Spectroscopic Studies of Structural Dynamics Induced by Heatingand Hydration: A Case of Calcium-Terephthalate Metal−OrganicFrameworkMatjaz Mazaj,*,† Gregor Mali,†,‡ Mojca Rangus,† Emanuela Zunkovic,† Venceslav Kaucic,†,§

and Natasa Zabukovec Logar†,§

†National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia‡EN-FIST Centre of Excellence, Dunajska 156, 1000 Ljubljana, Slovenia§CO-NOT Centre of Excellence, Hajdrihova 19, 1000 Ljubljana, Slovenia

*S Supporting Information

ABSTRACT: Structural dynamics of Ca(BDC)(DMF)(H2O)with rhombic-shaped channels and 44 net topology uponheating and hydration were elucidated by using complemen-tary methods of diffraction (XRD) and spectroscopy (FT-IR,MAS NMR, EXAFS, XANES). During heating the Ca(BDC)-(DMF)(H2O) framework underwent structural changes in twosteps. The first change at 150 °C includes breaking of Ca−Obonds with H2O and DMF molecules. In this step, DMF isremoved from the surface or near the surface of the crystals. The affected parts of the crystals are transformed to a newnonporous Ca-BDC(400) phase that prevents the diffusion of DMF from the cores of the crystals. Second transition at 400 °Cled to the complete transformation to Ca-BDC(400). This phase is reversibly transformed to a pseudo-3-D frameworkCa(BDC)(H2O)3 upon exposure to humid environment. We proposed mechanisms of Ca-BDC(RT) → Ca-BDC(400) and Ca-BDC(400) → Ca(BDC)(H2O)3 transformations, which include breaking of the bonds between Ca2+ and carboxylate groups,rotating of BDC ligand, and recoordination of COO− groups to Ca2+ centers. The crystal-to-crystal transformations are driven bythe tendencies to change the bonding modes between COO− and Ca2+ with the change of Ca2+ coordination number. Thus thedecrease in Ca2+ coordination number, which is usually a consequence of activation, does not lead to the expansion orcontraction of the pores, but it leads to pronounced structural rearrangement. Such behavior can explain the lack of porosity inCa-MOF systems.

■ INTRODUCTION

Much effort has been devoted to the development of newmetal−organic frameworks (MOFs) due to their potentialapplications in areas such as gas storage and gas separation,catalysis, drug delivery, electronics, and optics.1−17 MOFs canpossess rigid frameworks or they can exhibit distinctivestructure flexibility. Dynamics of flexible frameworks areamong the most interesting characteristics of metal−organicstructures, which can be exploited for various applications suchas sensing, separation, and adsorption. Structural dynamics canusually be triggered by external stimuli (most commonly byinclusion and exchange of guest molecules or by pressure andtemperature changes)18,19 and are possible in MOFs thatexhibit specific structural characteristics: (a) the presence ofweak interactions such as hydrogen bonds, π−π stacking, or vander Waals connections, which often support and stabilizecoordination polymer structures; (b) flexibility and versatility ofmetal-cation coordination environment (e.g., Jahn−Tellereffect, change of coordination number); and (c) flexibility ofligands, which include rotation, stretching, or bending oforganic molecules.20 Structural changes, which can be ingeneral classified to crystal-to-amorphous (CTA) and crystal-

to-crystal (CTC) transformations, are well-investigated fornumerous MOF structures.21−32 Adsorption of gases at highpressures in some cases induces structural transition andsignificantly increases the porosity at certain pressure point(“gate-opening” pressure).33 Some MOFs exhibit extensiveflexibility when exposed to a certain type of guest moleculesand show reversible structural dynamics upon adsorption/desorption processes (breathing effect).34−40 Temperaturechange is another very common external stimulus that cantrigger structural changes. In this case, structural dynamics areusually driven by the removal of solvents or dehydration uponheating.41−43

Structural dynamics are most frequently studied on MOFsbased on transition-metal cations and rarely on s-block metalcenters. However, Mg- and Ca-MOFs deserve special attention.They are advantageous for implementation and industrialapplications (bioapplications, gas storage) due to theirnontoxicity and relatively low densities. Despite the potential

Received: November 22, 2012Revised: March 15, 2013

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application opportunities that this group of MOFs offers, Ca-MOFs in particular are relatively poorly investigated. Moreover,the development of Ca-based metal−organic structures withpermanent porosity still remains a challenge due to the specificdifficulties in synthetic procedures, which is mainly caused bythe limited prediction of the desired structures, variability, andcontrol over coordination geometries.44 There are few papersdescribing rigid 3-D Ca-based MOF-type carboxylate struc-tures,45−52 but to our knowledge only work of Platero-Prats etal.32 has dealt with structural dynamic investigations of Ca-based MOF material.At the same time, terephthalate MOF-type materials based

on Cr, Al, and Ga with 44 networks (MIL-53 materials) areknown to exhibit extensive breathing upon guest exchange oradsorption/desorption of gases.53−55 Because the network ofCa(BDC)(DMF)(H2O) material reported by Liang et al.52

possesses topology of MIL-53, structural flexibility uponexternal stimuli can be expected for that material as well. Wereport on the structural changes studied by complementaryinvestigations using XRD and spectroscopic (NMR and XAS)methods. Furthermore, we explained the mechanism ofstructural dynamics of Ca(BDC)(DMF)(H2O) upon externalstimuli (heating and hydration/dehydration processes).

