Robust 2D Coordination Networks from a Two-Step Assembly Process

Article pubs.acs.org/crystal

Robust 2D Coordination Networks from a Two-Step Assembly Process with Predesigned Silver Cyclic Dimers and Hexamethylenetetramine Gema Durá,† M. Carmen Carrión,†,‡ Félix A. Jalón,† Blanca R. Manzano,*,† Ana M. Rodríguez,§ and Kurt Mereiter∥ †

Universidad de Castilla-La Mancha, Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Ciencias y Tecnologías Químicas-IRICA, Avda. C. J. Cela, 10, 13071 Ciudad Real, Spain ‡ Fundación Parque Cientı ́fico y Tecnológico de Castilla-La Mancha (Fundación PCTCLM), Bulevar Rio Alberche s/n, 45007 Toledo, Spain § Universidad de Castilla-La Mancha, Departamento de Química Inorgánica, Orgánica y Bioquímica, Escuela Técnica Superior de Ingenieros Industriales, Avda. C. J. Cela, 3, 13071 Ciudad Real, Spain ∥ Faculty of Chemistry, Vienna University of Technology, Getreidemarkt 9/164 SC, A-1060 Vienna, Austria S Supporting Information *

ABSTRACT: A two-step self-assembly methodology has been used to obtain 2D coordination polymers with an egg-crate structure of trigonal symmetry and space group R3̅m. The polymers have the molecular formula [Ag3(L1)3(hmt)]X3 (L1 = bis(pyrazol-1-yl)(pyridin-4-yl)methane; hmt = hexamethylenetetramine; X = BF4, ClO4, PF6). In the first step, the previously reported box-like cyclic dimers [Ag L1]2X2, in which the ligands L1 are in a head-to-tail disposition, were obtained and these were subsequently reacted with the base hexamethylenetetramine to form Ag− N(hmt) bonds. The dimers act as linear connectors and the bases as triconnected nodes to give slightly undulating hexagons that form honeycomb sheets. The sheets follow an ABCABC stacking arrangement along [001] so that between them two types of large cavities arise which are occupied by disordered solvent, e.g., DMF inherited from synthesis. The anions, which participate in the formation of the cavities, interact with the sheets through hydrogen bonds. The polymers can be activated by removing the solvents without collapsing, which enabled an evaluation of these materials for use as gas adsorption, anion exchange, and proton capture systems.

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INTRODUCTION

must be removed for most applications, without structural collapse, to generate permanent porosity. The common components of MOMs are metal ions, which usually have a preference for a particular coordination environment, and ligands, which not only provide the donor atoms in the required positions for coordination, but also can participate with different interaction sites to generate noncovalent interactions. Recently, the concept of secondary building units (SBUs) has been introduced, and according to Yaghi,65 these are molecular complexes and cluster entities in which ligand coordination modes and metal coordination environments can be utilized to transform these fragments into extended porous networks using polytopic linkers. SBUs are important in the design of directionality to build MOMs and they usually give rise to robust frameworks.65,66 Interesting examples of the use of this concept have been described in the

The self-assembly of organic and metallic components via the “node and spacer” approach1−7 for the construction of one-, two-, and three-dimensional inorganic/organic hybrid networks (MOMs, metal organic materials), including coordination polymers and metal organic frameworks (MOFs), is an area of intense current interest in inorganic crystal engineering.8−13 MOMs combine a modular nature with unprecedented levels of porosity and the possibility of introducing molecular functionalities.14 This activity has led to numerous materials with potential applications15−17 in areas such as gas storage and separation18−28 and even for the capture of toxic gases29 or harmful volatile organic compounds (VOCs),30,31 catalysis,17,32−41 chemical sensing,42−48 drug delivery,49−53 biomedical imaging,54,55 and ion exchange.16 These materials can also have applications due their magnetic17,56,57 or luminescence properties17,58−62 and their semiconductivity or metallic conductivity.58−60,63,64 The pores of the supra-structures obtained are usually occupied by solvent molecules and these © XXXX American Chemical Society

Received: March 27, 2015 Revised: May 28, 2015

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DOI: 10.1021/acs.cgd.5b00428 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

literature.67−75 More recently, it has been proposed that the term ‘supermolecular building block’ (SBBs) should be adopted with reference to much larger scale metal−organic polyhedra (MOPs) for the construction of MOMs or MOFs.9,76−78 Interestingly, the organic components can, in some cases, also be considered as SBUs.79 However, it is necessary to consider that although the concept of SBU is frequently used, MOFs are usually obtained by a “one-pot” procedure80 and there are very few examples where the SBUs are formed beforehand.70−72,81 The two-step methodology has the advantage that simple modifications can be made to the construction blocks, which in turn lead to an effect in the final supramolecular structure and therefore expand its possible uses. As a continuation of our previous studies on different bis(pyrazol-1-yl)-methane ligands that behave as chelates in monodentate species,82−88 we recently described the formation of box-like cyclic dimers containing the ligands bis(pyrazol-1yl)(pyridin-4-yl)methane (L1) and bis(3,5-dimethylpyrazol-1yl)(pyridin-4-yl)methane (L2) (Chart 1), which act as bridges

