Two New Series of Coordination Polymers and Evaluation of Their

Article pubs.acs.org/crystal

Two New Series of Coordination Polymers and Evaluation of Their Properties by Density Functional Theory Fredrik Lundvall,*,† Ponniah Vajeeston,† David S. Wragg,†,‡ Pascal D. C. Dietzel,§ and Helmer Fjellvåg†,‡ †

SMN - Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1126, N-0318 Oslo, Norway ‡ inGAP − Innovative Natural Gas Processes and Products, Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norway § Department of Chemistry, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway S Supporting Information *

ABSTRACT: Five new coordination polymers (CPs), CPO68-M (M = Zn, Mn, and Co) and CPO-69-M (M = Ca and Cd), were synthesized by solvothermal methods using 4,4′dimethoxy-3,3′-biphenyldicarboxylic acid as the organic linker. The three-dimensional frameworks are formed by metal carboxylate chains that are separated by the linker. Structural analysis reveals dense networks with narrow rhombic channels and sra topologies for both CPO-68-M and CPO-69-M. The major structural difference between the two series of CPs is in the metal coordination polyhedra, which are four- and eight-coordinated in CPO-68-M and CPO-69-M, respectively. The CPs are highly crystalline, robust, and have good thermal stability (> 350 °C). On the basis of the topological similarities with MIL-53, we tested whether the CPs would exhibit a similar flexible structure response to gas stimulus. Density functional theory (DFT) modeling was used to evaluate the CPs’ potential as gas adsorption materials over a large range of pressures. The DFT analysis concluded that the CPs are ill-suited for gas adsorption due to their structural rigidity. However, electronic structure calculations reveal that CPO-68-M and CPO-69-M are indirect band gap semiconductors with an estimated band gap between 2.49 and 2.98 eV.

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INTRODUCTION Custom compounds with potential applications in varied fields such as gas sorption, catalysis, luminescence, or photovoltaics can be created by combining organic and inorganic building blocks.1−4 For this intriguing class of materials called coordination polymers (CPs), the vast range of interchangeable organic linkers and metal secondary building units (SBUs) give a unique flexibility in the synthesis and tailoring of new materials, not only to satisfy scientific curiosity and understanding, but also to develop functional materials with interesting properties for industrial applications. Large scale CO2 capture from the flue gas of power plants requires efficient and robust functional materials, among which metal−organic frameworks (MOFs) have been and are still being heavily explored. For large scale application, good diffusion properties are important to increase efficiency and minimize the pressure drop over the adsorption material. Furthermore, strong interactions between the gas and a large surface area of the adsorbent are desired to increase the amount of gas adsorbed per gram material. Additional parameters for commercialization are naturally connected to costs of production, toxicity of chemicals, and not least, mechanical strength and potential formation of byproducts and materials loss. Our group and others have previously published several articles describing the family of materials formed by CPO-27 and its isostructural MOF-74 analogue.5−13 These are based on © XXXX American Chemical Society

divalent metal cations (Zn, Co, Ni, Mg, Mn, Fe, and Cu) and 2,5-dihydroxyterephtalic acid (DHTP) and feature coordinatively unsaturated sites, so-called open metal sites, upon activation. These open metal sites create a strong host−guest interaction toward a number of gases (CO2, CO, NO, H2, and CH4).13−20 Recreating similarly strong host−guest interactions in a new series of MOFs would be highly interesting in the context of gas sorption/separation. More efficient gas diffusion into the pores compared to CPO-27/MOF-74 would also be beneficial. CPO-27/MOF-74 are based on a substituted benzene linker. The logical step is hence to improve the gas diffusion by increasing the length of the linker and thereby create larger pores in the structure. This way of tuning the pore dimensions (and functional properties) of known MOFs is commonly referred to as reticular design or the isoreticular approach21 and has already successfully been applied to create isoreticular CPO-27/MOF-74 type MOFs.22,23 In the current work, the 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid linker has been explored along with a series of divalent cations. Isomers of this linker have produced highly crystalline CPs with interesting topologies,24−28 albeit none being isostructural to CPO-27/MOF-74. Nevertheless, 4,4′Received: September 8, 2015 Revised: November 16, 2015

