Gas Adsorption, Magnetism and Single-Crystal to Single-Crystal Transformation Studies of a Three-Dimensional Mn(II) Porous Coordination Polymer

A porous coordination polymer {[Mn2(DBIBA)3]×(NO3)·3DMF·4H2O}n (1) [DBIBAH = 3,5-di(1H-benzo[d]imidazol-1-yl)benzoic acid] has been synthesized solvothermally and structurally characterized by single-crystal X-ray diffraction. This compound shows significant selective CO2 uptake at low temperature. 1 exhibits antiferromagnetic properties below 17 K, confirmed by magnetic susceptibility measurements. Four new coordination polymers: {[Mn2(DBIBA)3]·ClO4·3DMF·3H2O}n (2), {[Mn2(DBIBA)3]×Cl×DMF×H2O}n (3), {[Mn2(DBIBA)3]×NO3×CH3OH×7H2O}n (4) and {[Mn2(DBIBA)3]×NO3×2CH3COCH3×H2O}n (5), have been synthesized from 1 via anion/solvent exchange protocols at room temperature. Introduction Studies on the functional aspects of porous metal-organic framework materials such as ion exchange, 1 separation, 2 gas storage, 3 sensor2,4 and magnetism4,5 are rapidly emerging. A large number of highly porous MOFs have been synthesized which are capable of storing huge amount of CO2, as well as significant amounts of other gases.6 Separation of CO2 from other gases have high technological and industrial importance.7 Ability to synthesize MOFs with various organic linkers and metal nodes provides tremendous flexibility in tailoring the porous material to acquire specific physical characteristics and chemical functionalities. For an effective optimized adsorption of CO2, a periodic porous structure is desirable where uptake and release are fully reversible and flexible, by which chemical functionalization and molecular level fine-tuning can be attained.8 Meanwhile, carboxylate bridged Mn(II) species are of particular interest, since such systems are known to exist at the active centers of some Mn(II) containing enzymes.9 Besides, high spin Mn(II) contain up to five unpaired electrons; thus the assembly with multicarboxylate is seemingly inclined to the formation of large clusters and extended solids.10,11 Bulk magnetic properties mainly depend on the bridging modes and the bridging geometry. In recent years, various cyanide, azides, nitrogen-containing aromatic ligands, or carboxylates have been primarily used for the design of hybrid metal-organic compounds with interesting magnetic properties.12 A great deal of work is essential to understand the underlying structural features, that govern the exchange coupling between paramagnetic centres, since such a relationship is complex and has so far remained elusive.13 Self-assembly and its fascinating progress have been highly influenced by several factors, such as the varying counter-ions, temperature and solvent, in addition to the nature of metal/ligand.14 Anion exchange has attracted increasing attention in recent years, because it opens up novel avenues to constitute such frameworks potentially attractive as anion exchange materials.15,16 When the framework-open space is occupied by molecular guests, the existing interactions between the host-framework and these guest species are expected to provide useful relationships between the concerned structure and gas sorption properties.17 Single-crystal to single-crystal transformation is extremely desirable owing to the systematic study of gas storage and separation, since it allows exact monitoring of how the crystal structure, location and orientation of guest molecules in the voids is changing during transformation process in the crystalline phase.18 This can be potentially important to in using the coordination space for applications.19 Herein, we report the synthesis, structural characterization, gas sorption and magnetic properties of a 3D-coordination polymer {[Mn2(DBIBA)3]×(NO3)·3DMF·4H2O}n (1), synthesized by utilizing a bifurcated ligand consisting of benzimidazole moieties as