Guest Exchange Reactions in Isostructural 3D Porous Coordination

Crystal Growth & Design · Advanced Search .... Publication Date (Web): September 10, 2014 .... Chao-Jie Li , Sheng-Run Zheng , Zhi-Yuan Chen , Wei-Don...
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Guest Exchange Reactions in Isostructural 3D Porous Coordination Polymers of Ni(II), Co(II), and Mn(II) Rashmi A. Agarwal* and Parimal K. Bharadwaj Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India S Supporting Information *

ABSTRACT: A series of isostructural compounds, {[Ni2(DBIBA)3]·(BF4)· 6H2O}n (1), {[Ni2(DBIBA)3]·(ClO4)·3H2O}n (2), {[Ni2(DBIBA)3]·(NO3)· 3H2O}n (3), {[Ni2(DBIBA)3]·Cl·C2H5OH}n (4) {[Co2(DBIBA)3]·(ClO4)· 3H2O}n (5), {[Co2(DBIBA)3]·Cl·3C2H5OH}n (7), {[Co2(DBIBA)3]·Cl· 3DMF}n (8), {[Co2(DBIBA)3]·Cl·3CH3COCH3·7H2O}n (9), and {[Mn2(DBIBA)3]·Cl·3DMF·3H2O}n (10), have been afforded via anion and solvent exchanges of previously reported coordination polymers {[Ni2(DBIBA) 3 ]·Cl·18H 2 O} n , {[Co 2 (DBIBA) 3 ]·Cl·9H 2 O} n , and {[Mn 2 (DBIBA) 3 ]·Cl·3H 2 O} n (DBIBA = 5-di(1H-benzo[d]imidazol-1-yl)benzoate). Anion-exchanged compounds including {[Co2(DBIBA)3]· (NO3)·3H2O}n (6) have also been synthesized under solvothermal conditions by employing metal salts and the corresponding acid hydrolysis of the cyanide functional group of 5-di(1H-benzo[d]imidazol-1-yl) benzonitrile (DBIBN). These structures are obtained with retaining crystallinity, and only insignificant changes are observed in bond lengths and bond angles involving the metal ion. However, noncovalent supramolecular interactions (anion−π, π−π, and hydrogen bonding) are utterly affected or changed. Interestingly, in complex 9, the chloride ion moves from the node of the parent complex to the center of the cavity, where it is surrounded by a hexameric water cluster revealing strong H-bonding, which is very rare to be seen. Exchange of solvent molecules leaves the size of the voids unaltered, but anion exchanges are accompanied by shrinking of the voids, except in the case of NO3− exchange, where there is no change in volume. All are isoreticular structures with a binodal 4,6-connected stp net topology. These compounds have been characterized by single-crystal X-ray crystallography, IR spectroscopy, TGA, PXRD, and elemental analysis.

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greater insight into such exchanges can be made possible.8 On the other hand, solvent exchanges in the SC-SC mode are also important as such exchanges afford to study if a structure is dynamic and susceptible to alter its overall architecture upon solvent exchange with the goal(s) of directing guests into the voids. In the present paper, we describe several guest exchange reactions on previously reported polymers9 to obtain new ones. Besides, anion exchange polymers are also synthesized solvothermally under different experimental conditions.

INTRODUCTION Current interest in novel metal−organic frameworks (MOFs) is rapidly growing because such materials are potentially important in various applications. Single-crystal to single-crystal (SC-SC) transformation1 in MOFs, which involve cooperative movements of atoms, has emerged as an interesting solid-state phenomenon that can be exploited to enhance the functionalities of MOFs. Direct observation of guest molecules occupying the voids in a coordination polymer can offer ways to modify the coordination space for directing multiple small molecules cooperatively to bring about chemical transformations,2 besides effecting guest exchange,3 separation of geometrical isomers,4 and so on. The study of hosts for anionic guest exchanges has become an area of intense research activity.5 Anions are implicated in several diseases besides posing different environmental problems. As an example, the perchlorate ion can adversely affect human health by interfering with iodide uptake into the thyroid. Perchlorate (ClO4−) and chromate (CrO42−) are listed as EPA (USA) priority pollutants.6 In addition, anions are known to play important roles in directing the self-assembly processes.7 When they are trapped in the cavity via weak interactions with the cationic framework, they can be exchanged with different anions, and when these exchanges take place without losing crystallinity, a © 2014 American Chemical Society

