Exploiting Synthetic Conditions to Promote Structural Diversity within

Article pubs.acs.org/crystal

Exploiting Synthetic Conditions to Promote Structural Diversity within the Scandium(III)/Pyrimidine-4,6-dicarboxylate System Javier Cepeda,*,†,‡ Sonia Pérez-Yáñez,† Garikoitz Beobide,† Oscar Castillo,*,† Antonio Luque,† Paul A. Wright,§ Scott Sneddon,§ and Sharon E. Ashbrook§ †

Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología and ‡Departamento de Ciencia y Tecnología de Polímeros, Facultad de Ciencias Químicas, Universidad del País Vasco, UPV/EHU, Spain § EaStCHEM School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St. Andrews, Fife, KY16 9ST, United Kingdom S Supporting Information *

ABSTRACT: An exhaustive study of the factors governing the hydro- and solvothermal reaction of ScCl3 with pyrimidine-4,6dicarboxylic acid (H2pmdc) has led to five Sc(III)-organic architectures, namely {[Sc(μ-pmdc)(μ-ox)0.5(H2O)2]·3H2O}n (1), {[Sc(μ-pmdc)(μ-OH)(H2O)]·H2O}n (2), {(NH4)[Sc(μ-ox)2]·2H2O}n (3), {(tma)2[ScK(μ-form)6]}n (4), and [Sc2(pmdc)(OH)3Cl]·DMF·2H2O (5) (where ox = oxalate, tma = tetramethylammonium, form = formate, DMF = dimethylformamide). In this system, the pmdc anion has proven to be particularly sensitive to changes in the synthetic conditions in the presence of Sc(III), generating in situ byproducts that are incorporated into the final compounds. In fact, oxalate anions join the Sc-pmdc arrays into 2D sheets in 1 whereas ammonium ions fill the channels of the open 3D scandium oxalate architecture of 3. Making the reagent mixture a bit more basic prevents such decomposition and allows hydroxides to replace oxalate as ligands for the Sc, thus modifying the topology of layers in 2. Formate ions coming from the hydrolysis of DMF prevent the coordination of pmdc and give rise to the framework of 4. Structure 5 is an as yet unknown scandium pyrimidine-4,6-dicarboxylate framework which displays permanent porosity. CO2 adsorption isotherms on 5 reveal an uptake of 3 mmol g−1 and strong adsorbate···framework interactions. 45Sc MAS NMR has afforded additional information on the metal environments, providing further characterization of compound 5.

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INTRODUCTION The strategy of “reticular chemistry”,1−8 based on the classical concept of “nodes and spacers”,9−15 has laid the foundations of the design of metal−organic frameworks (MOFs), which is known as “crystal engineering”.16,17 This area has been particularly developed during the past decade owing to the huge number of MOFs reported. 18−20 In fact, many compounds had been published before such crystal structures were eventually regarded as forming via the predesigned selfassembly of building blocks, including secondary building units and molecular building blocks (SBUs or MBBs) or tectons from coordination chemistry and supramolecular points of view.21−23 Therefore, even though the main interest in MOFs concerns their intrinsic porosity and related properties, the research that is focused on unraveling the key factors governing the self-assembling process from the crystallographic and © XXXX American Chemical Society

topological perspective of polytopic linkers is no less important.24−27 In this regard, the role played by species in the reaction media that direct the construction of the architectures through weak interactions, via templating effects, is vital. In fact, many reports cover the use or in situ formation of structure directing agents to guide the assembly process irrespective of whether the template remains incorporated in the structure after crystallization.28 As well as the quaternary organic ammonium salts commonly used in the synthesis of zeolites, organic solvents and compounds, coordination complexes, gas molecules, surfactants or even inorganic Received: February 3, 2015 Revised: March 12, 2015

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DOI: 10.1021/acs.cgd.5b00154 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Coordination environment polyhedron and ORTEP view of 1 with the atomic numbering scheme.

