05; H, 2.42; N, 9.96%. Found: C, 32.10; H, 2.50; N, 10.05%. {[Cd(L)(L2)]}n (3). A mixture containing H2L (20 mg, 0.05 mmol), L2 (8.5 mg, 0.05 mmol), Cd(NO3)2·4H2O (30 mg, 0.1 mmol) taken in ethanol (2 mL) and water (1 mL) was sealed in a


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Teflon-lined autoclave and heated under autogenous pressure to 180° C for 72 h and then allowed to cool to RT at the rate of 1°C per minute Block shaped colorless crystals of 3 were collected in ~30% yield. IR(KBr, cm-1): 3434(br), 2939(m), 1598(s), 1535(s), 1395(s), 1336(s), 1217(s), 913(s), 816(s), 718(s). Anal. calcd. for C27H18N6O12Cd: C, 44.38; H, 2.48; N, 11.50. Found: C, 44.50; H, 2.50; N, 11.60 %. {[(Cd)1.5(L)1.5(L3)]}n (4). A solution containing H2L (20 mg, 0.05 mmol), L3 (8.5 mg, 0.05 mmol) and Cd(NO3)2·4H2O (30 mg, 0.1 mmol) taken in ethanol (2 mL) and water (1 mL) was sealed in a Teflon-lined autoclave and heated under autogenous pressure to 180° C for 72 h and then allowed to cool to RT at the rate 1°C per minute. Block shaped colorless crystals of 4 were collected in ~50% yield. IR (KBr, cm-1): 3434(br), 3070(br), 1606(s), 1532(s), 1341(s), 922(s), 824(s), 722(s). IR(KBr, cm-1): 3429(br), 2916(s), 1670(s), 1600(s), 1562(s), 1382(s), 1260(s), 1030(s). Anal. calcd. for C66H32N16O36Cd3: C, 48.83; H, 4.59; N, 7.54%. Found: C, 48.50; H, 4.60; N, 7.50%.

{[Cd(L)(L4)(H2O)]}n (5). A solution containing H2L (20 mg, 0.05 mmol), phenanthroline (L4) (8.5 mg, 0.05 mmol) and Cd(NO3)2·6H2O (35 mg, 0.1 mmol) taken in ethanol (2 mL) and water (1 mL) was sealed in a Teflon-lined autoclave and heated under autogenous pressure to 130° C for 72 h and then allowed to cool to RT at the rate 1°C per minute. Block shaped colorless crystals of 5 were collected in ~40% yield. IR(KBr, cm-1): 3451(br), 3100(br), 1628(s), 1598(s), 1551(s), 1530(s), 1340(s), 841(s), 718(s). Anal. calcd. for C26H14N6O13Cd: C, 42.73; H, 1.93; N, 11.50%. Found: C, 42.60; H, 2.00; N, 11.60%.

RESULTS AND DISCUSSION



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All the MOFs are synthesized and reproduced in bulk amounts. Once isolated, they are insoluble in water and common organic solvents. The IR spectra of 1-5 (Figure S1-S6, Supporting Information) show strong absorption bands in the range, 1400-1620 cm−1 attributable to coordinated carboxylate groups70-72. The broad peak at 3390−3450 cm−1 indicates the presence of both coordinated and non-coordinated water molecules.73-74 The framework, {[Gd2(L)3·(DMF)4]·(4DMF)·(3H2O)}n (1) has been synthesized under solvothermal conditions in DMF (vide supra). The phase purity of 1 is confirmed by comparing the experimental and simulated PXRD patterns obtained from single crystal data (Figure S7, Supporting Information). The structure of 1 consists of a dimeric Gd2 (M···M =4.0481(11) Å) secondary building unit (SBU), constructed from two syn-syn bridging carboxylates, two terminal chelating carboxylates and two chelating as well as bridging carboxylates from six different L2- units (Figure. 1a). Each Gd(III) has 9-coordination where two DMF molecules are also coordinated giving rise to a distorted tri-capped trigonal prismatic geometry (Figure 1b). There are two types of channels named A (3.33 × 8.65 Å2) and B (2.40 × 8.24 Å2) running along crystallographic a axis (Figure S8, Supporting Information) (channel sizes are measured by considering van der Waals radii of constituting atoms). Channel A contains six DMF molecules while channel B having two DMF and six water molecules (Figure S8, Supporting Information).


