Two New Series of Coordination Polymers and Evaluation of TheirProperties by Density Functional TheoryFredrik Lundvall,*,† Ponniah Vajeeston,† David S. Wragg,†,‡ Pascal D. C. Dietzel,§ and Helmer Fjellvag†,‡

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

*S Supporting Information

ABSTRACT: Five new coordination polymers (CPs), CPO-68-M (M = Zn, Mn, and Co) and CPO-69-M (M = Ca andCd), were synthesized by solvothermal methods using 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid as the organic linker.The three-dimensional frameworks are formed by metalcarboxylate chains that are separated by the linker. Structuralanalysis reveals dense networks with narrow rhombic channelsand sra topologies for both CPO-68-M and CPO-69-M. The major structural difference between the two series of CPs is in themetal coordination polyhedra, which are four- and eight-coordinated in CPO-68-M and CPO-69-M, respectively. The CPs arehighly crystalline, robust, and have good thermal stability (> 350 °C). On the basis of the topological similarities with MIL-53, wetested 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 analysisconcluded that the CPs are ill-suited for gas adsorption due to their structural rigidity. However, electronic structure calculationsreveal that CPO-68-M and CPO-69-M are indirect band gap semiconductors with an estimated band gap between 2.49 and 2.98eV.

■ INTRODUCTION

Custom compounds with potential applications in varied fieldssuch as gas sorption, catalysis, luminescence, or photovoltaicscan be created by combining organic and inorganic buildingblocks.1−4 For this intriguing class of materials calledcoordination polymers (CPs), the vast range of interchangeableorganic linkers and metal secondary building units (SBUs) givea unique flexibility in the synthesis and tailoring of newmaterials, not only to satisfy scientific curiosity and under-standing, but also to develop functional materials withinteresting properties for industrial applications.Large scale CO2 capture from the flue gas of power plants

requires efficient and robust functional materials, among whichmetal−organic frameworks (MOFs) have been and are stillbeing heavily explored. For large scale application, gooddiffusion properties are important to increase efficiency andminimize the pressure drop over the adsorption material.Furthermore, strong interactions between the gas and a largesurface area of the adsorbent are desired to increase the amountof gas adsorbed per gram material. Additional parameters forcommercialization are naturally connected to costs ofproduction, toxicity of chemicals, and not least, mechanicalstrength and potential formation of byproducts and materialsloss. Our group and others have previously published severalarticles describing the family of materials formed by CPO-27and its isostructural MOF-74 analogue.5−13 These are based on

divalent metal cations (Zn, Co, Ni, Mg, Mn, Fe, and Cu) and2,5-dihydroxyterephtalic acid (DHTP) and feature coordina-tively unsaturated sites, so-called open metal sites, uponactivation. These open metal sites create a strong host−guestinteraction toward a number of gases (CO2, CO, NO, H2, andCH4).

13−20 Recreating similarly strong host−guest interactionsin a new series of MOFs would be highly interesting in thecontext of gas sorption/separation. More efficient gas diffusioninto the pores compared to CPO-27/MOF-74 would also bebeneficial. CPO-27/MOF-74 are based on a substitutedbenzene linker. The logical step is hence to improve the gasdiffusion by increasing the length of the linker and therebycreate larger pores in the structure. This way of tuning the poredimensions (and functional properties) of known MOFs iscommonly referred to as reticular design or the isoreticularapproach21 and has already successfully been applied to createisoreticular CPO-27/MOF-74 type MOFs.22,23

In the current work, the 4,4′-dimethoxy-3,3′-biphenyldicar-boxylic acid linker has been explored along with a series ofdivalent cations. Isomers of this linker have produced highlycrystalline CPs with interesting topologies,24−28 albeit nonebeing isostructural to CPO-27/MOF-74. Nevertheless, 4,4′-

Received: September 8, 2015Revised: November 16, 2015Published: November 23, 2015

Article

pubs.acs.org/crystal

© 2015 American Chemical Society 339 DOI: 10.1021/acs.cgd.5b01302Cryst. Growth Des. 2016, 16, 339−346

dimethoxy-3,3′-biphenyldicarboxylic acid has the potential tomimic the DHTP linker used in CPO-27/MOF-74 with regardto forming one-dimensional chain SBUs and thereby provides abasis 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 newseries of CPs named CPO-68-M (M = Zn, Mn, and Co) andCPO-69-M (M = Cd and Ca). Although their structures are ofa different type than targeted, they are nevertheless highlyinteresting. The topological similarities with flexible MOFs suchas MIL-53 led to an investigation into whether or not the CPswould exhibit a flexible behavior when subjected to pressurizedgas. These issues were additionally evaluated by DFT modeling,with a focus on gas adsorption at high pressures, as well as theelectronic properties of the CPs.