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RESULTS AND DISCUSSION A series of hydrothermal and solvothermal syntheses were performed, using ScCl3 and H2pmdc as reagents and varying the synthetic conditions. X-ray diffraction analyses revealed the occurrence of five different product phases, with crystal structures based on high dimensional architectures of Sc(III) and different organic ligands. Whereas only three contain the intact pmdc species, all compounds contain at least one molecule coming from either the hydrothermal decomposition of pmdc or the solvents employed for the solvothermal reaction. The oxalate and ammonium ions present in compounds 1 and 3, respectively, are known byproducts of the pmdc thermal degradation reaction and act as coligand and counterions. Moreover, taking into account that oxalate ions compete as ligands for scandium(III) atoms, they can even replace all pmdc molecules in the product MOF when their concentration (as breakthrough products) exceeds a critical threshold. By contrast, formate and tetramethylammonium ions are formed during the thermal decomposition of DMF, and captured in the 3D framework of 4. Finally, making the reagent mixture a bit more basic gives rise gives rise to hydroxide anions that act as coligands, giving rise to the 2D and 3D-connected structures of compounds 2 and 5. A section including further details on the synthetic conditions that lead to crystallization of each compound is later described in the Experimental Section. Structural Description of {[Sc(μ-pmdc)(μ-ox)0.5(H2O)2]· 3H2O}n (1). Compound 1 displays a crystal architecture comprising neutral [Sc(μ-pmdc)(μ-ox)0.5(H2O)2] layers that are stacked with water molecules of crystallization in between them. This compound is isostructural to a previously reported family of lanthanide coordination compounds.46 Each Sc(III) cation coordinates to two bis-chelating pmdc ligands, a bischelating oxalate anion (generated from the partial decomposition of pmdc ligand) and two water molecules, which renders a N2O4Ow2 donor set with a geometry close to a triangular dodecahedron and a spherical biaugmented trigonal prism (Supporting Information) as revealed by the continuous shape measurements [SDD = 1.03; SSBTP = 1.64] (Figure 1, Table 1).47−50 The oxophilic nature of the Sc3+ ion is reflected by the significantly shorter Sc−O bond distances compared to those to nitrogen donor atoms. Room temperature 45Sc MAS NMR reveals (Figure 2) single site with a quadrupolar coupling constant, CQ, of 12.1 MHz, an asymmetry parameter, ηQ, of 0.7, and an isotropic chemical shift of 1.0 ppm. Comparison of the isotropic chemical shift with reported shifts for a ScO6

compounds have all been shown to act as templating agents.29−31 Under acid−base equilibrium conditions, MOFs crystallize through the reaction of a bridging ligand with a metal ion or cluster that has more than one vacant/labile site. The most commonly used ligands are rigid molecules or ions containing oxygen (e.g., hydroxide, alkoxide, alcohol, and carboxylate groups) or nitrogen (e.g., amine, pyridine, azide, and azole functions). 32−37 The pyrimidine-4,6-dicarboxylic acid (H2pmdc) has both of these chemical functions, which afford it an extraordinary coordination capacity with a wide range of metal centers in many different fashions.38−42 In particular, while one-dimensional polymers are mainly achieved with firstrow divalent ions (because of geometric and steric limitations),43 the use of metals that admit higher coordination numbers may permit a higher connectivity of the ligand. The increase of the metal oxidation state also permits a higher ligand:metal ratio. As a consequence, a route to open structures and porous compounds is opened up. In this sense, a trivalent metal ion not only ensures such an outcome, but also imposes a charge imbalance that can be fulfilled by either admitting monovalent ions (M3+ + M+, 2 × pmdc2−), allowing the presence of a coligand (M3+, pmdc2− + L−) or raising the ligand to metal ratio (M3+, 1.5 × pmdc2−) in the crystal structure. It is precisely after this stage that the presence of a templating agent can make a difference. Taking these ideas into consideration, we selected scandium(III) as a framework-forming metal cation since despite being a first-row transition element, it shares many chemical features with lanthanides.44 Examination of the CSD database reveals that its most usual coordination number ranges between 5 and 8 (see Supporting Information),45 while its oxophilic character is also apparent. Indeed, it establishes stronger coordination bonds that should be translated into a greater robustness of the resulting frameworks. Moreover, we have taken advantage of our knowledge of the behavior of both the pmdc ligand and some organic solvents under solvothermal conditions (pH, temperature, solvent, etc.) to explore the Sc(III)/pmdc system. In this way, five new compounds have been identified as well as chemically and structurally characterized. Thermal analyses of some of these compounds have been accomplished when appropriate. Finally, permanent porosity for one of the compounds is shown and analyzed by means of adsorption of different gases. B



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Table 1. Selected Bond Lengths (Å) for 1a Sc1−N11 Sc1−N13a Sc1−O171 Sc1−O181a

2.460(2) 2.443(2) 2.118(2) 2.170(2)