■ EXPERIMENTAL SECTIONSynthesis. For the preparation of Ca(BDC)(DMF)(H2O),

we modified the procedure published by Liang et al.52 Thisprocedure resulted in a higher yield of pure Ca-terephthalateproduct (ca. 70% with respect to BDC ligand). Synthesisstarted with separate dissolution of 0.26 g (1 mmol) ofCa(NO3)2·6H2O (99%, Sigma-Aldrich) in 2 mL of deminer-alized water and 0.18 g of terephthalic acid (95% BDC, AlfaAesar) in a mixture of 8 mL (100 mmol) of N,N′-dimethylformamide (99% DMF, Aldrich) and 0.25 mL (0.7mmol) of triethylamine (99% TEA, Aldrich). The latter wasused to deprotonate dicarboxylic acid. Two solutions weremixed together and stirred for a few minutes. White suspensionwas solvothermally treated under autogenous pressure in a glassvessel at 125 °C for 72 h. The time of crystallization could bedecreased to 24 h if the molar ratio of TEA/Ca2+ was increasedto 1.8. The obtained rod-like crystals with the average length of100 μm were continuously rinsed with ethanol and dried underambient conditions.For the dynamic investigation purposes the synthesized

material was heated to 400 °C and hydrated in a controlledhumid environment. X-ray analysis indicated that as-synthe-sized and hydrated samples have already known structures,52,56

whereas thermal treatment at 400 °C yielded a new phase,denoted as CaBDC(400).Characterization. X-ray powder diffraction data of as-

synthesized, thermally treated, and hydrated samples werecollected on a PANalytical X’Pert PRO high-resolutiondiffractometer with Cu Kα1 radiation (λ = 1.5406 Å) in therange from 5 to 60° 2θ with the step of 0.016° per 100 s using afully opened 100 channel X’Celerator detector. For the purposeof the structure determination of CaBDC(400) phase, the XRDpattern was collected using a wider 2θ range (from 5 to 90°)and step of 0.008° per 300 s. The XRD pattern ofCaBDC(400) was indexed with the DICVOL06 package57 forthe first 20 lines. Unit cells in Cc space group (no. 9) with theparameters of a = 18.8433(6), b = 5.3324(3), c = 6.9596(3),and β = 87.005(5)° were obtained with satisfactory figure ofmerit (F20 = 24, 0.02, 37). Whole pattern profile refinement

without structural parameters using the Le Bail method58

confirmed the adequacy of the unit cell. Structure determi-nation was performed by using the EXPO2009 package.59 Thefirst electron density map revealed positions of all Ca and Oatoms. C atoms were located by the difference Fourier mapanalysis, except for one atom belonging to the benzene ring(C10) and one atom belonging to the carboxylate group (C13).After the missing atoms were inserted onto the most probablepositions, the constructed model was relaxed and its geometrywas optimized using the ab initio density functional theoryoptimization (with CASTEP code). The obtained structuralmodel with the space group C2/c (no. 15) was used in the finalRietveld refinement, which converged with acceptable agree-ment factors of Rexp = 4.1, Rp 7.4, and Rwp = 11.8. Details ofRietveld refinement along with the crystallographic parametersand crystal structure scheme for CaBDC(400) are available inthe Supporting Information.Elemental analyses for all samples were performed on a

CHNS analyzer (Perkin-Elmer 2400, Series II) and byinductive-coupled plasma atomic emission spectrometry onan Atom Scan 25 (Thermo Jarrell Ash) ICP-AES spectrometer.The thermal analysis (TG/DTG) was performed on a SDT

2960 Thermal Analysis System (TA Instruments, Inc.). Themeasurements were carried out in static air with the heatingrate of 10 °C/min.Structural changes during the heating were investigated on a

PANalytical X’Pert PRO high-resolution diffractometer with CuKα1 radiation (λ = 1.5406 Å) in the range from 5 to 60° 2θusing a step of 0.034° per 100 s. Diffraction patterns wererecorded in steps of 50 °C from room temperature to 500 °C inair flow.Fourier-transform infrared (FT-IR) measurements were

performed on a Perkin-Elmer Spectrum One FTIR spectrom-eter with resolution of 1 cm−1 from self-supporting KBr pellets.

1H CRAMPS (combined rotation and multiple pulsesequence), and 1H−13C CPMAS (cross-polarization magic-angle spinning) NMR spectra were recorded on a 600 MHzVarian NMR system, operating at 1H Larmor frequency of599.87 MHz and 13C Larmor frequency of 150.815 MHz.Sample rotation frequencies for 1H CRAMPS and 1H−13CCPMAS experiments were 10 and 16 kHz, respectively. One-dimensional CRAMPS measurement employed a supercycledwindowed DUMBO homonuclear decoupling scheme.60 Forthe decoupling, the strength of the radiofrequency field was 166kHz and the duration of the entire supercycle was 61.2 μs. Thesampling was performed after each of the two DUMBO blockswithin the supercycle. Duration of the sampling window was3.2 μs. The 1H−13C CPMAS experiment employed RAMP61

during CP block and high-power TPPM heteronucleardecoupling62 during acquisition. Chemical shifts of 1H and13C signals were in all experiments referenced to thecorresponding signals of tetramethylsilane, which was used asan external reference. The chemical shift axis for the CRAMPSspectra was scaled so that peak positions within these spectramatched the peak positions within the MAS spectra. Thescaling factor was 2.08.Two-dimensional 1H−1H homonuclear correlation NMR

spectra were obtained by first exciting protons by a 90° pulseand letting the magnetization to evolve under the homonuclearDUMBO decoupling during t1. Afterward magnetization wasrotated back to z axis. During a mixing delay of 1 ms spin-diffusion among the protons occurred. After the read-out 90°pulse the signal was detected under the windowed DUMBO