■ EXPERIMENTAL SECTIONMaterials and Methods. All starting materials and solvents were

obtained from commercial suppliers (Sigma-Aldrich and VWR) andwere used without additional purification. The 1H NMR spectra wererecorded on a Bruker DPX 300 MHz spectrometer at roomtemperature in the indicated solvents. Chemical shifts are expressedin parts per million (δ) using residual solvent protons as internalstandards (1H: CDCl3: δ 7.26 ppm; DMSO-d6: δ 2.49 ppm). Thesynthesis of the linker, 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid,is based on the procedure described by Wang et al.,29 and theexperimental 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 wereheated 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 APEX2CCD area-detector and using monochromatic MoKα1 radiation (λ =0.7093 Å) from a sealed tube source. The crystals were extracted fromthe mother liquor and mounted on thin glass rods with a small amountof epoxy glue. Data reduction was performed using SAINT, andabsorption correction was performed using SADABS.30 The structuresof CPO-68-Zn/Mn/Co and CPO-69-Ca were solved by directmethods using SIR9231 and refined with SHELXL-201232 asimplemented in the WinGX33 program suite. The structure of CPO-69-Cd was solved from a nonmerohedral twin and absorptioncorrection was performed with TWINABS.30 This structure wassolved with SHELXS-9732 and refined with SHELXL-201232 asimplemented in the WinGX33 program suite. Hydrogen atoms werepositioned 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) andUiso (H) = 1.5 Ueq (CH3).Powder X-ray diffraction data (P-XRD) collected at ambient

atmosphere and temperature were recorded on a Siemens D5000instrument with monochromatic CuKα1 radiation (λ = 1.5406 Å) anda Braun position sensitive detector operated in transmission geometry.Crystals of CPO-68-M and CPO-69-M were gathered, ground to apowder in a mortar, and sealed in glass capillaries for measurement. APawley fit was performed on the recorded patterns using the TOPAS34

software with the unit cell parameters determined by S-XRD to

confirm the absence of other crystalline phases in the samples (seeFigures S3−S7 in the SI). Because of fluorescence in the case of CPO-68-Co, this sample was measured on the Swiss-Norwegian Beamline(BM01A) at the European Synchrotron Radiation Facility usingmonochromatic synchrotron radiation (λ = 0.69396 Å). The setupuses a Huber goniometer and Dectris Pilatus 2M photon countingpixel area detector.35 The 2D diffraction data from a 90 s exposurewere converted to a 1D P-XRD pattern using FIT2D.36

Additional P-XRD data on selected samples were collected at 20 barpressure of CO2 and ambient temperature on a Bruker D8 instrumentwith monochromatic CuKα1 radiation (λ = 1.5406 Å) and a LynxEyeXE position sensitive detector operated in transmission geometry.

Diffuse reflectance UV−vis spectroscopy (DRS) was performed ona Shimadzu UV-3600 instrument with an integrating sphere usingcompacted BaSO4 as reference. The optical band gaps were estimatedby making a Tauc plot of [F(R)hν]1/2 versus the photon energy hν. Tothis end, the Kubelka−Munk function F(R) = (1 − R)2/2R wascalculated from the reflectance data obtained through the DRS UV−vis measurements.

Synthesis of CPO-68-M and CPO-69-M. The synthesisprocedure for CPO-68-Zn described below can be regarded as ageneral procedure for all members of the CPO-68-M and CPO-69-Mseries. 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, 0.2mmol) was weighed in and dissolved in a mixture of DMF (2.0mL) and deionized water (0.1 mL). The mixture was heated in a 5 mLglass vial at 120 °C for 48 h and then cooled to room temperature.This procedure yielded colorless needle crystals of sufficient quality forS-XRD analysis.