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DOI: 10.1021/acs.cgd.5b01302 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Summary of Synthesis Details for CPO-68-M and CPO-69-M name

CPO-68-Zn

CPO-68-Mn

CPO-68-Co

CPO-69-Ca

CPO-69-Cd

metal source metal amount linker amount solvent 1 solvent 2 temperature (°C) reaction time (h)

Zn(NO3)2·6H2O 59.5 mg, 0.2 mmol 60.4 mg, 0.2 mmol DMF, 2.0 mL H2O, 0.1 mL 120 48

MnCl2·4H2O 39.6 mg, 0.2 mmol 60.4 mg, 0.2 mmol DMF, 2.0 mL H2O, 0.1 mL 100 8

Co(NO3)2·6H2O 58.2 mg, 0.2 mmol 60.4 mg, 0.2 mmol DMF, 2.0 mL H2O, 0.1 mL 100 48

Ca(NO3)2·4H2O 47.2 mg, 0.2 mmol 60.4 mg, 0.2 mmol DMF, 2.0 mL none 120 24

Cd(NO3)2·4H2O 61.7 mg, 0.2 mmol 60.4 mg, 0.2 mmol DMF, 2.0 mL H2O, 0.1 mL 120 48

confirm the absence of other crystalline phases in the samples (see Figures S3−S7 in the SI). Because of fluorescence in the case of CPO68-Co, this sample was measured on the Swiss-Norwegian Beamline (BM01A) at the European Synchrotron Radiation Facility using monochromatic synchrotron radiation (λ = 0.69396 Å). The setup uses a Huber goniometer and Dectris Pilatus 2M photon counting pixel area detector.35 The 2D diffraction data from a 90 s exposure were converted to a 1D P-XRD pattern using FIT2D.36 Additional P-XRD data on selected samples were collected at 20 bar pressure of CO2 and ambient temperature on a Bruker D8 instrument with monochromatic CuKα1 radiation (λ = 1.5406 Å) and a LynxEye XE position sensitive detector operated in transmission geometry. Diffuse reflectance UV−vis spectroscopy (DRS) was performed on a Shimadzu UV-3600 instrument with an integrating sphere using compacted BaSO4 as reference. The optical band gaps were estimated by making a Tauc plot of [F(R)hν]1/2 versus the photon energy hν. To this end, the Kubelka−Munk function F(R) = (1 − R)2/2R was calculated from the reflectance data obtained through the DRS UV− vis measurements. Synthesis of CPO-68-M and CPO-69-M. The synthesis procedure for CPO-68-Zn described below can be regarded as a general procedure for all members of the CPO-68-M and CPO-69-M series. The individual synthesis parameters are summarized in Table 1. Synthesis of CPO-68-Zn. Zn(NO3)2·6H2O (59.5 mg, 0.2 mmol) and 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid (60.4 mg, 0.2 mmol) was weighed in and dissolved in a mixture of DMF (2.0 mL) and deionized water (0.1 mL). The mixture was heated in a 5 mL glass vial at 120 °C for 48 h and then cooled to room temperature. This procedure yielded colorless needle crystals of sufficient quality for S-XRD analysis. Computational Details. The quantum-mechanical calculations were performed in the framework of density functional theory (DFT) using the generalized gradient approximation (GGA) 37,38 as implemented in the VASP code.39,40 The interaction between the ion and electron is described by the projector augmented wave method.41,42 For the calculations presented here we have used planewave cutoff energy of 600 eV which give well converged results with respect to the basis set. The k-points were generated using the Monkhorst−Pack method with a grid size of 2 × 6 × 6, 2 × 6 × 8, and 2 × 8 × 4 for the CPO-68/69 materials, low temperature MIL-53(Cr) _lt and high temperature MIL-53(Cr)_ht, respectively. Iterative relaxation of atomic positions was stopped when the change in total energy between successive steps was less than 1 meV/cell. With this criterion, the maximum forces generally acting on the atoms were found to be less than 10 meV/Å. The initial atomic coordinates of the CPO-68/69 were taken from the presented refined X-ray structures and for the MIL-53(Cr)_lt and MIL-53(Cr)_ht framework the coordinates are taken from work by Serre et al.43 In our theoretical simulation, we have relaxed the atomic positions and cell parameters globally using force-minimization techniques fixed to the experimental volume. Then the theoretical equilibrium volume is determined by varying the cell volume within ±10% of the experimental volume. Finally the calculated energy versus volume data are fitted into the universal-equation-of-state fit, and the equilibrium cell parameters are extracted. The theoretically obtained structural parameters and the positional parameters are in very good agreement with the corresponding experimental findings. The calculated cell parameters are within 2.1% of the experimental values.