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decoupling. The number of increments along the indirectlydetected dimension was 160, and the number of scans for eachincrement was 4. The experiment was carried out in ahypercomplex mode63 at 10 kHz sample rotation frequency.X-ray absorption spectra of analyzed materials were

measured in the Ca K-edge energy region (4039 eV) in thetransmission detection mode at the XAFS beamline at theELETTRA synchrotron facility in Basovizza, Italy. A Si(111)double-crystal monochromator with ∼0.5 eV resolution at CaK-edge was used. Higher-order harmonics were effectivelyeliminated by slightly detuning the second monochromatorcrystal, keeping the intensity at 50% of the rocking curve withthe beam stabilization feedback control. The intensity of themonochromatic X-ray beam was measured by three consecutiveionization chambers filled with (1) 85 mbar N2 and 1915 mbarHe, (2) 460 mbar N2 and 1540 mbar He, and (3) 880 mbar N2and 1120 mbar He. The samples were prepared ashomogeneous self-supporting pellets with the total absorptionthickness (μd) of about two above the Ca K-edge and mountedon a sample holder between the first and the second ionizationdetectors. The absorption spectra of the samples weremeasured in the energy region from 250 to 1000 eV abovethe Ca K-edge with the integration time of 1 s per step. In theXANES region, equidistant energy steps of 0.25 eV were used,whereas for the EXAFS region equidistant k steps (k = 0.03 Å)were adopted. Exact energy calibration was established with thesimultaneous absorption measurements on Sn metal foilinserted between the second and third ionization cells. Theanalysis of EXAFS spectra was performed with the IFEFFITprogram packages64 using FEFF6 code65 in which photo-electron scattering paths were calculated ab initio from apresumed distribution of neighboring atoms

■ RESULTS AND DISCUSSION

Structural Changes upon Heating. Ca(BDC)(DMF)-(H2O) material is built up from chains of edge-sharingpolyhedra with Ca2+ centers with eight-fold coordination.Polyhedra can be described as distorted bicapped prismsfrequently found in Ca-based carboxylates.35,36,44 Ca2+ cationsare coordinated to two bridging oxygen atoms in monodentatefashion and four chelating oxygen atoms all coming fromcarboxylate groups (Figure 1a). The remaining two oxygenatoms are coming from water and DMF molecules, respectively(Figure 1b). Inorganic chains running along three a directionsare connected with terephthalate ligands along the bc plane.The planes of the benzene rings form the angles of ∼67° withrespect to each other, thus forming a 3-D structure with parallel1-D rhombic channel system with the 44 net and morphology

similar to the already observed V-, Cr-, Al-, Fe-, In-, and Ga-based terephthalates (Figure 1c).37−42 Coordinated planarDMF molecule is perpendicular to the channel direction, fillingthe majority of the free channel space, whereas coordinatedwater occupies the cis position with respect to the DMFmolecule.Thermogravimetric analysis of Ca(BDC)(DMF)(H2O)

(Figure 2) shows weight losses up to 680 °C in four distinctive

steps. The first step with the loss of 3.2 wt % up to 80 °C is dueto the physisorbed or surface water removal. The second stepwith the loss of 15.4 wt % up to 200 °C should correspond tothe removal of coordinated water. However, theoretical loss ofmass due to the removal of one water molecule fromCa(BDC)(DMF)(H2O) is 5.9 wt %, which suggests that alsosome DMF is removed at the temperature up to 200 °C.Weight loss of 14.7 wt % in the third step, which occursbetween 200 and 400 °C, is due to the removal of theremaining quantity of DMF. The sum of the removed DMF inthe second and third steps is 23.2 wt %, which is in goodagreement with theoretically predicted loss of DMF fromCa(BDC)(DMF)(H2O) material (24.0 wt %). The last stepwith the weight loss of 33.8 wt % in the range between 440 and670 °C is attributed to the decomposition and removal ofterephthalate ligand. To explain the phenomena of DMFremoval in relatively discrete steps, further investigations ofthermally treated samples at 150 and 400 °C, that is, at thetemperatures where the major weight changes occur, wasrequired. The obtained samples are denoted as Ca-BDC(150)

Figure 1. (a) Ca2+ environment in Ca(BDC)(DMF)(H2O) framework. (b) Chain of Ca-based edge-sharing polyhedra with DMF moleculescoordinated perpendicular to the chain direction. (c) Structure along the a axis revealing 1-D rhombic channels with coordinated DMF molecules.Dark-blue circles, Ca atoms; red circles, O atoms; light-blue circles, N atoms; gray circles, H atoms.

Figure 2. TG (full line) and DTG (dotted line) curves ofCa(BDC)(DMF)(H2O) material.

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and Ca-BDC(400), respectively, whereas as-prepared materialis marked as Ca-BDC(RT).CHN and ICP analyses confirmed the findings derived from

TG measurements. In the Ca-BDC(RT) sample, the measuredelemental composition of 13.3 wt % Ca, 44.4 wt % C, 4.9 wt %H, and 4.2 wt % N agreed well with the calculated values forCa(BDC)(DMF)(H2O) (13.6 wt % Ca, 44.7 wt % C, 4.4 wt %H, and 4.3 wt % N). In the Ca-BDC(150) sample, themeasured composition of 16.9 wt % Ca, 47.4 wt % C, 3.7 wt %H, and 2.4 wt % N suggests that ∼55 wt % of the total amountof DMF molecules diffuse from the material at 150 °C. FromTG analysis, we assumed that all coordinated water is alreadyremoved at this temperature. Slightly higher contribution ofhydrogen found in Ca-BDC(150) in comparison with thetheoretical value for the Ca(BDC)(DMF)0.45 (3.0 wt %) ismost probably due to the water, which readily adsorbs on thesurface of the material under ambient conditions in the humidair. In the Ca-BDC(400) sample the nitrogen is no longerdetected by CHN analysis and the measured composition of

