Group 13 Metal Carboxylates: Using Molecular Clusters As Hybrid

Article pubs.acs.org/crystal

Group 13 Metal Carboxylates: Using Molecular Clusters As Hybrid Building Units in a MIL-53 Type Framework Michael T. Wharmby,*,†,‡ Malte Snoyek,† Timo Rhauderwiek,† Knut Ritter,† and Norbert Stock*,† †

Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Strasse 2, 24118 Kiel, Germany Diamond Light Source Ltd., Diamond House, Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom

‡

S Supporting Information *

ABSTRACT: Systematic investigation of the reactions of the system AlCl 3 ·6H 2 O/pyridine-2,4,6-tricarboxylic acid (H3PTC)/pyridine in water yielded two new compounds, both containing the dimeric {AlPTC(μ-OH)(H2O)}22− unit. With long reaction times, the framework compound [Al(μOH){AlPTC(μ-OH)(H2O)}2]·2H2O (CAU-16, compound 1) is obtained, the first example of a framework compound with a metal−organic cluster linker, and bearing the MIL-53 network. Although the compound does not breathe, as other MIL-53 compounds do, it has a maximum uptake of CO2 of 1.76(2) mmol g−1 at 196 K. With shorter reaction times, the molecular compound {Al(HPTC)(μ-OH)(H2O)}2 (2) was prepared, leading to the proposal of a crystallization scheme for the Al3+-pyridine-2,4,6,-tricarboxylic acid system. To determine whether further framework compounds bearing hybrid metal cluster linkers could be prepared, systematic high-throughput investigations of pyridine-2,4,6-tricarboxylic acid in water with Ga3+ and In3+ were undertaken. These yielded two chain-type compounds, GaPTC(H2O)2 (3) and InPTC(H2O)2 (4), with different coordination chemistries. Optimized syntheses for compounds 1, 2, and 4 are reported.

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been extensively investigated with lanthanide cations,20,21 and a number of compounds have also been reported with groups 1 or 2 or transition metal cations.22,23 To date there have been no structures reported using group 13 metals: Al, Ga, In. These elements are known to form an extraordinary range of framework compounds with the trimesate anion (Al;16,24,25 Ga;26−28 In29−34). The presence of the pyridinyl N atom may lead to the crystallization of isoreticular structures, with markedly different adsorption or other properties;35−37 or due to the possibility of chelation, entirely new structures all together. We report here a systematic study of group 13 metals with pyridine-2,4,6-tricarboxylic acid. This led to a new variant of the MIL-53 network, with Al3+ as the framework-forming cation, wherein the organic linker molecule is replaced by a selfassembled molecular dimeric aluminum oxide cluster. This new phase is labeled CAU-16 (Christian-Albrechts-Universität porous material number 16).

INTRODUCTION In recent years, coordination polymers have been a focus of much research.1,2 Their structures are built up from inorganic and organic building units, which together determine the topology. Variation of the organic linker in some cases leads to isoreticular structures. Three approaches to variation of the linker are commonly used: variation of the organic linker, both in terms of its size and the functional groups decorating the molecule,3−5 and use of metal complexes capable of engaging in coordinative bonding to form a framework.6−8 This leads, for example, to altered adsorption properties4 and allows postsynthetic modification9 and introduction of catalytic capability.10 For applications, stability is also an important consideration, and to this end, the use of tri- or tetravalent metals in syntheses in combination with tri- or tetratopic linkers has led to the preparation of highly stable, porous compounds. Examples include NOTT-300,11 an Al3+-biphenyltetracarboxylate compound; soc-MOF, formed from M3+ (M = Sc, Fe, Ga, In)12−15 and azobenzenetetracarboxylic acid, displaying high-stability and porosity; and MIL-100, constructed from trimeric M3+ units (M = Al, Sc, Cr, Mn, Fe)12,16−19 and trimesic acid, which is one of the most stable, high-porosity framework compounds known. Pyridine-2,4,6-tricarboxylic acid (H3PTC) has an arrangement of carboxylic acid groups topologically identical to that of trimesic acid, but incorporates a pyridinyl N atom, forming a chelating pocket. The coordination chemistry of H3PTC has © 2014 American Chemical Society