present in the two silver centers of each dimer. It was predicted that, due to the presence of two silver centers and in contrast to the usual case of other examples with SBUs, in this case the dimers would behave as connectors and bridge two nodes, i.e., the molecular entity would play an “inverted role” as an SBU in the building of the network. As the organic component and potential node, we selected the polydentate base hexamethylenetetramine (hmt), in which the N atoms are tetrahedral and could lead to a supertetrahedral network. Cases have been described where this amine is di-, tri-, or tetra-connected.92−95 Three different weakly coordinating anions (BF4−, PF6−, ClO4−) were introduced in order to evaluate whether there was an effect on the global structure and on the possible applications of the complexes. We describe here the formation in a “two-step synthesis” of three MOMs with different anions. The structures obtained consist of layers formed by hexagons where the nodes are the hmt units and the cyclic metallic dimers function as linear linkers. These compounds have an egg-crate architecture and contain two types of large solventfilled cavities which do not collapse after removal of the solvents.

Chart 1. Ligands L1 and L2

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EXPERIMENTAL SECTION

General Comments. All syntheses described in this article were carried out in an air atmosphere. Solvents were dried with appropriate agents before use96 and were stored over 4 Å molecular sieves. Elemental analyses were performed with a Thermo Quest Flash EA1112 CHNS microanalyzer. IR spectra were recorded directly on the solids, with an ATR system, on a Shimadzu IRPrestige-21 (4000− 600 cm−1) spectrometer. Powder X-ray diffraction patterns were collected on a Philips X’Pert MPD diffractometer with Cu Kα radiation and Bragg−Brentano geometry. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out on an ATDTG SETARAM apparatus with a 92−16.18 graphite oven and CS32 controller. The analyses were performed without applying an initial vacuum and at a heating rate of 5 °C/min under an air flux in a platinum crucible. 1H and 19F{1H} NMR spectra were recorded at 298 K on Varian Gemini FT-400 (400 MHz for 1H and 376 MHz for 19F) and Inova FT-500 (500 MHz for 1H) spectrometers. Chemical shifts (ppm) are relative to tetramethylsilane (1H NMR) and CFCl3 (19F NMR). The starting material bis(pyrazol-1-yl)ketone (bpzCO)86 and the ligand89 L1 were prepared according to literature procedures. The metal salts AgBF4, AgPF6, and AgClO4 and hexamethylenetetramine (hmt) were purchased from Aldrich and were used without further purification. Ag(I) compounds should be stored with the exclusion of light in order to avoid reduction to Ag(0). Synthesis Safety Note: Transition metal perchlorates should be handled with caution as they are hazardous and explosive upon heating. Such problems were not encountered in the present study. X-ray Structure Determination for 1, 2, and 3. Crystal evaluation and data collection were performed at room temperature on a Bruker X8 APEX2 CCD (compounds 1 and 3) or a Bruker Kappa APEX2 CCD (compound 2) area detector diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å, sealed X-ray tube). Similar lattice dimensions and the common space group R3̅m indicated that the three compounds are isomorphous. Data were integrated using SAINT97 and an absorption correction was performed with the program SADABS.98 For all structures, a successful solution by direct methods provided most non-hydrogen atoms from the E-map.99 The remaining non-hydrogen atoms were located in an alternating series of least-squares cycles and difference Fourier maps using program SHELXL.99 All non-hydrogen atoms were refined with anisotropic displacement coefficients. Orientation disorder of the anion in 1 (BF4−) and 3 (PF6−) was taken into account, whereas ClO4− in 2 was ordered. All hydrogen atoms were included in the structure factor calculation at

Chart 2. Boxlike Cyclic Dimers Found with L1 or L2 and Silver Ions and the Simplification Used in Scheme 1a

a

X is a coordinating anion. This position is available for weak or noncoordinating anions.

with octahedral metallic centers89 or silver ions (Chart 2).90,91 In the former case significant differences were not found between the two ligands, but with silver a strong structural effect regarding the type of ligand was found. The use of L2 led to box-like cyclic dimers or 1D polymers depending on the coordinating ability of the anions and the crystallization conditions, whereas L1 invariably led to the formation of dimers. It was also concluded that the dimers with L1 were maintained in solution. In the solid state, when the anions had low coordinating ability the silver ions were found to be tricoordinated through bonding to three nitrogen atoms of two different ligands (in the case of the BF4− anion, two Ag−F interactions were found). With anions such as NO3− or OTf− the coordination led to tetracoordinated silver centers. We envisaged that these cyclic dimers with L1 containing weakly or non-coordinating anions could behave as SBUs for the construction of MOMs due to the open coordination sites B



