Control of the Crystallization Process and Structure Dimensionality of

Article pubs.acs.org/crystal

Control of the Crystallization Process and Structure Dimensionality of Mg−Benzene−1,3,5-Tricarboxylates by Tuning Solvent Composition Matjaž Mazaj,*,† Tadeja Birsa Č elič,† Gregor Mali,†,‡ Mojca Rangus,† Venčeslav Kaučič,†,§ and Nataša Zabukovec Logar†,§ †

National Institute of Chemistry, Hajdrihova 19, 1001 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: Four new magnesium 1,3,5-benzenetricarboxylate metal−organic framework materials (NICS-n; n = 3−6) were synthesized solvothermally in the presence of solvents with different EtOH/H2O ratios. We showed that the crystallization process of the Mg−1,3,5-benzentricarboxylate system strongly depends on the solvent composition, and that dimensionality of their structures can be tuned by changing the EtOH/water ratios in the reaction mixture. The presence of only water as a solvent yields the zero-dimensional molecular structure of Mg(H2BTC)2(H2O)4 (NICS-3). One-dimensional (1D) chainlike Mg3(BTC)2(H2O)12 (NICS-4) and two-dimensional (2D) layered Mg2(BTC)(OH)(H2O)4·2H2O (NICS-5) structures were crystallized from EtOH/H2O mixtures with molar ratios of 0.3 and 0.4−0.7, respectively. The crystallization in pure ethanol yields Mg3(BTC)2 material (NICS-6) with three-dimensional structure. Nuclear magnetic resonance investigations indicated that bulkier clusters of Mg species are formed in ethanol-rich solutions, even in the absence of the BTC ligand, and that the starting precursors formed with the reaction of Mg species and the BTC ligand at room temperature does not represent the final structures obtained by solvothermal reactions. NICS-4 and NICS-5 are formed from similar starting precursors but slightly different EtOH/H2O ratios causing the crystallization to go in two different directions. Systematic investigation of phase formation using different EtOH/H2O ratios, times, and temperatures of the synthesis along with the computational DFT studies confirmed that the 2D NICS-5 structure represents a thermodynamically more stable phase than 1D chainlike NICS-4. We showed that solvothermal reaction between Mg-precursors and the BTC ligand in EtOH/water mixture represents a complex and sensitive thermodynamic process.

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dimensionality of the final structure.23−26 Temperature of the synthesis is the external stimulus having an impact on framework topology and crystal structure dimensionality; however, the systematic studies of the temperature dependence on framework dimensionalities are rare.27−32 In general, higher reaction temperature increases the dimensionality of the framework due to the increased tendencies for M−O−M linkage formation.33−38 The nature of the ligand can also influence the dimensionality of the MOF framework. It was found out that higher flexibility of the ligand molecules and the presence of additional functional groups which enhance the acidity of the ligand molecules (e.g., fluorinated ligands) can contribute to the formation of the metal−organic frameworks with higher dimensionalities.39−42 The solvent used in the synthesis is another parameter which plays an important role in crystallization of MOFs since it governs reaction kinetics and thermodynamics during the coordination process.22 The

INTRODUCTION Metal−organic framework (MOF) materials have developed into an important class of porous materials due to their versatility in crystal structures and chemical compositions, which enable a wide range of applied properties such as adsorption, ion-exchange, separation, and catalysis.1−5 Ability to control the assembly of inorganic building blocks with organic linker molecules is beneficial for crystal engineering of MOFs and enables controlled design of framework structures with the desired properties.6 Even though the crystallization of MOFs generally undergo a complex thermodynamical process, it can be controlled by several synthesis parameters, such as the solvent type, pH of the reaction mixture, the presence of counterions, concentration of precursors, or temperature and time of reaction.7−22 The self-assembly of inorganic and organic building blocks is primarily influenced by the shape and connectivity of the ligand and coordination geometry of the metal cations. However, it was already shown that the abovementioned external parameters also play an important role in the packing of building blocks and can govern the © XXXX American Chemical Society