dimethoxy-3,3′-biphenyldicarboxylic acid has the potential to mimic the DHTP linker used in CPO-27/MOF-74 with regard to forming one-dimensional chain SBUs and thereby provides a basis for open metal sites in novel porous MOFs. In this work we report the solvothermal synthesis and structure determination of five new CPs that form two new series of CPs named CPO-68-M (M = Zn, Mn, and Co) and CPO-69-M (M = Cd and Ca). Although their structures are of a different type than targeted, they are nevertheless highly interesting. The topological similarities with flexible MOFs such as MIL-53 led to an investigation into whether or not the CPs would exhibit a flexible behavior when subjected to pressurized gas. These issues were additionally evaluated by DFT modeling, with a focus on gas adsorption at high pressures, as well as the electronic properties of the CPs.

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EXPERIMENTAL SECTION

Materials and Methods. All starting materials and solvents were obtained from commercial suppliers (Sigma-Aldrich and VWR) and were used without additional purification. The 1H NMR spectra were recorded on a Bruker DPX 300 MHz spectrometer at room temperature in the indicated solvents. Chemical shifts are expressed in parts per million (δ) using residual solvent protons as internal standards (1H: CDCl3: δ 7.26 ppm; DMSO-d6: δ 2.49 ppm). The synthesis of the linker, 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid, is based on the procedure described by Wang et al.,29 and the experimental details are described in the Supporting Information (SI). Thermogravimetric analysis (TGA) was performed on a Perkin−Elmer TGA 7 under a flow of N2-gas. The samples were heated from 30 to 800 °C in alumina crucibles, with a ramp rate of 2 °C/min. The TGA results are displayed in Figures S1 and S2 in the SI. Single-crystal X-ray diffraction data (S-XRD) were recorded at ambient temperature on a Bruker D8 instrument fitted with an APEX2 CCD area-detector and using monochromatic MoKα1 radiation (λ = 0.7093 Å) from a sealed tube source. The crystals were extracted from the mother liquor and mounted on thin glass rods with a small amount of epoxy glue. Data reduction was performed using SAINT, and absorption correction was performed using SADABS.30 The structures of CPO-68-Zn/Mn/Co and CPO-69-Ca were solved by direct methods using SIR9231 and refined with SHELXL-201232 as implemented in the WinGX33 program suite. The structure of CPO69-Cd was solved from a nonmerohedral twin and absorption correction was performed with TWINABS.30 This structure was solved with SHELXS-9732 and refined with SHELXL-201232 as implemented in the WinGX33 program suite. Hydrogen atoms were positioned geometrically at distances of 0.93 (CH) and 0.96 Å (CH3) and refined using a riding model with Uiso (H) = 1.2 Ueq (CH) and Uiso (H) = 1.5 Ueq (CH3). Powder X-ray diffraction data (P-XRD) collected at ambient atmosphere and temperature were recorded on a Siemens D5000 instrument with monochromatic CuKα1 radiation (λ = 1.5406 Å) and a Braun position sensitive detector operated in transmission geometry. Crystals of CPO-68-M and CPO-69-M were gathered, ground to a powder in a mortar, and sealed in glass capillaries for measurement. A Pawley fit was performed on the recorded patterns using the TOPAS34 software with the unit cell parameters determined by S-XRD to B



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Table 2. Crystallographic Data for CPO-68-M and CPO-69-Ma name

CPO-68-Zn

CPO-68-Mn

CPO-68-Co

CPO-69-Ca

CPO-69-Cd

formula formula weight T (K) crystal system space group (#) Z a (Å) b (Å) c (Å) β (deg) V (Å3) Dc (g cm−3) μ (mm−1) reflns collected reflns unique Rint crystal size (mm3) crystal color crystal shape F(000) residual density max/min (e·Å−3) GOF final R indices [I > 2σ(I)]