Computational Details. The quantum-mechanical calculationswere performed in the framework of density functional theory (DFT)using the generalized gradient approximation (GGA)37,38 asimplemented in the VASP code.39,40 The interaction between theion and electron is described by the projector augmented wavemethod.41,42 For the calculations presented here we have used plane-wave cutoff energy of 600 eV which give well converged results withrespect to the basis set. The k-points were generated using theMonkhorst−Pack method with a grid size of 2 × 6 × 6, 2 × 6 × 8, and2 × 8 × 4 for the CPO-68/69 materials, low temperature MIL-53(Cr)_lt and high temperature MIL-53(Cr)_ht, respectively. Iterativerelaxation of atomic positions was stopped when the change in totalenergy between successive steps was less than 1 meV/cell. With thiscriterion, the maximum forces generally acting on the atoms werefound to be less than 10 meV/Å. The initial atomic coordinates of theCPO-68/69 were taken from the presented refined X-ray structuresand for the MIL-53(Cr)_lt and MIL-53(Cr)_ht framework thecoordinates are taken from work by Serre et al.43 In our theoreticalsimulation, we have relaxed the atomic positions and cell parametersglobally using force-minimization techniques fixed to the experimentalvolume. Then the theoretical equilibrium volume is determined byvarying the cell volume within ±10% of the experimental volume.Finally the calculated energy versus volume data are fitted into theuniversal-equation-of-state fit, and the equilibrium cell parameters areextracted. The theoretically obtained structural parameters and thepositional parameters are in very good agreement with thecorresponding experimental findings. The calculated cell parametersare within 2.1% of the experimental values.

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 Zn(NO3)2·6H2O MnCl2·4H2O Co(NO3)2·6H2O Ca(NO3)2·4H2O Cd(NO3)2·4H2Ometal amount 59.5 mg, 0.2 mmol 39.6 mg, 0.2 mmol 58.2 mg, 0.2 mmol 47.2 mg, 0.2 mmol 61.7 mg, 0.2 mmollinker amount 60.4 mg, 0.2 mmol 60.4 mg, 0.2 mmol 60.4 mg, 0.2 mmol 60.4 mg, 0.2 mmol 60.4 mg, 0.2 mmolsolvent 1 DMF, 2.0 mL DMF, 2.0 mL DMF, 2.0 mL DMF, 2.0 mL DMF, 2.0 mLsolvent 2 H2O, 0.1 mL H2O, 0.1 mL H2O, 0.1 mL none H2O, 0.1 mLtemperature (°C) 120 100 100 120 120reaction time (h) 48 8 48 24 48

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■ RESULTS AND DISCUSSION

The solvothermal synthesis of 4,4′-dimethoxy-3,3′-biphenyldi-carboxylic acid and a range of divalent cations resulted in twonew series of isostructural CPs; CPO-68-M (M = Zn, Mn, andCo) and CPO-69-M (M = Cd and Ca) (Table 2). CPO-68-Mand CPO-69-M (Figure 1 and Figure 2) have at a first glancevery similar structures differing mainly in their metalcoordination polyhedra. Indeed, analyses of the structuresreveal 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 twoseries of CPs will therefore be discussed separately. Where it isnecessary for the sake of comparison, the structures of CPO-68-Zn and CPO-69-Ca have been chosen as representativeexamples of their series.

CPO-68-M. S-XRD analysis of CPO-68-M (M = Zn, Mnand Co) reveals that the CP crystallizes in the monoclinic spacegroup C2/c with one divalent metal and one deprotonatedlinker in the asymmetric unit. The structure features metalcarboxylate chains (Figure 3a) along the c-axis that are linkedby the biphenyl linker to form three-dimensional (3D) CPswith sra topology (Figure 5). The metal is four-coordinate witha slightly distorted tetrahedral coordination polyhedron, whichis defined by four oxygen atoms from four different biphenyllinkers. Thus, every linker is connected to four different metalatoms where the carboxylate-groups provide a bridging motifbetween two adjacent metal atoms (Figure 4). The M−O bonddistances in CPO-68-Zn range from 1.9468 (12) Å to 1.9835(13) Å and are in accordance with the expected bond lengthswhen applying the bond valence method.44 When the structure