Received: June 21, 2013

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dx.doi.org/10.1021/cg400929z | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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corresponding signals of 1 M MgCl2 and tetramethylsilane, respectively. Computational. Ground-state energies of NICS-3 and NICS-4 were calculated using the density functional theory in the generalized gradient approximation of Perdew−Burke−Ernzerhof (GGA PBE) with plane wave basis and ultrasoft pseudopotentials, as implemented in the CASTEP package (Accelrys). The plane-wave cutoff energies were 610 eV, and the reciprocal-space sampling was performed with the k-point grid of 2 × 2 × 4 points for NICS-3 and 2 × 2 × 1 points for NICS-4. For the initial cell parameters and atomic positions of NICS-3 and NICS-4, the data obtained by the crystallographic analysis were taken. The structures were then fully relaxed (cell parameters, cell volume, and atomic positions), and the final energies of the optimized geometries were recalculated so as to correct for the changes in the basis set of the wave functions during relaxation. In the next step, the relaxed structure calculations were performed at various constant volumes and the energy-volume data were fitted to parabola. Synthesis. All chemicals were commercially available and used as received. Magnesium acetate tetrahydrate [Mg(ac)2·4H2O, 99%] was purchased from Fluka; 1,3,5-benzenetricarboxylic acid (BTC, 95%) and ethanol (EtOH, 99%) were purchased from Aldrich. All crystalline solids were collected by filtration, continuously rinsed with deionized water, and dried at ambient conditions. Mg(H2BTC)2(H2O)4 (NICS-3). NICS-3 crystallized from the mixture of 0.65 g (3 mmol) of Mg(ac)2·4H2O and 0.65 g (3 mmol) of 1,3,5benzenetricarboxylic acid (BTC) in 10 mL of deionized water. Reaction mixture with molar composition of reagents Mg(ac)2·4H2O:BTC: 160 H2O was hydrothermally treated at 150 °C for 24 h. Elemental composition calculated for C18H18O16Mg: 43.5 wt % C, 51.6 wt % O, 4.9 wt % Mg. Found: 44.4 wt % C, 50.6 wt % O, 5.0 wt % Mg. IR (νmax/cm−1): 3522s, 3430s, 3142s, 3103s, 1705s, 1643m, 1613s, 1556s, 1426m, 1368w, 1229s, 975w, 759s, 715s, 566m, 528m, 470m. Mg3(BTC)2(H2O)12 (NICS-4). 0.65 g (3 mmol) of Mg(ac)2·4H2O and 0.65 g (3 mmol) of 1,3,5-benzenetricarboxylic acid were dissolved in 5.5 mL of deionized water and 8.8 mL of ethanol, respectively. Subsequently, both solutions were combined together. Obtained white gel with molar ratios of components Mg(ac)2·4H2O:BTC: 50 EtOH:100 H2O was solvothermally treated at 150 °C for 24 h. Elemental composition calculated for C18H30O24Mg3: 32.1 wt % C, 57.0 wt % O, 10.8 wt % Mg. Found: 30.6 wt % C, 5.0 wt %, 59.4 wt % O, 10.0 wt % Mg. IR (νmax/cm−1): 3479b, 3166b, 1686m, 1657m, 1609s, 1561s, 1528s, 1474s, 1436s, 1378s, 1138w, 850w, 743s, 681m. Mg2(BTC)(OH)(H2O)4·2H2O (NICS-5). NICS-5 material was prepared by analogous procedure as NICS-4, except that BTC was dissolved in 12.3 mL of ethanol. The reaction mixture with molar ratios of components Mg(ac)2·4H2O:BTC: 70 EtOH:100 H2O was solvothermally treated at 175 °C for 24 h. Elemental composition calculated for C9H16O13Mg2: 29.6 wt % C, 57.0 wt % O, 13.3 wt % Mg. Found: 30.6 wt % C, 54.4 wt % O, and 15.0 wt % Mg. IR (νmax/cm−1): 3479b, 3167b, 1681m, 1609s, 1566s, 1522s, 1474s, 1436s, 1378s, 1109w, 970w, 763s, 715s, 566m, 528m, 465m. Mg3(BTC)2 (NICS-6). NICS-6 was prepared using only ethanol as the solvent. 0.65 g (3 mmol) of Mg(ac)2·4H2O and 0.65 g (3 mmol) of 1,3,5-benzenetricarboxylic acid was separately dissolved in 5 and 12.5 mL of ethanol, respectively. The solutions were combined together under vigorous stirring. Obtained white gel with molar ratios of components Mg(ac)2·4H2O:BTC: 100 EtOH was solvothermally treated at 190 °C for 24 h. Elemental composition calculated for C18H6O12Mg3: 44.9 wt % C, 39.9 wt % O, 15.2 wt % Mg. Found: 44.4 wt % C, 39.1 wt % O, 16.5 wt % Mg. IR (νmax/cm−1): 3435b, 3060m, 1681m, 1623s, 1585s, 1441s, 1374s, 1119w, 955w, 816w, 773s, 711s, 566m, 460m. Crystal Structure Determination. Crystal structures of the MgBTC compounds were determined from single-crystal X-ray diffraction data collected on Nonius Kappa diffractometer with Mo Kα radiation (λ = 0.71073 Å). Single crystals were isolated and glued at the top of the glass fiber. The structures were solved by direct methods with the SHELXS-86 software package57 and subsequent difference Fourier map calculations. All of the nonhydrogen atoms were anisotropically