C16H12O6Zn 365.63 296 (2) monoclinic C2/c (15) 4 23.596 (2) 8.2484 (8) 7.6112 (7) 98.7920 (10) 1464.0 (2) 1.659 1,707 6139 1766 0.0143 0.50 × 0.20 × 0.15 colorless needle 744 0.274/−0.295 1.128 R1 = 0.0260 wR2 = 0.0649 R1 = 0.0292 wR2 = 0.0664

C16H12O6Mn 355.20 296 (2) monoclinic C2/c (15) 4 24.026 (11) 8.565 (4) 7.214 (3) 96.093 (6) 1476.0 (12) 1.598 0.923 2773 1104 0.054 0.15 × 0.08 × 0.05 brown plate 724 0.795/−0.977 1.152 R1 = 0.0621 wR2 = 0.1818 R1 = 0.0851 wR2 = 0.2105

C16H12O6Co 359.19 296 (2) monoclinic C2/c (15) 4 23.388 (3) 8.5369 (9) 7.3474 (8) 99.3740 (10) 1447.4 (3) 1.648 1.215 6082 1759 0.0125 0.28 × 0.15 × 0.09 purple plate 732 0.796/−0.453 1.081 R1 = 0.0273 wR2 = 0.0770 R1 = 0.0300 wR2 = 0.0785

C16H12O6Ca 340.34 296 (2) monoclinic C2/c (15) 4 24.721 (6) 8.000 (2) 7.5253 (19) 90.949 (2) 1488.1 (6) 1.519 0.451 5430 1463 0.0415 0.09 × 0.08 × 0.03 colorless needle 704 0.413/−0.224 1.091 R1 = 0.0505 wR2 = 0.1293 R1 = 0.0698 wR2 = 0.1400

C16H12O6Cd 412.66 296 (2) monoclinic C2/c (15) 4 24.514 (6) 8.401 (2) 7.300 (2) 92.548 (3) 1501.8 (7) 1.825 1.483 3986 3474 0.0416 0.50 × 0.20 × 0.16 yellow needle 816 1.188/−0.752 1.093 R1 = 0.0376 wR2 = 0.1107 R1 = 0.0411 wR2 = 0.1135

R indices (all data) a

Calculated standard deviations in parentheses.

Figure 1. Structure of CPO-68-Zn, viewed along the c-axis (Zn: teal, C: gray, O: red). Hydrogen atoms are omitted for clarity.

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CPO-68-M. S-XRD analysis of CPO-68-M (M = Zn, Mn and Co) reveals that the CP crystallizes in the monoclinic space group C2/c with one divalent metal and one deprotonated linker in the asymmetric unit. The structure features metal carboxylate chains (Figure 3a) along the c-axis that are linked by the biphenyl linker to form three-dimensional (3D) CPs with sra topology (Figure 5). The metal is four-coordinate with a slightly distorted tetrahedral coordination polyhedron, which is defined by four oxygen atoms from four different biphenyl linkers. Thus, every linker is connected to four different metal atoms where the carboxylate-groups provide a bridging motif between two adjacent metal atoms (Figure 4). The M−O bond distances in CPO-68-Zn range from 1.9468 (12) Å to 1.9835 (13) Å and are in accordance with the expected bond lengths when applying the bond valence method.44 When the structure

RESULTS AND DISCUSSION The solvothermal synthesis of 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid and a range of divalent cations resulted in two new series of isostructural CPs; CPO-68-M (M = Zn, Mn, and Co) and CPO-69-M (M = Cd and Ca) (Table 2). CPO-68-M and CPO-69-M (Figure 1 and Figure 2) have at a first glance very similar structures differing mainly in their metal coordination polyhedra. Indeed, analyses of the structures reveal that their underlying topologies are in fact the same. Although the two series of CPs share many features, there are also key differences. The crystallographic details of the two series of CPs will therefore be discussed separately. Where it is necessary for the sake of comparison, the structures of CPO-68Zn and CPO-69-Ca have been chosen as representative examples of their series. C



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Figure 2. Structure of CPO-69-Ca, viewed along the c-axis (Ca: light gray, C: gray, O: red). Hydrogen atoms are omitted for clarity.