Sc1−O21 Sc1−O23b Sc1−O1w Sc1−O2w

resulting 2D architecture is referred to as a Shubnikov hexagonal plane network, or in other words, a three-connected hcb topological layer with the (63) point symbol.52−54 The layers are stacked in a parallel fashion along the c axis creating distorted hexagonal channels by means of an extensive hydrogen bonding scheme involving both direct interactions (O1w−H11w···O182) between layers and larger paths mediated by crystallization molecules (Supporting Information). The latter occupy the accessible voids forming discrete D3 aggregates and account for 16.3% of the unit cell volume (Figure 3).55 Thermogravimetric analysis (see Supporting Information) of this compound reveals a first mass loss between room temperature and 77 °C, which is followed by a larger loss up to 115 °C. Both steps are attributed to the release of the water of crystallization (calcd 15.65, exp. 16.15%). Then, a consecutive mass loss accounting for the coordination water molecules takes place up to 250 °C, where the structure loses crystallinity. Upon heating, the anhydrous product remains stable up to 330 °C, after which decomposition leads to Sc2O3 as final residue at 600 °C, as shown by PXRD. Crystal Structure of {[Sc(μ-pmdc)(μ-OH)(H2O)]·H2O}n (2). This compound crystallizes in the monoclinic P21/c space group and exhibits a layered structure. Differently from compound 1, Sc-pmdc chains in 2 are connected through a double μ-hydroxydo bridge instead of the oxalate anion, in such a way that centrosymmetric [Sc2(μ-OH)2]4− dimers are established, imposing a short Sc···Sc distance of 3.23 Å. Within these units, Sc1 displays an N2O4Ow donor set that resembles a strongly distorted pentagonal bipyramid [SPBPY = 3.29] (Figure 4, Table 2). The 45Sc MAS NMR spectrum of compound 2 shows a single scandium site (CQ = 22.0 MHz, ηQ = 0.5, δiso = 55.0 ppm), consistent with the X-ray crystal structure that gives an asymmetric (large CQ) environment with five O atoms and weak interactions with the nitrogen atoms (see Figure 2b). The dimeric nodes are joined to each other through the bischelating pmdc linkers describing Sc-pmdc chains similar to those of 1, but linked because the Sc atoms are part of dimers. This arrangement disposes the pmdc anions belonging to adjacent chains slightly overlapping, but offset, permitting a weak lateral carboxylate···aromatic ring contact (O172···ring centroid of 3.32 Å). As a result, the dimer units are distributed over the sheet establishing tetragonal rings, which can be referred to as a sql Shubnikov planar network with the (44.62) point symbol (Figure 5). The bridging hydroxide groups are twisted with regard to the Sc-pmdc chain main plane and establish a hydrogen bonding interaction with the carboxylate nonbonded O172 atom. Coordinating O1w project toward neighboring sheets and are hydrogen bonded to O171 and O182 oxygen atoms, whereas unbound water molecules occupy the interlamellar region and reinforce the cohesiveness of the 3D crystal building by additional hydrogen bonding interactions (Supporting Information). The results of thermal analysis (TGA and thermodiffractometry) indicate that the loss of the crystallization H2O molecule promotes a structural rearrangement that renders the desolvated product of 2. The indexation of the diffractogram leads to a symmetry related unit cell [a = 6.47 Å, b = 15.94 Å, c = 10.27 Å, β = 108.9°], in which the a and c axis repeats slights contract and lengthen, respectively, whereas the b axis repeat increases strongly. This results from the rearrangement of the [Sc(μ-pmdc)(μ-OH)(H2O)] sheets in order to maximize intermolecular hydrogen bonding interactions. Upon heating, the bound water molecule

2.250(2) 2.269(2) 2.170(2) 2.208(2)

Symmetry codes: (a) −x + 3/2, y + 1/2, −z + 1/2; (b) −x + 3/2, −y + 1/2, −z + 1.

a

Figure 2. 45Sc (14.1 T, 40 kHz MAS) NMR spectra of compounds (a) 1 and (b) 2. Spectra are the result of averaging 1024 and 3360 transients for panels a and b, respectively, using a recycle delay of 0.5 s. Fits of the experimental spectra are shown in red.

environments in Sc2BDC3 (BDC = 1,4-benzendicarboxylate), 3.5 ppm, and a ScO4(OH)2 environment in MIL-53(Sc), 59 ppm,51 is consistent with a 6-fold O coordination and weak interactions with the more distant N atoms revealed by the crystal structure (see Supporting Information). Additionally, the shorter ionic radius of scandium(III) ion provides smaller unit cell parameters than those found in the lanthanide isostructural compounds.46 Metal atoms are joined to one another through μ-pmdc and μ-ox bridges that result in different Sc···Sc distances: ∼6.8 and 6.0 Å, respectively. The first linker gives rise to almost planar Sc-pmdc chains in which three consecutive scandium atoms are arranged with an angle of ca. 141.6° along the crystallographic b axis. The chains are linked to neighboring ones by ox ligands, which project perpendicularly to the chains, establishing sixmember rings that are arranged into a herringbone pattern. The C



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Figure 3. Crystal architecture of 1 showing the six-membered rings.

Figure 4. Coordination environment polyhedron and ORTEP view of 2.

Table 2. Selected Bond Lengths (Å) for 2a Sc1−N11 Sc1−N13a Sc1−O171 Sc1−O181a a

2.485(2) 2.677(2) 2.146(2) 2.118(2)

Sc1−O1 Sc1−O1b Sc1−O1w

Structure Description of {(NH4)[Sc(μ-ox)2]·2H2O}n (3). The crystal structure of 3 consists of a 3D [Sc(μ-ox)2]− framework that accommodates ammonium cations (formed as a byproduct of pmdc decomposition, see the synthetic method section for further details) and crystallization water molecules in between. The Sc1 atom occupies the center of a slightly distorted square antiprism (SSAPR = 0.65) built from the coordination of four bis-bidentate oxalate anions, which can be referred to as a strongly distorted tetrahedral connector given the mutual disposition of the surrounding Sc atoms. The crosslink of the building units knit a robust quartz type structure (qtz; 4/6/h1) that matches with the (64.82) point symbol. The

2.033(2) 2.041(2) 2.163(2)

Symmetry codes: (a) −x, y − 1/2, −z + 1/2; (b) −x, −y, −z + 1.