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 C16H12O6Zn C16H12O6Mn C16H12O6Co C16H12O6Ca C16H12O6Cdformula weight 365.63 355.20 359.19 340.34 412.66T (K) 296 (2) 296 (2) 296 (2) 296 (2) 296 (2)crystal system monoclinic monoclinic monoclinic monoclinic monoclinicspace group (#) C2/c (15) C2/c (15) C2/c (15) C2/c (15) C2/c (15)Z 4 4 4 4 4a (Å) 23.596 (2) 24.026 (11) 23.388 (3) 24.721 (6) 24.514 (6)b (Å) 8.2484 (8) 8.565 (4) 8.5369 (9) 8.000 (2) 8.401 (2)c (Å) 7.6112 (7) 7.214 (3) 7.3474 (8) 7.5253 (19) 7.300 (2)β (deg) 98.7920 (10) 96.093 (6) 99.3740 (10) 90.949 (2) 92.548 (3)V (Å3) 1464.0 (2) 1476.0 (12) 1447.4 (3) 1488.1 (6) 1501.8 (7)Dc (g cm−3) 1.659 1.598 1.648 1.519 1.825μ (mm−1) 1,707 0.923 1.215 0.451 1.483reflns collected 6139 2773 6082 5430 3986reflns unique 1766 1104 1759 1463 3474Rint 0.0143 0.054 0.0125 0.0415 0.0416crystal size (mm3) 0.50 × 0.20 × 0.15 0.15 × 0.08 × 0.05 0.28 × 0.15 × 0.09 0.09 × 0.08 × 0.03 0.50 × 0.20 × 0.16crystal color colorless brown purple colorless yellowcrystal shape needle plate plate needle needleF(000) 744 724 732 704 816residual density max/min (e·Å−3) 0.274/−0.295 0.795/−0.977 0.796/−0.453 0.413/−0.224 1.188/−0.752GOF 1.128 1.152 1.081 1.091 1.093final R indices [I > 2σ(I)] R1 = 0.0260 R1 = 0.0621 R1 = 0.0273 R1 = 0.0505 R1 = 0.0376

wR2 = 0.0649 wR2 = 0.1818 wR2 = 0.0770 wR2 = 0.1293 wR2 = 0.1107R indices (all data) R1 = 0.0292 R1 = 0.0851 R1 = 0.0300 R1 = 0.0698 R1 = 0.0411

wR2 = 0.0664 wR2 = 0.2105 wR2 = 0.0785 wR2 = 0.1400 wR2 = 0.1135aCalculated 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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is viewed along the c-axis, narrow rhombic channels arerevealed (Figure 1). The distances between the Zn-atoms in thechannels of CPO-68-Zn are 23.596 (2) Å along the a-axis and8.2484 (8) Å along the b-axis. These distances give anindication of the size of the channels and suggest a cross sectionsufficiently large for small guest molecules such as N2 or CO2.However, when the atoms of the linkers are taken into accountand the structure is viewed in a space filling model, the channelsare clearly too narrow for any guest molecules at ambient

conditions. All materials in the CPO-68-M series show goodthermal stability as well as long-term stability in ambientconditions. The TGA data show that the main thermaldecomposition of the materials occurs above 350 °C (FigureS1). Note that the TGA curve for CPO-68-Mn hints at a twostep decomposition, possibly due to the available oxidationstates of manganese when compared to zinc and cobalt. Theexact details of the decomposition and possible intermediatesare however yet to be determined experimentally.

CPO-69-M. S-XRD analysis of CPO-69-M (M = Ca andCd) shows that the CP crystallizes in the monoclinic spacegroup C2/c with one divalent metal and one deprotonatedlinker in the asymmetric unit. The structure features metalcarboxylate chains (Figure 3b) along the c-axis that are linkedby the biphenyl linker to form 3D CPs with sra topology. Theirregular coordination polyhedron comprises eight oxygenatoms originating from four different biphenyl linkers. Thelinkers are coordinated to the metal in two different modes.Two of the four ligands are coordinated with both oxygenatoms of the carboxylate-group and the remaining two ligandswith oxygen atom from the carboxylate group and the otherfrom the methoxy group (Figure 4). As expected, the M−Obond distances of CPO-69-M are significantly longer whencompared 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 thestructure is viewed along the c-axis, narrow rhombic channelsare revealed that cannot accommodate guest molecules atambient conditions (Figure 2). The distances between the Ca-atoms in the channels of CPO-69-Ca are 24.721 (6) Å alongthe 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 (FigureS2). The TGA curve of CPO-69-Cd also hints at a two stepdecomposition, but the nature of any decompositionintermediates is still unknown.To analyze the topology of CPs, it is necessary to reduce or

simplify the structure for analysis. For CPs containing finitemetal cluster SBUs, the structure is commonly simplified byreducing the linker and the metal SBU to single points or nodeswith a defined connectivity. However, when the metal SBU isan infinite one-dimensional (1D) chain, this method is notsuitable. To elucidate the underlying structures, we adopted theprocedure for analyzing CPs containing 1D rodlike SBUs

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.