N donor centres and carboxyl group as O donor centres (Scheme 1). The linker is particularly chosen because there is the possibility of rotation about the C—N bond that can allow cooperative movement of the metal-linker ensemble necessary for single-crystal to single-crystal (SCSC) transformations. Complex 1 affords four new coordination polymers (2–5) via anion/solvent induced SCSC transformations. Scheme 1. Representation of the ligand 3,5-di(1H-benzo[d]imidazol-1-yl)benzoic acid (DBIBAH) 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 protocols. Physical Measurements. Infrared spectra were recorded (KBr disk, 400-4000 cm-1) using a Perkin-Elmer equipment model spectrum version II. 1H NMR spectra were recorded on a JEOL-ECX 500 FT (500 MHz) in DMSO-d6 with Me4Si as the internal standard. ESI mass spectra were recorded on a WATERS Q-TOF premier mass spectrometer. Thermogravimetric analysis data plots were recorded using a Mettler Toledo (heating rate of 5 oC/min) TGA instrument. Powder X-ray diffraction (CuKa radiation, scan rate 3o/min, 293 K) was performed on a Bruker D8 Advance Series 2 powder X-ray diffractometer. Low pressure gas sorption measurements were performed using BelSorpmax (Bel Japan). All the gases used were of 99.999% purity. As-synthesized compound 1 was heated at 160 °C under vacuum for 10 h, to get guest free compound. Prior to adsorption measurement, the guest free sample was pretreated at 160 °C under vacuum for 4 h, using BelPrepvacII, and purged with He on cooling. Between the experiments with various gases, the out-gassing procedure was repeated for ca. 5 h. For low temperature measurements, N2 and H2 adsorption isotherms were monitored at 77 K, while CO2 gas sorption isotherm was monitored at 195 K. The adsorption isotherms at 298 K were monitored individually for the gases, namely CO2, N2, H2 and CH4. CO2 gas sorption isotherm was also measured at 273 K. Surface area and pore size distribution were calculated using BelMaster analysis software package. Magnetic measurements were carried out in the Unitat de Mesures Magnetiques (Universitat de Barcelona) on polycrystalline samples (ca. 30 mg) with a Quantum Design SQUID MPMS-XL magnetometer equipped with a 5 T magnet. Diamagnetic corrections were calculated using Pascal’s constant19 and an experimental correction factor for the sample holder was applied. Synthesis of 3,5-di(1H-benzo[d]imidazol-1-yl)benzoic acid (DBIBAH). In the first step, 5di(1H-benzo[d]imidazol-1-yl)benzonitrile (DBIBN) was synthesized as previously reported.20 In the second step, a mixture of DBIBN (2 g, 5.9 mmol) was hydrolyzed by refluxing it with an ethanolic solution of NaOH (0.35 g, 8.8 mmol) for 10 h. Finally, the resulting solution was carefully neutralized with dilute HCl to obtain a pale yellow precipitate. It was collected by filtration, washed thoroughly with water and air dried (Scheme 1). Yield: 1.8 g (90%). Melting point: 250 °C (uncorrected); IR(cm–1, KBr pellet): 3403(m), 3086(m), 2421(w), 1895(w), 1704(w), 1645(w), 1605(s), 1501(s), 1473(s), 1396(w), 1244(s), 1213(s), 1167(s), 1135(m), 1091(w), 896(w), 853(w), 782(m), 761(m), 731(s), 690(m), 622(w), 542(w), 489(w), 466(w), 427(w) (Figure S1, Supporting Information). 1H-NMR (DMSO-d6, 500 MHz) δ (ppm): 8.74 (s, 2H; HAr), 8.23 (d, 3H; HAr), 7.77 (m, 4H; HAr), 7.34 (m, 4H, HAr) (Figure S2, Supporting Information); 13C NMR (DMSO-d6) δ (ppm): 166.30, 144.21, 144.06, 138.17, 134.91, 133.26, 124.48, 123.49, 123.36, 123.08, 120.57, 111.35 (Figure S3, Supporting Information). ESI-MS: m/z [M-1] 353.10 (100%); calculated 354.13 (Figure S4, Supporting Information); Anal. calcd. for C21H14N4O2 (354.13): C, 71.16; H, 3.98; N, 15.82%. Found: C, 70.91; H, 4.07; N, 15.52%. Synthesis of {[Mn2(DBIBA)3]·NO3·3DMF·4H2O}n (1). A mixture containing Mn(NO3)2·4H2O (0.058 g, 0.22 mmol), DBIBAH (0.04 g, 