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EXPERIMENTAL SECTION

Materials. Reagent grade 3,5-difluorobenzonitrile and metal salts were acquired from Aldrich and used as received. All solvents, benzimidazole, and K2CO3 were procured from S. D. Fine Chemicals, India. Solvents were purified prior to use following standard procedures. Physical Measurements. Infrared spectra were obtained (KBr disk, 400−4000 cm−1) using a PerkinElmer model 1320 spectrometer. TGA were recorded using a Mettler Toledo (heating rate of 5 °C/ min) TGA instrument. Microanalyses for the compounds were obtained using a CE-440 elemental analyzer (Exeter Analytical Inc.). Powder X-ray diffraction (CuKα radiation, scan rate 3°/min, 293 K) Received: August 27, 2014 Published: September 10, 2014 6115

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patterns were obtained on a Bruker D8 Advance Series 2 powder X-ray diffractometer. Synthesis of 5-Di(1H-benzo[d]imidazol-1-yl) Benzonitrile (DBIBN). This ligand was synthesized as previously reported.9b Synthesis of {[Ni2(DBIBA)3]·(BF4)·6H2O}n (1), {[Ni2(DBIBA)3]· (ClO4)·3H2O}n (2), and {[Ni2(DBIBA)3]·(NO3)·3H2O}n (3). Crystals of {[Ni2(DBIBA)3]·Cl·18H2O}n synthesized earlier9a were dipped in 3 M aqueous solutions of NaX (X = BF4−, ClO4−, and NO3−) for about 1 week at room temperature, whereupon 1, 2, and 3 were obtained without losing crystallinity. These compounds had also been synthesized solvothermally by the following method: A mixture of Ni(NO3)2·6H2O (0.07 g, 0.24 mmol), DBIBN (0.04 g, 0.12 mmol), and dil. HX (X = BF4−, ClO4−, and NO3−) (0.1 mL) in EtOH:H2O (3 mL, 2:1 v/v) was placed in a Teflon-lined stainless steel autoclave and heated to 180 °C for 3 days under autogenous pressure. Upon cooling to room temperature at the rate of 1 °C/min, needle-shaped green crystals of 1, 2, and 3 were collected in ∼30, 35, and 30% yields, respectively. The crystals were repeatedly washed with ethanol and airdried. Anal. Calcd (%) for {[Ni2(DBIBA)3]·(BF4)·6H2O}n, C63H51N12O12BF4Ni2 (1): C, 55.13; H, 3.74; N, 12.25. Found: C, 55.17; H, 3.78; N, 12.20%. IR (cm−1): 3402(m), 3118(m), 1652(s), 1596(s), 1504(s), 1404(s), 1375(s), 1308(s), 1290(s), 1243(s), 1171(m), 1140(m), 1059(s), 911(m), 861(w), 795(m), 778(s), 765(s), 747(s), 724(s), 689(m), 523(m), 425(m) (Figure S1, Supporting Information). Anal. Calcd (%) for {[Ni2(DBIBA)3]·(ClO4)·3H2O}n, C63H45N12O13ClNi2 (2): C, 56.85; H, 3.41; N, 12.63. Found: C, 56.90; H, 3.45; N, 12.59%. IR (cm−1): 3419(m), 3101(m), 1652(s), 1597(s), 1504(s), 1474(s), 1404(s), 1375(s), 1325(m), 