19.3 wt % Ca, 47.5 wt % C, and 2.6 wt % H matched thecalculated values for Ca(BDC) (19.6 wt % Ca, 47.1 wt % C,and 2.0 wt % H).The dynamics of DMF and water molecules during the

heating of the Ca(BDC)(DMF)(H2O) structure was addition-ally investigated by infrared spectroscopy. The FT-IR spectra ofCa-BDC(RT), Ca-BDC(150), and Ca-BDC(400) in thewavenumbers region between 2800 and 3500 cm−1 areshown in Figure 3 a. The spectrum of Ca-BDC(RT) showsvery broad band in the range between 3500 and 3100 cm−1

with the peak and the knee at approximately 3370 and 3280cm−1, respectively. Two contributions indicate the presence ofdifferent water moieties in the sample. The broad band with thepeak at 3370 cm−1 is most probably due to the surface water,whereas the extensively overlapped band at 3280 cm−1 can beattributed to the coordinated water molecules. The band issignificantly narrowed in the Ca-BDC(150) spectrum andalmost disappears in the Ca-BDC(400) spectrum. The peak at3280 cm−1 is no longer observed in Ca-BDC(150), which

Figure 3. FT-IR spectra of the Ca-BDC(RT) (black), Ca-BDC(150) (red), and Ca-BDC(400) (green) materials shown at (a) higher and (b) lowerwavenumber regions.

Figure 4. 1H−13C CPMAS (a) and 1H CRAMPS (b) NMR spectra of as-prepared Ca-BDC(RT) (black), Ca-BDC(150) (red) and Ca-BDC(400)(green). Asterisks in (a) denote spinning sidebands.

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suggests that coordinated water is removed at 150 °C. Thesurface water is still present in the thermally treated materialsdue to the fact that these IR investigations were not performedin situ and the samples apparently adsorbed some water fromthe atmosphere, which may cause partial hydrolysis of thematerials. The IR spectra of the Ca-terephthalate samples in thelower wavenumbers region are shown in Figure 3 b. Thespectrum of Ca-BDC(RT) material exhibits some characteristicbands that are in agreement with the described structure.Strong bands at 1563 and 1400 cm−1 are assigned toasymmetric and symmetric stretching vibrations of carbonylgroups, respectively. The absence of the band in the rangebetween 1680 and 1800 cm−1 indicates the presence of onlydeprotonated carboxylic groups. The band at 1657 cm−1 isassigned to CO stretching of the DMF molecule.45,46

Carboxylate anions remain completely deprotonated afterthermal treatment up to 400 °C, as indicated by the absenceof the band at ∼1700 cm−1 in FT-IR spectra of Ca-BDC(150)and Ca-BDC(400). In the spectrum of Ca-BDC(150), the bandcorresponding to CO stretching of DMF is shifted for 13cm−1 to higher wavenumbers (1670 cm−1) with respect to thecorresponding band within the Ca-BDC(RT) spectrum. Thissuggests that in the temperature range between 150 and 400 °CDMF remains in the structure in the uncoordinated, freeform.47 This particular band disappears after the thermaltreatment at 400 °C, indicating that DMF is completelyremoved from the material at that temperature.

1H−13C CPMAS and 1H CRAMPS NMR spectra (Figure 4)confirm the above-described hypothesis and provide someadditional information. The 1H−13C CPMAS spectrum of theas-prepared material exhibits eight narrow contributions,showing that the environment of carbon nuclei within Ca-BDC(RT) is very well-defined and thus the material is wellcrystalline. The contribution resonating at 175.5 ppm belongsto carboxyl carbon atoms, and the contributions resonating at139.0, 138.2, 130.6, and 129.6 ppm belong to aromatic carbonatoms of the BDC ligands. The peaks at 165.3, 35.0, and 31.6ppm belong to carbon atoms of the DMF molecules. 1HCRAMPS spectrum of the as-prepared materials is also verywell resolved. We can distinguish signals from four differenthydrogen atoms from the aromatic ring resonating at 8.4, 7.9,

7.1, and 6.0 ppm. Protons from the two methyl groups of theDMF molecules give rise to two resolved peaks resonating at1.6 and 0.7 ppm. The signal of the proton from the formamidegroup overlaps with the signals of the protons from thearomatic ring. A shoulder at 2.5 ppm and a broad contributionat ∼4.9 ppm can further be assigned to protons from thecoordinated and the physisorbed water molecules, respectively.No signals belonging to protons from the carboxyl groups canbe found, showing that indeed all carboxyl groups of the BDCligands form bonds with Ca atoms.

1H−13C CPMAS and 1H CRAMPS NMR spectra of Ca-BDC(150) differ substantially from the spectra of the as-prepared material. We first note that the signal of the BDCligands in the carbon spectrum splits into several lines but doesnot broaden substantially. This implies that carbon nuclei inthis sample experience different environments than the carbonnuclei in the as-prepared material and also that these numerousenvironments are well-defined, at least on the short-range scale.We can also note that intensities of the DMF signals ascompared with the intensities of the BDC signals drop by∼35% in Ca-BDC(150). This matches very well with theamount of DMF molecules removed by thermal treatment at150 °C, as observed by thermal analysis. The resultant absenceof DMF molecules in some parts of the crystals might beresponsible for the generation of new environments of theBDC carbon nuclei and thus for the “splitting” of carbon peaksdiscussed above. The difference in resonance frequencies ofcarbon nuclei of the two methyl groups of the DMF moleculesin Ca-BDC(150) is larger (6.0 pm) than it was in the as-prepared materials (3.4 pm) and is closer to the differenceobserved in the solution NMR spectra of DMF.66 Also, 1HCRAMPS spectrum of Ca-BDC(150) exhibits a single peak dueto protons from the two methyl groups. Both observationsindicated that the arrangement of DMF molecules in Ca-BDC(150) is different than it was in the as-prepared material.The two methyl groups are in Ca-BDC(150); either they arefurther away from the framework or the DMF molecules aremore mobile, resulting in the fact that protons of the methylgroups no longer experience two substantially differentenvironment.