solvent molecule can act as a ligand or as a guest and has significant impact on building block assembly processes in both cases.43 The role of solvent on dimensionality of MOFs was noted by Yaghi et al.44 Since then, several works have been dealing with systematic studies of the solvent role on the dimensionality of MOF structures. For example, the influence of water content on MOF topologies was discussed for Co- and Cu-based pyridine-3,5-dicarboxylates.45 The effect of water/ methanol ratio on the MOF formation was investigated for Mnand Zn-based amino acid derivatives.46 Similarly, systematic exploration of water/dimethylacetamide ratio influence on coordination assembly was performed for a zinc terephthalate system.47 It was shown that topologies of Zn-MOFs built with trimesate ligands and their derivatives can be controlled by using various organic solvents.48,49 Among numerous coordination polymer structures, magnesium-based MOFs deserve special attention due to their potential advantages over the transition-metal-based MOFs. The Mg2+ cation possesses a similar radius as Fe2+, Co2+, Ni2+, and Zn2+ and preferably adopts a 6-fold coordination in the octahedral environment. Because Mg plays an important role in biological processes, biocompatible crystalline solids can be interesting for drug delivery applications. Moreover, magnesium is a light element contributing to low densities of MOF solids which can bring high uptake capacity values of small gas molecules such as hydrogen. In spite of the above-mentioned advantages, Mg-based MOFs are relatively unexplored compared with similar transition metal coordination polymers. Consequently, there is a lack of systematic studies dealing with influences of synthetic parameters on Mg-MOFs formation.32,50−54 In this contribution, we describe crystal structures of four new Mg-based 1,3,5-benzenetricarboxylate (BTC) materials and investigate the effect of solvent ethanol/water molar ratios on the dimensionalities of MOF structures. The self-assembly of MOF building units was additionally explained by 25Mg nuclear magnetic resonance (NMR) and 1H−13C CPMAS NMR investigations of the starting reaction precursors.

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EXPERIMENTAL SECTION

General Characterization Methods. Morphologies and size of the crystals of obtained phases were observed by scanning electron microscopy (SEM) measurements on a Zeiss Supra 3VP field-emission gun (FEG) microscope. Elemental analysis was performed by energy dispersive X-ray analysis (EDAX) with an INCA Energy system attached to the above-described microscope. TG analyses of Mg-BTC materials were performed on a SDT 2960 Thermal Analysis System (TA Instruments, Inc.). The measurements were carried out in nitrogen flow with a heating rate of 10 °C min−1. Fourier transform-infrared (FT-IR) measurements were performed on a Perkin-Elmer Spectrum One FTIR spectrometer. The samples were ground together with KBr and compressed into self-supported pellets. The resolution of the measurements was set to 1 cm−1. 25 Mg static and 1H−13C CPMAS (cross-polarization magic-angle spinning) NMR spectra were recorded on a 600 MHz Varian NMR system equipped with a 3.2 mm Varian NB Double Resonance HX MAS probe. Larmor frequencies for 25Mg and 13C nuclei were 36.71 and 150.815 MHz, respectively. 25Mg nuclei were excited by a single 90 degree pulse with a duration of 5.2 μs. Repetition delay was 0.25 s and number of scans was 16000. Sample rotation frequency for 1 H−13C CPMAS experiment was 16 kHz. The experiment employed RAMP55 during CP block and high-power XiX proton decoupling56 during acquisition. The contact time was 1 ms and repetition delay was 5 s. Chemical shifts of 25Mg and 13C signals were referenced to the B



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Table 1. Crystal Data and Structure Refinement Details for Mg-BTC Compounds empirical formula molar weight (g mol−1) crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z T (°C) wavelength (nm) Dcalc (g cm−3) F(000) R1 (all data) wR2 (all data) GOF

NICS-3

NICS-4

NICS-5

NICS-6

C18H18O16Mg 514.63 monoclinic P21/c (14) 5.1286(1) 12.9832(4) 15.1717(4) − 97.17(5) − 1002.32(5) 2 20 0.71073 Å (Mo Kα) 3.410 1064 0.0569 0.1086 1.152