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EXPERIMENTAL SECTION

Reactions were performed under solvothermal conditions in custombuilt high-throughput multiclaves, using either 400 or 2000 μL Teflon inserts.38 Initial characterization was performed using a Stoe Stadi P diffractometer in transmission geometry using Cu Kα1 radiation with data collected by an image plate detector. Synchrotron powder Received: August 8, 2014 Published: August 20, 2014 5310

dx.doi.org/10.1021/cg501189n | Cryst. Growth Des. 2014, 14, 5310−5317

Crystal Growth & Design

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diffraction data were collected at beamline P08 at PETRA III (DESY, Hamburg, Germany) in Debye−Scherrer geometry at a wavelength of 0.827164 Å with data collected using a Mythen detector (for full details of the experimental procedure, see Supporting Information [SI]). Single-crystal X-ray diffraction measurements were performed on a Stoe IPDS diffractometer equipped with a fine focus sealed tube (Mo Kα λ = 0.71073 Å). Infrared spectra were recorded on a Bruker ALPHA-P A220/D-01 FTIR spectrometer fitted with an ATR unit, over the spectral range 4000−400 cm−1. Thermogravimetric analysis was carried out using a NETSCH STA 429 CD analyzer with a heating rate of 4 K min−1 and under flowing air (flow rate 75 mL min−1) or a TA Instruments TGA Q500 with a heating rate of 5 K min−1 and under flowing air (60 mL min−1). Elemental analysis was performed using a EuroVector EuroEA elemental analyzer. NMR spectroscopy was performed using a Bruker DRX 500 spectrometer. N2 sorption experiments were performed using a BELSORP-mini volumetric porosimeter equipped with a liquid N2 bath, while CO2 sorption measurements were made using a Hiden-Isochema IGA gravimetric porosimeter, fitted with a dry ice/ethanol bath for measurements at 196 K or a Huber water bath for measurements at 298 K.

section S3). From the majority of reactions a highly crystalline powder later identified as CAU-16 ([Al(μ-OH){AlPTC(μOH)(H2O)}2], compound 1) was obtained, although in some cases this was mixed with a small amount of Böhmite impurity. From one reaction a single crystal of the phase {Al(HPTC)(μOH)(H2O)}2, referred to as compound 2 was also obtained. A focused high-throughput investigation of the phase space producing the most crystalline CAU-16 was performed to optimize the synthesis conditions. These conditions were then used for a single reaction in a 24 reactor multiclave, with a significantly reduced reaction time (12 h instead of 48 h). This gave nearly phase pure compound 2. Reactions of GaCl3, Ga(NO3)3·4H2O, and InCl3·4H2O with H3PTC and pyridine in water were also performed. Full details and results of these high-throughput syntheses are presented in the SI (section S3). Both Ga3+ salts were investigated using a focused high-throughput array using the optimized conditions determined for Al3+. This yielded exclusively mixed-phase products, and it was not possible to prepare any single phase in pure form. Nevertheless, across most of the reactions single crystals of GaPTC(H2O)2 (compound 3) were isolated. With InCl3·4H2O a high-throughput discovery array using a 24 reactor multiclave was undertaken, varying InCl3·4H2O/ H3PTC/pyridine ratios in water. Across a wide range of phase space, the phase InPTC(H2O)2 (compound 4) was obtained with some variation in crystallinity. The most highly crystalline compound 4 was obtained from the same phase region yielding the most crystalline CAU-16. Thus, a focused high-throughput array with stoichiometric ratios identical to those used in the Al3+ and Ga3+ systems was performed, which confirmed the optimized synthesis conditions. Optimized Reaction Conditions for [Al(μ-OH){AlPTC(μOH)(H2O)}2], Compound 1, CAU-16. Solid H3PTC (11 mg, 5.0 × 10−5 mol) was placed in a 400 μL Teflon insert, and to this was added in sequence water (63 μL), pyridine (6 μL, 7.5 × 10−5 mol), and aqueous AlCl3·6H2O solution (181 μL, 4.14 × 10−1 mol dm−3). The insert was placed in a 48 reactor, which was sealed and heated to 180 °C with a heating rate of 1.2 °C min−1 and kept at this temperature for 48 h. The reaction was cooled to room temperature over 24 h with a cooling rate of 0.1 °C min−1. Solids were separated by vacuum filtration and dried in air. The yield for this reaction was not determined due to the small scale of the reaction. Purity was confirmed by elemental analysis (Calc for [Al(μ-OH){AlPTC(μ-OH)(H2O)}2]: C = 30.98% H = 2.44% N = 4.52%; Found: C = 31.20%, H = 2.33%, N = 4.60%); IR spectra are included in the SI (section S5). Optimized Reaction Conditions for {Al(HPTC)(μ-OH)(H2O)}2, Compound 2. A reaction mixture with identical stoichiometry to that used in the preparation of CAU-16 (above) was heated to 180 °C at a rate of 3 °C min−1 and held at this temperature for 12 h instead of 48 h. The reactions was then cooled at a rate of 0.5 °C min−1 to room temperature. Solids were separated by vacuum filtration and dried in air. Large crystals of compound 2 were obtained from this reaction. Yield for this reaction was not determined due to the small scale of the reaction Composition of the crystals was confirmed by elemental analysis (Calc for {Al(HPTC)(μ-OH)(H2O)}2: C = 35.44% H = 2.23% N = 5.17%; Found: C = 34.5%, H = 2.4%, N = 5.0%); IR spectra are included in the SI (section S5). Optimized Reaction Conditions for InPTC(H2O)2, Compound 4. A mixture of H3PTC (10 mg, 4.8 × 10−5 mol), water (135 μL), pyridine (3 μL, 4.2 × 10−5 mol), and InCl3·4H2O solution in water (112 μL, 6.82 × 10−1 mol dm−3),

