Transformation of a Mother Crystal to a Daughter Crystal through

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Transformation of a Mother Crystal to a Daughter Crystal through Amorphous phase: De-assembly of Coordination Helices upon Heating and Re-assembly through Aquation Rajat Saha, Sanjoy Kumar Dey, Ssuobhan Biswas, Atish Dipankar Jana, and Sanjay Kumar Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400224a • Publication Date (Web): 20 Mar 2013 Downloaded from http://pubs.acs.org on March 22, 2013

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Crystal Growth & Design

Transformation of a Mother Crystal to a Daughter Crystal through Amorphous phase: De-assembly of Coordination Helices upon Heating and Re-assembly through Aquation Rajat Saha,a Sanjoy Kumar Dey,a Susobhan Biswas,a Atish Dipankar Jana*b and Sanjay Kumar*a a

Department of Physics, Jadavpur University, Jadavpur, Kolkata-700 032, India b

Department of Physics, Behala College, Kolkata-700 060, India

Dedicated to our beloved colleague Dr. Golam Mostafa [1962-2011].

Graphical Abstract:

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The NLO active chiral complex 1 [{Co(2,5-pdc)(H2O)2}H2O]n (2,5-pdc = 2,5-pyridine dicarboxylate) has been synthesized via solvothermal technique using achiral 2,5-pdc ligand. Complex 1 (phase 1), a 2D coordination polymer, undergoes crystalline to amorphous (phase 2) transformation upon de-aquation, which under re-aquation generates a new microcrystalline phase (phase 3). The crystal structure of phase 3 has been determined by powder X-ray diffraction analysis (PXRD) which reveals that the resultant microcrystalline phase 3 is an achiral complex consisting of 1D coordination chains. Phase 3 undergoes reversible structural transformation via amorphous phase (phase 2) upon dehydration and subsequent rehydration. This amorphous phase shows selective adsorption of water from water-DMF mixture and waterCCl4 mixture. Phase 1 to phase 3 structural transformation proceeds through selective bond breaking. The magnetic studies of the two crystalline and the amorphous phase reveal that phase 1 behaves as canted-antiferromagnet while both amorphous phase 2 and phase 3 show antiferromagnetism.

Keywords: 1) Crystalline to amorphous to crystalline transformation 2) Chiral metal-organic framework, 3) Selective adsorption, 4) Magnetism, 5) Photoluminescence.

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■ INTRODUCTION Guest dependent structural transformation of coordination polymers is very important in the prospect of functional material design.1 The phenomenon itself is of great importance from the point of view of basic understanding of the mechanism of structural transformation in the solid state. In the literature, there are three types of solid state structural transformations, viz, (i) single crystal to single crystal transformation,2 (ii) single crystal to microcrystalline transformation and (iii) single crystal to amorphous transformation.3 In case of coordination polymers, single crystal to single crystal transformation is a rare phenomena. While in most cases upon desolvation the long range crystalline order breaks down, resulting into microcrystalline form. In many cases, the role of guest solvent molecules is so vital that framework cannot withstand the high level of stress generated upon loss of guest molecules and the framework converts into amorphous phase. In very few cases, upon resolvation, the amorphous phase regenerates original structure i.e. reversible structural transformation.4 This kind of structural dynamism makes the frameworks as potential material for selective adsorption, catalysis, separation etc.5 Strong modulation of physical properties is generally associated with such transformations and leads to new materials with novel functionalities.6 In the present communication, we are going to report a very interesting framework, complex 1 [{Co(2,5-pdc)(H2O)2}H2O]n (phase 1) (2,5-pyridinedicarboxylate), that undergoes crystalline to amorphous (phase 2) transformation upon de-aquation, which under re-aquation generates a new microcrystalline phase (phase 3). This transformation proceeds through selective bond breaking mechanism accompanying with drastic changes in NLO, thermal, magnetic and optical properties. The crystal structure of phase 3 has been determined by PXRD analysis. It is further interesting that; phase 1 to phase 3 transformations via amorphous phase 2 is irreversible whereas transformation of phase 3 into the same amorphous phase 2 is reversible. Phase 3 upon heating transforms to phase 2 and reverts to phase 3 upon rehydration. A comparison of IR, UVVis, and thermal data shows that both the amorphous phases generated upon heating from phase 1 as well as phase 3 are actually the same material. It is also interesting to note that this amorphous phase shows selective adsorption of water from water-DMF mixture. While phase 1 is a NLO active chiral material, phase 3 is achiral. The magnetic study of the two crystalline and 3



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the amorphous phase reveals that phase 1 shows canted antiferromagnetic behavior while both the amorphous phase 2 and phase 3 show antiferromagnetic property. Finally, we want to illustrate ‘how this solid state structural transformation proceeds?’.