C18H30O24Mg3 704.36 monoclinic C2 (5) 17.2532(5) 12.9657(4) 6.6115(2) − 111.443(2) − 1376.62(7) 2 20 0.71073 Å (Mo Kα) 1.697 732 0.0465 0.0841 1.055

C9H16O13Mg2 380.84 triclinic P1̅(2) 7.8423(5) 8.7463(5) 10.7163(8) 101.274(3) 91.868(3) 96.042(3) 715.79(8) 2 20 0.71073 Å (Mo Kα) 1.758 392 0.1233 0.2508 1.143

C18H6O12Mg3 487.16 triclinic P1̅(2) 7.7922(2) 7.7932(2) 7.7929(2) 66.135(4) 66.148(4) 66.148(4) 379.06(2) 1 20 0.71073 Å (Mo Kα) 1.054 126 0.0854 0.2111 1.074

Figure 1. A view of a NICS-3 [MgO2(H2O)4] complex in the ORTEP-type drawing with an atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. refined. Hydrogen atom locations were inferred from neighboring oxygen and carbon sites or from difference Fourier maps and treated by a mixture of independent and constrained refinement. Crystal parameters, data collection and refinement results for all compounds are summarized in Table 1. All atomic coordinates, selected bond lengths, and angles along with atomic displacement parameters are available in the Supporting Information. The supplementary crystallographic data for this paper (CCDC 945045, 768993, 768994, 768995) can also be obtained free of charge from The Cambridge Crystallograpic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ality of the obtained products is increased with the increased amount of ethanol in the solvent mixture. Scanning electron micrographs (SEM) of all products are available in the Supporting Information. Crystal Structure Description. NICS-3 represents a zerodimensional (0D) complex structure with mononuclear MgO2(H2O)4 octahedra bonded to two crystallographically identical H2BTC ligands in monodentate fashion in the trans positions and four water molecules (Figure 1). Each trimesate ligand possesses one bonding and two protonated carboxylate groups, leading to a single negative charge on the ligand. The Mg−O distance of oxygen atom belonging to carboxylate groups (Mg1−O2) is 2.1031(1) Å, whereas Mg−O distances of two oxygen atoms from water molecules are somewhat shorter [Mg1−O7 = 2.0678(2) and Mg1−O8 = 2.0650(2) Å]. The carboxylate groups of adjacent H2BTC moieties are involved in strong hydrogen bonds via the O4 atom [O4···H5 = 1.6269(3) Å]. Other important hydrogen bonds are present between

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RESULTS AND DISCUSSION Four new Mg-based 1,3,5-benzenecarboxylates (NICS-n, n = 3−6) were obtained using the mixture of H2O/EtOH solvents with volume ratios 1:0, 1:1.6, 1:2.2, and 0:1, respectively, keeping the Mg/BTC ratio constant. We found that the solvent composition has a strong influence on the formation of crystal structures and their dimensionalities. The structure dimensionC



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Figure 2. (a) A view of a NICS-3 along the a axis showing a network of hydrogen bonds forming a Mg-BTC layer. (b) A view along the b axis showing stacking of layers. Hydrogen bonds are represented as dotted lines (red dots, oxygen; black dots, carbon; and light gray dots, hydrogen).

terminal coordinated water molecules and carboxylate groups [O6...H7(A) = 1.8552(4) and O2...H7(B) = 1.8559(3) Å]. This set of strong hydrogen bonds leads to a pseudo threedimensional (3D) structure (Figure 2). A similar arrangement of M(H2BTC)2(H2O)4 units are already known for Fe-, Co-, and Mn-based trimesate MOF-type materials.58−60 NICS-4 with structural formula Mg 2 (BTC)(OH)(H2O)4·2H2O is isostructural with cobalt- and nickel-based trimesates.61,62 Crystal structure is one-dimensional (1D) and consists of chains of trimesate anions and isolated octahedra with magnesium centers (Figure 3). Asymmetric unit with unit cell axes is shown in Figure 4. Magnesium cations occupy two different crystallographic sites (Mg1 and Mg2). Mg2 cations are coordinated to two μ1-oxygen atoms from carboxylate groups (O2 and O4) in monodentate fashion with the distances of 2.043(2) and 2.033(2) Å, respectively, and four oxygen atoms from water molecules (O9, O10, O11, and O12) with the

Figure 4. ORTEP-type drawing at 50% probability level and labeled atoms showing part of the NICS-4 structure with unit cell edges. Hydrogen atoms are omitted for clarity.