is immediately released, giving an anhydrous [Sc(pmdc)(OH)]n framework that is stable from 300 to 400 °C, decomposing after several exothermic processes to Sc2O3 at 540 °C. D



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three-dimensional void system enclosed (47% of the unit cell volume), but it is occupied by the bulky tma cations that are weakly anchored to the backbone by means of C−H···O hydrogen bonds. Comments on [Sc2(pmdc)(OH)3Cl]·DMF·2H2O (5). The stoichiometry of compound 5 has been derived from the elemental, thermogravimetric and EDX analyses, which allowed us to suggest its formula. FT-IR spectroscopy and 13C MAS NMR (see Supporting Information) confirms the presence of the pmdc ligand in the compound. X-ray powder diffraction (PXRD) pattern (Figure 8) shows that it is a very crystalline material that can be described by a tetragonal lattice having a = 16.38 and c = 18.45 Å; V = 4949.6 Å3 (based on a Le Bail refinement with the following figures of merit: Rwp =0.092 and Rp = 0.065). The extinctions observed in the diffractogram did not allow unambiguous determination of the space group. Density considerations [Dobs = 1.83(1) g cm−3, measured by flotation of a small fragment of a pressed pellet of the compound] lead to an estimated Z value of 12 [Dcalc = 1.82 g cm−3]. Solid state NMR spectra were carried out in order to afford further details of compound 5. 45Sc MAS NMR reveals the presence of two distinct scandium environments (Figure 9): symmetrical hexacoordinate species (CQ = 7.3 MHz, ηQ = 0.0, δiso = 60.2 ppm) and the other nonsymmetrical environment with a large chemical shift that may suggest the presence of the chloride ion (or some other no O species) in the Sc coordination shell (CQ = 18.0 MHz, ηQ, = 0.6, δiso = 109.5 ppm). The 13C NMR spectrum confirms the presence of the pmdc ligand in the crystal structure (Supporting Information). However, no structural model was obtained using either conventional reciprocal-space techniques or simulated-annealing direct space methods. Nevertheless, description and discussion of this compound is included because it possesses interesting porous properties. Upon heating sample 5, two distinct steps are observed below 380 °C, corresponding to the release of crystallization solvent molecules, to render the evacuated [Sc2(pmdc)(OH)3Cl] framework. The thermodiffractometric data indicate that the framework remains almost unchanged during the solvent release, whereas it collapses above 390 °C. Occurrence of Sc2O3 is observed above 650 °C. Scanning the Synthetic Method of Sc/pmdc System. Every rational design, in terms of crystal engineering, should be accompanied by a parallel synthetic study which explores conditions that may affect the chemistry of the building units: metal and ligand, thus enabling the different possibilities of selfassembly to be sampled. Hereafter, it allows defining and tuning the key factors governing the synthesis toward each compound that form the system. In the Sc/pmdc system, we have performed a thorough synthetic study. As can be inferred from Scheme 1, all compounds have been obtained under mild hydro/solvothermal conditions. We employed water as a solvent because of the adequate solubility of the pmdc. Nonetheless, this media is known to promote partial ligand decomposition to give oxalate anions in the product in the presence of lanthanides under hydrothermal conditions.56,57 The in situ oxalate formation has been reported for many related ligands and is thought to proceed by a ringopening mechanism via hydrolysis-oxidation of the resulting species at acidic pH and high temperatures.58,59 For this reason compound 1 is obtained employing a 1:2 stoichiometry of Sc:H2pmdc. However, when the Sc:H2pmdc:H2ox reagents ratio is shifted to agree with the formula of 1, a scandium-

Figure 5. View of 2D layers of 2: (a) connectivity along a single layer highlighting in orange the 1D chain and (b) packing of layers along the c axis.

metal−organic backbone leaves an inner 3D void system that reaches 31.6% of the lattice volume, in which ammonium ions and water molecules are placed (Figure 6). The organic cations are disorderedly sited along the main tubular channels of ∼3.6 Å of section along the c axis between two carboxylate moieties, such that they are strongly anchored to them through hydrogen bonding interactions. Despite the disorder affecting the water molecules (which precludes their correct assignment in the structure), the analysis of the main residual density peaks allows to determine that they occupy the smaller windows that interconnect the main channels. There, they may act as hydrogen bonding donors with two close oxalate oxygen atoms while two oxalate C−C bonds may interact with their lone pairs. Crystal Structure of {(tma)2[ScK(μ-form)6]}n (4). 4 exhibits an anionic open 3D framework of Sc3+ and K+ cations bonded through formate anions, in which tma cations occupy the voids. Both organic ions are well-known products of the thermal decomposition of DMF solvent under solvothermal conditions. Sc1 and K2 atoms are coordinated by six formate bridging ligands, thus rendering almost ideal MO6 octahedra (SOC of 0.01 and 0.11 for Sc1 and K2, respectively) (Figure 7). Each formate ligand joins both metal centers one another such that each Sc atom connects to six surrounding K atoms and vice versa, generating a metal−organic backbone based on the pilling of cubic cages. The topological analysis confirms that the network is of the pcu type (α-Po primitive cubic packing), which corresponds to the (412.63) point symbol. This framework is of particular interest given the considerable E



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Figure 6. Packing of 3 along c axis with hydrogen bonding interactions showing the molecules that occupy the voids.