Figure 5. Two-dimensional 1H−1H homonuclear correlation NMR spectra of (a) Ca-BDC(RT) and (b) Ca-BDC(150). Above each spectrum thereis a projection corresponding only to the area of the 2D spectrum that is depicted by the dotted line. Such projection enables easier comparison ofthe intensities of the cross-peaks and the diagonal peaks.

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NMR spectra of Ca-BDC(400) are more simple. The1H−13C CPMAS spectrum exhibits only three signals with aratio of intensities of 1:1:2. The spectrum shows that there isno DMF left in the sample and that the BDC molecules in theCaBDC(400) framework are in more symmetrical environmentthan the frameworks of Ca-BDC(RT) and Ca-BDC(150). The1H CRAMPS spectrum also confirms that DMF was completelyexpelled from the material.A further insight into the structure of Ca-BDC(150) sample

is obtained by the 2D 1H−1H homonuclear correlation NMRexperiment. The experiment provides unambiguous evidencethat DMF molecules interact much more weakly with theframework of Ca-BDC(150) than with the framework of Ca-BDC(RT) (Figures 5 and 6). Namely, the comparison of the

spectra of those two samples clearly shows that the cross-peaksbetween the protons of the DMF molecule and the protons ofthe BDC molecules are much stronger in Ca-BDC(RT) than inCa-BDC(150).After investigating the changes in the local environment of

the organic part of the materials (terephthalate and DMFmolecules), we also studied how the environment of Ca2+

changes during heating. The study was carried out using X-rayabsorption spectroscopy. Normalized Ca XANES spectra of thesamples were extracted by a standard procedure67 and areshown in Figure 7. The shapes of XANES spectra suggest thatduring the heating a change in coordination of Ca atoms occurswhen the sample is heated to 150 °C. Because the Ca-BDC(150) sample is a mixture of phases, we could notdetermine the exact coordination number of Ca in theconstituents using only XANES. However, in combinationswith other analyses done on the sample the results pointtoward an average coordination number of seven. A furtherdecrease in Ca coordination to six-fold can be observed for Ca-BDC(400).More information about the local environment of Ca ions

was obtained from the EXAFS part of the spectra, which werequantitatively analyzed for the coordination number, distance,type of neighboring atoms, and the Debye−Waller factors for

the nearest coordination shells of Ca atoms. Recorded spectraalong with the best fit EXAFS models are shown in Figure 8.In the EXAFS analysis of the Ca-BDC(RT) spectrum, the

crystallographic data were used as an input model. In theFigure 6. Two-dimensional 1H−13C heteronuclear correlation NMRspectrum of Ca-BDC(150). Horizontal dotted lines indicate thepositions of carbon signals that exhibit no cross-peaks with the DMFprotons. Leftmost is the projection of the 2D spectrum onto thecarbon axis. Compared with this projection is the 1H−13C CPMASNMR spectrum of Ca-BDC(400).

Figure 7. Normalized Ca K-edge XANES part of the absorptionspectra of Ca-BDC(RT), Ca- BDC(150), and Ca-BDC(400)materials. The changes in the absorption edge shape indicatingalterations in the local Ca environment are clearly visible.

Figure 8. k3-weighed Fourier-transformed Ca K-edge EXAFS spectraof Ca-BDC(RT) and the thermally treated Ca-BDC(150) and Ca-BDC(400). Experimental data (black line) are presented along withbest fit of the FT magnitude (red line) and imaginary part (blue line).

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model, all single-scattering (SS) paths and multiscattering (MS)paths with relative intensities of 10% or higher of the first Ca−O scattering path were used.68 Because the structure of Ca-BDC(RT) is known, we used EXAFS analysis of this sample toprove the validity of our approach and to determine parameterssuch as amplitude factor So = 1.17 and energy shift ΔE0 = 5.2eV, which were then kept constant for the analysis of theEXAFS spectra of Ca-BDC(150) and Ca-BDC(400). Thespectrum of Ca-BDC(RT) was analyzed in the range from 1 to4.8 Å in the R space and from 3.5 to 11.5 Å−1 in the reciprocalspace. The first coordination shell of Ca atoms (which in thiscase comprises eight O atoms) was fitted with separate distanceparameters and Debye−Waller (DW) factors to the rest of thescattering paths, which were also in the second coordinationshell: the carbon atom of the DMF molecule coordinated to Ca(CDMF) and the neighboring Ca atoms. While fitting, the rest ofthe crystal structure model was allowed to adapt to the EXAFSdata with the overall expansion parameter α and the overallDW factor σ2. The total number of O atoms in the firstcoordination shell was fixed to eight. The best fit gave two O

atoms at the distance of 2.32 Å and six O atoms at a longerdistance of 2.47 Å, which is consistent with the crystallographicdata.As it was suggested, coordination bonds between Ca2+ and

DMF molecule are already broken at 150 °C, but DMF stillremains trapped within the cores of the crystal. This results in aslightly deformed RT structure. For the EXAFS analysis of Ca-BDC(150), the starting model of Ca-BDC(RT) was adapted byusing seven O atoms in first coordination shell and by theremoval of CDMF atom and all MS paths. Because the O−Ca−Oangles were expected to change due to the subsiding of thechannels, the distance and DW-factor of C atoms closest to Cawere added to the fit. The remaining paths were allowed toadapt by an overall extension and DW factors. The fit of themodified model described the average environment of Ca2+

better than the one with the coordinated DMF molecules. Thebest fit was obtained with two oxygen atoms at a distance of2.30 Å and five oxygen atoms at a distance of 2.44 Å. It shouldbe noted that the EXAFS spectrum reflects an average Caenvironment and that it cannot distinguish the contributions