■ EXPERIMENTAL SECTION Materials and Methods. Co(II) nitrate-hexahydrate, 2,5-pyridinedicarboxylic acid and 4,4΄trimethylenebipyridine were purchased from Merck chemical company. All other chemicals used were of AR grade. Elemental analyses (CHN) were carried out using a Perkin-Elmer 240C elemental analyzer. The thermal analyses were carried out using a Mettler Toledo TGA-DTA 85 thermal analyzer under a flow of N2 (30 mlmin-1). The sample was heated at a rate of 10 ºCmin-1 with inert alumina as a reference. IR studies were done on Nicolet Impact 410 spectrometer between 400 and 4000 cm-1, using the KBr pellet method. Photoluminescence spectra were collected on a Shimadzu RF-5301PC spectrofluorometer. Variable temperature (2–300 K) magnetization data were acquired using a SQUID magnetometer (MPMS Quantum design Excel 7). The experimental susceptibility data were corrected for underlying diamagnetism by using data of tabulated Pascal’s constants. Powder XRD patterns of the compounds were recorded by using Cu-Kα radiation (Bruker D8; 40 kV, 40 mA). Solid state (KBr pellets) circular dichroism (CD) spectra were recorded on a JASCO J-810 spectropolarimeter. Second Harmonic Measurements. Q-switched Nd:YAG laser of fundamental wavelength 1064 nm (Spectra Physics, PROLAB 170, pulse width 10 ns and repetition rate 10 Hz) was used as the source of light for second harmonic generation (SHG) measurements. The beam from the laser was passed through a couple of mirrors specially designed for high energy laser and a long pass colored glass filter before being focused onto a glass capillary using a converging lens of 200 mm focal length. The incoherently scattered SH photons were collected in the transverse direction using a combination of a monochromator and a photomultiplier tube. The second harmonic signal was then sampled, averaged over 512 shots and recorded in a digital storage oscilloscope. Potassium dihydrogen phosphate (KDP) powder was used as the reference material.

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Gas adsorption analysis. Gas sorption isotherms for pressures in the range of 0-1 bar were measured using an Autosorb iQ (Quantachorme Inc., USA) gas sorption system. A sample of ca. 50 mg of as-synthesized material was introduced into a pre-weighed analysis tube (6 mm diameter, 6 cm3 bulb), capped with a gas-tight transeal to prevent leakage of air and moisture during transfer and weighing. The samples were evacuated under dynamic vacuum (10-3 torr) at desired temperature, until a constant weight was achieved. The analysis tube was then weighed again to determine the mass of the evacuated sample. For all isotherms, warm and cold free space correction measurements were performed using ultrahigh pure He gas (99.999% purity). N2 isotherm at 77 K was measured in a liquid nitrogen bath using ultrahigh pure (99.999% purity) gas source.

Synthesis of phase 1 (complex 1). {[Co(2,5-pdc)(H2O)2] ·H2O}n. 5 ml aqueous solution of 1 mmol CoCl2·6H2O (0.2384 g) was added to 5 ml methanolic solution of 0.5 mmol 4,4'trimethylenebipyridine (0.0998 g) and stirred for half an hour. Now, NaOH solution (1 bead in 5 ml water) was added dropwise to 5 ml aqueous solution of 1.5 mmol 2,5-pyridinedicarboxylic acid (0.2512 g) to maintain the pH of the solution at 7-8. This acid solution was added dropwise to the previous solution and stirred for another half an hour. An opaque orange solution is obtained. Now this solution was poured in a 25 ml Teflon coated autoclave and heated at 120 ºC for 24 hrs and afterward it was allowed to cool to room temperature. Pink colored block shaped crystals suitable for X-ray characterization was obtained. Yield: 50%. Anal. Cald. for C7H9CoNO7: C, 30.21%; H, 3.23%; N, 5.03%. Found: C, 30.18%; H, 3.20%; N, 5.01%. Synthesis of phase 2. Phase 1 was heated in vacuum at 250 ºC for 4 hrs and deep purple colored phase 2 was formed. Anal. Cald. for C7H3CoNO4: C, 37.5%; H, 1.33%; N, 6.25%. Found: C, 37.55%; H, 1.35%; N, 6.20%.

