Zone-Collapse Amorphization Mimicking the Negative Compressibility

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Zone-Collapse Amorphization Mimicking the Negative Compressibility of a Porous Compound Szymon Sobczak, and Andrzej Katrusiak Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01535 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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Crystal Growth & Design

Zone-Collapse Amorphization Mimicking the Negative Compressibility of a Porous Compound Szymon Sobczak and Andrzej Katrusiak* *Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan, Poland, [email protected]. KEYWORDS: metal-organic framework, high-pressure, volume compression, amorphization Supporting Information Placeholder

ABSTRACT: Strongly anisotropic architecture of stacked metal-organic grids, with pores arranged through their eyes, has been reveled for the new coordination polymer Ni(bipy)(hip)(H2O)2·DMF·CH3OH·H2O, where bipy is 4,4’-bipyridine and hip is 5hydroxyisophthalic acid. This porous crystal, when submerged in various non-penetrating oils used as the hydrostatic media, displays a negative volume compressibility. This counterintuitive effect results of the zone-amorphization injecting the contents of collapsing pores into the retained crystalline portions of the sample.

INTRODUCTION Metal-organic frameworks (MOFs, also known as porous coordination polymers, PCPs)[1] attract a considerable attention from chemists and materials engineers.[2-4] These advanced functional materials rapidly find numerus new applications in catalysis,[7] magnetic[8], luminescent materials,[9,10] drug delivery,[11,12] and sensing.[13-15] Particularly the gas storage capabilities of MOFs are highly efficient.[16-18] Owing to the large surface area, adjustable pore size and tunable pore-surface properties, MOFs are envisaged as ideal storages for the clean-energy applications,[19] such as fuel-gases (H2, CH4) storage,[20-22] and CO2 absorbers.[23,24] The maximized absorption of these gases has been a demanding challenge for designing new porous MOFs. New MOFs architectures are generated mainly through the application of new organic struts of different size and shape, as well as by introducing various functional groups.[25-27] In our search for new types of porous materials we have modified the conditions of reactions successfully applied for synthesizing the known structural motifs.[28,29] We are particularly interested in the patterns of square, rectangular and rhomboidal connections leading to 2-dimentional (2D) girds.[30-32] Such grids of the general formula Me(Linker1)(Linker2)(Ligand1)(Ligand2) can be further stacked into 3-dimentional (3D) architectures, where the third dimension of aggregation is controlled through some kind of cohesion forces, for example hydrogen bonds between ligands and linkers. The layer MOFs possess new properties associated with the flexibility of 2D grids and ‘anisotropic solubility’ considerably higher for the H-bonds than for the coordination bonds along the 2D grids, while the minimum dimensions of the pores within the gird eyes can be controlled by the conformation and orientation of linkers. This type of stacked grids was realized in piezochroMOF AMU-1,[33] the first strongly piezochromic MOF visibly changing its color. Herein we present a new pours metal-organic material Ni(bipy)(hip)(H2O)2, (hereafter AMU-2, where AMU abbreviates Adam Mickiewicz University), involving two different linkers, 4,4’-bipyridine (bipy) and 5-hydroxyisophthalic acid anion (hip), as well as water molecules coordinating Ni cations in 2D grid-like sheets tightly H-bonded into a 3D porous structure (Figure 1.). We have revealed an unpreceded elastic and sorption-elastic properties of this layer crystal at high-pressure. It appears that the crystal displays the effect of negative volume compressibility (NVC), which cannot be reconciled with the fundamental thermodynamic requirement, that any substance exposed to increased pressure reduces its volume. In the structures of two other analogous grid MOFs Co(bipy)(hip)(H2O)2 · bipy · DMSO and Co(bipy)(hip)(H2O)2· bipy · DMF reported by He and all [34,35] the grids of Co(II) coordinated by bipy and hip linkers are H-bonded and additionally supported by guest bipy molecules stacked between the bipy linkers oriented approximately parallel to the grids. It will be shown that these features are drastically different from AMU-2 described in this report.

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2+

Figure 1. (a) Cation Ni octahedrally coordinated by 4,4’-bipyridine (bipy), 5-hydroxyisophthalic acid (hip); and water molecules in MOF AMU-2; and (b) its block-scheme architecture.

EXPERIMENTAL SECTION Synthesis of AMU-2. NiNO3 (Sigma-Aldrich), 4,4’-bipy (Sigma-Aldrich) and H2hip (Sigma-Aldrich) were used as supplied without further purification. Single crystals of Ni(bipy)(hip)(H2O)2 were synthesized by the diffusion method: 1 mmol of NiNO3 (0,8720 g) was dissolved in 5 ml of MeOH and 1 mmol H2hip (0,5380 g) was added and stirred for 10 min; then the solution was layered carefully on top of the 5 ml bipy - DMF solution (2 mmol 0,9329 g). These two layers were separated with 2 ml of the DMF-MeOH mixture. After 1 week light-blue prismatic crystals precipitated (Figure S1).