expected longer distances [from 2.08(2) to 2.104(2) Å]. Mg− O distances correspond to the typical values for Mgcarboxylates.63,64 The Mg1 cation is coordinated to two oxygen atoms from the carboxylate group (O1) [Mg1−O1 = 2.131(2) Å] in bidentate fashion and four oxygen atoms corresponding to water molecules (O7 and O8) with the distances of 1.993(3) and 2.097(2) Å, respectively. Two of three trimesate ligands are fully deprotonated and connected to MgO2(H2O)4 octahedra in a monodentate way with two carboxylate groups and in a bidentate way with one carboxlate group. The remaining ligand is linked to magnesium with two carboxylate groups in a monodentate fashion, while one carboxylate group remains protonated. Oxygen atoms from carboxylate groups form several hydrogen bonds with terminal coordinated water molecules forming a pseudo 3-dimensional (3D) metal− organic framework. NICS-5 has a 2D layered crystal structure with structural formula Mg2(BTC)(OH)(H2O)4·2H2O. A major feature of the structure are corner- and edge-sharing tetramers of Mg-

Figure 3. A view of the chainlike NICS-4 along the c axis showing the connection of Mg-trimesate chains with hydrogen bonds, represented by dotted lines (red dots, oxygen; black dots, carbon; and light gray dots, hydrogen). D



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cis positions as terminal hydroxyl groups and water molecules with the distances Mg2−O10 = 2.080(6) Å and Mg2−O11 = 2.152(6) Å, respectively, whereas μ1-oxygen atoms coordinated to magnesium in edge-sharing octahedra occupy trans positions [Mg1−O8 = 2.142(7) Å, Mg1−O9 = 2.057(7) Å]. Neighboring Mg-trimesate layers are connected to each other by hydrogen bond connections established between unprotonated carboxylate groups and a coordinated hydroxyl group [O4...H14 = 1.9065(7) Å] as shown in Figure 7.

octahedra connected through a trimesate ligand forming 2D layers (Figure 5). Tetramer building unit consists of two edge-

Figure 5. A layer of Mg octahedral tetramers connected through trimesate ligands in the NICS-5 structure (red dots, oxygen; black dots, carbon; and light gray dots, hydrogen). Figure 7. A view of the NICS-5 structure showing stacking of Mgtrimesate layers connected through hydrogen bonds represented as dotted lines (red dots, oxygen; black dots, carbon; and light gray dots, hydrogen).

sharing Mg-centered octahedra connected with another two corner-sharing octahedra on each side of the edge. Such type of inorganic unit has already been observed in Ni2(BTC)F(H2O)4·2H2O coordination polymer,65 but to our knowledge, it is unique for magnesium carboxylates. The tetramer is linked with the bridging of four carboxylate groups in equatorial positions [Mg1−O2 = 2.047(6) Å, Mg1−O5 = 2.040(6) Å, Mg2−O1 = 2.018(6) Å, and Mg2−O6 = 2.020(6) Å] and two carbonyl groups in the trans positions (one above and one below) with slightly larger Mg−O distances [Mg2−O3 = 2.100(6) Å]. Connection of ligands with Mg-centered octahedral tetramer is represented in Figure 6. Magnesium in corner-sharing octahedra is coordinated to μ1-oxygen atoms in

NICS-6 with structural formula Mg3(BTC)2 consists of linear Mg-centered octahedra face-sharing trimers connected through a trimestate ligand, forming a 3D framework structure (Figure 8). Magnesium cations occupy two crystallographic sites (Mg1 and Mg2). Mg1 is coordinated to three μ1 oxygen atoms from the carbonyl group [Mg1−O1 = 1.975(5) Å, Mg1−O3 and Mg1−O5 = 1.978(5) Å] and three μ2 oxygen atoms with longer Mg−O distances [Mg1−O2 = 2.186(5) Å, Mg1−O4 = 2.186(5) Å, and Mg1−O6 = 2.197(5) Å]. Mg2 atom is coordinated to six μ2 oxygen atoms [Mg2−O2 = 2.072(4) Å, Mg2−O4 = 2.078(5) Å, and Mg2−O6 = 2.073(4) Å]. Although linear MgO6 trimer building units are quite common

Figure 6. ORTEP-type drawing at 50% probability level with labeled atoms showing a building unit of Mg-centered octahedral tetramer connected with trimesate ligands (hydrogen atoms are omitted for clarity).

Figure 8. 3D Network of NICS-6 structure along the a axis. E



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Figure 9. (a) Linear Mg3 face-sharing building unit in the NICS-6 structure and (b) a view of the structure showing connectivity of trimesate ligand to Mg atoms. Atoms are drawn in ORTEP-type at the 50% probability level.