Synthesis of phase 3. Phase 2 was soaked in water, the color of the sample changes from deep purple to orange. After 24 hrs, orange colored residue (phase 3) was filtered off. Anal. Cald. for C7H15CoNO10: C, 25.30%; H, 4.51%; N, 4.21%. Found: C: 25.34%; H: 4.54%; N: 4.18%.

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Synthesis of phase 2 from phase 3. Phase 3 was heated in vacuum at 250 ºC for 4 hrs and deep purple colored phase 2 was formed. Anal. Cald. for C7H3CoNO4: C, 37.5%; H, 1.33%; N, 6.25%. Found: C: 37.56%; H: 1.30%; N: 6.22%. Single Crystal X-ray Crystallography: Crystallographic Data Collection and Refinement. Suitable single crystal of the complex was mounted on a Bruker SMART diffractometer equipped with a graphite monochromator and Mo-Kα (λ = 0.71073 Ǻ) radiation. The structure was solved using Patterson method by using the SHELXS97. Subsequent difference Fourier synthesis and least-square refinement revealed the positions of the remaining non-hydrogen atoms. Non-hydrogen atoms were refined with independent anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and their displacement parameters were fixed to be 1.2 times larger than those of the attached non-hydrogen atom. Successful convergence was indicated by the maximum shift/error of 0.001 for the last cycle of the least squares refinement. All calculations were carried out using SHELXS 97,7 SHELXL 97,8 PLATON 99,9 ORTEP-3210 and WinGX system Ver-1.6411. Data collection and structure refinement parameters and crystallographic data for complex 1 were given in Table S1. Selected coordination bond lengths, bond angles and non-covalent interaction parameters are summarized in Table S2 & S3. It is to be noted that crystal structure of 1 has already been reported by several groups.12 Here, we have re-discussed the structure in detail in the context of structural transformation of 1 into amorphous form 2 and therefrom to crystalline 3. Coordination bond breaking upon heating of 1 leads to separation of helical coordination chains constituting 2D coordination layers in it. Crystalline 3 results from reassembly of the separated helices and the three forms are closely related with each other structurally. We also report for the first time the NLO activity of 1. Powder X-ray crystallography: Crystallographic Data Collection and Refinement. X-ray powder diffraction data were recorded with a Bruker D8 Advance powder diffractometer using Cu-Kα radiation (λ= 1.5418 Å). The diffraction pattern was scanned with a step size of 0.02° and counting time 7s/step over an angular range 7–60°(2θ) using the Bragg–Brentano geometry. The X-ray powder diffraction pattern was indexed with the program TREOR90 using the first 20 observed Bragg reflections. The best solution indicated a monoclinic cell with a = 10.898 6