Table 1. Summary of EXAFS Refinement Parameters Obtained on the Ca-BDC(RT) (Δk = 3.5−11.5 Å−1; ΔR = 1.0−4.8 Å), Ca-BDC(150) (Δk = 3.8−11.5 Å−1; ΔR = 1.0−4.0 Å), and Ca-BDC(400) (Δk = 3.5−11.5 Å−1; ΔR = 1.0-3.9 Å) in Comparison withCrystallographic Data Obtained by XRD Analysisa

Ca-BDC(RT)

crystallographic data EXFAS experiment

path N R (Å) path N R (Å) σ2 (Å2)

Ca−O1a 2 2.372(3)Ca−O1b 1 2.376(6) Ca−O1 2 2.324(9) 0.005(1)Ca−O1c 1 2.418(6)Ca−O1d 2 2.518(4) Ca−O2 6 2.469(5) 0.010(1)Ca−O1a 2 2.565(4)Ca−C1 1 2.879(6)Ca−C2 1 2.881(6)Ca−CDMF 1 3.36(1) Ca−CDMF 1 3.27(5) 0.009(7)Ca−C2 2 3.597(2)Ca−Ca 2 3.9388(9) Ca−Ca 2 3.89(3) 0.016(3)

α 0.001(7)σ2 0.017(5)

Ca-BDC(150)

EXFAS experiment

path N R (Å) σ2 (Å2)

Ca−O1 2 2.292(9) 0.004(1)Ca−O2 5 2.438(7) 0.009(1)Ca−C1 1 3.22(4) 0.013(6)Ca−C2 1 3.22(4) 0.013(6)Ca−C2 2 3.93(1) 0.013(6)Ca−Ca 2 3.73(3) 0.017(4)α 0.012(7)σ2 0.006(3)

Ca-BDC(400)

crystallographic data EXFAS experiment

path N R (Å) path N R (Å) σ2 (Å2)

Ca−O1 2 2.234(2) Ca−O1 2 2.30(1) 0.002(1)Ca−O2 4 2.430(4) Ca−O2 4 2.45(2) 0.007(1)Ca−C 6 3.256(9) Ca−C 6 3.27(4) 0.017(5)Ca−Ca 2 3.832(2) Ca−Ca 2 3.86(6) 0.014(6)

α −0.01(1)σ2 0.016(8)

aUncertainty of the last digit is given in the parentheses.

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from the two domains, which were proposed to exist in Ca-BDC(150) by NMR analysis.The analysis of the EXAFS spectra of Ca-BDC(400)

confirmed the six-fold coordination of Ca atoms proposed byRietveld refinement of the powder X-ray diffractogram,described later in the text. In the first coordination shell twoO atoms were found at a distance of 2.30 Å and four O atomswere found at a distance of 2.45 Å from the Ca cations. Carbonatoms from the carboxylic groups coordinated to Ca2+ werefound at 2.27 Å. The distances between Ca2+ cations andneighboring atoms obtained by EXAFS analysis along with therefinement parameters for Ca-BDC(RT), Ca-BDC(150), andCa-BDC(400) are given in Table 1. The results are comparedwith the crystallographic data extracted from XRD analysis.Note that the phase transformation at 150 °C is nothomogeneous and that the Ca-BDC(150) does not representa uniform phase. Therefore, the crystallographic data for Ca-BDC(150) are not given in the Table. However, findings ofEXAFS analysis for this transformation are still importantbecause they provided very strong indication that the bondbetween DMF and Ca2+ breaks at 150 °C. The more detailedexplanation of the phase transformation at 150 °C will bedescribed later in the text.Complementary spectroscopic investigations (FT-IR, NMR,

XAS) along with the TG measurements offered good insightinto local structural changes of Ca(BDC)(DMF)(H2O)material during thermal treatment. Coordination bondsbetween Ca2+ ions and oxygen atoms belonging to H2O andDMF molecules are broken at approximately the sametemperature (ca. 150 °C). Spectroscopic results suggest thatCa-BDC(150) contains either two separate phases or two typesof domains within the crystals. Because the prolonged heatingtime at 150 °C did not enhance the removal of DMF, theoccurrence of different domains in one crystal is more probable.At 150 °C, a slight framework rearrangement seems to occur,because of which only water molecules can completely desorbfrom the channels. DMF molecules with larger kinetic diameterand lower mobility than water diffuse more slowly. Partialstructural changes of Ca(BDC)(DMF)(H2O) that occur at 150°C seem to block the channels within the material and allowonly the desorption of the DMF molecules, which are on orclose to the surface of the crystals. The rest of the free DMF

molecules remain trapped within the blocked channels and onlyafter the next structural rearrangement at 400 °C doesdesorption of the remaining DMF occur. The DMF moleculeslocated in the cores of the crystals are released after thecracking of the crystals, which apparently appear at ∼350 °Cand were confirmed by SEM observations. (See the SupportingInformation.)Information about the changes of the local environment of