conditions. All materials in the CPO-68-M series show good thermal stability as well as long-term stability in ambient conditions. The TGA data show that the main thermal decomposition of the materials occurs above 350 °C (Figure S1). Note that the TGA curve for CPO-68-Mn hints at a two step decomposition, possibly due to the available oxidation states of manganese when compared to zinc and cobalt. The exact details of the decomposition and possible intermediates are however yet to be determined experimentally. CPO-69-M. S-XRD analysis of CPO-69-M (M = Ca and Cd) shows that the CP crystallizes in the monoclinic space group C2/c with one divalent metal and one deprotonated linker in the asymmetric unit. The structure features metal carboxylate chains (Figure 3b) along the c-axis that are linked by the biphenyl linker to form 3D CPs with sra topology. The irregular coordination polyhedron comprises eight oxygen atoms originating from four different biphenyl linkers. The linkers are coordinated to the metal in two different modes. Two of the four ligands are coordinated with both oxygen atoms of the carboxylate-group and the remaining two ligands with oxygen atom from the carboxylate group and the other from the methoxy group (Figure 4). As expected, the M−O bond distances of CPO-69-M are significantly longer when compared to CPO-68-M and range from 2.302 (2) Å to 2.589 (2) Å in the case of CPO-69-Ca. As with CPO-68-M, when the structure is viewed along the c-axis, narrow rhombic channels are revealed that cannot accommodate guest molecules at ambient conditions (Figure 2). The distances between the Caatoms in the channels of CPO-69-Ca are 24.721 (6) Å along the a-axis and 8.000 (2) Å along the b-axis. Like CPO-68-M, the materials of the CPO-69-M series also exhibit good stability, with thermal decomposition occurring above 350 °C (Figure S2). The TGA curve of CPO-69-Cd also hints at a two step decomposition, but the nature of any decomposition intermediates is still unknown. To analyze the topology of CPs, it is necessary to reduce or simplify the structure for analysis. For CPs containing finite metal cluster SBUs, the structure is commonly simplified by reducing the linker and the metal SBU to single points or nodes with a defined connectivity. However, when the metal SBU is an infinite one-dimensional (1D) chain, this method is not suitable. To elucidate the underlying structures, we adopted the procedure for analyzing CPs containing 1D rodlike SBUs

Figure 3. (a) The metal carboxylate chain of CPO-68-Zn (tetrahedral coordination). (b) The metal carboxylate chain of CPO-69-Ca. (c) The zigzag ladder formed by linking the carboxylate C atoms of CPO69-Ca (Zn: teal, Ca: light gray, C: gray, O: red).

Figure 4. Coordination modes of 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid in CPO-68-Zn (left) and CPO-69-Ca (right).

is viewed along the c-axis, narrow rhombic channels are revealed (Figure 1). The distances between the Zn-atoms in the channels of CPO-68-Zn are 23.596 (2) Å along the a-axis and 8.2484 (8) Å along the b-axis. These distances give an indication of the size of the channels and suggest a cross section sufficiently large for small guest molecules such as N2 or CO2. However, when the atoms of the linkers are taken into account and the structure is viewed in a space filling model, the channels are clearly too narrow for any guest molecules at ambient D



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Figure 5. sra topology formed by linking the carboxylate C atoms of CPO-68-Zn. One metal carboxylate chain is added for context (Zn: teal, C: gray, O: red).

described by O’Keeffe and Yaghi.45 Using the TOPOS46,47 software, the carboxylate C atoms of CPO-68/69 were selected as the nodes of extension and these were connected to create a uninodal four-connected network of zigzag ladders and rhombohedral channels (Figure 5). The network was subsequently run through the classification procedure of TOPOS to reveal the point symbol 42.63.8 and vertex symbol 4.6.4.6.6.8(2). This corresponds to the sra topology, following the three letter codes recommended by RCSR.48