0.11 mmol) in DMF:H2O (4 mL, 3:1 v/v) was placed in a Teflon-lined stainless steel autoclave and heated to 80 oC for 2 days under autogenous pressure. Then it was allowed to cool to room temperature at the rate of 1 oC/min. Small light brown crystals of 1 were collected in ~52% yield. The crystals were repeatedly washed with ethanol and air-dried. Anal. calcd. for C72H68N16O16Mn2: C, 56.75; H, 4.50; N, 14.72%. Found: C, 56.22; H, 4.65; N, 14.82%. IR (cm–1, KBr pellet): 3436(m), 3091(m), 3065(m), 2848(w), 2926(w), 1664(s), 1644(s), 1596(s), 1584(s), 1500(s), 1474(s), 1448(m), 1407(s), 1377(s), 1321(m), 1306(s), 1291(s), 1241(s), 1224(s), 1170(m), 1151(m), 1142(m), 1094(m), 1062(w), 1009(w), 906(m), 872(m), 793(m), 780(s), 766(s), 755(s), 721(s), 689(m), 657(w), 626(w), 610(w), 562(w), 520(w) (Figure S5, Supporting Information). Synthesis of {[Mn2(DBIBA)3]·ClO4·3DMF·3H2O}n (2). Crystals of 1 were immersed in 3M aqueous solutions of NaClO4 for one week at room temperature, upon which 2 was obtained without losing crystallinity. Anal. calcd. for C72H66N15O16ClMn2: C, 56.02; H, 4.28; N, 11.67%. Found: C, 56.4; H, 4.5; N, 11.8%. IR (cm–1, KBr pellet): 3539(m), 3093(m), 1696(m), 1663(m), 1645(s), 1596(s), 1585(s), 1502(s), 1409(s), 1378(s), 1307(s), 1291(s), 1242(s), 1225(s), 1092(s), 765(s), 755(s) (Figure S12). Synthesis of {[Mn2(DBIBA)3]×Cl×DMF×H2O}n (3). Crystals of 1 were immersed in 3M aqueous solutions of NaCl for one week at room temperature to obtain 3 without losing crystallinity. Anal. calcd. for C66H48N13O8ClMn2: C, 61.09; H, 3.70; N, 14.03%. Found: C, 61.53; H, 3.78; N, 14.27%. IR (cm–1, KBr pellet): 3417(m), 3067(m), 1663(s), 1643(s), 1595(s), 1499(s), 1404(s), 1375(s), 1241(s), 743(s) (Figure S13). Synthesis of {[Mn2(DBIBA)3]×NO3×CH3OH×7H2O}n (4). Crystals of 1 were dipped in dry methanol for four days to obtain 4 without losing crystalinity. Anal. calcd. for C64H57N13O17Mn2: C, 55.25; H, 4.10; N, 13.09%. Found: C, 55.64; H, 4.18; N, 13.24%. IR (cm–1, KBr pellet): 3370(s), 3117(s), 3067(s), 1644(s), 1598(s), 1585(s), 1501(s), 1403(s), 1367(s), 1220(s), 1171(s), 1050(s), 1011(s), 906(s), 882(s), 794(s), 781(s), 746(s), 719(s), 426(s) (Figure S14). Synthesis of {[Mn2(DBIBA)3]×NO3×2CH3COCH3×H2O}n (5). Crystals of 1 were dipped in dry acetone for four days to obtain 5 in a SCSC transformation. Anal. calcd. for C69H53N13O12Mn2: C, 60.63; H, 3.95; N, 13.32%. Found: C, 60.9; H, 4.11; N, 13.62 %. IR (cm–1, KBr pellet): 3118(m), 1698(w), 1723(w), 1643(s), 1596(s), 1585(s), 1501(s), 1474(s), 1460(s), 1451(s), 1368(s), 1304(s), 1293(s), 1241(s), 1220(s), 1171(m), 1149(m), 1141(m), 1107(m), 906(m), 873(m), 794(m), 781(s), 764(s), 744(s) (Figure S15). Single Crystal X-ray Studies. Single crystal X-ray data for complexes 1-5 were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromated MoKα radiation (λ = 0.71073 Å) as described earlier.20 Squeeze refinement have been performed for 2–4 complexes using PLATON. For compound 5 acetone molecules were located but water molecules could not be located. Contributions from all atoms of the solvent molecules have been incorporated in both the empirical formulae and formula-weights as shown in the crystal and structure refinement data Table. In 1, all water molecules, nitrate anion and atoms O8, N9, C37, C38, and C39 while in 4, atoms C12, C13 and C20 were refined isotropically. Hydrogens could not be located in the difference maps for the three lattice water molecules and a distorted DMF molecule in 1. Several DFIX and DANG commands were given for nitrate anion of 1 to fix bond distances and bond angles. To fix bond distance of perchlorate anion in 3 and between C9 and C10, C7 and C12 in 4, DFIX commands were given. The crystal and refinement data are collected in Table 1 and 2, while selective bond angles and bond distances are given in Table S1 in the