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CHEMICALS All chemicals were obtained from commercial sources and used without further purification. Pyridine-2,4,6-tricarboxylic acid (H3PTC) was synthesized following the reported method of Syper et al., by the oxidation of 2,4,6-trimethylpyridine with potassium permanganate.39 Purity of the synthesized ligand was confirmed by NMR spectroscopy in d6-DMSO (SI, section S2).

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HIGH-THROUGHPUT SYNTHESES An initial high-throughput discovery reaction was performed using a 48 reactor multiclave38 varying the ratios of AlCl3· 6H2O/pyridine-2,4,6-tricarboxylic acid (H3PTC)/pyridine in water, with a total reaction volume of 250 μL. The reactor was heated at a ramp rate of 2.5 °C min−1 to 180 °C and held at this temperature for 48 h, before cooling at 0.125 °C min−1 to room temperature. Figure 1 shows the outcome of these reactions (full details of all high-throughput syntheses are given in the SI,

Figure 1. Crystallization diagram showing the compositional space probed by the discovery array used in the initial investigation of the AlCl3/pyridine-2,4,6-tricarboxylic acid (H3PTC)/pyridine (py) system. Gray circles represent reactions yielding amorphous or poorly crystalline products; green squares are reactions from which highly crystalline [Al(μ-OH){AlPTC(μ-OH)(H2O)}2]·2H2O (CAU-16) was obtained. Circled region indicates compositions for which crystalline products were obtained. 5311



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Table 1. Summary of Crystallographic Results for the Four Compounds Reporteda cmpd

1

2

3

4

space group (crystal system) a/Å b/Å c/Å β/deg R1/wR2 Rwp/RBragg

C2/c (monoclinic) 37.1330(13) 11.5278(4) 6.51553(18) 109.266(3) − 0.0680/0.0300

P21/c (monoclinic) 13.6630(8) 11.7211(8) 6.4221(4) 99.700(5) 0.0562/0.1103 −

P21/c (monoclinic) 10.0568(3) 17.3408(5) 12.1952(3) 111.985(2) 0.0437/0.1093 −

Pbcn (orthorhombic) 9.60049(12) 9.03411(12) 11.17851(15) 90 − 0.0678/0.0355

a

Data for compounds 2 and 3 were obtained from single-crystal X-ray diffraction data, while compounds 1 and 4 were analyzed by synchrotron powder X-ray diffraction. Compound 1: [Al(μ-OH){AlPTC(μ-OH)(H2O)}2]·2H2O or CAU-16; 2: {Al(HPTC)(μ-OH)(H2O)}2; 3: GaPTC(H2O)2; 4: InPTC(H2O)2.