Figure 7. Packing of 4 along c axis showing metal coordination environments.

F



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fixed to 2.8−4.6 by adding NaOH, allowing hydroxide anions to be taken up by the Sc-pmdc framework to give 2. In a final attempt using water as solvent, we repeated the synthesis by employing a 1:1:0.5 Sc:H2pmdc:H2ox ratio but increasing the hydrothermal temperature. This resulted in a 3D {(NH4)[Sc(μox)2]·2H2O}n (3) open structure in which the coordinated water molecules present in compound 1 have been released, in agreement with an entropy driven dehydration process61−63 together with the templating effect that exerts the ammonium ion. The appearance of ammonium cations in the structure may seem unexpected, but they are known to form through the ringopening of the pmdc decomposition.56,57 A DMF/MeOH mixture was also employed for the synthesis of Sc-pmdc compounds. Compared to water, the solvent mixture gives the opportunity of rendering high-dimensional porous crystal structures because of the bulkier size of DMF molecule and the lower coordination capacity of MeOH. Thus, the whole coordination sphere is expected to be available for the bridging ligand, increasing, in this way, the connectivity of the network. Additionally, the large steric hindrance of DMF disfavors its coordination whereas it can play a templating effect promoting the inclusion of solvent occupied voids inside the crystal structure. This strategy leads to compound 5, whose adsorption measurements indicate a porous nature. Finally, when the synthesis temperature is raised above 160 °C, DMF undergoes a hydrolysis64,65 that gives rise to formate ions and a mixture of methylated ammonium ions.66 Under these conditions, tetramethylammonium cations counterbalance the negative charge of the Sc-formate open 3D framework in compound 4. As previously stated, the oxophilic nature of Sc3+ ion promotes the coordination of formate anions over N/O chelating pmdc ligands. Gas Adsorption Measurements. The permanent porosity of 5 was studied by means of N2 adsorption isotherms at 77 K (Figure 10) upon the activated sample, which was dried under vacuum at 150 °C for 6 h to eliminate the solvent guest molecules prior to measurement. However, the shape of the adsorption curve shows a knee at low relative pressures (P/P0 ≈ 0.01) followed by a not completely flat plateau (which deviates from those of type I isotherms), in which capillary condensation is observed at high relative pressures due to the small particle size of the samples. The evacuation of DMF from the microporous structures is frequently not complete and it can block the channel system leading to lower adsorptions. With the aim of ensuring a complete activation of the sample while its microporous nature is preserved, freshly prepared assynthesized sample was soaked in MeOH for 24 h to force the solvent exchange prior to the outgassing. N2 adsorption curve by the soaked sample shows almost double the uptake of the assynthesized sample (70 versus 116 cm3 g−1) because of the removal of residual solvent. Results of the porosity analysis are shown in Table 3. CO2 adsorption isotherms were measured at 298, 273, and 196 K, covering the low pressure range (0−1 bar) (Figure 11) given that this is the region to be optimized by adsorbents as to be considered efficient in postcombustion CO2 capture from flue gases,21,67,68 particularly at very low relative pressures. As a consequence, the main goal of MOFs regarding CCS (carbon capture and sequestration) technologies development focuses on maximizing the guest···host interactions at the aforementioned conditions. Compound 5 reveals a good adsorption uptake considering its moderate surface area, reaching up to 1.10, 1.54, and 3.04 mmol g−1 at the measured temperatures.

Figure 8. Pattern-matching analysis of polycrystalline sample of 5.

Figure 9. (a) 45Sc (14.1 T, 40 kHz MAS) NMR spectrum of compound 5. Spectrum is the result of averaging 2048 transients using a recycle delay of 3 s. (b) 45Sc (14.1 T, 26.67 kHz triple-quantum (sheared) MAS) NMR spectrum of compound 5. The spectrum is the result of averaging 840 transients using a recycle delay of 0.5 s for each of 128 t1 increments of 12.5 μs. In panel a, the red and green lines indicate fits of the individual components, where a ratio of 1:1.6 is observed. The asterisk indicates an impurity in the sample. In panel b, the sum projection is shown along the isotropic dimension.

oxalate, {[Sc2(μ-ox)3(H2O)2]·3H2O}n (AOXSCH in CSD database),60 is crystallized instead. This is explained according to the oxophilic character of the scandium(III) ion, in such a way that it prefers to coordinate to oxalate anions and a H2pmdc excess is required to ensure its presence in the final structure. An efficient way to avoid the oxalate decomposition is to slightly increase the pH of the solution such that the cleavage of the aromatic ring is inhibited. To that end, the pH value was G



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Scheme 1. Summary of the Best Synthetic Conditions under Hydro/Solvothermal Synthesis for Sc/pmdc Systema

a

Sc/H2pmdc/H2ox reagent ratio has been indicated in the dotted rectangular boxes.