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Å, b = 14.1467 Å, c = 9.7311 Å, β = 96.098° and V = 1492 Å3. Statistical analysis of powder data using the FINDSPACE module of EXPO200913 indicated the most probable space group as P21/a, which was used for subsequent structure analysis. The structure solution procedure was carried out using FOX, an ab-initio reverse Monte Carlo structure solution program. The initial molecular geometry was optimized by hyperchem using Polak-Ribiere algorithm. The optimized molecular geometry and lattice parameters were imported in FOX14. The profile parameters, zero-point and interpolated background calculated by powder-pattern decomposition based on the Le Bail algorithm. The atomic coordinates obtained from the parallel tempering procedure of FOX were used as the starting model for the Rietveld refinement using the program GSAS15 with an EXPGUI16 interface. The 2θ range of 7.5-60.0° was used in the Rietveld refinement. The lattice parameters, the background coefficients and profile parameters were refined initially followed by the refinement of positional coordinates of all non hydrogen atoms adopting bond length, bond angle and planar restraints. The background was described by the shifted Chebyshev function of first kind with 24 points regularly distributed over the entire 2θ range. The isotropic displacement parameter of 0.04 Å2 for all non-hydrogen atoms was introduced. Hydrogen atoms were placed in the calculated positions with Biso = 0.06 Å2. Data collection and structure refinement parameters and crystallographic data for complex 3 are given in Table S4. Selected coordination bond lengths and bond angles are summarized in Table S5. ■ RESULTS AND DISCUSSION Crystal and supramolecular structure of complex 1. Complex 1 (phase 1) has been synthesized via solvothermal technique from mixture of CoCl2·6H2O with achiral ligand 2,5-pdc (2,5-pyridinedicarboxylate) and bpp (4,4'-trimethylenebipyridine). Single crystal X-ray diffraction (SC-XRD) analysis shows that complex 1 is a 2D coordination polymer and crystallizes in chiral P212121 space group. The asymmetric unit contains one Co(II) ion, one 2,5pdc ligand, two coordinated water molecules and one guest water molecule, Figure S2. Co(II) has a distorted octahedral coordination geometry. N1 and O1 atoms of one 2,5-pdc ligand and two water molecules (O1W and O2W) form the basal plane and two carboxylic oxygen atoms O3** (**= -1/2+x, 1/2-y, 1-z) and O4* (*= x, 1+y, z) of two different 2,5-pdc ligands occupy the trans axial coordination sites. 7



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Complex 1 is a 2D coordination polymer constituted by Co(II) metal nodes and 2,5-pdc spacers. 2,5-pdc simultaneously binds successive Co(II) centers both along crystallographic aaxis as well as b-axis. Helical coordination chains are formed along crystallographic a-axis and linear coordination chains are formed along b-axis, Figure 1. All coordination helical chains posses right handed helicity while the linear coordination chains act as chirality transformer i.e. stereochemical information among the helical 1D coordination chains are transferred through the linear coordination chains. Hence, the 2D coordination sheets become homochiral in nature. Adjacent 2D coordination sheets are joined by water (O2W) mediated O-H···O hydrogen bonding [O2W-H1W2···O2*(*= 1/2-x,1-y,-1/2+z)] interactions leading to the formation of 3D supramolecular framework with channels along crystallographic b-axis, Figure S3. The other coordinated water molecule (O1W) forms intra layer hydrogen bonds (Table S3). Both the coordinated water molecules (O1W & O2W) establish hydrogen bonds with the guest water molecules (O3W) and help it to stabilize in the interlayer space (Figure 2). Hydrogen bonding interactions among three water molecules form 1D supramolecular helical chains along crystallographic b-axis with right handed helicity. One can also visualize another type of supramolecular helical chains of right handed helicity along crystallographic caxis through O2W-H1W2···O2 hydrogen bonding interactions (Figure S4). These hydrogen bonding interactions transfer identical stereochemical information between 2D homochiral coordination sheets and thus the overall 3D supramolecular framework becomes homochiral. Solid state CD spectral study proves the chirality of complex 1, Figure 3. Complex 1 shows negative Cotton effects at 277 nm. Thermal and PXRD analysis. Thermo-gravimetric analysis shows that between 170-250 ºC one guest water and two coordinated water molecules (theo:19.42wt%; exp: 19.31 wt%) are removed and the resultant anhydrous framework is stable upto 320 ºC, Figure S5. Phase purity of the as synthesized complex is checked by PXRD analysis, Figure 4. When 1 is heated in vacuum at 250 ºC for 4 hrs, a deep purple colored amorphous (confirmed through PXRD analysis) specimen is obtained (phase 2). This amorphous material is kept in open air for three days, the color of the amorphous material turns gradually to pink but still remains amorphous. But, when phase 2 is 8