the carbon atoms and calcium ions during heating enabled us togo one step further and to explain the entire crystal structuretransformations. Structural changes during heating weremonitored in situ by XRD measurements. XRD patternsmeasured at given temperatures are shown in Figure 9. Phasetransitions at 150, 400, and 550 °C are in accordance withweight losses observed by TG measurements. An attempt todetermine the crystal structure of Ca-BDC(150) failed becausethe reflections in XRD pattern could not be indexed with theproper reliability due to the presence of different domains. Thediffraction pattern of Ca-BDC(150) namely shows thereflections corresponding to Ca-BDC(RT), which are shiftedtoward higher angles, and also the reflections belonging to Ca-BDC(400). As already proposed, the outer domains of thecrystals, where DMF is initially removed, are most probablytransformed to the Ca-BDC(400) phase, which will bedescribed later in the text. The cores of the crystals, whereDMF is still trapped within the structure, seem to be subjectedto only minor framework rearrangement. As indicated by theshifting of the corresponding XRD reflections, the rhombic-shaped channels contract along one direction and expand alongthe other; however, the presence of free DMF molecules withinthe channels prevents substantial channel subsiding andcomplete transformation to the Ca-BDC(400) phase. Theprocess is completed only at 400 °C, when the crystals crackand DMF leaves the channels.The crystal structure of Ca-BDC(400) phase, determined by

the support of NMR and EXAFS findings, was successfullyrefined by Rietveld method. The structure contains infinitechains of edge-sharing CaO6 octahedra running along the b axis.Note that Ca2+ occurring in octahedral environment is a rarityin Ca-based metal−organic frameworks, and it has beenreported only a few times.49,51 CaO6 octahedra are bondedthrough terephthalate linkers along the a axis. Such arrange-

Figure 9. High-temperature XRD patterns of Ca(BDC)(DMF)(H2O) material measured at every 50 °C. Broad peak at ∼7° 2θ is due to the foilcovering during the measurements.

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ment of terephthalate ligands and CaO6 building unit exhibitsnonporous 3-D crystal structure.Once we knew the structure of Ca-BDC(400) we were able

to predict the mechanism of structural transformation, whichoccurs between 350 and 400 °C. The removal of free DMFmolecules trapped within the channels enables further closingand subsiding of the channels. Consequently, 1-D CaO6 chainsmove closer and closer to each other along the b direction.Terephthalate ligand that is initially linked to two neighboringCaO6 chains in bidentate, chelating fashion rotates for ∼90°around the axis, which runs through the carboxylate carbonatoms in ortho positions of the benzene ring, and recoordinateswith one carboxylate oxygen atom on each side of the ligand totwo additional CaO6 edge-shared chains. Thus, after thetransformation the BDC ligand connects four Ca-based chainsin monodentate fashion. The proposed mechanism of crystalstructure transformations during the thermal treatment isschematically represented in Figure 10.

Structural Changes upon Hydration. Although the Ca-BDC(400) phase possesses a nonporous framework, whichseems to be relatively rigid, it still shows some structuralflexibility upon hydration/dehydration processes. When it isexposed to the controlled humid environment (relativehumidity of 80%) for 24 h, it completely transforms to ahydrated Ca(BDC)(H2O)3 phase that was already previouslyreported.56 The transformation is indicated by the XRDpatterns shown in Figure 11. Treatment of the hydratedphase in vacuum at slightly elevated temperatures (ca. 50 °C)causes reversible transformation back to the Ca-BDC(400)phase. The reversibility of the hydration/dehydration process isclearly also demonstrated by 1H−13C CPMAS NMR spectros-copy (Figure 11). Ca2+ in the octahedral environment, which isgenerated in Ca-BDC(400) after the removal of coordinatedwater and DMF molecules, obviously has a high tendency tobond additional ligands (water molecules) to increase itscoordination number. This presumption can be supported by

Figure 10. Schematic representation of structural transformations of Ca-terephthalate from Ca-BDC(RT) to Ca-BDC(400) with the correspondingmeasured and calculated XRD patterns.

Figure 11. X-ray powder patterns (left) and 1H−13C CPMAS NMR spectra (right) of (a) Ca-BDC(400), (b) hydrated sample Ca(BDC)(H2O)3 atrelative humidity of 80%, and (c) dehydrated sample of panel b in vacuum at 50 °C.

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the fact that the majority of Ca-based carboxylate structurespossess Ca2+ centers with higher coordination numbers thensix. High tendency for additional coordination with water maybe the driving force for the transformation into the hydratedphase. Ca-BDC(400) ↔ Ca(BDC)(H2O)3 structural trans-formation involves breaking and reformation of Ca−O bonds.More precisely, the process involves rotation of the BDCligand, disconnection, and protonation of the carboxylate groupon one side and partial disconnection and reformation of Ca−O bond on the other side of the BDC ligand. Thus,terephthalate ligand that connects four CaO6 chains inmonodentate fashion in Ca-BDC(400) structure becomesconnected to one CaO8 chain in a bidentate way withdeprotonated COO− group on one side, whereas thecarboxylate on the other side becomes protonated. Theoccurrence of two distinct carboxylate groups upon hydrationis clearly demonstrated by 1H−13C CPMAS NMR spectrosco-py. The single carboxyl peak in the spectrum of Ca-BDC(400)splits into two equally strong and resolved peaks in thespectrum of Ca(BDC)(H2O)3. This structure is stabilized topseudo 3-D structure by hydrogen bonds between coordinatedwater and protonated carboxylates. Dehydration obviouslytriggers the reverse structural transformation mechanism:disconnection of Ca−O bonds from one carboxylate group,rotation of BDC ligand, deprotonation of carboxylate group,and reformation of Ca−O bond. The scheme of the proposedmechanism of Ca-BDC(400) ↔ Ca(BDC)(H2O)3 structuraltransformation is shown in Figure 12.