with a total reaction volume of 250 μL, was placed in a 400 μL Teflon insert and heated under the same conditions as the other reaction. Again, yield was not determined due to the small size of the reaction. Purity was confirmed by elemental analysis (Calc for InPTC(H2O)2: C = 26.76% H = 1.68% N = 3.90%; Found: C = 26.48%, H = 1.82%, N = 4.06%); IR spectra are included in the SI (section S5).

the reaction time; 12 h reaction times produced large, single crystals of compound 2, whereas 48 h reactions produced CAU-16 as a microcrystalline powder. This indicates that compound 2 may be an intermediate in the synthesis of CAU16. To confirm this, syntheses were performed using compound 2 as a reagent, with additional AlCl3 solution added to give the correct stoichiometric ratio for CAU-16. Reaction times of 16 h yielded phase-pure CAU-16 (full details of the reaction are given in the SI, section S3). The relationship between compound 2 and CAU-16 can be clearly understood from their crystal structures (Figures 2 and

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STRUCTURAL STUDIES Crystal Structure Determination of {Al(HPTC)(μ-OH)(H2O)}2 and GaPTC(H2O)2. The structures of {Al(HPTC)(μOH)(H2O)}2 (2) and GaPTC(H2O)2 (3) were determined from laboratory single-crystal X-ray diffraction data. Structures were solved using the direct methods routines of the Sir2011 package40 and refined using the SHELXL least-squares routines implemented in the SHELX-2013 package.41 Results of the final refinement are summarized in Table 1 while a complete set of results, including R-factors for both structures, are given in the SI (Table S8.1). Crystal Structure Determination of [Al(μ-OH){AlPTC(μOH)(H2O)}2] and InPTC(H2O)2. The structures of [Al(μOH){AlPTC(μ-OH)(H2O)}2] (CAU-16−compound 1) and InPTC(H2O)2 (compound 4) were determined from synchrotron powder X-ray diffraction data collected at beamline P08 at PETRA III (DESY, Hamburg, Germany).42 The structure of CAU-16 was solved using an approach combining the parallel tempering routines of the FOX43 package and direct space modeling in Materials Studio.44 The structure of compound 4 was solved using the direct methods routines of Expo200945 and completed using direct space modeling in Materials Studio.44 Both structures were refined by the Rietveld Method using TOPAS-Academic v5.46 Full details of the experimental procedure, the structure solution, refinement (including Rietveld plots) and crystallographic information are given in the SI (section S7 and Table S8.2), while the results of the final refinements, including cell parameters and R-factors, are summarized in Table 1.

Figure 2. Crystal structure of {Al(HPTC)(μ-OH)(H2O)}2, compound 2. Aluminum shown in gold; carbon, black; nitrogen, blue; oxygen red; and protons are shown in pink.

3). The asymmetric unit of compound 2 consists of one HPTC2− unit, an Al3+ cation, an OH− anion, and a water molecule. Al3+ cations are octahedrally coordinated by two O atoms from monodentate carboxylic acid groups and one pyridinyl N atom, all from the same HPTC2− unit. The coordination sphere is completed by a terminal water molecule and two μ-OH groups. Although the coordination chemistry of Al3+ is dominated by oxophilic interactions,47,48 coordination by N in this structure is enforced by the chelating nature of the rigid O/N/O pocket of the H3PTC linker. Similar coordination schemes have also been reported in the literature with amines having two α-carboxylic acid groups.49 Monomeric {Al(HPTC)(μ-OH)(H2O)} units are linked through μ-OH, with AlO5N octahedra edge-sharing to form dimeric clusters (Figure 2). Such a dimeric motif is common in Al3+ solution chemistry, with the [Al2(μ-OH)2(H2O)8]4+ cluster observed by NMR spectroscopy as a product of the hydrolysis of the [Al(H2O)6]3+ complex under slightly basic conditions and as a precursor to the formation of Keggin ions.50−52 In compound 2, the dimeric clusters crystallize to form an extensive H-bonding network. The structure of CAU-16 ([Al(μ-OH){AlPTC(μ-OH)(H2O)}2], compound 1) was solved from synchrotron powder diffraction data and is shown in Figure 3. CAU-16 is isoreticular with MIL-53(Al),53 with the metal−organic dimeric cluster