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DFT CALCULATIONS PERFORMED ON CPO-68-M AND CPO-69-M In order to understand the electronic properties of the CPs we have calculated the electronic structure of these compounds. The calculated band structure and total density of states (DOS) at the equilibrium volume for CPO-68-M and CPO-69-M with GGA-PBE level is displayed in Figures S8−S12. The calculated band gaps (Eg) of CPO-68-M and CPO-69-M series are indirect and range between 2.49 and 2.98 eV. In general, CPO69-M seems to yield larger Eg than the CPO-68-M CPs. The calculated band gap of CPO-68/69 is close to the IRMOF-1049 and smaller than that of MOF-5.50 Band gap (Eg) values of solids obtained from usual DFT calculations are often systematically underestimated (commonly 30−50%) due to discontinuity in the exchange correlation potential. In our recent contribution, however,51 we found that the DFT calculations on MOF-5 gave a band gap value (3.5 eV) which was in unexpectedly good agreement with that observed from experimental studies.52,53 To evaluate the accuracy of the DFT calculations in this work, we performed optical measurements using diffuse reflectometry UV−vis spectroscopy (DRS). These measurements were used in a Tauc plot of [F(R)hν]1/2 as a function of photon energy to estimate the band gap of selected samples (Figure 6).54−56 The experimentally determined band gaps are in relatively good agreement with the calculated values, albeit with some underestimation in the DFT values (eV calc/ exp for CPO-68-Zn: 2.49/2.95 and CPO-69-Ca: 2.98/3.04).

Figure 6. Tauc plot of [F(R)hν]1/2 as a function of photon energy in eV. The dashed lines indicate the linear sections of the plot that were used to estimate the band gaps of CPO-68-Zn (red) and CPO-69-Ca (blue).

structure or “breathing” effect upon uptake or removal of solvent guest molecules.43,57,58 The CPs reported herein were initially evaluated for gas adsorption by analyzing them with the SOLV function in PLATON.59 This analysis indicated that the CPs have no solvent accessible void in the structure and that any surface area would originate from the surface of the particles. Indeed, if the structures are drawn with a space filling model, it becomes obvious that the rhombic channels are too narrow to accommodate gas or solvent at ambient conditions. However, the rhombic nature of the channels bears great resemblance to the MIL-53 MOF which has the ability to expand under moderate (1−10 bar) pressure.43,60,61 Recently, Gustafsson et al. reported a family of flexible MOFs, the SUMOF-6-Ln family, which is based on lanthanides and bipyridine dicarboxylates.62 These MOFs differ from CPO68-M or CPO-69-M due to the more linear nature of the linker and the trivalent metals used in the synthesis. However, the underlying topologies are very similar, and thus a comparison is interesting. Furthermore, the SUMOF-6-Ln MOFs demonstrate a similar reversible flexibility as MIL-53 upon desorption and readsorption of the synthesis solvent. In this way, the MOFs provide precedence for flexible MOFs with sra topology and biphenyl linkers.

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HOST−GUEST INTERACTION IN FLEXIBLE COORDINATION POLYMERS Several MOFs reported in the literature show interesting properties with regard to host−guest interactions. Notable examples are MIL-53 and MIL-88, which exhibit a flexible E