1308(m), 1289(m), 1243(s), 1171(m), 1140(m), 1093(s), 1011(w), 911(m), 860(w), 794(m), 778(s), 765(s), 747(s), 724(s), 689(m), 650(w), 622(m) (Figure S2, Supporting Information). Anal. Calcd (%) for {[Ni2(DBIBA)3]·(NO3)·3H2O}n, C63H45N13O12Ni2 (3): C, 58.5; H, 3.51; N, 14.08. Found: C, 58.55; H, 3.56; N, 14.11%. IR (cm−1): 3453(m), 3113(w), 3073(w), 1809(w), 1695(m), 1651(s), 1599(s), 1504(s), 1473(s), 1448(m), 1403(s), 1375(s), 1324(m), 1308(s), 1290(s), 1242(s), 1225(s), 1202(m), 1170(m), 1143(m), 1104(m), 779(s), 766(s), 755(s), 724(s), 689(m) (Figure S3, Supporting Information). Synthesis of {[Ni2(DBIBA)3]·Cl·C2H5OH}n (4). Crystals of {[Ni2(DBIBA)3]·Cl·18H2O}n were immersed in absolute ethanol for 4 days at room temperature, whereupon 4 was obtained in SC-SC transformations. Anal. Calcd (%) for {[Ni2(DBIBA)3]·Cl·C2H5OH}n, C65H45N12O7ClNi2: C, 62; H, 3.60; N, 13.35. Found: C, 61.82; H, 3.52; N, 13.28%. IR (cm−1): 3639(m), 3000(m), 2923(w), 1649(s), 1598(s), 1584(s), 1504(s), 1474(s), 1450(s), 1400(s), 1372(s), 1306(s), 1292(s), 1242(s), 1221(s), 941(m), 909(w), 861(w), 795(m) 777(s), 764(m), 745(s), 723(s) (Figure S4, Supporting Information). Synthesis of {[Co2(DBIBA)3]·(ClO4)·3H2O}n (5). Crystals of {[Co2(DBIBA)3]·Cl·9H2O}n were immersed in 3 M NaClO4 solution for 1 week at room temperature to afford compound 5 in an SC-SC transformation. This compound is also obtained by hydrothermal reaction using dil. HClO4 as described for 1−3 complexes. Anal. Calcd (%) for {[Co2(DBIBA)3]·(ClO4)·3H2O}n, C63H45N12O13ClCo2: C, 57.09; H, 3.42; N, 12.68. Found: C, 57.01; H, 3.36; N, 12.58%. IR (cm−1): 3419(w), 1648(s), 1595(s), 1503(s), 1472(s), 1404(s), 1375(s), 1323(m), 1307(s), 1290(s), 1242(s), 1226(s), 1171(w), 1140(m), 1091(s), 779(m), 765(m), 748(s), 720(m) (Figure S5, Supporting Information). Synthesis of {[Co2(DBIBA)3]·(NO3)·3H2O}n (6). Compound 6, on the other hand, was obtained solvothermally by heating a mixture of Co(NO3)2·6H2O (0.07 g, 0.24 mmol), DBIBN (0.04 g, 0.12 mmol), and dil. HNO3 in EtOH:H2O (3 mL, 2:1 v/v) at 180 °C for 3 days under autogenous pressure. Upon cooling to room temperature at the rate of 1 °C/min, needle-shaped dark pink crystals of 6 could be isolated in ∼40% yield. The crystals were repeatedly washed with ethanol and air-dried. Anal. Calcd (%) for {[Co2(DBIBA)3]·(NO3)· 3H2O}n, C63H45N13O12Co2: C, 58.48; H, 3.50; N, 14.07. Found: C, 58.32; H, 3.34; N, 14.0%. IR (cm−1): 3441(m), 3107(w), 3069(w), 1696(m), 1650(s), 1598(s), 1503(s), 1473(s), 1449(s), 1403(s), 1376(s), 1323(s), 1307(s), 1290(s), 1241(s), 1224(s), 1170(m),