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soaked in water the color of the sample instantly changes to light orange along with a sharp ‘hishis’ sound. The PXRD data of the filtered light orange residue (phase 3) shows that phase 3 is crystalline material (Figure 4). Astonishingly, this PXRD pattern is different from that of phase 1, which indicates that phase 3 is a completely different crystalline material than phase 1. That means under de-aquation crystal 1 (mother crystal) becomes amorphous and upon re-aquation it turns into a new crystalline form 3 (daughter crystal). So a crystalline to amorphous to a newcrystalline transformation has taken place, which is a very interesting phenomenon indeed. IR spectra of different phases. IR spectroscopic studies also indicate the absence of coordinated water molecules in phase 2 (Figure S6). For phase 1 and phase 3, the bands in the region 800-770 cm-1 indicate the presence of coordinated water molecules.17 The broad nature of band at 3243 cm−1 in phase 1 and 3253 cm-1 in phase 3 and a peak between 887 and 955 cm-1 in the complexes indicate the presence of lattice water molecules.18 In comparison to IR spectra of phase 1 and 3, the IR spectra of phase 2 show relatively sharp peak around 3399 cm-1 and absence of peak around 700-850 cm-1 suggests the absence of any water molecule.

Crystal structure of phase 3. The crystal structure of microcrystalline phase 3 has been determined by powder X-ray diffraction analysis, the corresponding Rietveld plot is given in Figure 5. The structural analysis reveals that phase 3 is a achiral complex with 1D coordination chains. The asymmetric unit contains one Co(II) ion, one 2,5-pdc ligand, two coordinated water molecules and four guest water molecules (Figure S7). Co(II) is in distorted square pyramidal (‘τ’ value 0.11)19 coordination environment. N1 and O1 atoms of one 2,5-pdc ligand and two water molecules (O1W and O2W) form the basal plane and one carboxylic oxygen atom O3* (*= 3/2-x, 1/2+y, 1-z) of another 2,5-pdc ligand is at the apical position. In 3, Co(II) metal nodes are bridged by 2,5-pdc spacers leading to 1D helical coordination chains running along crystallographic a-axis. The achiral nature of complex 3 is due to the presence of both left handed and right handed helical chains within it (Figure 6) which are interwoven by hydrogen bonding interaction.

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Thermal and PXRD analysis of phase 3. Thermo-gravimetric analysis of phase 3 shows that within 210 ºC all the guest and coordinated water molecules (theo: 32.50 wt%; exp: 33.36 wt%) are removed and the resultant anhydrous framework is stable upto 320 ºC, Figure S8. When, phase 3 is heated in vacuum at 250 ºC for 4 hrs, a deep purple colored specimen is obtained (similar to phase 2) which is also amorphous in nature (confirmed by PXRD analysis, Figure 7 and S9). This anhydrous-amorphous component was rehydrated for 24 hrs and the resulting sample was characterized again by PXRD analysis which establishes that phase 3 is regenerated. So, phase 3 undergoes reversible crystalline to amorphous to crystalline transformation upon dehydration and rehydration. Selective adsorption and separation. The N2 adsorption isotherm (Figure S10) indicates that phase 2 is non-porous (10 ml/g), only surface adsorption occurs. But, this amorphous phase shows selective adsorption of water from water-DMF and water-CCl4 mixture. The amorphous phase 2 was soaked in polar DMF and in non-polar CCl4, but it remains amorphous (Figure 7) upto 24 hrs. When, this amorphous phase is soaked in water-DMF as well as water-CCl4 mixture, then phase 3 (Figure 7) is regenerated through selective adsorption of water. SHG-NLO activity. Non-centrosymmetric MOFs have attracted much attention for their SHG non-linear optical activity.20 Due to their high thermal stability, neutral non-centrosymmetric frameworks could be considered as potential NLO-active materials. For evaluation of their NLO activity, we have performed second harmonic generation (SHG) measurements on microcrystalline powder samples of complex 1 using the method adopted by Kurtz and Perry.21 The chiral metal-organic framework 1 is weak NLO active (SHG intensity 0.675 with respect to KDP) material due to its lower polarity. Photoluminescence spectra of different phases. The emission spectra of all the three phases were studied in solid state at room temperature. It can be observed that intense emissions occurring at 390 nm (λex~314 nm) for phase 1, 420 nm (λex~314 nm) for phase 2, and 413 nm (λex~308 nm) for phase 3 (Figure 8). The fluorescence arises due to ligand to metal charge transfer spectra. This result suggests that complexes may be an excellent candidate for