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During initial tests, the CPs were subjected to moderate pressures of N2 gas while recording the P-XRD patterns to reveal any changes in the unit cell parameters. However, problems with the experimental setup meant that these results had to be regarded as inconclusive. Nonetheless, the topological similarities of CPO-68-M and CPO-69-M with flexible MOFs reported in the literature prompted a more thorough investigation of the potential of these materials for gas adsorption at high pressure. Since the materials are nonporous at ambient conditions, it was necessary to evaluate at what pressureif anythe materials would respond to gas stimulus. DFT modeling has proven to be a versatile and strong method for evaluating physical properties of solid state materials and was therefore selected for the investigation. Furthermore, recent publications in the literature have started to elucidate the effects of extreme pressures on MOFs and MOF-like materials.63−66 The results are intriguing and demonstrate that even supposedly rigid frameworks can respond to guest molecules. The adsorption modes are generally different from flexible networks and phase transitions or distortions of the network are common. In particular, the relationship between framework compression and guest molecule inclusion is interesting.67 Since DFT modeling is not limited by the pressure tolerances of hardware, the method proved even more fitting for our needs. For the gas adsorption isotherm calculation we have used the sorption module implemented in the Material Studio 6.068 package. Sorption is used to simulate sorption of small molecules (guest molecules) into porous 3D frameworks. A loading curve is generated using series of fixed pressure (grand canonical ensemble) calculations performed over a series of fugacities. For the sorption computation, we have used the optimized structures obtained from the VASP calculation as input for the starting model. The adsorption isotherm displays the adsorption in molecules per cell at each fugacity. In a typical adsorption isotherm the curve will rise toward a saturation point value beyond which no more molecules can be adsorbed. We have tested for N2, H2, CO2, CO, N2O, and CH4, and there is no significant uptake by the CPO-68/69 materials up to the pressure of 0.1 GPa. This finding is consistent with the experimental gas adsorption observations where selected samples were subjected to moderate gas pressures (∼20 bar) of CO2, while recording the P-XRD patterns. In order to validate our theoretical approach we have carried out the CO2 adsorption isotherm calculation on MIL-53(Cr) as a model with various unit-cell volumes. Our calculated CO2 isotherm for the MIL-53(Cr) MOF in two different polymorphs (lt and ht) as a function of cell parameters vs saturation point is displayed in Figure 7. The calculations successfully reproduce the reported difference in CO 2 adsorption in the lt and ht polymorphs. Further details on the calculation method are provided in the Supporting Information. The above finding clearly demonstrates that the presented type of approach is valid. From our theoretical adsorption simulation and from experimental study we conclude that the CPO-68/69 CPs are not suitable candidates for gas adsorption applications and that they are nonporous at all pressures. On the other hand, based on the magnitude of the band gap as well as the high stability, these CPs might have potential application in the photovoltaics industry. More research is needed in this direction.

Figure 7. Calculated CO2 adsorption isotherm as a function of the cell volume for MIL-53(Cr).

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A POSSIBLE RATIONALE FOR THE LACK OF FLEXIBILITY IN CPO-68/69 As discussed above, MIL-53 and CPO-68/69 share the same sra topology. Since the difference in flexibility cannot be attributed to the topology, it must originate from other structural factors. In MIL-53, the O···O axis of the carboxylate group acts as a form of hinge between the organic linker and the metal carboxylate chain. As the structure opens up, the dihedral angle between the Cr−O−O−Cr and O−C−O planes changes from 139° (lt) to 180° (ht). Moreover, the expansion of the structure is accompanied by a rotation of the benzene moiety of the linker (Figure 8).69 This rotation has a relatively small energetic barrier due to the single bond between the benzene moiety and the carboxylate groups.

Figure 8. Molecular “hinges” of MIL-53(Cr)_lt (left) and CPO-69-Ca (right) (Cr: green, Ca: light gray, C: gray O: red).

This is in contrast to CPO-68/69, where the O···O hinge is oriented in an unfavorable direction. In fact, the hinge is off by almost 40° relative to the c-axis. Furthermore, the necessary rotation of the linker is severely hindered (Figure 8). We propose that these two structural features provide a rationale for the difference in flexibility between MIL-53 and CPO-68/ 69.

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CONCLUSIONS Two series of coordination polymers with the sra topology, CPO-68-M (M = Zn, Mn, and Co) and CPO-69-M (M = Cd and Ca), were synthesized by solvothermal methods. The CPs F



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have good thermal stability as well as good stability against degradation in ambient conditions. The CPs were evaluated for gas sorption by DFT, which indicated that the materials are nonporous even at high pressures and do not have a flexible structure. A possible explanation for the lack of flexibility in the structure is provided. DFT calculations as well as UV−vis DRS measurements reveal that the CPs have band gaps that might make them interesting in photovoltaic applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01302. Synthesis details for the ligand, TGA, P-XRD of the structures, and additional details regarding the DFT calculation. (PDF) Accession Codes

CCDC 1437170−1437174 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +47 92292547; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from the Research Council of Norway (Project No. 190980), inGAP and the Departments of Chemistry at UiO and UiB. P.V. acknowledges the Research Council of Norway for providing computing time at the Norwegian supercomputer facilities. The skillful assistance from the staff at the Swiss−Norwegian Beamlines, ESRF, Grenoble, is highly acknowledged. We acknowledge use of the Norwegian national infrastructure for X-ray diffraction and scattering (RECX).

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REFERENCES

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Two New Series of Coordination Polymers and Evaluation of Their

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