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RESULTS AND DISCUSSION Al3+/Pyridine-2,4,6-tricarboxylic Acid/Pyridine. Highthroughput investigations of the reaction system AlCl3·6H2O/ pyridine-2,4,6-tricarboxylic acid (H3PTC)/pyridine with water as solvent, using our in-house developed 48 reactor multiclave,38 showed that the phases [Al(μ-OH){AlPTC(μ-OH)(H2O)}2] (denoted CAU-16 or compound 1) and {Al(HPTC)(μ-OH)(H2O)}2 (denoted compound 2) could be prepared across a wide range of reaction stoichiometries (Figure 1 and SI, section S3). Both phases are formed using the same starting stoichiometry, but the final product depends on 5312



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by strut length but also by the breathing properties of the framework.60 MIL-53(Al) undergoes a reversible phase transition, but for compounds containing longer linkers (MIL-69/DUT-4; DUT-5; MOF-253) no breathing effect is observed, and some of the compounds show little or no porosity to gas molecules. MIL-69 and CAU-16 both have narrow/closed pore structures and have similar thermal properties. Thermogravimetric analysis (TGA) of both compounds show a similar pattern of weight losses (Figure 4a, TGA of CAU-16: see SI for

Figure 3. Structure of CAU-16, [Al(μ-OH){AlPTC(μ-OH)(H2O)}2]· 2H2O, (a) shows a detail of the linker, with the dimeric Al2(μOH)2(H2O)2 cluster at the core and two PTC3− ions extending outward to coordinate further Al sites (shown as polyhedra); (b) view along the c-direction, showing the rhombic channels delimited by the metal−organic cluster linkers. The unit cell is indicated (black dashes), while the blue-dashed ellipse indicates the portion of the structure shown in (a). Aluminum shown in gold; carbon in black; nitrogen, blue; and oxygen, red.

units observed in compound 2 directly replacing the terephthalic acid of the prototypical MIL-53 structure. The asymmetric unit of CAU-16 contains two Al sites, one PTC3− unit, two μ-OH units, one terminal water molecule, and two isolated physisorbed water molecules. Coordination about the first Al site is identical to that described for compound 2, forming {AlPTC(μ-OH)(H2O)}22− metal−organic dimeric clusters. Unlike in compound 2, however, each 4-position carboxylate group coordinates two symmetry-equivalent Al sites in a bridging mode. The octahedral coordination environment about this second Al site consists of four carboxylate O atoms, from four different metal−organic linkers and is completed by two μ-OH− ions, forming trans corner-sharing chains analogous with that found in MIL-53(Al).53 Bond valence sums indicate that both the μ-O sites of the dimers and the trans cornersharing chains are OH groups (O7:1.25; O9:1.17).54,55 Further evidence of two distinct hydroxide environments is provided by the infrared spectra, which show two resonances at 3673 and 3562 cm −1 (Supporting Information, section S5). By comparison with other MIL-53-like structures,53,56 the higher wavenumber band is assigned to the μ-OH of the chains (O9) and therefore the lower band corresponds to the μ-OH of the dimers (O7). The rhombic unidirectional channels delimited by the metal−organic cluster linkers have a cross-section of ∼8.5 × 8.0 Å and are occupied by physisorbed water molecules, which form an H-bonding network. The MIL-53 motif exhibited by CAU-16 has been reported in an isoreticular series of compounds in which different linear dicarboxylate ions form the framework: terephthalic acid (MIL53(Al)53); naphthalenedicarboxylic acid (MIL-6957 and DUT458); biphenyldicarboxylic acid (DUT-558); bipyridinedicarboxylic acid (MOF-25359); and trans-1,4-cyclohexanedicarboxylic acid (CAU-1356). Of these compounds, CAU-16 has the longest strut (distance measured between carboxylate C atoms), CAU-16:14.36 Å; next longest, DUT-5:10.20 Å). However, porosity in these compounds is determined not only

Figure 4. Thermal analysis of CAU-16, with the principal weight losses indicated on the TGA (a). Variable temperature powder X-ray diffraction data (b) indicate no large structural changes on dehydration, with changes in intensity of the diffraction peaks consistent with the loss of physisorbed water molecules from the channels.