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potentially photoactive material and this overall structural transformation upon dehydrationrehydration is accompanied by a change in color (thermochromism). Magnetic study of different phases. As the coordination environment around metal ions as well as the structure is changed drastically, a significant change in the magnetic property is expected and thus the magnetic studies of phase 1, 2 and 3 have been carried out in the temperature range of 3-300 K. For phase 1, the χm value is 0.011 cm3 mol-1 at 300 K, upon cooling χm value increases very slowly to 0.074 cm3mol-1 at 34 K and afterward increases sharply to reach a maximum value of 0.86 cm3 mol-1 at 3 K, Figure 9. In order to determine the nature of magnetic interaction, the susceptibility data above 34 K were fitted to the Curie–Weiss law. Curie and Weiss constants are of 3.43 cm3mol–1K and θ = -16.52 K, respectively, Figure S11. The negative value of the Weiss constant indicates the presence of antiferromagnetic interaction between Co(II) ions. The χmT value is 3.30 cm3mol-1K at 300 K which is larger than the calculated spin-only value (1.9 cm3mol-1K) for one Co(II) (S = 3/2) ions, indicating the important orbital contribution arising from the high-spin octahedral Co(II). Upon cooling χmT value decreases smoothly to reach a value of 2.61 cm3mol-1K at 34 K due to antiferromagnetic interaction between Co(II) centers, Figure 9. Upon further cooling, χmT value increases to 2.81 cm3mol-1K at 15 K, suggesting an appreciable but weak ferromagnetic exchange interaction between Co(II) centers. The sudden decrease in χmT to 2.60 cm3mol-1K at 3 K may be attributed to zero-field splitting (ZFS) effect. The high-temperature magnetic behavior indicates that the magnetic interactions between Co(II) ions are dominated by antiferromagnetic coupling while the low temperature magnetic behavior suggests that weak ferromagnetism exists within the system. For an antiferromagnetic system, such ferromagnetic correlation can be attributed to spin canting.22 The isothermal magnetization (M) vs field (H) of phase 1 shows hysteresis loop at 5 K, Figure S12, strongly supports the weak ferromagnetic like ordering in complex 1. The hysteresis loop shows values of the coercive field (HC) and remnant magnetization (MR) of 10O e and 6.3x10-6Nβ mol-1, respectively, being characteristic of a very soft magnet. The canting angle is 0.1º. Thus magnetic behavior between the temperature range of 34-3 K suggests the occurrence of weak ferromagnetic ordering in complex 1 which arises from the presence of canting between 11



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the antiparallel alignments of the spins within the 2D coordination sheets. The saturation value of the magnetisation is well below the expected one for a parallel alignment of the spins of Co(II) ions and the magnetisation does not saturate up to 5T, Figure S13. Thus the weak ferromagnetism observed in complex 1 is due to spin canting that prevents complete cancellation of the spins of the Co(II) ions. It is well known that the occurrence of spin canting is usually caused by either single-ion magnetic anisotropy or antisymmetric exchange in magnetic entities. As there is no inversion center in the Co(II) entity and Co(II) ions have a large magnetic anisotropy, the observation of the spin canting in complex 1 should arise from the antisymmetric exchange in the 2D coordination polymer together with the anisotropy of the Co(II) ions. For phase 2, the χm value at 300 K is 0.0059 cm3 mol-1, upon cooling χm value increases very slowly to 0.033 cm3mol-1 at 35 K and afterward increases sharply to reach a maximum value of 0.20 cm3mol-1 at 3K, Figure 10. To determine the nature of magnetic interaction, the susceptibility data were fitted to the Curie–Weiss law. Curie and Weiss constants are of 1.89 cm3mol–1K and θ = -20.87 K, respectively, Figure S14. The negative value of the Weiss constant indicates the presence of antiferromagnetic interaction between Co(II) ions. The χmT value is 1.77 cm3mol-1K at room-temperature and then the χmT values decreases smoothly to reach minimum value of 0.61 cm3 mol-1K at 3K due to antiferromagnetic interaction between Co(II) centers. The phase 2 does not exhibit any hysteresis loop at 5 K. This clearly indicates that phase 2 is antiferromagnetic in nature. It is interesting that a material in amorphous phase (like phase 2) shows antiferromagnetic behavior. For phase 3, χm value at 300 K is 0.0082 cm3 mol-1, upon cooling χm value increases very slowly to 0.051 cm3mol-1 at 33 K and afterward increases sharply to reach a maximum value of 0.46 cm3mol-1 at 3 K, Figure 11. Curie and Weiss constants of the sample are of 2.55 cm3mol–1 K and θ = -19.42 K, respectively, Figure S15. The negative value of the Weiss constant indicates antiferromagnetic interaction between Co(II) ions. The χmT value is 2.46 cm3mol-1K at roomtemperature and then the χmT values decreases smoothly to reach minimum value of 1.39 cm3mol-1K at 3 K due to antiferromagnetic interaction between Co(II) centers. For phase 3 no hysteresis loop has been detected at 5 K and this corroborates the susceptibility data.