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Figure 1. The perspective view of (a) coordination environment around the Gd2 dimer in compound 1, (b) geometry around metal center (hydrogen atoms are omitted for clarity).

Reversible Single-Crystal to Single-Crystal Transformation of 1 to 1a. Ethylacetoacetate (EAA) exists mostly in its keto form (S) in the gas phase and in solution75 with the enol form (S′) present only in minute quantities (Scheme 2). Tautomerism plays important roles in modern organic chemistry, biochemistry, medicinal chemistry as well as pharmacology76 Keto–enol tautomerism is a fundamental process in bioorganic chemistry and is responsible for most condensation reactions77−79. However, the existence of the unconjugated enol form (S'') has not been confirmed.

Scheme 2. Schematic representation of tautomeric forms of ethyl acetoacetate When a single crystal of 1 (mother crystal) is dipped in ethylacetoacetate at ambient condition for 5 h, compound 1a, {[Gd(L)1.5(DMF)2(H2O)2](S)(H2O)}n (S =ethyl 3-oxobutanoate) is formed without losing crystallinity (Figure 2).



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Figure 2. single-crystal to single-crystal loading of EAA guest in compound 1, forming the daughter compound 1a. The noticeable rearrangement of the framework channels is depicted. DMF and water molecules have been removed for clarity. The disorder nitro groups have been deleted in 1a to increase the clarity of the image. Single crystal X-ray data reveals that the space group changes to C2/c (Table S2, Supporting Information) and two ethylacetoacetate (S= ethyl 3-oxobutanoate) molecules of the keto form (Figure 3) are trapped inside each cavity of 1 (Figure 2). The photograph of a single crystal show shape retention and ruling out a dissolution−recrystallization pathway during this transformation, (Figure S9, Supporting Information).

Figure. 3 Perspective view of (a) trap of keto form of ethyl acetotate inside the framework 1 and (b) its bond distances. The formation of 1a is accompanied by a number of bond breaking/formation processes compared to its mother, 1. Interestingly, the lattice water molecules moves towards the metal center. During this mode of operation the bridging carboxylate (η2-bridging) in the parent


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framework undergoes a noticeable ‘carboxylate shift’ and convert into a mono dentate fashion (η1-carboxylates) (Figure 4). As a result there is a significant increase in the Gd···Gd distance from 4.048 (11) Å to 4.929 (11) Å (Figure 4). Besides, in a drastic change of the binding mode of the carboxylate group, again there is a terminal chelating in 1 open up into a mono dentate mode (η1-carboxylates) in 1a. Therefore, the coordination number of each metal center in 1a decreases to eight which is fulfilled by coordination of two η1-carboxylates, two syn carbxylates, two water molecules and two DMF molecules (Figure 4).

Figure. 4 The carboxylate shift process at the metal centre, occurring through the reversible SCSC transformation from 1 to 1a and vice versa. It is worth noting that while a number of SC-SC structural transformations have been reported; those involving changes in the first coordination sphere are fewer80−82. A careful look at the crystal structure of 1a shows a number of weak host-guest interactions (C−H···O range, 2.434(3)−3.820(4) Å) that are responsible for stabilizing the guest molecule (Figure S10, Table S3, Supporting Information). The incoming guest molecule forms an intermolecular hydrogen bonding with the oxygen atoms of the carboxylates that became unbound to the metal center due to “carboxylate shift” (Figure S10, Supporting Information). The channel of 1 undergoes drastic distortion of the channel dimensions during this trapping process giving values of 7.98 × 10.86 Å2 (channel A) and 2.01× 5.83 Å2 (channel B) (Figure 2). Two ‘S’ molecules occupy each



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channel in 1a and they form complementary C–H····O (3.128(22) Å) (Figure S11, Supporting Information) interactions between themselves. The IR spectrum of 1a shows a sharp peak at 1717 cm-1 (Figure S2, Supporting Information) attributable to the C=O stretching vibration. The phase purity of 1a is confirmed by matching the PXRD pattern with the simulated pattern obtained from the single crystal data (Figure 5).

Figure 5. PXRD patterns of (a) simulated of 1a and (c) as synthesize of 1a.