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Scheme 1. Synthetic Route for Derivatives 1−3

mmol). Since only a small amount of crystals of 3 was obtained with this procedure the only feasible characterization was by X-ray diffraction. Anion Exchange Processes. Compound 1 was used as the starting material to study the selectivity in anion exchange reactions. In each experiment, 20 mg (1.49 × 10−5 mol) of compound 1 was suspended in 5 mL of MeOH and 3 equiv (4.48 × 10−5 mol) of the corresponding salt, namely, (TBA)CF3SO3 (16.7 mg) or (TMA)PF6 (9.4 mg) (TBA = tetrabutylammonium, TMA = tetramethylammonium), were added and dissolved in the methanolic suspension. In the case of tertiary systems, 4.48 × 10−5 mol of each salt was added. The reaction mixture was stirred at room temperature for 1 week with protection from light and changes were not observed. After this time, the solid was filtered off and washed with three portions of MeOH (3 mL) to remove all traces of the salts. The resulting solid was dissolved in DMSO-d6 and a 19F-NMR spectrum was registered to calculate the ratio of anions. All experiments were run in duplicate to evaluate the reproducibility of the results. In order to determine accurately the amount of anions in the sample, a calibration was performed for the relative integration of the anions in the 19F-NMR spectra. Thus, a correction factor was applied for the ratio of the intensities found in the anion exchange reactions. In order to calculate this factor, solutions with 4.28 × 10−5 mol of each of the three salts employed for the study ((TBA)BF4, (TBA)CF3SO3, and (TMA)PF6) in 0.5 mL of DMSO-d6 were prepared. The amounts of each salt used and the experimental integrals for each component used to calculate the correction factor are collected in Table S9 in the Supporting Information. Proton Capture. As supplement to the proton capture experiment the stability of 1 in water and dilute HBF4 was checked and verified with powder X-ray diffraction and NMR spectroscopy. Powder X-ray patterns recorded with a native sample, a 20 mg sample suspended for 1 h in 10 mL of water, and a sample after proton capture were identical and gave no indication for a decomposition (see Supporting Information). The stability of 1 in water and in acid solution was further verified by NMR. For this purpose a HBF4 solution in D2O was prepared with 14.0 mg of a diethyl ether solution of HBF4 (54%) in 5 mL of D2O. 0.1 mL of this solution and 0.9 mL of D2O were used for the study. Two milligrams of 1 were stirred in 1 mL of D2O or in 1 mL of the HBF4/D2O solution for 1 h. No organic compound could be detected in these solutions. For the proton capture experiment, a tetrafluoroboric acid solution was prepared with 28 mg of a commercial diethyl ether solution of HBF4 (54%) and HPLC quality deionized water in a 10 mL volumetric flask. One milliliter of this solution and 9 mL of water were used for the study. The pH was measured before and after the addition of 20 mg (1.43 × 10−5 mol) of compound 1 and thereafter every 15 min for the 1 h. The solid was then filtered off, washed with 5 mL of water, and dried in the air. See Supporting Information for the table and a graphic

idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. All of the compounds contained disordered solvent of crystallization in large cavities, according to NMR experiments, mainly DMF. Because it was impossible to identify any discrete solvent molecular entities, the contribution of the solvents to the structure factors was removed with procedure SQUEEZE of program PLATON and then the final refinement proceeded with the modified structure factors in the usual fashion.100 The distances and angles for the weak interactions (hydrogen interactions, π stacking, etc.) were evaluated using the program Mercury.101 Syntheses of the New Derivatives. Complexes 1−3 were obtained by slow diffusion of reactants, where each component is added in a different solvent and allowed to diffuse. An intermediate phase of a clean solvent is usually used to make the diffusion slower in order to obtain better quality crystals. The time to get crystals was 24− 48 h. These reactions were carried out at room temperature in glass tubes with a threaded cap. The dimensions of the tubes were 1.5 cm diameter and 16 cm length. The dimers used for the synthesis could be isolated as reported previously.90,91 [Ag3(L1)3(hmt)](BF4)3, 1. In a glass tube, dimer I, [Ag2(L1)2](BF4)2,90 (55.9 mg, 0.066 mmol) was dissolved in DMF (1.5 mL) as the lower phase. An intermediate phase of MeOH (0.5 mL) was added. For the upper phase, hmt (6.22 mg, 0.044 mmol) was dissolved in MeOH (1.5 mL) and the solution was added carefully to the tube on top of the other two layers. The crystallization tube was protected from light and allowed to stand until crystals had grown. The resulting light brown crystals were separated from the solution by filtration and dried in air. The solvents contained in the crystals (mainly DMF) were completely removed by immersion of the sample in acetone for 24 h and subsequent heating at 110 °C under vacuum for a further 24 h. Yield: 32.3 mg, 52%. Anal. Calcd for C42H45Ag3B3F12N19: C, 36.03; H, 3.24; N, 19.01. Found: C, 36.64; H, 3.27; N, 18.41. IR (ATR) ν/cm−1: 1676 ν(CN, CC); 1008, 1051, 1243, 1443 (hmt); 1039, 1076 (BF4−). Crystals of 1 suitable for X-ray diffraction were obtained by this synthetic methodology but they were only lightly dried and stored under oil. [Ag3(L1)3(hmt)](ClO4)3, 2. The synthetic procedure was the same as described for compound 1. The amounts were as follows: dimer II, [Ag2(L1)2](ClO4)2,91 (57.6 mg, 0.066 mmol), hmt (6.22 mg, 0.044 mmol). Yield: 22.6 mg, 36%. The solvent removal was performed by the procedure described for 1. Anal. Calcd for C42H45Ag3Cl3N19O12: C, 35.08; H, 3.15; N, 18.51. Found: C, 34.80; H, 3.20; N, 18.56. IR (ATR) ν/cm−1: 1676 ν(CN, CC); 1010, 1053, 1242, 1444 (hmt); 623, 1053, 1097 (ClO4−). Crystals of 2 suitable for X-ray diffraction were obtained as described above. [Ag3(L1)3(hmt)](PF6)3, 3. The synthetic procedure was the same as described for compound 1. The amounts were as follows: dimer III [Ag2(L1)2](PF6)2,91 (63.6 mg, 0.066 mmol) and hmt (6.22 mg, 0.044 C



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representation for the pH change and the powder X-ray diffractogram of the solid obtained at the end.

Table 1. Selection of Bond Lengths and Angles for 1, 2, and 3

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1

RESULTS AND DISCUSSION Synthesis and General Characterization of the New Metallic Derivatives. As stated, the main goal of this work was the synthesis of supramolecular structures, in a rational manner, by the “two step self-assembly” methodology. The first step involved the synthesis of the molecular dimers and these were then reacted with the organic component hmt in the second step to yield the final structure. The synthetic route for the silver complexes is shown in Scheme 1. The synthesis of dimers I−III has been described previously and the structure of I was determined by X-ray diffraction.90,91 Slow diffusion of DMF solutions containing the dimer and the hmt component dissolved in methanol, with these two phases separated by an interlayer of methanol, led to the formation of crystalline samples of the new derivatives 1−3. As described in detail below, the product solids consist of 2D networks formed by the assembly of the boxlike dimers and hmt regardless of the nature of the anion present. The stoichiometry of the polymer is [Ag3(L1)3(hmt)]nX3n. The polymers were characterized by different techniques (see below). The hexafluorophosphate species 3 was only studied by X-ray diffraction as the yield of the reaction was very low and insufficient sample was obtained for characterization by other techniques. The new derivatives are highly insoluble in most common organic solvents except for DMSO, in which the polymer is cleaved to give the free ligand L1 and hmt separately. As a consequence, these polymers could not be characterized by solution NMR spectroscopy. However, it was possible to calculate the ratio between L1 and hmt from the integration of their corresponding signals in the 1H NMR spectrum in DMSO-d6 solution. The ratio is consistent with the formulas determined for the crystals by X-ray diffraction (L1:hmt = 3:1). In addition, it was possible to estimate the approximate amounts of solvents present in the crystals, a result that was not obtainable by X-ray diffraction due to the presence of serious solvent disorder. The deduced L1:hmt:DMF ratio was 3:1:1.4 for 1 and 3:1:2 for 2. Negligible amounts of MeOH were detected. In the case of polymer 1 a 19F-NMR spectrum was recorded to verify the presence of the BF4− anion (resonance around −148 ppm). Elemental analysis data are consistent with the molecular formulas (1 and 2). The IR spectra of 1 and 2 are consistent with the presence of ligand L1 and the hmt component. Bands due to the anions were also observed. The stretching bands of the anions appear as split indicating a possible decrease in the tetrahedral symmetry in the solid state (see Supporting Information). This situation could be due to interactions between the fluorine or oxygen atoms and different parts of the structure, as observed by X-ray diffraction (see below). Solid State Characterization of Complexes 1−3. Crystals of 1−3 were studied by single crystal X-ray diffraction. Crystallographic data and bond lengths and angles are given in the Supporting Information and a selection of bond lengths and angles in Table 1. The structures consist of cationic 2D coordination polymers formed by two kinds of construction units. On one hand, there are diconnected boxlike cyclic dimers (I−III) that contain two silver centers and two L1 ligands in a head-to-tail disposition,