Figure 11. CO2 adsorption isotherms for compound 5 at all studied temperatures.

Figure 10. N2 adsorption isotherms at 77 K for samples of compound 5 with and without the soaking.

using the modified Clausius−Clapeyron equation by fitting adsorption isotherms at 273 and 298 K (Figure 12),76 which approaches to 90 kJ mol−1 at zero coverage. Qst decreases up to

Table 3. Porosity Results for 5 (6 h, 1 × 10−4 atm) sample

SBETa

Smicrob

Sextb

Vtc

Vmicrob

Vextd

as-synth. soaked

258 458

211 424

47 34

0.121 0.186

0.081 0.160

0.040 0.026

BET specific surface area (m2 g−1). bMicropore surface area (Smicro) and volume (Vmicro) and external surface area (Sext) are estimated from the t-plot calculation (solely pores < 2 nm contribute to the microporosity calculation). cTotal specific pore volume at P/P0 = 0.93. d External pore volume estimated from the Vt − Vmicro subtraction. a

These values result in higher gravimetric adsorption uptakes (4.82%, 6.76%, and 13.4% for 298, 273, and 196 K, respectively) than those reported at similar conditions for many well-known MOFs [such as IRMOF-6, MIL-101(Cr), MOF-1(Mg), SNU-9(Cu), or MOF-5]69−73 and most of the ZIFs [for instance ZIF-25, ZIF-97, or ZIF-8].74,75 The shape of the curve at 196 K, with a very steep slope followed by a plateau close to saturation at high pressures (∼0.9 bar), also resembles a type I isotherm. A reference parameter to evaluate the interaction between the adsorbed CO2 molecules and the host framework is the adsorption enthalpy (Qst). The Qst value for 5 was calculated

Figure 12. CO2 adsorption enthalpies for 5 upon increasing loading. H



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a loading of 0.4 mmol g−1, where the preferred adsorption sites of the framework surface seem to be occupied by the guest molecules. The Qst remains almost constant at ∼28 kJ mol−1 during the remaining loading range. This value tends toward the highest reported adsorption enthalpies for porous metal− organic compounds associated with strong amine···CO2 interactions and is similar to the values calculated for compounds bearing incompletely coordinated metal cations exposed to the pores (Table 4).77−81 However, the lack of its crystal structure prevents any further explanation for the observed interactions.

also introduce new species into the reaction mixture. In particular, despite the presence of H2pmdc in all reagent mixtures, the formation of different ligands containing solely oxygen donor atoms, such as oxalate and formate, compete for coordinating to scandium(III) centers, leading to compounds in which they act as coligands or even the only ligands. Moreover, it has also been demonstrated that both ammoniumlike and alkaline ions (generated in the reaction mixture or added expressly) can also assist Sc(III) atoms in rendering novel open architectures. One of the compounds has revealed a permanent porosity according to the N2 and CO2 uptakes. The estimation of the isosteric heats of adsorption from the isotherms at 298 and 273 K seems to indicate a strong interaction of CO2 with the framework. 45Sc MAS NMR has been decisive to confirm the presence of two independent metal environments in compound 5.

Table 4. Zero-Coverage Heats of CO2 Adsorption in MOFs compounda

functionality type

Cu-BBTri-mmen Cu-BBTri-en [Sc2(pmdc)(OH)3Cl]

amines amines b

Zn(TPDC)(DABCO) MIL-100(Cr) TBA@bio-MOF-1 CAU-1 bio-MOF-11(Co) MOF-74(Mg) CdMn(pmdc)

amines exposed cations amines amines amines exposed cations amines + exposed cations

−Q (kJ mol−1) 96 90 91 77 62c 55 48 45 39 28

ref 82 83 this work 84 85 86 87 88 89 90

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EXPERIMENTAL SECTION

Chemicals. All the chemicals were of reagent grade and were used as commercially obtained. The starting material pyrimidine-4,6dicarboxylic acid was prepared following the previously reported procedure.91 Some compounds are obtained at different solvothermal set point temperatures and pH values, so temperature and pH ranges have been indicated when appropriate (see Scheme 1 for more details). Synthesis of {[Sc(μ-pmdc)(μ-ox)0.5(H2O)2]·3H2O}n (1). An aqueous solution of ScCl3 (207 μL, 1.45 M, 0.3 mmol) was added over 15 mL of an aqueous solution containing H2Pmdc (0.1224 g, 0.6 mmol), giving rise to a white powder. The mixture (pH = 0.2) was kept under vigorous stirring for 1 h at 60 °C. Colorless block-shaped single crystals of 1 were obtained by placing the solution on a 45 mL Teflonlined stainless steel autoclave under autogenous pressure at 110 °C for 3 days and then slowly cooled to room temperature (2 °C/h). Yield 85−90% (based on metal). Synthesis of {[Sc(μ-pmdc)(μ-OH)(H2O)]·H2O}n (2). Single crystals of 2 were obtained by rising the pH value of the latter solution to 2.8− 4.6 and setting the hydrothermal temperature between the 110−160 °C range for 3 days and then slowly cooled to room temperature (2 °C/h). Yield 90−95% (based on metal). Synthesis of {(NH4)[Sc(μ-ox)2]·2H2O}n (3). The addition of 0.15 mmol of H2ox (0.0378 g), dissolved in 5 mL of water, to 20 mL of an aqueous solution containing 0.3 mmol of ScCl3 (207 μL of 1.45 M) and 0.3 mmol of H2Pmdc (0.0612 g) led immediately to a white precipitate (pH = 0.2). Applying the same hydrothermal procedure of 1 to this mixture, block-shaped single crystals of 3 were achieved. Yield 80−85% (based on metal).