Figure 10. TG (solid lines) and DTG (dashed lines) curves of (a) NICS-3, (b) NICS-4, (c) NICS-5, and (d) NICS-6 materials.

Thermal Behavior. Thermogravimetric analysis of NICS-3 (Figure 10a) shows weight loss of 13 wt % up to 250 °C due to the coordinated water removal, which is in good agreement with theoretical water loss for Mg(H2BTC)2(H2O)4 (14.0 wt %). Interestingly, NICS-4 and NICS-5 exhibit similar thermal behavior as can be indicated from thermograms shown in Figure 10 (panels b and c), respectively, but the similarity is only coincidental. In both cases, first weight losses of approximately 30 wt % occur between 100 and 250 °C and correspond to the removal of coordinated and adsorbed water

in Mg-based carboxylate structures, they are usually edgeshared,66 while the NICS-6 structure represents to our knowledge the first example of linear MgO6 trimer building units with common faces (Figure 9 a). Each oxygen atom in the trimer belongs to the carbonyl group of one trimesate ligand [C8−O3 = 1.255(7) Å and C8−O4 = 1.261(8) Å]. Twelve trimesate ligands connected to one Mg3 unit and, consequently, six Mg3 units connected to one trimesate ligand (Figure 9 b) give rather a dense, nonporous structure, which was also confirmed by N2 sorption analysis (SBET = 5.0 m2 g−1). F



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compound was observed even at higher EtOH/H2O ratios (0.7). Three-dimensional Mg3(BTC)2 (NICS-6) crystallizes only in pure ethanol in wide ranges of temperature (150−200 °C) and crystallization times (from 1 to 3 days). In syntheses conditions, which were not included in marked areas shown in the phase diagram (Figure 11), the formation of unidentified phases or a mixture of the described phases were observed. Influence of the Solvent on Crystallization. To gain some understanding about how solvent influences the formation of different crystal structures (with different dimensionalities of the metal−organic networks), we performed NMR experiments. NMR spectroscopy provides information on the local environment of atomic nuclei and does not require a crystalline material. First we inspected 25Mg NMR spectra of Mg−acetate solutions in solvents with the EtOH/H2O ratios of 0, 0.3, 0.7, and 1 [i.e., in the same solvents as the ones used for the preparation of NICS-n (n = 3−6) materials], respectively. In Figure 12, we can clearly see that the

molecules. The weight losses are in good agreement with theoretical water content deduced from structural formula Mg3(BTC)2(H2O)12 of NICS-4 (30.7 wt %) and Mg2(BTC)(OH)(H2O)4·2H2O of NICS-5 (28.4 wt %). Second, weight loss of about 30 wt % in the range between 500 and 700 °C is in both cases attributed to decomposition of the ligand and removal of its fragments. NICS-6 shows much higher thermal stability than other described compounds due to the dense 3D framework structure. Thermogravimetric analysis shown in Figure 10d indicates the presence of water adsorbed on the surface (2.9 wt %). Ligand decomposition and thus degradation of the framework occurs in one major step in the temperature range from 500 to 700 °C, showing weight loss of 42.6%. Investigations of Phase Formation. The solvothermal reaction between magnesium cations and 1,3,5-benzenetricarboxylic acid as a ligand seems to be a relatively complex and sensitive thermodynamical process. Crystallization of Mgtrimesates is strongly dependent on polarity of water/ethanol mixture (ethanol vs water content). In order to obtain the proper conditions at which individual Mg-trimesate phases are formed, systematic investigation of EtOH/water ratio versus temperature and time of crystallization was performed. Phase diagram shown in Figure 11 indicates that all compounds

Figure 12. 25Mg NMR spectra of Mg(ac)2 solutions in solvents with the EtOH/H2O ratios of 0, 0.3, 0.7, and 1 (labeled with 0D-, 1D-, 2D, and 3D-MgMOF, respectively). Figure 11. Phase diagram of EtOH/water vs crystallization temperature dependency on formation of Mg-trimesate compounds: NICS-3 (blue ■), NICS-4 (purple ■), NICS-5 (orange ■), and NICS-6 (red ■). Colored areas indicate the conditions at which pure phases can be obtained.