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

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

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described by O’Keeffe and Yaghi.45 Using the TOPOS46,47

software, the carboxylate C atoms of CPO-68/69 were selectedas the nodes of extension and these were connected to create auninodal four-connected network of zigzag ladders andrhombohedral channels (Figure 5). The network wassubsequently run through the classification procedure ofTOPOS to reveal the point symbol 42.63.8 and vertex symbol4.6.4.6.6.8(2). This corresponds to the sra topology, followingthe three letter codes recommended by RCSR.48

■ DFT CALCULATIONS PERFORMED ON CPO-68-MAND CPO-69-M

In order to understand the electronic properties of the CPs wehave 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 withGGA-PBE level is displayed in Figures S8−S12. The calculatedband gaps (Eg) of CPO-68-M and CPO-69-M series areindirect and range between 2.49 and 2.98 eV. In general, CPO-69-M seems to yield larger Eg than the CPO-68-M CPs. Thecalculated band gap of CPO-68/69 is close to the IRMOF-1049

and smaller than that of MOF-5.50 Band gap (Eg) values ofsolids obtained from usual DFT calculations are oftensystematically underestimated (commonly 30−50%) due todiscontinuity in the exchange correlation potential. In ourrecent contribution, however,51 we found that the DFTcalculations on MOF-5 gave a band gap value (3.5 eV) whichwas in unexpectedly good agreement with that observed fromexperimental studies.52,53 To evaluate the accuracy of the DFTcalculations in this work, we performed optical measurementsusing diffuse reflectometry UV−vis spectroscopy (DRS). Thesemeasurements were used in a Tauc plot of [F(R)hν]1/2 as afunction of photon energy to estimate the band gap of selectedsamples (Figure 6).54−56 The experimentally determined bandgaps 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).

■ HOST−GUEST INTERACTION IN FLEXIBLECOORDINATION POLYMERS

Several MOFs reported in the literature show interestingproperties with regard to host−guest interactions. Notableexamples are MIL-53 and MIL-88, which exhibit a flexible

structure or “breathing” effect upon uptake or removal ofsolvent guest molecules.43,57,58

The CPs reported herein were initially evaluated for gasadsorption by analyzing them with the SOLV function inPLATON.59 This analysis indicated that the CPs have nosolvent accessible void in the structure and that any surface areawould originate from the surface of the particles. Indeed, if thestructures are drawn with a space filling model, it becomesobvious that the rhombic channels are too narrow toaccommodate gas or solvent at ambient conditions. However,the rhombic nature of the channels bears great resemblance tothe MIL-53 MOF which has the ability to expand undermoderate (1−10 bar) pressure.43,60,61

Recently, Gustafsson et al. reported a family of flexibleMOFs, the SUMOF-6-Ln family, which is based on lanthanidesand bipyridine dicarboxylates.62 These MOFs differ from CPO-68-M or CPO-69-M due to the more linear nature of the linkerand the trivalent metals used in the synthesis. However, theunderlying topologies are very similar, and thus a comparison isinteresting. Furthermore, the SUMOF-6-Ln MOFs demon-strate a similar reversible flexibility as MIL-53 upon desorptionand readsorption of the synthesis solvent. In this way, theMOFs provide precedence for flexible MOFs with sra topologyand biphenyl linkers.

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).