full analysis). Both compounds show an initial loss of physisorbed water molecules, which in MIL-69 is complete below 100 °C, whereas for CAU-16 it is complete by 110 °C. In the variable-temperature X-ray diffraction (VT-PXRD) data for CAU-16 (Figure 4b), it is accompanied by changes in the intensities of the diffraction peaks (most noticeably a relative increase of the 200 and 110 reflections). However, unlike MIL53(Al) no breathing-type structural transition is observed. CAU-16 shows an additional weight loss, not observed for any of the other MIL-53 compounds, over the range 220−250 °C which may be attributed to the loss of the coordinating water 5313



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by another PTC molecule and subsequent chelation and crystallization steps leads to compound 2. However, compound 2 represents a kinetic product or intermediate phase, since it forms with a much shorter reaction time and can be used as a direct starting material for the synthesis of CAU-16 (see SI, section S3, Figure S3.3). The free 4-position carboxylate group is able to react with further [Al2(μ-OH)2(H2O)8]4+ or [Al(H2O)6]3+ species in solution, leading to the final crystallization of CAU-16 (compound 1). This proposed process is summarized in Scheme 1. Ga3+ or In3+/Pyridine-2,4,6-tricarboxylic Acid/Pyridine. To investigate the possibility of further MIL-53 analogues with the same dimeric linker unit as CAU-16, the systems M3+/H3PTC/pyridine (M = Ga, In) were studied. Optimization arrays identical to that employing Al3+ as the metal source were performed, using Ga(NO3)3, GaCl3, and InCl3·4H2O. The Ga3+/H3PTC/pyridine system yielded a multitude of phases, and although further syntheses were performed, it was not possible to produce any one of these phase-pure and in the bulk. However, single crystals of the phase GaPTC(H2O)2 (compound 3) were isolated. For the InCl3/H3PTC/pyridine system, only one phase, InPTC(H2O)2 (compound 4), was obtained across the entire region investigated. The optimized conditions for CAU-16 and for compound 4 are relatively similar, the synthesis of CAU-16 requiring more pyridine to yield the most crystalline product. H3PTC is relatively insoluble in water, and thus it is likely that one role of pyridine in the reaction is in solubilizing the linker. Furthermore, in the synthesis of CAU-16, the pyridine is likely to play a second role in the basic hydrolysis of the [Al(H2O)6]3+ complex, stabilizing the [Al2(μOH)2(H2O)8]4+complex.50,51 The crystal structures of compounds 3 and 4 were determined from single-crystal and synchrotron powder diffraction data, respectively. In contrast to the Al3+ system, both exhibit -M-PTC-M-PTC- chains though with slightly different connectivities. The asymmetric unit of GaPTC(H2O)2 (compound 3) consists of two independent residues, each composed of a Ga3+ cation, a PTC3− unit, and two coordinating water molecules. Each Ga3+ cation is octahedrally coordinated in a configuration similar to that of the Al site in compound 2. The [Ga(H2O)6]3+ cation undergoes hydrolysis less readily than [Al(H2O)6]3+, forming [Ga(OH)4]− (and this only at high pH values) rather than dimeric units of the sort observed in compounds 1 (CAU16) and 2.50,62,63 Instead, a monodentate 4-position carboxylate group from a neighboring PTC3− anion along the b-direction coordinates to the metal, with the GaO5N coordination environment completed by two terminal H2O molecules. Coordination by the 4-position carboxylate forms sinusoidal -Ga-PTC-Ga-PTC- chains (Figure 6). Chains are held together by H-bonding interactions between the terminal water molecules and uncoordinated carboxylate O atoms. The structure is dense and therefore not expected to show any degree of porosity. The asymmetric unit of InPTC(H2O)2 (compound 4) consists of one In3+ cation, half a PTC3− anion, and a coordinating water molecule. The larger cationic radius of In3+ (80.0 pm)64 compared to that of Al3+ (53.5 pm) or Ga3+ (62.0 pm) allow it to adopt a seven-fold coordination state. The coordination environment of the In3+ site is very similar to that of the Ga3+ in compound 3, with an additional coordinative

molecules. VT-PXRD shows that crystallinity is retained by the compound to more than 300 °C. CAU-16 undergoes a final decomposition step from about 330 °C, which is at significantly lower temperature than the other long linker MIL-53-like structures. Gas adsorption experiments were performed on CAU-16, activating the material under dynamic vacuum overnight at 150 °C. CAU-16 was found to be nonporous to N2 at 77 K (SI, section S10), which is not unusual for MIL-53 type materials reported with other metals,12 or those with longer struts and closed/narrow pore structures.57 Adsorption experiments were also performed using CO2 as the adsorbate at two temperatures: 196 K, allowing access to the widest p/p0 range possible and thus measurement of the most complete isotherm; and 298 K, to ensure kinetic effects did not inhibit adsorption or flexible behavior. At both temperatures, the material is porous to CO2 (Figure 5) showing a type I isotherm, consistent with a