Materials and Measurements. Single-crystal X-ray diffraction data were measured at room temperature on a 4-circle diffractometer Oxford-Diffraction Xcalibur Eos with graphite monochromated MoKα radiation (0.71073 Å) and a CCD detector. The unit-cell dimensions were obtained by fitting observed reflections in in CrysAlisPro;[36] the structure was solved by direct methods with Shelxs[37] and refined by last-squares with Shelxl[38] implemented in Olex2.[39] Crystallographic data and experimental details of the structural analyses are summarized in Table 1. The H-atoms were located according to the molecular geometry and right H2O model was assumed. High-pressure experiments were performed in a Merrill-Bassett diamond-anvil cell (DAC), modified by mounting the diamond anvils directly on the steel supports with conical windows.[40] The gaskets were made of steel foil, 0.3 mm thick, with spark-eroded and pre-indented 0.4 mm holes. Daphne 7373 oil (of Idemitsu Kosan Co. Ltd.), Fluorinert FC-77 (of 3M) and NVH oil (of MiTeGen LLC) were used as the pressure-transmitting media. The pressure in the DAC was calibrated by the R1 ruby-line shift, measured by a Photon-Control Spectrometer of enhanced resolution, affording the accuracy of 0.02 GPa.[41] The DAC was centered by the gasket-shadow method.[42] The high-pressure structure of AMU-2 has been refined starting from its ambientpressure model by full-matrix least squares with SHELXL.[37,38] The contents of pores in high-pressure structures were treated by the SQUEEZE instruction.[43] High-pressure powder X-ray diffraction was measured for the samples enclosed in the DAC on a four-circle X-ray diffractometer KUMA-KM04 CCD. The beam was collimated to a dimeter smaller than the gasket hole, so the images were not contaminated with the steel gasket rings. The images were recorded for ω rotated ±10° at χ positioned at equal to 0°, 45° and 90°, each at φ=0° and 180°. The background of images recorded for the samples was accounted for by subtracting the images recorded, at the same positions, for the empty DAC. Then, after removing the diamonds reflections, the recorded images were integrated to produce I(2θ) plots in CrysAlisPro. Simultaneous thermal analysis (STA) with coupled Fourier transform-infrared spectroscopy (FT-IR) measurements were performed on a Perkin Elmer model STA 6000 machine with coupled FTIR Frontier spectrometer (cf. Supporting Information). Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker AXS D8 Advance diffractometer equipped with a Johansson monochromator (λCuKα1 = 1.5406 Å) and a silicon-strip LynxEye detector (cf. Supporting Information). FT-IR spectra of solid state samples (KBr pellets) were collected on a Jasco 4000 FTIR apparatus in the range from 399 to 4000 -1 cm (cf. Supporting Information).

RESULTS AND DISCUSSION Framework and Pores in AMU-2 The highly porous AMU-2 crystal of orthorhombic space group Pccn (Figure 1, Table 1) is built of Ni(hip)(bipy)(H2O)2 grids ex2+ tending parallel to axes [y, z] and H-bonded along [x]. The grid is formed of the Ni cations linked by hip dianions along direction [y] and by bipy molecules along [z]. The hydrogen bonds OH···O are formed between the hip and water ligands. The coor2+ dination octahedron of the Ni cation is only slightly distorted: the Ni-O distance is 2.068(2) Å, Ni-Owater 2.079(2) Å and Ni-N 2+ 2.105(2) Å; all neighboring and opposite O-Ni-N, O-Ni-O and N-Ni-N angles are close to 90° and 180°, respectively. The Ni 2+ cation is located at Wyckoff special position d. Its symmetry requires that the Ni cation and its bipy ligands lie on the same 2fold axis parallel to [z]. The pores extend along [x] and they include Wyckoff special positions a and b, both of site symmetry Ci,


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separated by distance a/2. Consequently, all the channel pores in AMU-2 are equivalent and without polar directions nor enantiomorphic preferences (Figure 2).