Bond distances (Å) Ag(1)−N(1) Ag(1)−N(3) Ag(1)−N(4) Bond angles (deg) N(1)−Ag(1)−N(1) N(1)−Ag(1)−N(3) N(1)−Ag(1)−N(4) N(3)−Ag(1)−N(4)

2× 1× 1× 1 2 2 1

× × × ×

2

3

2.341(5) 2.361(6) 2.329(5)

2.347(4) 2.360(5) 2.343(4)

2.327(6) 2.313(9) 2.333(7)

83.8(2) 114.0(1) 121.3(13) 102.4(2)

83.17(18) 115.76(13) 119.58(11) 103.18(15)

85.0(3) 115.5(2) 118.5(2) 104.0(3)

which act as linkers. On the other hand, triconnected hmt bases act as nodes by being bonded to the silver centers of the dimeric units. The remaining nitrogen of the hmt base (N5) is not coordinated. Figure 1 shows the disposition of one node in complex 1. The three compounds 1−3 crystallize in the trigonal space group R3̅m and the cationic structure is very

Figure 1. (a) [(Ag)3(L1)3(hmt)](BF4)3 fragment of 1 viewed along the c axis with hmt in middle and three [Ag2L12]2+ dimers and BF4 groups around. The fragment has point symmetry 3m. Hydrogen atoms are omitted for clarity. (b) Side view of the fragment. D



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similar, regardless of the anion, and only small differences in the distances or angles are observed. In all three structures the Ag atoms, the ligands L1, and the anions adopt point symmetry m, whereas hmt adopts symmetry 3m. Thus, the atoms Ag, C4, C5, N3, N4, C8, C9 and B, F1, F2 in 1 or Cl, O1, O2 in 2 are located on mirror planes, N5 on a threefold axis, and the rest (N1, N2, C1, C2, C3, C6, C7, F3, and O3) are in general position with point symmetry 1. A superposition of the [(Ag)3(L1)3(hmt)]3+ fragments of 1−3 is shown in the Supporting Information. The very modest mismatch of the three is largely due to differences in the unit cell dimensions of 1−3. The Ag atoms exhibit distorted tetrahedral coordination figures by two pyrazole, one pyridine, and one hmt nitrogen atom (Figure 1). The Ag−N bond lengths are relatively uniform and almost irrespective of the different types of N; they range from 2.329(5) to 2.361(6) Å (Table 1) and are in reasonable agreement with those of comparable Ag complexes. The N−Ag−N bond angles are small between the two chelating pyrazole N atoms (N1−Ag−N1 = 83.17(18)− 85.0(3)°) and big for the rest. The τ4 value,102 a measure of the distortion of the AgN4 tetrahedron, is 0.83 for 1, 0.86 for 2, and 0.87 for 3. Thus, the coordination of Ag in 1−3 is similar to that of other previously reported dimers91 with tetrahedral silver centers (nitrate or triflate coordination) and to that of I,90 except for the additional coordination of the hmt group to the silver center. The geometric parameters of the L1 ligand in 1−3, such as the bite angle (see above) and the pz−pz dihedral angle between the two pyrazolyl rings (121.2−124.2°), are only slightly higher than those observed for the isolated silver cyclic dimers (see Table S5 of Supporting Information).90,91 However, the difference in the MNN/N4(pz) angle (see Chart 3) (159.5−162.5°) is more pronounced and the values

Figure 2. Noncovalent interactions in one [Ag2L12]2+ dimer in compounds 1−3. π−π stacking in red and CH−π interactions in purple. The dimer has point symmetry 2/m.