Direct synthesis of 1a by mixing ethylacetoacetate with H2L and Gd(III) in proper stiochiometric ratio remained unsuccessful. Interestingly, this SC-SC transformation is found to be reversible. When 1a is dipped in DMF for 3 days it reverts back to the mother framework (Table S4 Supporting Information). The absence of the guest molecules is also confirmed by the disappearance of a peak at 1717 cm-1 in the IR spectrum (Figure S12, supporting information). Furthermore, PXRD pattern of this product is in good agreement with the pattern of 1 (Figure 6) showing bulk phase transformation.


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Figure 6. PXRD pattern (a) simulated of 1, (b) experimental of 1 and (c) regenerated of 1 from 1a.

Structural Description of 2-5. We have synthesized four different MOFs using different type of co-linkers. Compound 2 crystallizes in the triclinic chiral space goup P1 (Table S1 supporting information). The asymmetric unit consists of four crystallographically independent Cd(II) ions, four L2- and two HL1-1 ligands, one DMF and two water molecules (Figure 7a). Out of four metal ions, Cd1 and Cd4 show hepta-coordination while Cd2 and Cd3 exhibit hexa-coordination (Figure 7b). The Cd1 is ligated by three carboxylates oxygen atoms (Cd-O = 2.206(9)-2.538(8) Å) from three L2ligands, two oxygen atoms from two water molecules (Cd-O=2.281(1)-2.363(9) Å) and two oxygen atoms from two co-ligands, HL1-1. The Cd4 ion is bonded to five oxygen atoms (Cd-O = 2.231(6)-2.609(1) Å) from five L2- ligands and two oxygen atoms from two co-ligands, HL1-1.



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Figure 7. The perspective view of (a) asymmetric unit of 2 and (b) geometry around metal centers (hydrogen atoms are omitted for clarity). The hexa-coordination of Cd3 is completed by six O atoms (Cd-O = 2.243(8)-2.279(7) Å) from five L2- and one O atom from a HL1-1 co-ligand. In case of Cd2 ion, it is ligated by four O atoms from four L2- ligands, one O atom each from a HL1-1 co-ligand and a DMF molecule. All Cd−O bond distances are comparable to those reported earlier.83


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Figure 8. (a, b, d, e, f and g) coordination modes of L2- complexes 2-5 and c is the coordination mode of HL1-1 in 2. The carboxylate groups are ligated to the metal ions in different fashion, µ5:η2:η2:η2:η1, µ4:η1:η1:η1:η1, µ 3:η1:η2:η1 (Figure 8a-c). The connectivity pattern of these carboxylate groups creates a repeated Cd-C-O chain along the crystallographic b direction (Figure 9b). These repeating chains further connected by the L2- to the neighboring chains to a 3D framework (Figure 9a).

Figure 9. The perspective view of (a) 3D view of 2 and (b) Cd-C-O chain (hydrogen atoms are omitted for clarity). Complex 3 crystallizes in the monoclinic space group P21/c (Table S1, Supporting Information). The asymmetric unit contains a Cd(II) ion, one L2- ligand and one L2 co-ligand (Figure S13, Supporting Information). This is an two dimentional framework (Figure 10a) and the structure has bimetallic cadmium cluster as a secondary building unit where each metal ion is



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heptacoordinated (Figure 10b) with ligation from five O atoms (Cd-O=2.316(3)-2.745(4) Å) from three L2- ligands and two N atoms (Cd-N=2.279(5)-2.291(5) Å) from two L2 co-ligands. The coordination mode of L2- ligand is (µ 3:η1:η1:η1:η2) (Figure 8d). All the carboxylate groups and the metal ions are coplanar while the axial positions are occupied by N atoms of L2 leading to intramolecular π···π (3.517(7) Å) interactions between the phenyl rings (Figure 10b).