1142(m), 792(m), 779(s), 765(s), 755(s), 720(m), 689(m) (Figure S6, Supporting Information). Synthesis of {[Co2(DBIBA)3]·Cl·3C2H5OH}n (7), {[Co2(DBIBA)3]· Cl·3DMF}n (8), and {[Co2(DBIBA)3]·Cl·3CH3COCH3·7H2O}n (9). Crystals of {[Co2(DBIBA)3]·Cl·9H2O}n were immersed for 4 days at room temperature in absolute ethanol, dry DMF, and dry acetone to obtain 7, 8, and 9 respectively in SC-SC transformations. Anal. Calcd (%) for {[Co2(DBIBA)3]·Cl·3C2H5OH}n, C69H57N12O9ClCo2 (7): C, 61.32; H, 4.25; N, 12.44. Found: C, 61.24; H, 4.11; N, 12.10%. IR (cm−1): 3549(w), 3404(w), 3137(w), 3073(w), 3056(w), 3002(w), 2922(w), 1913(w), 1861(w), 1647(m), 1597(s), 1503(s), 1474(m), 1451(m), 1401(s), 1374(m), 1304(m), 1293(m), 1242(s), 1222(m), 777(m), 763(w), 744(s), 719(m) (Figure S7, Supporting Information). Anal. Calcd (%) for {[Co2(DBIBA)3]·Cl·3DMF}n, C72H60N15O9ClCo2 (8): C, 60.36; H, 4.22; N, 14.67. Found: C, 60.10; H, 4.11; N, 14.54%. IR (cm−1): 3421(w), 3092(w), 3068(w), 2925(w), 2849(w), 1807(w), 1661(s), 1594(s), 1501(s), 1473(m), 1402(s), 1374(s), 1322(w), 1307(m), 1290(m), 1242(s), 1224(m), 779(m), 766(s), 755(s) (Figure S8, Supporting Information). Anal. Calcd (%) for {[Co2(DBIBA)3]·Cl·3CH3COCH3·7H2O}n, C72H71N12O16ClCo2 (9): C, 57.13; H, 4.73; N, 11.10. Found: C, 57.10; H, 4.21; N, 11.02%. IR (cm−1): 3451(m), 3108(w), 3069(w), 2997(w), 1935(w), 1806(w), 1695(m), 1649(s), 1595(s), 1503(s), 1472(s), 1403(s), 1374(s), 1322(m), 1307(s), 1291(s), 1241(s), 1224(s), 1170(m), 1143(m), 779(m), 765(s), 755(s), 720(m) (Figure S9, Supporting Information). Synthesis of {[Mn2(DBIBA)3]·Cl·3DMF·3H2O}n (10). Crystals of {[Mn2(DBIBA)3]·Cl·3H2O}n were immersed in dry DMF for 4 days at room temperature to obtain 10 in an SC-SC transformation. Anal. Calcd (%) for {[Mn2(DBIBA)3]·Cl·3DMF·3H2O}n, C72H66N15O12ClMn2: C, 58.48; H, 4.5; N, 14.21. Found: C, 58.34; H, 4.62; N, 14.15%. IR (cm−1): 3415(w), 3065(w), 2993(m), 1665(s), 1643(s), 1596(s), 1501(s), 1474(s), 1403(s), 1375(s), 1293(s), 1240(s), 1220(s), 1171(m), 1142(m), 1089(m), 905(m), 794(m), 780(s), 766(s), 751(s), 720(m), 427(m) (Figure S10, Supporting Information). Single-Crystal X-ray Studies. Single-crystal X-ray data were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromated MoKα radiation (λ = 0.71073 Å) as described earlier.9b Because of the disorder contents of the large pore in the framework, identification of solvent entities within the voids was impossible, so squeeze refinement has been done for all compounds except 9 using PLATON10 to remove diffuse electron density associated with badly disordered solvent molecules. A few atoms had to be refined isotropically: C2, C12, C15 in 8, BF4− in 1, and NO3− in 3. Besides, several DFIX and DANG commands were given to fix bond angles and bond distances for the anion or solvent molecules in some of the structures. Hydrogen atoms could not be located near oxygen of the water molecule in 9. Contributions of all atoms of the solvent molecules have been incorporated in both the empirical formula and the formula weight in Table S1 (Supporting Information). Complex 10 did not diffract well at high angles, and the data completeness was found to be 0.93 due to incomplete scans. For 1−3 crystal structures, the maximum residual density value remains high because of the poor crystal quality and large amount of disordered caused by dynamic guest moieties. It was tried with several crystals from different batches, but every crystal was exhibiting the same problem. The crystal and refinement data are shown in Table S1, while selective bond angles and bond distances are given in Table S2 (Supporting Information).