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

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During initial tests, the CPs were subjected to moderatepressures of N2 gas while recording the P-XRD patterns toreveal any changes in the unit cell parameters. However,problems with the experimental setup meant that these resultshad to be regarded as inconclusive. Nonetheless, thetopological similarities of CPO-68-M and CPO-69-M withflexible MOFs reported in the literature prompted a morethorough investigation of the potential of these materials for gasadsorption at high pressure. Since the materials are nonporousat ambient conditions, it was necessary to evaluate at whatpressureif anythe materials would respond to gas stimulus.DFT modeling has proven to be a versatile and strong methodfor evaluating physical properties of solid state materials andwas therefore selected for the investigation. Furthermore,recent publications in the literature have started to elucidate theeffects of extreme pressures on MOFs and MOF-likematerials.63−66 The results are intriguing and demonstratethat even supposedly rigid frameworks can respond to guestmolecules. The adsorption modes are generally different fromflexible networks and phase transitions or distortions of thenetwork are common. In particular, the relationship betweenframework compression and guest molecule inclusion isinteresting.67 Since DFT modeling is not limited by thepressure tolerances of hardware, the method proved even morefitting 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 smallmolecules (guest molecules) into porous 3D frameworks. Aloading curve is generated using series of fixed pressure (grandcanonical ensemble) calculations performed over a series offugacities. For the sorption computation, we have used theoptimized structures obtained from the VASP calculation asinput for the starting model. The adsorption isotherm displaysthe adsorption in molecules per cell at each fugacity. In a typicaladsorption isotherm the curve will rise toward a saturationpoint value beyond which no more molecules can be adsorbed.We have tested for N2, H2, CO2, CO, N2O, and CH4, and thereis no significant uptake by the CPO-68/69 materials up to thepressure of 0.1 GPa. This finding is consistent with theexperimental gas adsorption observations where selectedsamples 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) asa model with various unit-cell volumes. Our calculated CO2

isotherm for the MIL-53(Cr) MOF in two differentpolymorphs (lt and ht) as a function of cell parameters vssaturation point is displayed in Figure 7. The calculationssuccessfully reproduce the reported difference in CO2

adsorption in the lt and ht polymorphs. Further details onthe calculation method are provided in the SupportingInformation. The above finding clearly demonstrates that thepresented type of approach is valid. From our theoreticaladsorption simulation and from experimental study weconclude that the CPO-68/69 CPs are not suitable candidatesfor gas adsorption applications and that they are nonporous atall pressures. On the other hand, based on the magnitude of theband gap as well as the high stability, these CPs might havepotential application in the photovoltaics industry. Moreresearch is needed in this direction.

■ A POSSIBLE RATIONALE FOR THE LACK OFFLEXIBILITY IN CPO-68/69

As discussed above, MIL-53 and CPO-68/69 share the samesra topology. Since the difference in flexibility cannot beattributed to the topology, it must originate from otherstructural factors. In MIL-53, the O···O axis of the carboxylategroup acts as a form of hinge between the organic linker andthe metal carboxylate chain. As the structure opens up, thedihedral angle between the Cr−O−O−Cr and O−C−O planeschanges from 139° (lt) to 180° (ht). Moreover, the expansionof the structure is accompanied by a rotation of the benzenemoiety of the linker (Figure 8).69 This rotation has a relativelysmall energetic barrier due to the single bond between thebenzene moiety and the carboxylate groups.

This is in contrast to CPO-68/69, where the O···O hinge isoriented in an unfavorable direction. In fact, the hinge is off byalmost 40° relative to the c-axis. Furthermore, the necessaryrotation of the linker is severely hindered (Figure 8). Wepropose that these two structural features provide a rationalefor the difference in flexibility between MIL-53 and CPO-68/69.

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

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

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

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have good thermal stability as well as good stability againstdegradation in ambient conditions. The CPs were evaluated forgas sorption by DFT, which indicated that the materials arenonporous even at high pressures and do not have a flexiblestructure. A possible explanation for the lack of flexibility in thestructure is provided. DFT calculations as well as UV−vis DRSmeasurements reveal that the CPs have band gaps that mightmake them interesting in photovoltaic applications.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.cgd.5b01302.

Synthesis details for the ligand, TGA, P-XRD of thestructures, and additional details regarding the DFTcalculation. (PDF)

Accession CodesCCDC 1437170−1437174 contains the supplementary crys-tallographic data for this paper. These data can be obtained freeof charge via www.ccdc.cam.ac.uk/data_request/cif, or byemailing data_request@ccdc.cam.ac.uk, or by contacting TheCambridge Crystallographic Data Centre, 12, Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +47 92292547; e-mail: fredrik.lundvall@smn.uio.no.NotesThe authors declare no competing financial interest.

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

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