Figure 5. Isotherms for the adsorption of CO2 by CAU-16 (compound 1) measured at 196 K (green circles - adsorption; red squares - desorption) and 298 K (blue triangles - adsorption; orange stars - desorption). Inset shows an enlargement of the region p/p0 0− 0.016.

nonbreathing material. The low temperature isotherm shows a hysteresis loop, which is attributed to retention of adsorbates within the framework due to kinetic effects, since no hysteresis is observed in the higher-temperature measurement. Dubinin− Radushkevich analysis61 indicated a maximum loading of CO2 at 196 K of 1.76(2) mmol g−1. (Full details of the sorption experiments and analysis, including diffraction patterns of the sample before and after adsorption, are given in the SI, section S10.) The crystallization process occurring during reactions of Al3+ and H3PTC can be interpreted by considering the similarities of the crystal structures and relative reaction times producing compounds 1 (CAU-16) and 2, as well as by considering the solution chemistry of Al3+ in water (Scheme 1). Dimeric [Al2(μ-OH)2(H2O)8]4+ complexes are known to form from the hydrolysis of the [Al(H2O)6]3+ complex.52 Following substitution of one of the H2O ligands of the dimer by a PTC ligand, the chelation by PTC and substitution of a further two H2O molecules would be expected to follow rapidly. Substitution of another H2O ligand on the second Al3+ cation 5314



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Scheme 1. Proposed Crystallization Process for [Al(μ-OH){AlPTC(μ-OH)(H2O)}2]·2H2O, CAU-16 (compound 1)a

a Aluminum hexaaqua ions undergo basic hydrolysis in solution to form the dimers. Ligand substitution of three water molecules by a PTC3− anion forms a hypothetical intermediate, while reaction with a second equivalent of H3PTC forms dimeric compound 2, {AlPTC(μ-OH)(H2O)}2. Reaction with a further equivalent of Al3+ from solution leads to the final product, CAU-16.

-M-PTC-M-PTC- chain structures are common in the coordination chemistry of H3PTC with transition metals. Reactions of Mn3+, Fe2+, Fe3+, or Co2+ with H3PTC result in chain structures very similar to that of of compound 4.23,65,66 Chain structures with monodentate carboxylate groups, similar to that observed in compound 3, have been reported with Cu2+.67 It is instructive to compare the coordination chemistries of Al3+, Ga3+, and In3+ in the presence of benzene-1,3,5,tricarboxylic acid (H3BTC) and pyridine-2,4,6-tricarboxylic acid (H3PTC). Only MIL-9626 has been reported with Ga3+ and H3BTC; MIL-96 is also known with Al3+, In3+, and also Cr3+.24,29,68 With Al3+, the mesoporous compound MIL100(Al)16 and near mesoporous MIL-11025 are also reported, consisting of trimeric μ3-oxo clusters linked in three dimensions by BTC3− ligands. While the coordination chemistry of Al3+ and Ga3+ in the presence of BTC3− anions is dominated by trimeric μ3-oxo metal clusters, the chemistry of In3+ with H3BTC is more diverse, featuring a range of other building blocks in addition to the trimeric μ3-oxo metal building blocks.30,31,33,34 However, the results of our syntheses seem to indicate that such wide structural diversity is not available in the system In3+/ H3PTC/water, since the highly favorable coordination of the metal in the O/N/O pocket of the ligand limits the options available to form an extended network.

Figure 6. Chain structures adopted by Ga3+ (above) and In3+ (below) on reaction with H3PTC. In GaPTC(H2O)2, compound 3, chains are formed by linking of octahedrally coordinated Ga site through monodentate 4-position carboxylate groups and the O/N/O pocket of two PTC3− ions; the sinusoidal topology of the chain is indicated (purple dashes). In InPTC(H2O)2 the 4-position carboxylate is bidentate and so In sites are seven-fold coordinated, while chains have a linear configuration. Ga shown in green; In in purple; carbon, black; nitrogen, blue; oxygen red; and protons are shown in pink.