a

H3BBTri =1,3,5-tri(1H-1,2,3-triazol-4-yl)benzene; mmen = N,N′dimethylethylenediamine; en = ethylendiamine; TPDC = p-terphenyl4,4′-dicarboxylate; DABCO = 1,4-diazabicyclo[2,2,2]octane; MIL-100 = [Cr3O(H2O)3F(1,3,5-benzenetricarboxylate)2]n; TBA = tetrabuthylammonium; bio-MOF-1 = [Zn 8 (adenine) 4 (bipheny ldicarboxylate) 6 O 3 ] n ; CA U-1 = [Al4(OH)2(OCH3)4(H2N−BDC)3]n; MOF-74(Mg) = [Mg2 (dobdc)] n ; bio-MOF-11 = {[Co 2 (adenine) 2 (CO 2 CH3 ) 2 ]· 2DMF·0.5H2O}n. bIt cannot be provided the functionality responsible for the high interaction given the lack of crystal structure for this compound. cDetermined experimentally by microcalorimetry.

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CONCLUSIONS In the present work, five scandium(III) 2D and 3D coordination polymers have been structurally and chemically characterized as a result of a thorough study of various synthetic parameters (solvents, pH, temperature set point) upon the Sc/ pmdc system. All these variables not only alter the kinetic parameters and the position of thermodynamic equilibrium but

Table 5. Crystallographic Data and Structure Refinement Details of Compounds 1−4 emp. form. form. weight space group a (Å) b (Å) c (Å) β (deg) V (Å3) GOF (S)a R1b/wR2c [I > 2σ(I)] R1/wR2 [all data]

1

2

C7H12N2O11Sc 345.15 C2/c 18.493(1) 12.875(1) 13.834(1) 131.71(1) 2458.9(2) 1.075 0.0366/0.0834 0.0477/0.0917

C6H7N2O7Sc 264.10 P21/c 6.9719(2) 13.4790(3) 10.0899(2) 103.570(2) 921.72(4) 1.074 0.0364/0.0745 0.0500/0.0789

3 C4H4NO8Sc 239.09 P6222 8.5940(8) 8.5940(8) 12.4441(5) 795.95(9) 1.147 0.1106/0.3226 0.1142/0.3258

4 C14H30KN2O12Sc 502.46 P21/n 9.5369(2) 9.1544(1) 13.3131(2) 90.483(1) 1162.25(3) 1.068 0.0314/0.0830 0.0360/0.0880

S = [∑w(F02 − Fc2)2/(Nobs − Nparam)]1/2. bR1 = ∑∥F0| − |Fc∥/∑|F0|. cwR2 = [∑w(F02 − Fc2)2/∑wF02]1/2; w = 1/[σ2(F02) + (aP)2 + bP] where P = (max(F02,0) + 2Fc2)/3 with a = 0.0332 (1), 0.0271 (2), 0.1765 (3), 0.0435 (4), and b = 2.6936 (1), 0.4690 (2), 9.5678 (3), 0.2256 (4). a