Figure 2. Solvent-accessible channel pore in AMU-2, modeled by program MOFomics,[44] with the positions of guest molecules as determined at ambient conditions. According to the calculations by program MOFomics[42] the pores in AMU-2, include two void-cavities, the larger about 6.6 Å and smaller 6.0 Å in diameter, separated by a somewhat narrower section about 2 Å long (Figure 2). Thus all the channel volume is easily accessible, and the as-prepared crystal in its pores contains DMF, methanol and water molecules. The DMF guest molecules are released in three stages, at 40˚C, 80˚C and 164˚C, which can be associated with three cross sections of pores distinguished by the MOFomics calculations (cf. Figure S6 in Supporting Information). The AMU-2 structure is most rigid along [-Ni-bipy-] linkers aligned along 2-fold axes in direction [z]. The [-Ni-hip-]n strands are more flexible due to the general position of the hip linkers and their wavy modulation in the crystal along [y] (Figure 3a). In high pressure this sinusoidal feature is straightened and parameter b becomes longer, with initially, to about 0.12 GPa, the negative linear compressibility clearly visible. Above 0.12 GPa the b compression is positive.

Table 1. Selected crystal data of AMU-2 compressed in oil Daphne 7373: crystal symmetry orthorhombic, space group Pccn, Z = 8 Z ’= 1. The initial stoichiometry Ni(hip)(bipy)(H2O)2·H2O·CH3OH·DMF assumed for Dx . Pressure a (Å)

0.1 MPa 13.722(2)

0.08(2) GPa 13.789(9)

0.3(2) GPa 13.530(16)

0.5(2) GPa 13.235(18)

1.0(2) MPa 12.61(2)

1.67(2) GPa 12.23(3)

b (Å)

19.1020(17)

19.340(3)

19.337(3)

19.248(3)

19.086(4)

19.025(6)

22.516(3)

22.514(2)

22.512(2)

22.459(3)

22.360(3)

22.377(5)

c (Å) 3

Volume (Å ) 3

Dx (g/cm )

5895.6(13)

6004(4)

5890(7)

5721(8)

5381(9)

5205(14)

1.105

1.085

1.106

1.139

1.211

1.252



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Figure 3. Crystal framework of Ni(hip)(bipy)(H2O)2 projected along crystal directions: (a) [x]; (b) [z]; (c) [y], with the solventaccessible channels drawn for the probing-sphere radius of 1.2 Å and 0.2 Å steps.[45,46] The crystal is softest along direction is [x], owing to the hydrogen bonds, weaker than the coordination bonds Ni-O and Ni-N. Moreover, the compression along [x] strongly depends on the rotation of hip linkers. The hip and bipy rotations have little or no effect on the crystal compression along directions [y] and [z], however these rotations can confine or widen up the cross section of the pores. Thus the voids volume strongly depends on the easiest-compressed direction [x] as well as on the rotations and conformations of the linkers. The grids are connected by two types of OH···H bonds. Both of them are included in the cyclomer of O···H-O-H···O···H-O bonds 1 1 1 between carboxylate oxygen O4 , water H2O1W, carboxylate oxygen O2, and hydroxyl O5 H ( symmetry code -½ + x, 1 – y, ½ - z, 3 Figure 4). The graph descriptor for this cyclomer is R 3(11).[47,48].


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Figure 4. The lengths of H-bonds in AMU-2 as a function of pressure. The inset shows the H-bonded cyclomer of one water molecule and two hip anions between the neighboring grids. The highlights joining the points are guiding the eye only.

Negative Compression of AMU-2 The compression of AMU-2 (Figure 5) confirms the stiffness of the grids and softness of their H-bonded spacing. When compressed hydrostatically in Fluorinert FC-77, Daphne 7373 oil and NVH oil, the Ni(hip)(bipy)(H2O)2 · H2O · CH3OH · DMF crystal initially (starting from 0.1 MPa to about 0.1 GPa) increases its volume by about 2%, which indicates that some transport increasing the content of pores in the bulk of crystal takes place. This is a most surprising result for a crystal compressed in nonpenetrating oils, built of molecules considerably larger than the cross section of the pores. This unprecedented property of AMU-2 has been discussed separately below. Above 0.1 GPa the crystal compression becomes positive and at 0.3 GPa the crystal volume becomes comparable to that at 0.1 MPa, and at still higher pressure the volume strongly decreases, to about 90% at 1 GPa. As predicted, the least affected by pressure are directions [z] and [y] along the grid; the lengthening and subsequent shortening of direction [y] is due to the changing positions of hip linkers; and the strongest changes along [x] are due to the compression of H-bonds and rotations of the hip linkers. It is characteristic that the crystal volume initially increases mainly due to the elongation of b, whereas the volume compression above 0.12 MPa are practically regulated by the linear shortening of a. The negative volume compressibility (NVC) to 0.1 GPa is surprising, counterintuitive, and cannot be reconciled with the strict thermodynamic requirement that pressure must reduce the volume of any compound. The only explanation of the NVC is the transport of some additional compounds pushed into the bulk of the crystal, either its pores or isolated voids.[49] However, in our experiments the crystals stored in dry environment for weeks were later submerged in oils, the large molecules of which block the entrance to pores. Therefore no transport of the fluid from outside to the bulk of crystal through its surface, could occur. This well-known sealing property of oils was often evidenced for various types of porous materials.[50] After the initial series of experiments with Daphne 7373, in order to eliminate the possibility that some of its components are capable of penetrating into pores of AMU-2, we have repeated the compression measurements in Fluorinert FC-77 and NVH oil. Moreover, the microscopic observations of the compressed samples revealed their visible compression along [y] (Figure 6).