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CONCLUSION Four group 13 metal pyridine-2,4,6-tricarboxylate (H3PTC) coordination compounds were synthesized, using a highthroughput multiclave setup, with water as the solvent and in the presence of pyridine. With Al3+ two phases were identified and their syntheses optimized: porous [Al(μ-OH){AlPTC(μOH)(H2O)}2] (CAU-16 - Christian-Albrechts-Universität porous materials number 16, compound 1) and {Al(HPTC)(μ-OH)(H2O)}2 (compound 2). It was found that both CAU16 and compound 2 could be prepared from reactions with the same initial composition, with compound 2 obtained after 12 h

interaction provided by the bidentate 4-position carboxylate group (cf. monodentate in compound 3), to give an InO6N coordination environment. Linear -In-PTC-In-PTC- chains are aligned parallel to the b-direction (Figure 6) and connected by H-bonding. As for compound 3, the structure is dense and therefore does not show any porosity. 5315



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(University of Kiel) is thanked for helpful discussions and assistance with solution of structures from powder data. Sylvia Williamson (University of St. Andrews) is thanked for collection of gas adsorption data. Prof. Anthony Cheetham is thanked for assistance with funding during writing of the paper.

and CAU-16 after 48 h. Compound 2 could also be used as a reagent in the synthesis of CAU-16. Thus, compound 2 is an intermediate in the synthesis of CAU-16 and a reaction process was proposed. Compound 2 is a molecular compound, consisting of a dimeric Al3+ cluster as its core with one uncoordinated carboxylate group on each of the two PTC ligands. In CAU-16, this metal−organic cluster unit acts as a rigid linker, coordinating trans corner-sharing Al-OH-Al chains through the 4-position carboxylate groups, forming a structure isoreticular with MIL-53(Al). CAU-16 has a narrow pore structure which does not breathe. It is not porous to N2; however, it shows an uptake of 1.76 mmol g−1 CO2 at 196 K. Similar hydrothermal investigations of the Ga3+ and In3+/ H3PTC/pyridine systems yielded two new chain compounds, GaPTC(H2O)2 and InPTC(H2O)2. Within the Ga3+ system, several other phases were identified, but it was not possible to obtain them phase pure, whereas with In3+ only one phase was observed, the synthesis of which was optimized. In all of the compounds, the metal cation is coordinated by the O/N/O pocket of the PTC3− anion. It seems likely that coordination of the metal in this pocket occurs rapidly and, due to the tridentate nature of the binding, irreversibly. Such a coordination environment limits the possible network topologies that may form with this linker, unless a second ligand is incorporated. Alternatively, it might be possible to form the Noxide to block this site during synthesis. Synthetic investigations in these areas are currently underway.

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ASSOCIATED CONTENT

* Supporting Information S

Lists of reagents and methods; characterization of the ligand; ternary diagrams and ratios used in high-throughput reactions; thermal and elemental analysis and FTIR spectra of optimized phases; experimental details and results of single-crystal and Xray powder diffraction experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel. +49-431-880-1675. Fax: +49-431-880-1775. E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

K.R. first synthesized compounds 1 and 2. M.S. and T.R. prepared the linker and first synthesized compounds 1 and 4; T.R. also optimized synthesis of compound 2. M.T.W. prepared compound 3 and completed optimization of compounds 1 and 4; performed structure solution and refinement on all four phases. N.S. and M.T.W. jointly wrote the text of this article. Funding

M.T.W. and N.S. thank the Deutsche Forschungsgemeinschaft (DFG, SPP 1362 “Porous Metal-Organic Frameworks”) and the European Union (Seventh Framework Program FP7/ 2007−2013, Grant Agreement No. 228862) for their financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank DESY for beamtime at beamline P08, PETRA III, with particular thanks to Dr. Carsten Deiter and Kathrin Pflaum for assistance in setting up the experiments. Dr. Mark Feyand 5316



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Group 13 Metal Carboxylates: Using Molecular Clusters As Hybrid

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