■ CONCLUSIONSWe described the structural dynamics of Ca(BDC)(DMF)-(H2O) MOF-type material with rhombic-shaped channels and44 net topology upon heating and hydration process.Mechanisms of structural changes were elucidated in situ byXRD analysis supported by complementary spectroscopictechniques (FT-IR, 1H−13C CPMAS NMR, 1H CRAMPSNMR, and XAS). During the heating, Ca(BDC)(DMF)(H2O)framework undergoes two phase transitions before the finalframework decomposition. First structural change to Ca-BDC(150) at 150 °C causes breaking of Ca−O(H2O) and

Ca−O(DMF) bonds and leads to the nonuniform phasetransition. DMF molecules diffuse from the outer parts of thecrystals, and the emptied domains are transformed to the Ca-BDC(400) phase. The cores of the crystals seem to consist ofdifferent domains. Nonporous nature of Ca-BDC(400)structure prevents the removal of DMF from the cores of thecrystals. Here the free DMF molecules that are trapped withinthe channels disable the transformation to Ca-BDC(400) andallow only minor subsiding of the channels. The removal of theremaining DMF from the cores of the crystal occurs at 400 °C,where the transformation to Ca-BDC(400) is completed.Exposure of Ca-BDC(400) to humid environment leads to theformation of Ca(BDC)(H2O)3. The structural changes uponhydration/dehydration were found to be reversible.We showed that for the elucidation of structural dynamics of

MOFs induced by external stimuli the complementaryspectroscopic methods are an indispensable support todiffraction analysis, particularly in the cases where the latterdata are not of sufficient quality for ab initio studies. Thedetermination of structural dynamics of Ca(BDC)(DMF)-(H2O) upon external stimuli not only enabled us to proposethe mechanisms of transformations but also enabled us toelucidate the reason for such behavior. Even though Ca(BDC)-(DMF)(H2O), like MIL-53, possesses the 44 net topology, itdoes not show any breathing effect upon heating or hydration.Instead, it undergoes significant crystal-to-crystal transforma-tions leading to the new nonporous phases. The reason for thedifferent behavior of Ca(BDC)(DMF)(H2O) lies in thetendency to change the carboxylate-to-Ca2+ coordinationmode when the coordination number of Ca2+ is changed. Asit was shown by Williams et al.,49 chelating−bridgingcoordination mode of COO− groups preferentially occurswhen Ca2+ is in environment with higher coordination number(7 to 9), whereas the monodentate−bridging mode is found inoctahedral geometry. In the Ca-BDC(RT) structure, Ca2+

occurs in eight-fold coordination and COO− are indeedbidentately coordinated to Ca cations in chelating−bridgingmode. Heating to 400 °C removes the coordinated water andDMF molecules, and consequently the coordination number ofCa2+ in Ca-BDC(400) is reduced to 6. In such an environment,COO− groups tend to coordinate to Ca2+ in monodentate−bridging mode, which causes the rotation of terephthalateligand for ∼90° and recoordination in monodentate fashionconnecting two neighboring edge-sharing CaO6 chains.However, because of its ionic radius (106 pm), Ca2+ prefers ahigher coordination number than 6. Thus in MOFs, Ca cationsin octahedral environment have a high tendency for additionalligand bonding (typically water molecules). Indeed, Ca-BDC(400) is stable only at elevated temperatures, in inertatmosphere, or in vacuum. In humid environment, Ca2+ willcoordinate to an additional three water molecules andtransform to Ca(BDC)(H2O)3. Nine-fold coordination ofCa2+ favors the carboxylate-to-Ca2+ coordination in thebridging−chelating mode, and thus Ca-BDC(400) → Ca-(BDC)(H2O)3 transformation undergoes a reversible pathwayto the one observed in the Ca-BDC(RT) → Ca-BDC(400)transformation.The explanation of structural transformations by specific

carboxylate-to-Ca2+ interactions can be projected also to otherCa-MOF systems. Such transformations are most probably thereason for the lack of porosity of those systems. In the majorityof Ca-MOF structures, the inorganic building units representchains of edge- or face-shared polyhedra with seven-, eight-, or

Figure 12. Schematically depicted mechanism of the reversiblestructural transformation of Ca-BDC(400) upon hydration/dehydra-tion. Hydrogen bonds in Ca(BDC)(H2O)3 are represented by dottedlines.

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nine-fold coordination of Ca2+. In such configuration ofinorganic units, coordination of COO− to metal is preferablyin the chelating−bridging mode and Ca2+ is additionallycoordinated to solvent molecules. The removal of solvent andthus the decrease in the coordination number to six does notcause only the change of the pore dimensions (contraction orexpansion), as in the case of the flexible MIL-53 system, but itleads to the significant irreversible crystal-to-crystal trans-formation due to the tendency to change the coordinationmode between Ca2+ and carboxylate group.

■ ASSOCIATED CONTENT*S Supporting InformationRefinement details and crystallographic data of atomicpositions, bond lengths and angles for Ca-BDC(400),ORTEOP scheme of Ca-BDC(400) structure, simulated andmeasured XRD patterns of Ca-BDC(400), SEM micrographs ofCa-BDC(RT), Ca-BDC(150), and Ca-BDC(400). This ma-terial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: matjaz.mazaj@ki.si.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Slovenian Research Agencyresearch programme P1-0021.

■ ABBREVIATIONSMOFs, metal−organic frameworks; SBU, secondary buildingunit; CTA, crystal-to-amorphous; CTC, crystal-to-crystal;BDC, 1,4-benzenedicarboxylic acid; TEA, triethylamine;DMF, N,N′-dimethylformamide; NMR, nuclear magneticresonance; XRD, X-ray diffraction; CPMAS, cross-polarizationmagic-angle spinning; CRAMPS, combined rotation andmultiple pulse sequence; TG, thermogravimetric analysis;DTG, derivative thermogravimetric analysis; XAS, X-rayabsorption spectroscopy; XANES, X-ray absorption near-edgespectroscopy; EXAFS, extended X-ray absorption finestructure; MIL, Materials of Institute Lavoisier

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