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RESULTS AND DISCUSSION All compounds are found to be air-stable and insoluble in water as well as common organic solvents. Strong absorption bands between 1454 and 1608 cm−1 in the IR spectrum of all the complexes are indicative of the presence of coordinated carboxylates.11 To examine the thermal stabilities of the compounds, they were subjected to thermogravimetric analyses. In the case of 1, 2, 3, and 4, the weight losses were found to be as follows: 6116

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∼8.7% (calcd, 8%) range, 140−250 °C (Figure S11), ∼4.4% (calcd, 4.06%) range, 33−90 °C (Figure S12), ∼4.5% (calcd, 4.17%) range, 30−220 °C (Figure S13), and ∼3.4% (calcd, 3.7%) range, 90−180 °C (Figure S14) (Supporting Information), respectively, attributable to the loss of lattice solvent molecules. Decomposition of the compounds could be achieved beyond 400 °C. For 5 and 6, the weight losses were found as follows: ∼3.9% (calcd, 4.1%) range, 30−73 °C (Figure S15), ∼4.6% (calcd, 4.2%) range, 35−335 °C (Figure S16) (Supporting Information), respectively, attributable to the loss of three water molecules. In these two cases, decomposition starts at 300 and 340 °C, respectively. Compound 7 showed a weight loss of ∼10% (calcd 10.2%), range, 40−140 °C, corresponding to the loss of three ethanol molecules (Figure S17, Supporting Information). For 8, a weight loss of ∼15% in the range, 30−240 °C accounted for the loss of three DMF molecules (Figure S18, Supporting Information) that matched well with the calculated value of ∼15.3%. Both 7 and 8 decomposed after 380 °C. A weight loss of ∼17.5% (calcd 19.8%) for 9 was observed in the wide temperature range of 25−400 °C, corresponding to three acetone and seven water molecules (Figure S19, Supporting Information). For 10, the weight loss was found to be ∼18% (calcd 18.5%) between 25 and 300 °C (Figure S20, Supporting Information). Crystal structures of all of these (1−10) complexes are similar as previously reported compounds9a,b {[Ni2(DBIBA)3]· Cl·18H 2 O} n , {[Co 2 (DBIBA) 3 ]·Cl·9H 2 O} n , and {[Mn 2 (DBIBA)3]·Cl·3H2O}n. The parent complex crystallizes in the trigonal space group P3̅1c, and the asymmetric unit consists of one M(II) ion (Ni, Co, Mn) with 1/3 occupancy, a half ligand (DBIBA−), one chloride ion with 1/6 occupancy, and different number of lattice water molecules. The structure contains a binuclear M(II) unit built through three bridging carboxylates, while the other three coordination sites of each metal ion are occupied by three benzimidazole N atoms forming a distorted octahedral geometry. The framework is quite rigid with strong π−π (between two benzene rings of two different benzimidazole moieties in the core of the cavity) and anion−π (between the anion and electron-deficient imidazole ring) interactions in the framework of Ni(II) and Co(II) parent crystals, but these are weaker for the Mn(II) mother crystal. All three parent compounds were investigated systematically for reversible anion and solvent exchanges. In crystal structures of all of these complexes, the chloride anions are involved in weak Hbonding interactions (Figure S21) with the framework, and the presence of OH stretching peak in IR spectra for noncoordinated water molecule (Figure S22) (Supporting Information) prompted us to attempt anion/solvent exchange studies. In this study, a crystal is chosen as the mother crystal (Figure S23, Supporting Information) and used in transformation reactions. Immersion of a crystal of the parent Ni(II) complex in a 3 M aqueous solution of NaBF4 for 7 days at 298 K led to complete replacement of Cl− by the BF4− anion, giving complex 1 without losing its morphology and crystallinity. If the crystal is taken out from the solution after 4 days and its structure is determined, no anion exchange is found to have taken place. The IR spectrum shows a strong peak at 1059 cm−1, which originated from free BF4− (Figure S1). The overall structure of the framework remains the same, although the positions of anions and supramolecular π−π bonding (2.926− 3.15 Å) in hexagonal cavities and anion−π (2.2−3.664 Å) interactions are drastically altered (Figure 1). To investigate the reversibility of the anion exchange process, the exchanged