I



Article

Synthesis of {(tma)2[ScK(μ-form)6]}n (4). ScCl3 (0.3 mmol, 207 μL of 1.45 M solution) were added over 20 mL of DMF/MeOH (1:1) mixture containing 0.6 mmol of H2Pmdc (0.1224 g) to lead to a colorless and transparent solution (pH = 1.5). Then, 0.0224 g of KCl dissolved in 5 mL of DMF/MeOH (1:1) solution were poured to the mixture, which was placed into a Teflon-lined vessel and subjected to the general solvothermal procedure with a set point temperature of 170−180 °C. Prismatic shaped single crystals of 4 were obtained. Yield 85−90% (based on metal). Synthesis of [Sc2(pmdc)(OH)3Cl]·DMF·2H2O (5). Twenty-five milliliters of a DMF/MeOH (1:1) mixture (pH = 1.5) containing 0.3 mmol of ScCl3 (207 μL of 1.45 M solution) and 0.45 mmol of H2Pmdc (0.0918 g) was submitted to the general solvothermal treatment but fixing the set point between 110−160 °C. Polycrystalline white powder of 5 was collected from the cold solution. Yield 80− 85% (based on metal). X-ray Diffraction Data Collection and Structure Determination. Single crystal diffraction data of 1, 2, 3, and 4 were collected at 100(2) K on an Agilent Technologies Supernova diffractometer (λMo−Kα = 0.71073 Å). The data reduction was done with the CrysAlisPro program.92 All crystal structures were solved by direct methods using the SIR92 program93 and refined by full-matrix leastsquares on F2 including all reflections employing the WINGX crystallographic package.94,95 Compound 3 presents a strong disorder that affects ammonium nitrogen atom and crystallization water molecules. The disorder has been modeled by splitting the ammonium ion into two positions. Moreover, the electron density at the voids of this structure was subtracted from the reflection data by the SQUEEZE procedure as implemented in PLATON program due to the presence of disordered solvent molecules.96 Details of the structure determination and refinement of all compounds are summarized in Table 5. The powder X-ray diffraction (PXRD) patterns were collected on a Phillips X’PERT powder diffractometer with Cu−Kα radiation (λ = 1.5418 Å) over the range 5 < 2θ < 50° with a step size of 0.02° and an acquisition time of 2.5 s per step at 25 °C. Indexation of the diffraction profiles were made by means of the FULLPROF program (patternmatching analysis).97 Variable-temperature powder X-ray diffraction measurements of compounds 1, 2, and 5 were run under ambient atmosphere with heating rates of 5 °C min−1 and measuring a complete diffractogram every 20 °C. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publication nos. CCDC 1046998−1047001. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, U.K. (Fax +44−1223−335033, e-mail deposit@ ccdc.cam.ac.uk, or http://www.ccdc.cam.ac.uk). Physical Measurements. Elemental analyses (C, H, N) were performed on an Euro EA Elemental Analyzer, whereas the metal content, determined by inductively coupled plasma (ICP-AES) was performed on a Horiba Yobin Yvon Activa spectrometer. The IR spectra (KBr pellets) were recorded on a FTIR 8400S Shimadzu spectrometer in the 4000−400 cm−1 spectral region. Thermal analyses (TG/DTA) were performed on a TA Instruments SDT 2960 thermal analyzer in a synthetic air atmosphere (79% N2/21% O2) with a heating rate of 5 °C min−1. Samples were activated in vacuo for 6 h at 150 °C. Nitrogen physisorption data for 5 was recorded with a Micromeritics Tristar II 3020. The specific surface area was calculated from the adsorption branch in the relative pressure interval using the Brunauer−Emmett−Teller (BET) method98 and the consistency criteria proposed by Walton and Snurr that is commonly applied for MOFs,99−101 while the micropore volume was estimated by fitting the measured N2 isotherms with the t-plot method.102 The volumetric carbon dioxide physisorption data were recorded in a Micromeritics ASAP 2020 porosity analyzer at 298 and 273 K, and in a Hiden IGA automatic gravimetric porosimeter at 196 K (ethanol/dry ice mixture). Scanning electron microscopy (SEM) measures were made with a Carl Zeiss EVO-40, using the attached Oxford EDX analyzer to identify the presence of metals. Solid-state NMR spectra were recorded using a BrukerAvance III spectrometer equipped with a wide-bore super-

conducting magnetic with operating at a magnetic field strength of 14.1 T, with Larmor frequencies 150.9 and 145.78 MHz for 13C and 45 Sc, respectively. Samples were packed into 1.9 mm ZrO2 MAS rotors and rotated at MAS rates of between 16 and 40 kHz. Chemical shifts are reported in ppm relative to TMS for 13C and 0.2 M ScCl3 (aq) for 45 Sc. For 13C, spectra were acquired using cross-polarization (CP), with a contact pulse (ramped for 1H) of 3 ms duration and 1H decoupling (TPPM-15 with ω1/2π = 100 kHz) applied throughout signal acquisition. 45Sc MAS spectra were acquired using a spin echo pulse sequence with a rotor-synchronized delay of 25 μs. 45Sc triplequantum MAS NMR experiments were recorded using an amplitudemodulated z-filtered pulse sequence. The scale of the isotropic axis of the sheared MQMAS spectrum is plotted according to the conventions described elsewhere.103,104

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ASSOCIATED CONTENT

S Supporting Information *

Analysis of the coordination geometries of Sc(III) compounds, additional structural material, FT-IR analysis, PXRD data, chemical characterization and thermal behavior, EDX, gas adsorption isotherms, NMR spectra, 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 Authors

*E-mail: [email protected] *E-mail: [email protected]. Fax: +34-94601-3500. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by Ministerio de Economı ́a y Competitividad (MAT2013-46502-C2-1-P), Eusko Jaurlaritza/ Gobierno Vasco (Grant IT477-10, S-PE13UN016), and Universidad del Paı ́s Vasco/Euskal Herriko Unibertsitatea (EHUA14/09, Grant UFI11/53, postdoctoral fellowship for J.C. and S.P.Y.). The authors thank for technical and human support provided by SGIker of UPV/EHU. S.E.A. and S.S. gratefully acknowledge EPSRC-DTG for funding. We also want to thank Sylvia Williamson (University of St. Andrews) for gas adsorption measurements.

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REFERENCES

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