Zone-collapse Model We have connected the puzzling NVC of AMU-2 in non-penetrating liquids with the observation of significantly decreasing intensity of X-ray reflections (Figure 5) and with the clear pattern of stripes clearly visible on the crystal faces (010) and (001) perpendicular to direction [x]. It is plausible that these stripes originate from a collapse of the crystal portions parallel to the Ni(hip)(bipy)(H2O)2 grids. Furthermore, microscopic observation of the compressed sample shows that it shrinks along direction [x] by about 30% at 1 GPa, whereas the diffraction measurements show that the corresponding unit-cell dimension a shrinks by about 9% (Figure 5). It means that a considerable portion of the crystal collapsed along direction [x] much stronger than the compression in the remaining part responsible for the diffraction data testifying that the crystal compression is much smaller than visually observed size reduction of the sample. The collapse concerns mainly the softest H-bonded connections between the grids and are likely to induce shear strains of subsequent layers at random directions. Consequently, the translational symmetry along [x] is lost and the collapsed regions become amorphisized. This is consistent with the considerably reduced intensity of X-ray reflections in high-pressure experiments (Figure 6 inset and Figure S2 in Supplementary Information). These reflections are diffracted on the sample portions which have not collapsed and retain their crystallinity. Another consequence of the collapsed structure is that the volume of pores is considerably reduced and their guest contents is pushed out along the channels to the pores within the crystalline regions. This increased contents of pores initially ‘inflates’ the crystalline portions. We have observed the increased volume of the compressed sample by X-ray diffraction experiment, because only the crystalline ‘inflated’ portions of the sample diffract. At still higher pressure, above 0.1 GPa, the transport of the guests contents along the pores is slowed and stopped, and the crystal resumes a ‘conventional’ compression of its volume. In this pressure range the ‘inflated’ pores counteract the external pressure and prevent the collapse of the crystalline regions. Such an effect of pressure-induced gusts preserving the sample crystallinity was observed in zeolites.[49]



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Figure 5. Relative changes in the unit-cell volume (top) and parameters (bottom) for AMU-2 isothermally compressed in oils: Daphne 7373, NVH and Fluorinert FC-77. The top insets show the visible compression of the sample crystal and the pressure dependence of the intensity of reflection 220: the bottom insets show the layer 1kl of the diffraction data of AMU-2 measured at 0.08 GPa and 1.67 GPa. The compression of AMU-2 in non-penetrating oils can be termed as a ‘zone-collapse’ effect. The framework material, with its pores sealed off by large molecules of oil, at some points collapses under the external pressure, which triggers the transport of guest molecules pushed to the crystalline (not collapsed) regions nearby. In this manner the material is modified by an increased contents of the guests generating a higher internal pressure in the pores, capable of compensating the effect of the external pressure and preventing the structure from collapsing. However, the formation of such ‘enhanced’ crystalline regions depends on the collapsed zones, triggering the transport of the guest molecules and externally generating the pressure in the preserved parts. In order to better understand this mechanism of ‘zone-collapse’ compression, we have performed the compression of AMU-2 in glycerin and the mixture CH3OH : DMF (1:1 mol). It occurs that the single-crystal samples can be kept in this mixture with no visible changes of their quality, however a small increase of pressure of about 0.1 GPa causes the disintegration of crystals. It confirms that transport of guest molecules in AMU-2 is very sensitive to relatively small changes of external pressure. It appears that the ‘zone-collapse’ in AMU-2 is associated with its specific structure, built of grids, securing the flow of guests along the pores, and ‘soft’ inter-grids connections, which are sensitive to the pressure within and without the pores. Figure 6 schematically illustrated the compression of this porous material. The overall crystal compressibility, βV, βV= -1/V dV/dp= -1/V (V-Vo)/dp

(1)

can be decomposed into the compressibility components of the framework and the pores:


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βV = -1/(VF + VP) d(VF + VP) /dp

Crystal Growth & Design (2)

where V is the crystal volume, VF is the framework volume and Vp is the volume of pores: V=VF + VP. Irrespective of the pores contents, the volume of the framework, VF (nodes and linkers), is handly changed. If we assume that dVF

Zone-Collapse Amorphization Mimicking the Negative Compressibility

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