Supporting Information. Space for Tables 1 and 2 Results and discussion Complex 1 was synthesized under solvothermal condition at 80 oC by utilizing DBIBAH ligand. Once isolated, it is found to be stable in air and insoluble in common organic solvents. Single crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic space group C2/c and the asymmetric unit consists one Mn(II) ion, 1.5 DBIBA ligand, one nitrate anion with half occupancy, one lattice water molecule with full occupancy and two free water molecules with half occupancy, one non-coordinated DMF molecule with full occupancy and Figure 1 Showing selected planes (N1 Mn1 O1, N4 Mn1 O2 and N6 Mn1 O3) in 1. another DMF molecule with half occupancy. This DMF molecule is distorted and having translational symmetry passing through its nitrogen atom. 1 is iso-structural to previously reported coordination polymers of Ni(II)21, Co(II)20 and Mn(II)21; but bond angles, torsional angles and dihedral angles (Figure 1) for 1 vary considerably compared to the reported Mn(II) coordination polymer. Dihedral angle between the two planes of N1 Mn1 O1 and N6 Mn1 O3 is 56.25o, between N1 Mn1 O1 and N4 Mn1 O2 is 81.7o, while that between N4 Mn1 O2 and N6 Mn1 O3 is 86.8o. Torsion angles N1–Mn1–O1–C1, N4–Mn1–O2–C23 and N6–Mn1–O3– C23 are -78.9o, 104.7o and -51.2o respectively. Along the crystallographic c axis, 1D channels of diameter ~5.8 Å (considering van der Waals radii) are present revealing strong π–π interactions between two benzene rings of two different benzimidazole moieties. Between the two benzene rings of different ligands, DMF molecules are located (Figure 2a). In the centre of the cavity, four nitrate anions with six water molecules are present (Figure 2c). These water molecules show significant intermolecular hydrogen bonding interactions Figure 2 (a) View along c axis showing location of distorted DMF molecules and nitrate anion, (hydrogen atoms are omitted because of clarity) (b) intermolecular hydrogen bonding of framework with the guest water and DMF molecules and (c) a perspective view of the central part of cavity. with the benzene rings of benzimidazole and hydrogen of methyl group of DMF. Nitrogen atoms of DMF molecule are also involved in hydrogen bonding with benzene ring hydrogens (Figure 2b). An examination of the packing diagram of 1 along crystallographic c axis shows the existence of strong hydrogen bonding between nitrate anions, and water molecules (Figure 3). (a) (b)


Introduction
Studies on the functional aspects of porous metal-organic framework materials such as ion exchange, 1 separation, 2 gas storage, 3 sensor 2,4 and magnetism 4,5 are rapidly emerging. A large number of highly porous MOFs have been synthesized which are capable of storing huge amount of CO2, as well as significant amounts of other gases. 6 Separation of CO2 from other gases have high technological and industrial importance. 7 Ability to synthesize MOFs with various organic linkers and metal nodes provides tremendous flexibility in tailoring the porous material to acquire specific physical characteristics and chemical functionalities. For an effective optimized adsorption of CO2, a periodic porous structure is desirable where uptake and release are fully reversible and flexible, by which chemical functionalization and molecular level fine-tuning can be attained. 8 Meanwhile, carboxylate bridged Mn(II) species are of particular interest, since such systems are known to exist at the active centers of some Mn(II) containing enzymes. 9 Besides, high spin Mn(II) contain up to five unpaired electrons; thus the assembly with multicarboxylate is seemingly inclined to the formation of large clusters and extended solids. 