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Solid State Reaction Mechanism. Now, it is worthy to explore the overall transformation mechanism of crystalline phase 1 to amorphous phase 2 and therefrom to a new crystalline phase 3. A close comparison of the structures of the two crystalline phases reveals that both 1 and 3 consist of helical coordination polymers. While in 1 the homochiral coordination helices are cross linked through an additional coordination bridging with the carboxylate group of 2,5-pdc ligand, in 3 same helices are not cross linked by coordination bonds, instead these are organized side by side due to interchain weak hydrogen bonding interactions leading to achirality in 3. This conversion from chiral arrangement of the helices in phase 1 to the achiral in phase 3 is through the separation of the cross linked helices in 1 due to a probable selective bond breaking process (Scheme 1) at 250 ºC leading to the unlocking of individual helices in the amorphous phase 2.

Scheme 1. Selective bond breaking during structural transformation from phase 1 to 3 through amorphous phase 2 13



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In 1, each 2,5-pdc ligand forms a chelate ring at the Co(II) center by simultaneously binding through one of the carboxylate O (O1) atom and the pyridine N (N1) atom while the two O atoms (O3 and O4) of the other carboxylate group is engaged in trans axial bridging between two adjacent Co(II) centers. Due to the extra binding energy provided by the chelate effect, at 250 ºC it is the axial bond (Co1-O4) that is selectively broken and the helices are separated from each other. As the helices become independent from each other and due to their random arrangement, phase 2 is amorphous in nature. Upon rehydration, the helices are reorganized by hydrogen bonding forces and due to centrosymmetric cyclic hydrogen bonding synthon (Figure S16) formed by the coordinated water molecules from oppositely aligned helices the generated phase 3 becomes achiral. When 3 is heated above 210 ºC, the weakly attached hydrogen bonded helices in 3 become independent and the dehydrated phase (identical to phase 2) becomes amorphous which upon rehydration regenerates phase 3. So, achiral phase 3 undergoes reversible crystal to amorphous to crystal transformation in contrast to chiral phase 1 which only shows one way transformation. It is to be noted that chirality of phase 1 results from hydrothermal synthesis. It is already mentioned that crystalline phase 1 consisting of 2D coordination polymers turns into amorphous phase 2 upon removal of coordinated water molecules by heating. Amorphous phase upon rehydration generates crystalline phase 3 consisting 1D helical coordination polymers, the structure of which is determined by powder method. It is quite interesting to contemplate how the amorphous phase 2 can show weak antiferromagnetic behavior that requires magnetic ordering. The antiferromagnetic behavior of phase 2 can be explained if we assume that the helical coordination chains that are cross linked in the 2D coordination sheets of 1 gets separated from each other due to thermally induced coordination bond breaking (Scheme 1) and these free 1D coordination chains are randomly oriented in the amorphous phase. The magnetic ordering that prevails in the amorphous phase is due to the interactions among the Co(II) centers through 2,5-pdc bridges within the coordination chains. This view is further strengthened as the magnetic behavior of crystalline phase 3 is exactly similar in nature with that of phase 2. And also, the canted-antiferromagnetic behavior of phase 1 is in accordance with its crystal structure where the syn-anti bridging by carboxylate groups 14