between those found in other silver and in octahedral cages, and this points to an expected relationship between these two parameters. In addition, CH−π interactions between H3 and H5 of the pyridine and pyrazole rings of the same ligand are present. A complete set of parameters for the noncovalent interactions for all three complexes is given in the Supporting Information. As stated above, the [Ag12L2]2+ dimers are the linear connectors between two hmt units. These units, which are the nodes, are bonded to three silver ions of three dimers in a pyramidal fashion, with one hmt nitrogen atom remaining uncoordinated. In this way hexagons are formed with all the vertexes occupied by hmt molecules, which have two different orientations, i.e., with the noncoordinating nitrogen atom alternately on one or the other side of the sheet (Figure 3). A consequence of this arrangement is that the centroids of the six hmt units are not located in the same plane and the hexagons are undulating and show a boat conformation (see Figure 4). The distance between the two planes defined by the centers of mass of the hmt units of each three vertexes is 1.67 Å. The hexagons are shared and a cationic warped honeycomb sheet is formed that extends along the ab plane (see Figures 4 and 5). The [(Ag)3(L1)3(hmt)]3+ sheet shown in Figure 3a is centrosymmetric with the symmetry centers on the midpoints of the linker units. The sheet is distinctly undulating and has a maximum thickness of 0.54 × c = 11.8 Å assuming point atoms. In the unit cell, the sheets are stacked along the c-axis according to the rules of a trigonal R-lattice, i.e., separated by a distance of c/3 = 7.4 Å and consecutive sheets are mutually displaced by a distance of a/3 and −b/3. The mean planes of the sheets in the unit cell are at heights of z = 0, z = 1/3, and z = 2/3. If we begin with one sheet (first sheet, purple in Figure 5), there are vertexes of the second sheet (green in Figure 5) which protrude into the centers of the open hexagons of the first sheet, and vice versa because of centrosymmetry. The third sheet (pink in Figure 5) is different again and contains vertexes in the center of the hexagons of the second sheet. Thus, there is an ABCABC··· arrangement of the sheets that is analogous to that found in rhombohedral graphite, which crystallizes in the same space group.103 The differences between the two structures are that in our case the sheets are undulating and penetrate each other partially whereas in the allotropic form of graphite the sheets are flat. The mutual coherence between the sheets in 1− 3 perpendicular to their layer planes is furnished by the anions via Coulomb forces and hydrogen bonds (C−H···F or C−H··· O) involving the anions and several hydrogen atoms of the ligands L1 and the hmt units of different sheets (SI Tables S6− S8). The locations of the anions are very similar in the three compounds and they are arranged in three layers per unit cell parallel to (001).

Chart 3. Representation of the MNN/N4 Angle

obtained are intermediate between those observed for the tetrahedral or tricoordinated silver units (147.1−155.6°) and those found in dimers that contain only octahedral metallic centers (173.3−178.8°).89 In the latter case, steric hindrance between the pyrazolyl rings and the substituent cis to them is proposed as the reason for the high MNN/N4(pz) values, which allow the metallacyle to be better considered as a half chair. In the case of 1−3, the size of the hmt unit and the steric hindrance with the pyrazolyl rings may account for the increases in these values. There are several internal noncovalent interactions in the [Ag2L12]2+ dimers of 1−3 that were already present in all the previously investigated dimers with silver or other metal centers, and these may contribute to their stability and shape (Figure 2). The two pyridine rings are parallel and they exhibit a π−π stacking interaction with dCt···Ct distances between 3.65 and 3.71 Å, which is indicative of a stronger interaction than in the previously described silver complexes. In analogy to the MNN/N4(pz) angle, the dCt···Ct distances are intermediate E



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Figure 3. (a) Cationic 2D polymer sheet of complex 1 in the ab plane, with a honeycomb architecture. (b) Schematic representation of the structure of the cationic 2D polymer. The pale and dark circles represent the two types of hmt orientation with uncoordinated hmt nitrogen either up or down.

by 2 × 6 pyrazole rings, by a belt of 6 F1 atoms of the BF4 groups, and by two six-membered rings of CH2 and N of two hmt units. The void is quite spherical in shape and has a solvent accessible void volume of approximately 431 Å3 in 1 (calculated100 with standard van der Waals radii and a probe radius of 1.2 Å). One py void (see Figure 6b,c) is located at x,y,z = 0,0,1/2. The walls of this void are defined by 2 × 3 pyridine rings, by 2 × 3 F2 atoms of the BF4 groups, and by two N(CH2)3 fragments of hmt units which protrude distinctly into the cavity and make the void space disk-like. The solvent accessible void volume of approximately 256 Å3 in 1 is therefore notably smaller than for the pz void. The total void volume of the three pz and py voids per unit cell is 2061 Å3 in 1, 2121 Å3 in 2, and 2649 Å3 in 3, which represent 21.5%, 21.6%, and 24.5% of the corresponding unit cell volume, respectively. The spatial arrangement of the voids and how they are embedded between the [(Ag)3(L1)3(hmt)]3+ sheets is presented in Figure 7. Additional packing diagrams of the highly symmetric and simultaneously complicated crystal structure are given in the Supporting Information together with a commentary on the solvent content of the voids. Comparison of the structures of 1−3 with those of other hmt−Ag systems (with anions but with no other ligands)92,104 shows that there are significant differences due to the lack of stereochemical rigidity of the silver ion. For example, structures that are also composed of hexagons have been described but in some cases alternate nodes of hmt and Ag(I) ions, both triconnected, are present.92,105−108 In other examples silver centers act as linear spacers, as do the dimers in our case, and the concomitant presence of tri- or tetra-coordinated silver ions is observed and 2D or 3D networks with different topologies are obtained.92,95 The molecular entities used as connectors in this work clearly show a much more predictable diconnected behavior. The nature of the counteranion also has a more marked effect in the Ag−hmt systems than that observed in the derivatives described in this work. Thermogravimetric and Powder XRD Analysis. Thermogravimetric analyses were carried out on complexes 1 and 2. As an example, the result for complex 2 is presented in Figure 8.