10,11 Bulk magnetic properties mainly depend on the bridging modes and the bridging geometry. In recent years, various cyanide, azides, nitrogen-containing aromatic ligands, or carboxylates have been primarily used for the design of hybrid metal-organic compounds with interesting magnetic properties. 12 A great deal of work is essential to understand the underlying structural features, that govern the exchange coupling between paramagnetic centres, since such a relationship is complex and has so far remained elusive. 13 Self-assembly and its fascinating progress have been highly influenced by several factors, such as the varying counter-ions, temperature and solvent, in addition to the nature of metal/ligand. 14 Anion exchange has attracted increasing attention in recent years, because it opens up novel avenues to constitute such frameworks potentially attractive as anion exchange materials. 15,16 When the framework-open space is occupied by molecular guests, the existing interactions between the host-framework and these guest species are expected to provide useful relationships between the concerned structure and gas sorption properties. 17 Single-crystal to single-crystal transformation is extremely desirable owing to the systematic study of gas storage and separation, since it allows exact monitoring of how the crystal structure, location and orientation of guest molecules in the voids is changing during transformation process in the crystalline phase. 18 This can be potentially important to in using the coordination space for applications. 19 Herein, we report the synthesis, structural characterization, gas sorption and magnetic properties of a 3D-coordination polymer {[Mn2(DBIBA)3]×(NO3)·3DMF·4H2O}n (1), synthesized by utilizing a bifurcated ligand consisting of benzimidazole moieties as N donor centres and carboxyl group as O donor centres (Scheme 1). The linker is particularly chosen because there is the possibility of rotation about the C-N bond that can allow cooperative movement of the metal-linker ensemble necessary for single-crystal to single-crystal (SCSC) transformations. Complex 1 affords four new coordination polymers (2-5) via anion/solvent induced SCSC transformations.

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 protocols.
Physical Measurements. Infrared spectra were recorded (KBr disk, 400-4000 cm -1 ) using a Perkin-Elmer equipment model spectrum version II. 1 H NMR spectra were recorded on a JEOL-ECX 500 FT (500 MHz) in DMSO-d6 with Me4Si as the internal standard. ESI mass spectra were recorded on a WATERS Q-TOF premier mass spectrometer. Thermogravimetric analysis data plots were recorded using a Mettler Toledo (heating rate of 5 o C/min) TGA instrument. Powder X-ray diffraction (CuKa radiation, scan rate 3 o /min, 293 K) was performed on a Bruker D8 Advance Series 2 powder X-ray diffractometer. Low pressure gas sorption measurements were performed using BelSorpmax (Bel Japan). All the gases used were of 99.999% purity. As-synthesized compound 1 was heated at 160 °C under vacuum for 10 h, to get guest free compound. Prior to adsorption measurement, the guest free sample was pretreated at 160 °C under vacuum for 4 h, using BelPrepvacII, and purged with He on cooling.
Between the experiments with various gases, the out-gassing procedure was repeated for ca. 5 h. For low temperature measurements, N2 and H2 adsorption isotherms were monitored at 77 K, while CO2 gas sorption isotherm was monitored at 195 K. The adsorption isotherms at 298 K were monitored individually for the gases, namely CO2, N2, H2 and CH4. CO2 gas sorption isotherm was also measured at 273 K. Surface area and pore size distribution were calculated using BelMaster analysis software package. Magnetic measurements were carried out in the Diamagnetic corrections were calculated using Pascal's constant 19 and an experimental correction factor for the sample holder was applied.

Synthesis of 3,5-di(1H-benzo[d]imidazol-1-yl)benzoic acid (DBIBAH).