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transmits magnetic interaction. But, this syn-anti bridging breaks down upon heating and the consequent µ-2 bridging of 2,5-pdc generates antiferromagnetic coupling in phase 3, Figure S17. ■ CONCLUSIONS In conclusion, we have found an interesting system where a mother chiral crystal gives rise to closely related daughter crystal through an amorphous phase upon dehydration and rehydration. All the phases, which we landed upon here, are functionally active. Mother crystal 1 is NLO active and canted-antiferromagnetic in nature and astonishingly the amorphous phase 2 is antiferromagnetic in nature and possesses selective adsorption property towards water. Last but not the least phase 3, besides possessing antiferromagnetic property, it also shows reversible crystalline to amorphous to crystalline transformation. This system is an interesting example where controlled bond breaking through dehydration of a homochiral 2D coordination polymer consisting of coordination helices leads to unlocking of the helices and rehydration leads to their rearrangement into a new achiral crystalline form. In summary, we have presented a rare system consisting of three phases each possessing multiple functionalities and that can be achieved in a controllable fashion. And also, such transformations will offer a better understanding of solid state structural transformation mechanism.

■ ASSOCIATED CONTENT Supporting Information: Figures, tables for crystallographic data and structural parameters are given in supporting information file. CCDC 787144 and 897154 contains the supplementary crystallographic data for phase 1 and 3, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html. This information 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] (S. K.); [email protected] (A. D. J.) ■ Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS

RS would like to acknowledge CSIR for Fellowship under the sanction no (09/096(0565)2008EMR-I). Thanks to Prof. P. K. Das and his student Mr. Ravindra Pandey, Indian Institute of Science, Bangalore, India, for the powder SHG measurement. We are grateful to Dr. D. Das, UGC-DAE-CSR Kolkata center for magnetic study. Thanks to Prof. A. Ghosh, Dept. of Chemistry, Calcutta University for his kind assistance. S. K. Dey acknowledges NITMAS, South 24 paraganas, West Bengal-743368, India. A. D. Jana acknowledges UGC for research grant under the post doctoral research fellowship, sanction letter NO. F. 30-1/2009(SA-II).

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Figure 1. 2D coordination sheet of complex 1; helical coordination chains are formed along a-axis and linear chains are formed along b-axis

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Figure 2. Supramolecular channels are filled up by guest water molecules in complex 1

Figure 3. CD spectral study complex 1

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Figure 4. Irreversible transformation of phase 1; (a) simulated pattern of 1; (b) as synthesized of 1; (c) pxrd pattern after dehydration; (d) phase 3: after rehydration.

Figure 5. Rietveld plot for phase 3 22


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P-helix

M-helix

Figure 6. Phase 3 contains both P-helix and M-helix

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Figure 7. (a) Phase 3; (b) after dehydration of phase 3; (c) after rehydration of the amorphous phase obtained by dehydration of phase 3; (d) PXRD pattern of phase 2 after 24 hrs soaking in DMF; (e) PXRD pattern of phase 2 after 24 hrs soaking in CCl4; (f) After soaking in Water-DMF mixture phase 3 is regenerated through selective adsorption of water.

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Figure 8. Photoluminescence spectra of three different phases

Figure 9. χmvsT and χmTvsT plots of phase 1 25



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Figure 10. χmvsT and χmTvsT plots of phase 2

Figure 11. χmvsT and χmTvsT plots of phase 3 26


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“For Table of Contents Use Only”

Transformation of a Mother Crystal to a Daughter Crystal through Amorphous phase: De-assembly of Coordination Helices upon Heating and Re-assembly through Aquation Rajat Saha, Sanjoy Kumar Dey, Susobhan Biswas, Atish Dipankar Jana* and Sanjay Kumar*

Crystalline-amorphous-crystalline phase transformation has been observed in a chiral 2D coordination polymer with modulation of functional properties.

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Transformation of a Mother Crystal to a Daughter Crystal through

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