Figure 10. The perspective (a) 2D view of 3 and (b) the geometry around metal centers and the π···π interaction between the benzene rings (hydrogen atoms are omitted for clarity). Compound 4 crystallizes in the monoclinic space group C2/c (Table S1, Supporting Information). The asymmetric unit contains two crystallographically independent Cd(II) ions (one full occupancy and the other half occupancy), one and half of the ligand L2- and one L3 coligand (Figure S14, Supporting Information). The hexa-coordinated Cd1 is ligated by four O atoms (Cd-O=2.2261(3)-2.234(3) Å) from four different carboxylate groups and two nitrogen atoms (Cd-N=2.321(4) Å) from two different L3 co-ligands. The hepta-coordinated Cd2 is bounded by six O atoms (Cd-O=2.199(3)-2.700(3) Å) from four different L2- ligand while the seventh coordination is satisfied by a pyridine N of the co-ligand L3. The mode of binding of carboxylate groups towards the metal ions in different fashion are µ6:η2:η2:η2:η2, µ3:η2:η1:η1


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(Figure 8e-f). The carboxylate connectivity leads to the creation of Cd-C-O chain along the crystallographic c direction (Figure 11b). The L3 ligand makes a connection between the Cd-C-O chains and ultimately leads to an overall 3D structure (Figure 11a). Compare to complex 3, the growth along the axially is blocked due to the linker L2 leads to 2D structure whereas in 4, the presence of linker L3 allow to growing the framework axially and afford a 3D structure.

Figure 11. The perspective (a) 3D view of 4 and (b) Cd-C-O chain (hydrogen atoms are omitted for clarity). Complex 5 crystallizes in the monoclinic space group C2 (Table S1, Supporting Information). The asymmetric unit consists of two Cd(II) ions each having half occupancy, one L2- ligand, one L4 co-ligand and one water molecule. The framework is a bimetallic cluster where each metal ion is six coordinated. The Cd1 ion is ligated by four O atoms from two carboxylate (of two L2- ligands) plus two water molecules and one L4 co-ligand while Cd2 is ligated by four O atoms from four carboxylates (of four L2- ligand) and one L4 co-ligand (Figure



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S15, Supporting Information). The L2- ligand forming a bridge between the metal ion (or cluster) and the coordination mode of carboxylate ligand is µ 3:η1:η1:η1:η0 (Figure 8g). This mode of carboxylate binding leads to the creation of Cd-C-O chains where each chain is connected by the ligand L2- (Figure 12a). Moreover, each chain bearing L4 ligand in alternate position and involves a weak π···π interaction (4.001 Å) (closest distance) and results in the 3D structure (Figure 12b).

Figure 12. The perspective view of (a) coordination environment around each metal center, (b) Cd-C-O chain connected by ligand H2L and (c) π···π interaction between the ligand L4 ligand (hydrogen atoms are omitted for clarity).

PXRD and Thermal Stability Analyses. Phase purity of 2−5 are confirmed by powder X-ray diffraction (PXRD) patterns, which are in excellent agreement with the corresponding simulated patterns obtained from single crystal data confirming phase purity of all the complexes (Figures S16−S19, Supporting Information). Thermal stabilities of all the compounds were examined by thermogravimetric analysis (TGA) (Figures S20−S24, Supporting Information). The TGA curve of 1a shows a gradual weight loss of ~ 29.57% (expected = 30.51%) due to expulsion of guest molecules in the lattice followed by metal-bound DMF and water molecules in the temperature range, 30-260° C. Beyond that


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temperature, complete decomposition of the framework takes place. TGA curve of 2 exhibits a loss of lattice water and DMF molecules in the temperature range 30-250° C and beyond that temperature the framework collapses due to the removal of coordinated solvent molecules. Complex 3 and 4 are stable up to 330 °C and their decomposition takes place beyond this temperature. TGA of 5 shows weight loss of ~2.46% (expected, 2.5 %) in the temperature range 30-90 °C due to the removal of coordinated water molecule and the framework is stable upto 330 °C. Ferroelectric Study of Complex 2. Since complex 2 crystalized in the chiral triclinic space group P1, associated with the point group C1 (one of the 10 polar point groups; C1, Cs, C2, C2v, C4, C4v, C3, C3v, C6, C6v) it fulfills the essential requirement for ferroelectric behavior. To carry out ferroelectric measurements, the powdered sample was compressed and sintered into a pellet. The plot of polarization (P) against applied electric field (E) of magnitude 3.5 kV measured at room temperature exhibited electric hysteresis loop for 2 (Figure 13a). At room temperature, the remnant polarization (Pr) of 2 is ca. 0.16 µC cm−2 with a coercive field (Ec) of ca. 1.1 kV cm−1. The saturation spontaneous polarization (Ps) of 2 is ca. 5.7 µC cm−2. The Ps value of this compound is higher than a typical ferroelectric compound, KH2PO4 (KDP) (Ps = 5.0 µC cm-2) but smaller compared to some chiral MOFs.84−85 It can be seen that the P-E loop at room temperature differs from the classic ferroelectric loop and conforms to a quasi-linear dielectric. Increasing the field further resulted in dielectric breakdown of the pellet. Further, we have investigated the variation of dielectric constant (ε’) as a function of frequency (Figure 13b) at room temperature. The dielectric constant (ε’) of complex 2 initially decreases from 12 to 8 up to 104 Hz and subsequently remains constant (ε~8) upto ~5 MHz beyond which a dielectric relaxation is observed (Figure 13b). On