nature of the solvent influences the line width of the 25Mg NMR signal. In water-rich solvents, like the ones from which NICS-3 and NICS-4 were obtained, the 25Mg signal is narrow, whereas in ethanol-rich solvent mixtures, like the ones from which NICS-5 and NICS-6 were obtained, the 25Mg signal is broader. The width of the signal in the solution NMR spectrum can be related to the mobility of the dissolved species, and this can be further related to the size of the species. The narrow 25 Mg NMR lines in the solvents used for the preparation of NICS-3 and NICS-4 thus suggest that Mg2+ ions are welldispersed throughout the solution and that no Mg−oxo clusters are formed. On the contrary, the wide 25Mg NMR lines in the solvents used for the preparation of NICS-5 and NICS-6 indicate that Mg2+ ions form larger species, most probably Mg− oxo clusters, which after the addition of BTC and upon hydrothermal treatment lead to frameworks in which several MgO6 octahedra are connected to each other. In the second step, we combined the Mg(ac)2 and BTC components in EtOH/H2O solvent mixtures and, thus, prepared and inspected the initial gels used for the synthesis of the four different Mg-BTC structures. The species that are

crystallize in relatively strict syntheses conditions. Complex compound (NICS-3) crystallizes only in the complete absence of ethanol and in a wide range of temperatures (between 150 and 190 °C) and times (between 1 and 3 days). Onedimensional Mg2(BTC)(OH)(H2O)4·2H2O (NICS-4) is formed at 150 °C after 1 day at EtOH/H2O molar ratio values between 0.2 and 0.3. When increasing the temperature up to 175 °C, conditions of 1D Mg-trimesate become even more restricted. At this temperature, the NICS-4 phase was observed only at an EtOH/H2O molar ratio of 0.3. At longer crystallization times NICS-4 did not form even at lower temperatures. T wo-dim ensional Mg 2 (BTC)(OH)(H2O)4·2H2O (NICS-5) is formed in the same temperature range as NICS-4, but preferably at longer crystallization times (3 days) and in slightly higher EtOH/H2O molar ratios (0.4− 0.5). When increasing temperature up to 200 °C, this G



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It is also interesting to note that the carbon NMR spectra of the initial gels for the synthesis of NICS-4 and NICS-5 are almost identical. It seems that similar or identical precursors are formed in both cases and only slight differences in synthesis conditions causes the crystallization process to go in two different directions. This similarity is in agreement with the observations presented in the phase diagram (Figure 14), which

formed in the gels seem to be substantially larger and less mobile than the species within the above-discussed solutions of pure Mg(ac)2. Because of the low mobility, the 25Mg NMR signals became much broader and, unfortunately, 25Mg NMR spectra could not be measured. Namely, 25Mg is a lowabundance low-gamma spin−5/2 quadrupolar nucleus, whose spectrum is extremely difficult to detect, especially in immobile disordered systems such as investigated gels. Fortunately, there were no difficulties with the detection of spectra of 13C nuclei. Carbon spectra, obtained by 1H−13C cross-polarization, are presented and compared to the spectra of the final crystalline materials in Figure 13. We can see that for the four distinct

Figure 14. Calculated ground-state energies for NICS-4 (open □) and NICS-5 (filled ■).

shows that the range of conditions for the crystallization of one form is very close to the range of conditions for the crystallization of the other form. For example, both NICS-4 and NICS-5, can be obtained using the solvent with the EtOH/ H2O ratio of about 0.35. Otherwise, NICS-4 tends to crystallize in the solvent that contains less ethanol, shorter crystallization times, and lower temperatures than NICS-5. A possible explanation for the described tendency is that NICS-5 is thermodynamically more stable than NICS-4 and that when crystallization is slower and lasts longer, the probability for obtaining the stable form is larger. To verify the hypothesis, the DFT-based quantum mechanical calculations were carried out. The results are presented in Figure 14 and indeed show that NICS-5 is more stable than NICS-4. One thing that could affect the rate of crystallization of Mg-BTC materials is the solubility of the starting precursors. With the increasing amount of ethanol, BTC ligand is getting more soluble, so the ligand exchange and thus the coordination of carboxylate groups with the Mg species becomes more favorable, leading to the crystallization of structures with higher dimensionalities. On the other hand, the Mg(ac)2 precursor becomes less soluble with an increasing fraction of EtOH, and thus the crystallization might be getting slower and slower. With an increasing fraction of EtOH in the solvent, therefore, the probability of obtaining NICS-5 (2D) instead of NICS-4 (1D) is increasing. At a given EtOH/H2O ratio, the solubility can be increased by increasing the temperature of the synthesis. Indeed, one can see that NICS-5 can be obtained from the solvent with the EtOH/H2O ratio of 0.4 in the temperature range between 150 and 190 °C, from the solvent with the EtOH/H2O ratio of 0.5 in the temperature range between 175 and 190 °C, and from the solvent with the EtOH/H2O ratio of 0.7 only at the synthesis temperature of 190 °C. The increasing ethanol content in the

Figure 13. 1H−13C CPMAS NMR spectra of different initial gels and corresponding final crystalline Mg-BTC materials.