In the first step, 5di(1H-benzo[d]imidazol-1-yl)benzonitrile (DBIBN) was synthesized as previously reported. 20 In the second step, a mixture of DBIBN (2 g, 5.9 mmol) was hydrolyzed by refluxing it with an ethanolic solution of NaOH (0.35 g, 8.8 mmol) for 10 h. Finally, the resulting solution was carefully neutralized with dilute HCl to obtain a pale yellow precipitate. It was collected by filtration, washed thoroughly with water and air dried (Scheme 1). Yield: 1.8 g (90%). Melting  166.30, 144.21, 144.06, 138.17, 134.91, 133.26, 124.48, 123.49, 123.36, 123.08, 120.57, 111.35  Single Crystal X-ray Studies. Single crystal X-ray data for complexes 1-5 were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromated MoKα radiation (λ = 0.71073 Å) as described earlier. 20 Squeeze refinement have been performed for 2-4 complexes using PLATON. For compound 5 acetone molecules were located but water molecules could not be located. Contributions from all atoms of the solvent molecules have been incorporated in both the empirical formulae and formula-weights as shown in the crystal and structure refinement data Table. In 1, all water molecules, nitrate anion and atoms O8, N9, C37, C38, and C39 while in 4, atoms C12, C13 and C20 were refined isotropically. Hydrogens could not be located in the difference maps for the three lattice water molecules and a distorted DMF molecule in 1. Several DFIX and DANG commands were given for nitrate anion of 1 to fix bond distances and bond angles. To fix bond distance of perchlorate anion in 3 and between C9 and C10, C7 and C12 in 4, DFIX commands were given. The crystal and refinement data are collected in Table 1 and 2, while selective bond angles and bond distances are given in Table S1 in the Supporting Information.
Space for Tables 1 and 2

Results and discussion
Complex 1 was synthesized under solvothermal condition at 80 o C by utilizing DBIBAH ligand. Once isolated, it is found to be stable in air and insoluble in common organic solvents.
Single crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic space group C2/c and the asymmetric unit consists one Mn(II) ion, 1.5 DBIBA ligand, one nitrate anion with half occupancy, one lattice water molecule with full occupancy and two free water molecules with half occupancy, one non-coordinated DMF molecule with full occupancy and another DMF molecule with half occupancy. This DMF molecule is distorted and having translational symmetry passing through its nitrogen atom. 1 is iso-structural to previously reported coordination polymers of Ni(II) 21 , Co(II) 20 and Mn(II) 21 ; but bond angles, torsional angles and dihedral angles ( Figure 1) for 1 vary considerably compared to the reported Mn(II) coordination polymer. Dihedral angle between the two planes of N1 Mn1 O1 and N6 Mn1 O3 is 56.25º, between N1 Mn1 O1 and N4 Mn1 O2 is 81.7º, while that between N4 Mn1 O2 and N6 Mn1 O3 is 86.8º. Torsion angles N1-Mn1-O1-C1, N4-Mn1-O2-C23 and N6-Mn1-O3-C23 are -78.9º, 104.7º and -51.2º respectively. Along the crystallographic c axis, 1D channels of diameter ~5.8 Å (considering van der Waals radii) are present revealing strong π-π interactions between two benzene rings of two different benzimidazole moieties. Between the two benzene rings of different ligands, DMF molecules are located (Figure 2a). In the centre of the cavity, four nitrate anions with six water molecules are present (Figure 2c). These water molecules show significant intermolecular hydrogen bonding interactions  The solvent accessible volume is ~28.3 % of the unit cell volume, calculated from the crystal structure using the PLATON program. 22 ( Figure S6).
Thermogravimetric analysis of 1 shows a weight loss of ~19% in the temperature range 35-290 o C, which corresponds to the loss of four water molecules and three DMF molecules (calcd 19.1%). After 370 o C the complex starts decomposing ( Figure S7).