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the other hand, 2 exhibit a dielectric loss (tan δ = 0.5) in the frequency range (103-104 Hz) which falls further (tan δ ~ 0.01) and remains constant till highest measurement frequency. These measurements reveal that the complex 2 can be better described as a linear dielectric material with low losses resulting in almost linear dependence of polarization with applied electric field till the breakdown voltage.

Figure. 13 (a) Polarization (Pr) vs. applied electric field (Ec) plots of complex 2 at room temperature and (b) frequency dependence dielectric constant of 2 at room temperature.

Photoluminescence Studies. The luminescence properties of MOFs of d10 metal ion with nitrogen and carboxylate donor ligands are already reported in the literature.86−89 The solid state luminescence of complexes 2-5 and metal-free ligand H2L were investigated at room temperature (Figure 14). It can be seen that the metal-free H2L exhibits an emission band at 410 nm upon excitation at 260 nm (Figure S25, Supporting Information). This emission band of the free ligand is attributable to the π→π* transition.90−91 The irradiation of complex 2-5 with ultraviolet light (λex=260 nm) at room temperature results a broad emission bands similar to the free ligand H2L could be attributed to the intra-ligand (π–π*) transition. Upon complexation with Cd(II) ion (2-5), intensity of the band


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increases significantly (Figure 12) as the ligand becomes more rigid and that blocks the deactivation pathway.92−93

Figure 14. Solid state luminescence spectra at room temperature.

CONCLUSIONS By utilizing a linear ligand H2L, we have synthesized a flexible Gd(III) coordination polymer (1) that acts as a container for ethylacetoacetate molecules in the exclusively keto form showing reversible, SC-SC transformations. The SC-SC transformations involve the phenomenon of “carboxylate shift” as in enzymatic processes. The ligand H2L in presence of different nitrogen and carboxylate donor co-ligands afford four new Cd(II) containing MOFs one of which is a chiral framework. The chiral MOF shows ferroelectric behavior while all the Cd(II) MOFs exhibits enhanced luminescence spectra compared to the free ligand due to intra-ligand π–π* transition.

The successful synthesis of the frameworks (1-5) improves the idea of crystal

engineering which can help us design of ligands (or MOFs) for various purposes.



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ASSOCIATED CONTENT Supporting Information. X-ray crystallographic data in CIF format, table for selected bonds and distances for complexes 1− 5 and complete data for IR, TGA analysis, PXRD and solid state UV/vis. This material is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We gratefully acknowledge the financial support received from the Department of Science and Technology, New Delhi, India (to RK, AG and PKB) and SRF from the CSIR to TKP.

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Metal Organic Frameworks Built from a Linear Rigid Dicarboxylate and Different Co-linkers: Trap of the Keto form of Ethylacetoacetate, Luminescence and Ferroelectric Studies Tapan K. Pal,a Rajesh Katoch,b Ashish Gargb and Parimal K. Bharadwaj*a a b

Department of Chemistry, Indian Institute of Technology Kanpur, 208016, India Department of Materials Science and Engineering, IIT Kanpur, Kanpur, 208016, India Graphical Abstract

We describe a 3D flexible Gd(III)-metal organic framework, constructed from a linear dicarboxylate ligand, which is found to be an excellent host to keto form of ethylacetoacetate in single crystal to single crystal (SC-SC) transformation associate with the structural change.



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270x83mm (96 x 96 DPI)


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