Figure 4. Complex 1. Three sheets in ABC-stacking along [001], constituted by undulating hexagons. Spheres symbolize hmt nodes and connecting lines the [Ag2L12]2+ linkers.

Figure 5. Complex 1. (a) Relative positions of three different [(Ag)3(L1)3(hmt)]3+ sheets. (b) Schematic representation. The pale and dark circles represent the up or down orientation of the hmt units. Here sheets are slightly displaced in order to provide visibility.

The outlined shape and stacking of the sheets and anions creates two kinds of large cavities that are named according to their principal wall forming constituents: (i) pz voids for pyrazole and (ii) py voids for pyridine (Figure 6). Both voids have point symmetry 3̅ and are confined by pairs of [(Ag)3(L1)3(hmt)]3+ sheets. One pz void (see Figure 6a and c) is located at x,y,z = 0,0,0. The walls of this void are defined F



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Figure 6. Complex 1: (a) pz void, (b) py void, (c) consecutive pz and py voids at x,y,z = 0,0,0 (pz) and 0,0,1/2 (py).

Figure 7. Complex 1: (a) unit cell with the voids in blue, (b) view along the b axis showing the disposition of the pz and py voids. pz voids are located at z = 0, 1/3, 2/3, and 1; py voids at z = 1/6, 1/2, and 5/6.

per formula unit, respectively, corresponding to solvent mass portions of 7.5% and 10.1%, respectively. The stability of the polymers was evaluated by powder X-ray diffraction. The patterns for complex 1 under different conditions are shown in Figure 9. The powder X-ray pattern of Figure 9, curve (a), is a simulation calculated with program Mercury and the X-ray single crystal structure data of compound 1, which corresponds to a desolvated structure; native solvent content of the structure would decrease the intensities of the first and third peaks by about 30%, whereas the rest remain practically unaffected. Curve (b) in Figure 9 is an experimental powder X-ray diffraction pattern measured on a native sample of 1. The two patterns, (a) and (b), are sufficiently similar to conclude that the crystal of 1 analyzed by single crystal X-ray structure analysis is representative for the bulk crystalline sample. In particular, no phase impurity or amorphous matter can be seen. The integrity of the structure of 1 after desolvation was also analyzed. The solvents were removed by immersing the sample in acetone for 24 h followed by treatment of the solid under vacuum for a further 24 h at 110 °C. It was verified by 1H NMR that neither DMF, which was initially observed, nor acetone was present in the sample after the treatment. Good agreement was observed between this pattern (pink in Figure 9c) and patterns in (a) and (b), from which it can be concluded that the structure of the polymer is maintained after activation to remove the solvents without a collapse of the structure. This finding implies that the structure has good stability, which could be related with the higher stability usually observed for MOM derivatives with molecular building units.65,66

Figure 8. TG (blue) and DTA (brown) analysis of compound 2. Peaks up are exothermic.

It can be seen that there is a 7% mass loss up to 208 °C and this could correspond to the solvent molecules. At about 240 °C there is a mass loss of 30%, and above this temperature, and up to approximately 300 °C, there is a significant and sharp mass loss associated with an exothermic peak that should correspond to decomposition. A similar result was obtained for complex 1, where a mass loss of 6.5% was observed up to 205 °C, which could be due to the loss of solvent molecules present in the cavities of the polymer. An intense loss of mass occurs above 205 °C, and at around 340 °C, the loss of mass is greater than 50% with a complex system of DTA peaks (see figure in Supporting Information). The solvent contents of native samples of 1 and 2 determined by NMR spectroscopy were ca. 1.4 and 2 DMSO G



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Figure 9. Powder X-ray diffractograms of 1. (a) Simulated with X-ray single crystal structure. Experimental powder X-ray patterns of (b) native sample, (c) desolvated sample, and (d) desolvated sample after N2 adsorption.