Gas adsorption studies
Gas adsorption measurements for N2, H2, and CO2 gases were performed in a relative pressure range from 10 -4 to 1 atm at 77 K (N2 and H2) and 195 K (CO2) using activated compound prepared from 1. Interestingly, the activated compound presented differential adsorption behaviour towards CO2 (195 K), as compared to those for N2 (77 K) and H2 (77 K). The CO2sorption profile was of typical type-I nature with a significant uptake amount of 155 mL g −1 at 195 K, whereas both the N2 and H2-sorption plots refer to typical type-III nature (with negligible gas-uptake excluding the high-pressure induced surface adsorption at P/P0>0.87) ( Figure 4). Here, size selective uptake can certainly be ruled out because of the bigger pore size compared to the kinetic diameters of the probe-adsorptive gas species (CO2 = 3.3 Å, N2 = 3.64 Å and H2 = 2.8 Å). 23
The type-III adsorption plots for N2 and H2 suggests of a notably weak interaction between the sample surface of 1 and these adsorbate gas molecules possessing zero dipole moment. On the contrary CO2, having much larger quadruple moment (1.34 x 10 -39 Cm 2 ), known to interact selectively with polar functionalities as compared to other gases registers a much higher adsorption-uptake with type-I profile, characteristic of strong interaction and also confirming the microporous nature of 1. This striking low-temperature CO2-selectivity further prompted to investigate the sorption performances for this material at room temperature (298 K), since the CO2-adsorption amount at 273 K was experimentally found to be substantially high (73 mL g −1 ) ( Figure 5). The uptake amounts for all the four gaseous probe species, namely; CO2, N2, H2 and CH4 could evidently present CO2-selective adsorption phenomena even at room temperature, since the amounts adsorbed were 50 mL g −1 for CO2, 14.6 mL g −1 for N2, 23 mL g −1 for CH4 and 3 mL g −1 for H2 ( Figure 6).  Since the adsorption amounts of other gases (H2, N2 and CH4) are much smaller compare to CO2, the compound 1 can prove expedient for separation of CO2 from gas mixture at low to ambient temperature range. The CO2/H2 selectivity can be exploited in pre-combustion CCS (carbon capture and storage) technology, while The CO2/N2 selectivity makes it a prospective candidate for application in post-combustion CCS technology. BET Surface area as calculated from CO2 adsorption isotherm at 195 K was found to be 365.82 m 2 g −1 . The H-K (Haworth-Kawazoe) plot for CO2 sorption at 195 K ( Figure S8) reveals an effective pore diameter of 8.4 Å, which is relatively larger than kinetic diameter for any of the probe gases. This further confirms the fact that size-selectivity between different probe gases is not the deciding factor; rather the selective uptake of CO2 by 1 can be ascribed to the electrostatic interactions of CO2 with nitrate anions and the carboxylate oxygens of the coordinated ligand.
The isosteric heat of adsorption for CO2 is found to be 27.4 kJ/mol for low loading ( Figure S9), as measured from CO2-adsorption data plots at 273 K and 298 K. Phase purity of the sample was proved by PXRD measurements before and after activation ( Figure S10). After activation, framework is devoid of any guest molecules.
Magnetic susceptibility data at an applied dc field of 0.3 T for 1 were collected in the 2-300 K temperature range. The plots of χT vs T and χ vs T are shown in Figure 7. The χT product has values of 9.16 cm 3 K mol -1 at 300 K. The observed value is slightly higher than that expected for two non-interacting Mn(II) ions with g = 2.0 and S = 5/2, of 4.375 x 2 = 8.75 cm 3 K mol -1 . As the temperature decreases, the χT product declines slightly, due to the  There are three well-defined structural motifs for 1, which will have a great influence in the magnetic property. First, the binuclear Mn(II) unit is bridged by three syn-syn carboxylate groups with a distorted configuration (Figure 8), which are bent with torsion angles Mn-O-C-O'-Mn' between 60 o and 70 o . The lack of planarity will affect the super-exchange process through the delocalized π orbitals of carboxylate group, making it less effective and weaker.
The second structural motif is, the unique arrangement of chains assembled by meta-bisimidazole phenyl groups, constituted from the linking of the binuclear units (as presented in Figure 8). It is known, that this type of meta-substituted phenyl rings can lead to weak ferromagnetic coupling, although this is unlikely with Mn(II). 24 Finally as shown in Figure 3a, each one of these chains is a node in the basic hexagonal network.