Figure 1. (a) BF4− involved in hydrogen bond formation with the framework of 1 and (b) packing diagram in 1, where BF4− is sandwiched between two benzene rings of benzimidazole.

crystal 1 was dipped in a 3 M aqueous solution of NaCl at room temperature for 4 days; because of the smaller size of the chloride, the anion exchange rate was faster. Crystal quality was intact, with absolutely no effect on crystal color, shape, and its size, and it had the same cell parameters like the Ni(II) parent crystal. Elemental analysis, PXRD (Figure S24), and the IR spectrum (Figure S25) (Supporting Information) also support that it is a completely reversible process. In the IR spectrum, there is no peak due to BF4− and it has completely disappeared. Similarly, the syntheses of 2 and 3 were accomplished by putting the parent Ni(II) crystal in a 3 M aqueous solution of NaClO4 and NaNO3, respectively, for 7 days. An intense band around 1093 cm−1 is due to free ClO4− in 2 (Figure S2), while 3 shows peaks in the region of 1305−1380 cm−1 (Figure S3), which correspond to free NO3− anions. The metal-to-anion distance is increased in 1 and 2 because anions are placed at different positions in the framework compared to the parent Ni(II) complex, and in the case of 3, nitrate anions occupy the same position of the chloride anion. When viewed along the a axis, perchlorate and nitrate anions form hydrogen bonds with the framework (Figures S26 and S27, Supporting Information), and the packing diagram along the c axis exhibits hexagonal 6117

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the Ni mother crystal toward different anions, a solution was prepared consisting of 3 M NaClO4, NaBF4, and NaNO3 in which the crystal was immersed for 7 days. After 7 days, the crystal was not diffracted because of its poor quality, so IR spectra have been taken. Intense bands around 1084 and 1121 cm−1 originated from the ClO4− anion (Figure S30, Supporting Information). It can be concluded that the parent Ni complex is selective for the ClO4− anion. Its mechanism is not very clear, but anion exchange is accompanied by bond formation or dissociation in some cases.12 In anion resins, selectivity is to be governed by the hydration energy of the anion, which is a function of radius and charge.14 Coming to the parent crystal of the Co(II) complex, keeping it in a 3 M aqueous solution of NaClO4 affords compound 5 (Figure 3); a strong peak arising in the IR spectrum at 1091

cavities decorated with benzimidazole moieties where perchlorate anions are sandwiched between two benzene rings of two different benzimidazoles (Figure 2) and anions take part in anion−π interactions with the electron-deficient imidazole ring.

Figure 3. View along the c axis showing the position of ClO4− in 5.

cm−1 is due to free ClO4− anions (Figure S5). This exchange was also found to be reversible when 5 was immersed in an aqueous solution of NaCl. Cell parameters were identical with those of the parent Co(II) complex; further confirmation was done through the IR spectrum, where ClO4− stretching disappeared (Figure S31, Supporting Information). However, other anions cannot replace the chloride anion from the parent compound in the SC-SC mode. It shows that the parent Co(II) complex is selective only for ClO4−, but for the parent Mn compound, the chloride anion cannot be replaced by any other anion when crystals of the compound are kept in an aqueous solution of an appropriate salt for more than 14 days at room temperature. Complexes 1−3 and 5 can be synthesized by a solvothermal route as well, which is already described, whereas 6 is obtained only at high temperature (Figure S32), not by single-crystal to single-crystal transformation. In this complex, strong Hbonding and anion−π interactions are present involving anions and the framework (Figures S33 and S34) (Supporting Information). Solvent exchange reactions are found to undergo at greater speed compared to the speed of anion exchange reactions. For probing the possible SC-SC ethanol exchange reaction, a parent crystal of the Ni compound is dipped in ethanol for 4 days at room temperature. All the water molecules are removed to afford the compound 4 giving an intense band at 3638 cm−1 for

Figure 2. Packing diagram of (a) 2 showing ClO4− sandwiched between two benzimidazole moieties and (b) 3 consisting of NO3−.