MgMOFs, the spectra of the initial gels differ substantially from the spectra of the final crystalline products. This indicates that the “precursors” formed within the starting gels do not yet resemble the final frameworks. A crucial element of the MgBTC networks is the connectivity between the carboxyl group of the BTC molecule with the MgO6 octahedron. The nature of this connectivity/bond most strongly affects 13C NMR signals in the range between 165 and 185 ppm. The differences in these parts of the spectra of the initial gels and of the final products show that during the synthesis, the connectivities between the carboxyl groups and Mg species are being rearranged. Except for the NICS-3 case, the signals of the carboxyl atoms of the final products are on average shifted to larger chemical shifts, as compared to the signals of the carboxyls of the initial gels. This suggests that, on average, the number of connections of the carboxyl groups increases with crystallization. As mentioned, the single exception is the NICS3 material, however, for which even in the crystalline material there is only one C−O−Mg connection out of six possible connections of this kind per one BTC molecule. H



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ACKNOWLEDGMENTS This work was supported by the Slovenian Research Agency research program P1-0021.

solvent mixture also has an impact on the Mg coordination environment established during the solvothermal process. With the increased amount of ethanol, the number of water molecules acting as terminal ligands decreases in the Mgoctahedra. This increases the ability to form multinuclear clusters as secondary building units which enables the formation of structures with higher dimensionalities.

ABBREVIATIONS MOFs, metal−organic frameworks; BTC, 1,3,5-benzenetricarboxylic acid; NMR, nuclear magnetic resonance; IR, infrared spectroscopy; XRD, X-ray diffraction; CPMAS, cross-polarization magic angle spinning; TG, thermogravimetric analysis; DTG, derivative thermogravimetric analysis; ZIF, zeolitic imidazolate framework; HKUST, Hong Kong University of Science and Technology; MIL, Materials of Institut Lavoisier

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CONCLUSIONS Four new Mg-trimesate MOF materials with different structure dimensionalities (from 0D to 3D) were synthesized solvothermally from different water/ethanol mixtures. Dimensionality of the final structure can be tuned by varying the water/ethanol ratio used in the synthesis. A Mg(H2BTC)2(H2O)4 (NICS-3) 0D complex can be synthesized in hydrothermal conditions. A 1D chainlike Mg3(BTC)2(H2O)12 (NICS-4) is crystallized from a EtOH/H2O mixture with molar ratio of 0.3, while crystallization undergoes a different process at slightly higher EtOH/H2O ratios (from 0.4 to 0.7), yielding a 2D-layered Mg2(BTC)(OH)(H2O)4·2H2O (NICS-5) structure. Finally, a 3D structure with the formula Mg3(BTC)2 (NICS-6) is synthesized in pure ethanol. The influence of solvent on crystallization of Mg-BTC systems was more deeply investigated by NMR spectroscopy. By 25Mg NMR, we showed that mobility of Mg species in pure Mg2+ solutions, in the absence of the BTC ligand with its decreased increased amount of ethanol, indicates that in ethanol-rich solutions, bulkier clusters are already formed. When the BTC ligand is added to the Mg2+ solution, the components start to react already at the very beginning, forming crystalline phases which do not resemble the final structures obtained after hydrothermal treatment. Moreover, it seems that NICS-4 and NIC-5 phases crystallize from similar precursors but slightly different EtOH/H2O ratios direct the crystallization process in different paths. A sensitive thermodynamic process which directs the formation of these two phases can be correlated by solubility properties and coordination behavior of Mg2+ in solutions with different EtOH/H2O composition. By systematic investigation of the dependence of EtOH/water ratio versus temperature and time of crystallization on phase formation supported by DFT calculations, we additionally confirmed that NICS-5 represents a thermodynamically more stable structure in comparison with NICS-4. We can conclude that the crystallization of the MgBTC system under solvothermal conditions is a complex thermodynamic process strongly dependent on synthesis parameters, where the solvent plays a crucial role in the selfassembly of secondary building units.

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ASSOCIATED CONTENT

S Supporting Information *

Scanning electron micrographs, calculated and measured XRD patterns, IR spectra, and tables with crystallographic data of NICS-n (n = 3−6) materials, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +38614760215. Fax: +38614760300. Notes

The authors declare no competing financial interest. I



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