The susceptibility and magnetization data were fitted together using the software package PHI. 25  In the crystal structure of 1, nitrate anions, DMF and water molecules are involved in significant hydrogen bonding interactions with the framework, rendering the framework an ideal candidate for performing anion/solvent exchange studies. A crystal of 1 of suitable size is dipped in 3 M aqueous solution of NaClO4 for 7 days at room temperature. It leads to complete replacement of NO3ˉ with the ClO4ˉ anion affording compound 2 with retention of crystallinity. A strong peak at 1,090 cm -1 in the IR spectrum of 2 suggests the presence of free ClO4ˉ anion, ( Figure S12). Similarly, when a crystal of 1 is dipped in 3 M aqueous solution of NaCl for 7 days at room temperature, complete replacement of NO3ˉ by Clˉ occurs to afford compound 3 with retention of crystallinity. This is further supported by the IR spectra where complete disappearance of the NO3ˉ peak at 1350 cm -1 suggests conversion of 1 into 3 ( Figure   S13). Likewise, compounds 4 and 5 can be obtained from 1 in SCSC fashion by dipping a crystal of 1 in dry methanol and dry acetone respectively. The IR spectra of 4 and 5 are indicative of the presence of methanol ( Figure S14) and acetone ( Figure S15) respectively.
Snapshots were taken before and after each SCSC transformation reaction ( Figure S16).
However, in each case of these SCSC transformation reactions, the crystal quality is maintained there is no change in colour and shape.
Crystallographic space group of 1 underwent a clear change from monoclinic C2/c to triclinic P-1, in both 2 and 3. For compound 2, when viewed along the c axis, two perchlorate anions which are opposite to each other in a single cavity remain sandwiched between two benzene rings of different benzimidazole moieties; while in 3, the chloride ions are present between binuclear cluster units along the a axis ( Figure 9). Both π-π and anion-π interactions are present in the hexagonal cavity. In 3, a view along the b axis shows chloride ions to be involved in hydrogen bonding interactions with the central benzene ring of the ligand (Figure 10), but no such hydrogen bonding interactions is present in 2 ( Figure 11). In both the structures, metal to anion distance is decreased significantly compare to that of the framework 1.

Figure 10
Showing hydrogen bonding interactions of chloride ions with the framework along b axis in 3.

Figure 11
View along a axis showing perchlorate anions in 2.
The solvent accessible volume in 3 is increased up to ~34% of the unit cell volume (Figure 12), calculated from the crystal structure using the PLATON program [22].  Also, unlike in 1, no appreciable π-π interactions can be observed between benzene rings of benzimidazole in 4 after methanol inclusion (Figure 14), while strong π-π interactions are present in 5 along the a axis ( Figure 15). Metal to anion distance gets decreased in case of 4 and the anion NO3ˉ is placed between two binuclear clusters revealing strong hydrogen bonding with the hydrogens of imidazole moiety and central benzene ring of the ligand (Figure 16).
Metal to anion distance gets increased in 5 in comparison to 1 and anion is located between two benzimidazoles of two different ligands, and oxygen atom of acetone molecule is involved in hydrogen bonding interaction with hydrogen of central benzene ring (Figure 17).

Figure 14
View along c axis showing no π-π interactions in cavity of 4.

Figure 15
Strong π-π interactions along a axis in cavity of 5.

Figure 16
Hydrogen bonding between nitrate anion and the framework in 4.

Figure 17
Involvement of acetone molecules in hydrogen bonding with the framework in 5.
The solvent accessible volume in 4 is increased to ~33.6% ( Figure S17), while in 5 it remains almost constant as in 1.
Guest-exchange reactions were monitored by powder X-ray diffraction technique. The similar PXRD patterns observed for the exchanged products are indicative of the fact that the entire framework is retained, throughout the SCSC transformations mediated by solvent and anion-exchange processes ( Figure 18).  coordinaton polymers (2-5) in single-crystal to single-crystal (SCSC) fashion, with precise crystalline framework-retention during which supramolecular non-covalent interactions play extremely crucial role. Guest-free activated phase of complex 1 exhibits the intriguing selective CO2-sorption phenomena at low to ambient temperature and at low pressure regime. Magnetic measurements on 1 show that it also behaves as a weak antiferromagnetic material below 17 K.