To check the reversibility of the anion exchange process, the exchanged crystals 2 and 3 were dipped in an aqueous solution of NaCl to regenerate the parent Ni(II) complex. Crystal quality was utterly maintained. Further elemental analysis, PXRD (Figure S24), and IR spectra (Figures S28 and S29, Supporting Information) of regenerated compounds were identical to those of the parent Ni(II) crystal. Interestingly, any compound (1−3) in the series can be transformed without losing crystallinity from any one by simply putting the crystal in the appropriate aqueous salt solution at room temperature (Scheme S1, Supporting Information). During all these transformations, there is no change in shape, size, and color of the crystal, but a few water molecules are also lost from the lattice to make room for the bigger anions. In several coordination polymers, selectivity of anion exchanges has been observed.12,13 To check the selectivity of 6118

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and no hydrogen bonding of chloride anions with the framework, but acetone molecules are involved, forming hydrogen bonds with the framework, and strong π−π interactions are present in the cavity. Any of the solventexchanged compounds can be converted into one another by simply putting a crystal in the appropriate solvent for 4 days at room temperature (Scheme S2, Supporting Information). Coordination polymer 10 is acquired after sinking a crystal of the parent Mn(II) compound in DMF. A sharp peak at 1665 cm−1 is due to CO stretching in DMF molecules, and the broad band at 3415 cm−1 is because of lattice water molecules (Figure S10). In this case, solvent is not exchanged but new solvent molecules (DMF) are added due to high polarity, forcing the space group to change from trigonal P31̅ c to monoclinic C2/c. The cavity is not highly ordered in this case (Figure 6). Unlike other cases, this transformation is irreversible, which is confirmed when solvent-exchanged complex 10 was poured in water for 4 days. IR peaks due to DMF and water molecules were still present, and the IR was identical with the IR of complex 10 (Figure S10). Framework integrity was confirmed through PXRD measurement (Figure S40, Supporting Information). During all these transformations, there is absolutely no change in shape, size, and color of the crystal. The bond distances and bond angles involving the metal ion remain unaltered within statistical errors upon guest exchange reactions. Having high bond distances involving metal ions and weak supramolecular interactions in the parent Mn(II) polymer make this framework more flexible compared to those of the parent Ni(II) and Co(II) coordination polymers. Conclusively, entrance of a guest moiety completely depends upon the stability (rigidity/flexibility) of the framework. The guest exchange phenomenon shows insolubility and stability of porous coordination polymers. In coordination polymers, the anion exchange process probably depends on the size and shape of the cavity and anions. Cavity size should be sufficient so that new anions can have the proper entry and existing anions can easily exit. The order of crystallographic radii of different anions is as follows: ClO4− (2.40 Å) > BF4− (2.29 Å) > Cl− (1.81 Å) > NO3− (1.29 Å). 1D hexagonal channels in all complexes have a diameter of ∼6.3 Å (considering the van der Waals radii). Interestingly, mother crystals (Ni, Co, and Mn) have almost the same void volume (∼32%) and cavity size, but all are showing different properties. These are mainly depending on noncovalent supramolecular interactions (anion−π, π−π, and hydrogenbonding interactions), and those are much stronger for the mother Ni(II) and Co(II) polymers, but very weak for Mn(II). Besides this bond distances are also longer for the Mn(II). Other probable reasons for this may be the torsion angle N2 M1 O1 C1 and the dihedral angle between two planes N1 M1 O1 (M = Ni, Co, and Mn), which is higher (64.6°, 65.07°) for Mn(II) and lowest (51.8°, 58.22°) for Ni(II) mother crystals, and Co(II) lies in between. This inconsistent feature can also be rationalized by the hydrophilic/hydrophobic character of the anions. BF4− is the most hydrophilic16 than any other anions, and it will be more hydrated and surrounded by more water molecules. It should behave as a bigger anion, which would hinder the inclusion in channels of the Co(II) parent crystal because the important thing to notice here is the order of the ionic radii: Mn(II) > Co(II) > Ni(II). On the other hand, ClO4− is more hydrophobic; it can easily get entry. There are many factors that can explain all of these things. All the key