Gas Storage. As mentioned previously, the polymers retain their structure even after being treated at 110 °C to remove the solvent molecules that were previously present in the cavities. It was therefore decided to explore the use of the desolvated polymers of 1 and 2 in the gas storage of N2 and CO2 as adsorbates. The corresponding figures are gathered in the Supporting Information. Although the uptake of CO2 is bigger than that of N2, in both cases the values are low and the materials may be considered essentially nonporous. This means that although there is void space in the structures, the pores must be inaccessible due to the absence of windows or pore openings and the material is essentially nonporous. It was verified by powder X-ray diffraction that the structure was maintained after the gas storage study. See Figure 9d that reflects a good agreement with the pattern of Figure 9b. Proton Capture Materials. As described previously, in complexes 1−3 there is one nitrogen atom in each hmt ligand that is not coordinated. This fact led us to consider the possibility of using these complexes to capture small-sized acid molecules. A study was carried out to evaluate the behavior of 1 as a proton capture agent for protons from an aqueous solution of HBF4. For this purpose, the crystalline polymer in the solid state was suspended in a solution with a known initial pH (2.76) and the pH value was registered at different times to quantify the amount of protons that had been captured. After 60 min the moles of protons that had disappeared from the solution were nearly equal to the moles of the hmt units in the polymer (99.9% of protonated nitrogen atoms). The data and graphical representation of the pH change with time is shown in the Supporting Information. As outlined in the Experimental Section it was previously ascertained that the compound was stable in water or in HBF4 solution for this time. Reactivity. Selectivity in Anion Exchange. The anions in the crystal structures of 1−3 are not bonded to the Ag atoms but are embedded between the 2D polymeric cationic sheets and interacting with them through weak hydrogen bonds. We decided therefore to explore whether it is possible to carry out anion exchange in a solid−liquid interface and also if any level of selectivity exists. The experiments were carried out with a suspension of compound 1 in methanol in which tetrabutylammonium (TBA) or tetramethylammonium (TMA) salts of other anions

were dissolved. Compound 1 is insoluble and stable in methanol. Once the exchange reaction had taken place, the crystalline product was filtered off and the ratio of the anions was determined by 19F NMR spectroscopy. For this reason, fluorinated anions (CF3SO3− or PF6−) and compound 1 were chosen for the study. The results obtained for the different combinations are presented in Table 2. Both binary and ternary exchange reactions, with two or three different anions present in the medium, respectively, were carried out. Table 2. Results for Anion Exchange Reactions with 1 as Starting Material and Different Fluorinated Salts, Using MeOH as Solvent and a Reaction Time of 1 Week 19

F NMR results

entry

starting anion

kind of reaction

1 2 3

BF4− BF4− BF4−

Binary Binary Ternary

exchange salts (TBA)CF3SO3 (TMA)PF6 (TBA)CF3SO3 + (TMA)PF6

% (BF4)

% (CF3SO3)

% (PF6)

100 92 90

0 0

8 10

Exposure of compound 1, which contains tetrafluoroborate anions, for 1 week to the triflate salt (entry 1) did not lead to interchange and with hexafluorophosphate (entry 2) the anion exchange was very low (8%). The exchange did not proceed any further over longer times. A ternary system was used (entry 3) with the initial compound 1 along with (TBA)CF3SO3 and (TMA)PF6 salts and, after 1 week of reaction, 10% of the BF4− anion had been exchanged by PF6− but the CF3SO3− anion did not enter into the solid. With the aforementioned results in mind, it can be concluded that the polymer has a high selectivity for tetrafluoroborate (BF4−) anions and it is possible to introduce PF6− anions but with low selectivity. However, the triflate ion (CF3SO3−) did not enter into the solid under the conditions used in the experiments. This fact may indicate the selectivity of the structure for anions with a spherical shape, such as BF4− and PF6−, particularly the tetrahedral BF4− anion. H



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CONCLUSIONS The reaction in a two-step process of the previously reported box-like cyclic dimers [AgL1]2X2 (L1 = bis(pyrazol-1-yl)(pyridin-4-yl)methane) with the polydentate base hexamethylenetetramine, hmt, led to the formation of 2D coordination polymers of stoichiometry [Ag3(L1)3(hmt)]X3 (X = BF4, 1, X = ClO4, 2, X = PF6, 3) and trigonal symmetry, space group R3̅m. The [Ag L1]22+ dimers behave as linear connectors, i.e., as SBUs with an “inverted role”, and the hmt bases as triconnected nodes. Undulating [(Ag)3(L1)3(hmt)]3+ sheets with a honeycomb architecture are formed. These sheets are stacked along [001] in an ABCABC disposition and are held together with the help of the anions. Between the sheets and confined also by the anions two types of alternating and closed cavities with total void volumes of about 21.5−24.5% do exist. The polymers are robust and can be activated to remove the solvents without collapse of the structure. However, the ability of these networks to adsorb N2 or CO2 after this activation is poor. It was, however, possible to carry out an anion exchange in the solid−liquid interface with a high degree of selectivity (BF4 ≫ PF6 > OTf). In fact, the triflate anion did not enter into the solid at all. The new derivatives also behave as proton capture systems.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format and ORTEP representations of the structures. Tables with crystallographic data, distances and angles and noncovalent interactions. Absorption isoterms, TG/DTA and IR and NMR spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00428.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone:++34926295300. Fax: ++34926295318. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MICINN of Spain (CTQ2011-24434) and the Junta de Comunidades de Castilla-La Mancha-FEDER Funds (PEII11-0214). We thank the MEC of Spain for a FPU grant (GD) and INCRECYT program (contract to MCC).

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Robust 2D Coordination Networks from a Two-Step Assembly Process

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