OH stretching in ethanol (Figure S4). To check the reversibility of solvent-exchanged complex 4, it was immersed in water to regenerate the parent Ni(II) complex. Crystal quality was completely maintained. Further elemental analysis, PXRD (Figure S24), and the IR spectrum (Figure S35, Supporting Information) of the regenerated compound were identical to those of the parent Ni(II) crystal. When the mother crystal was put in DMF or other common organic solvents, the crystal quality was not good, so the data cannot be collected. In contrast, the water molecules in the parent Co(II) compound can be replaced in a SC-SC fashion by ethanol, DMF, as well as acetone when a parent crystal is poured in any of these solvents for 4 days at room temperature. The intense band at 3550 cm−1 is due to OH stretching (Figure S7), the band at 1660 cm−1 is because of CO stretching in DMF (Figure S8), and the sharp peak at 1695 cm−1 is due to the acetone molecule (Figure S9). To investigate the reversibility of the solvent exchange processes, all of these three 7−9 complexes were submerged in water. After 4 days, all crystals were diffracted, achieving identical cell parameters like the mother crystal. Elemental analysis, PXRD (Figure S36), and IR studies (Figures S37− S39) (Supporting Information) further confirmed that this is a reversible reaction. In the case of complex 8, the metal-to-anion distance is the same, but the position of the anion is changed and there is no more π−π bonding between two benzene rings of benzimidazole in the hexagonal cavity (Figure 4). However,

Figure 4. Cavity predicting no π−π bonding interactions in 8.

in the case of 9, after insertion of acetone molecules, O1W molecules take the position of chloride anions and chloride anions move to the center of the hexagonal cavity. Surrounding the chloride ion, there are six O2W molecules in the shape of a hexameric core showing strong noncovalent interactions (Figure 5a). The core is stabilized by strong hydrogen bonding between O2W water molecules. A variety of clusters formed by the water molecules are reported in the literature.15 Acetone places between two benzene rings of different benzimidazole moieties and shows hydrogen bonding with the hexanuclear water cluster (Figure 5b). There are no anion−π interactions, 6119

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Figure 5. (a) Hexameric cluster in 9 and (b) core showing hydrogen-bonding interactions with hydrogens of acetone molecules as well as benzene ring hydrogens in 9.

Mn(II) not showing reversibility because of weak anion−π and π−π interactions in the cavity attributing flexibility to the framework. Therefore, we can say that the nature of the cavity has an important role in deciding the properties of a compound. Our future work will be based on the modification of DBIBN to synthesize new functional coordination polymers.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format, table for selected bonds and distances for 1−10, and complete data for IR, TGA analysis, and PXRD. 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 The authors gratefully acknowledges the financial support received from the Department of Science and Technology, New Delhi, India.

Figure 6. Showing a view of the cavity consisting of different ranges of π−π interactions in 10.

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parameters that are changing after guest inclusion have been summarized in the form of a table (Table S3, Supporting Information).

REFERENCES

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CONCLUSION A series of isostructural coordination polymers 1−10 have been obtained via anion or solvent exchange reactions. In the case of anion exchange reactions, the anions move slightly in the cavity to seek their optimum position. All anion/solvent exchange reactions are not only reversible but also completely interchangeable without losing crystallinity. The space group is maintained in all exchange reactions except in 10, where it changes from trigonal P3̅1c to monoclinic C2/c. In 9, the chloride anion shifted to the center of the cavity where it is surrounded by six water molecules, making a hexameric core. Mother crystals Ni(II) and Co(II) reveal a reversible SC-SC phenomenon probably due to the rigid nature of the cavity consisting of strong anion−π and π−π bonding and the parent 6120

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