Modular Assembling of [Zr(C2O4)4]4- and [DabcoH2]2+ Units in Supramolecular Hybrid Architectures Including an Open Framework with Reversible Sorption Properties (Dabco ) 1,4-Diazabicyclo[2. 2. 2]octane)
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1870–1877
Franck The´tiot, Carine Duhayon, Thengarai S. Venkatakrishnan, and Jean-Pascal Sutter* Laboratoire de Chimie de Coordination du CNRS, UniVersite´ Paul Sabatier, 205 route de Narbonne, 31077 Toulouse, France ReceiVed October 5, 2007; ReVised Manuscript ReceiVed February 28, 2008
ABSTRACT: An emerging trend toward development of metal-organic frameworks (MOF) consists in using preformed complexes as building blocks able to assemble via coordination chemistry and/or charge-assisted H-bonding. Within this framework and depending only on the experimental conditions, reactions of the [Zr(C2O4)4]4- anion with the [DabcoH2]2+ dication (Dabco ) 1,4-diazabicyclo[2. 2. 2]octane) selectively afford the compounds {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O (1), {(DabcoH2)2[Zr(C2O4)4]} · 3H2O (2), {(DabcoH2)2[Zr(C2O4)4]} · 5.5H2O (3), or {K2(DabcoH2)2[Zr2(C2O4)7]} · 6H2O (4). Three association schemes exhibiting drastically different network topologies have been characterized ranging from 1D to 3D; the fourth involving the in situ generated bimetallic {Zr2(C2O4)7}6- moiety. For all four compounds, selective synthetic processes have been developed. In addition, compound 1 exhibits an open framework able to release solvates without collapsing. Besides the reversible sorption of small molecules like EtOH and MeCN, the open framework 1 was found to adsorb CO2 at ambient temperature and pressure (ca. 30 cm3 of gas g-1 at 298 K), whereas its propensity to take up H2O from air is low. Introduction Within the area of supramolecular chemistry, the design and synthesis of open frameworks with remarkable structural, chemical and physical properties have drawn an increasing interest over the past few years. Among these, the 3D coordination polymers known as metal-organic frameworks (MOF) are the most investigated. The interest for supramolecular frameworks is not only for their prospective applications as functional materials1 but also for their intriguing and diverse molecular topologies and assembling processes. 2–12 The approach toward development of this kind of functional architectures consists in using molecular building blocks able to assemble via coordination chemistry and/or H-bonding. Compared to the “classical” inorganic materials, this flexible approach exhibits a rational and more resourceful design methodology to achieve supramolecular architectures with specifically tailored properties. Given the diversity and versatility of building units (inorganic, organometallic, or organic) potentially available and their intrinsic properties, such materials can display an extensive variety of customized physical or chemical properties, such as gas storage,13–15 sensors,16 sorption and separation processes,17,18 catalysis,16 ion-exchange,20 luminescence,21 guest-driven magnetic behaviors,22–25 etc.5,26 Until now, the widely used route to form a metal-organic framework has consisted of the direct assembly of a metal ion with a bridging ligand. The resulting network relies on the formation during the association process of the so-called secondary building units. In contrast, the use of a preformed coordination compound as a molecular building block remains hardly considered for the construction of porous frameworks. 16,21,27–40 However, the efficiency of this latter route has already been demonstrated in the conception of numerous molecule-based materials like bimetallic magnets. 41,42 Typically, such a building block consists of a metal center surrounded by ligands that are able to connect to a second building unit or * Corresponding author. E-mail:
[email protected]. Web address: http:// www.lcc-toulouse.fr/lcc/accueil.php3?lang)en. Fax: (33) 561 55 30 03.
metal ion by available functional groups. This may take place either via covalent (metal-ligand) or hydrogen bonds. As a result, the use of such preformed building blocks selected according to their geometry and linking abilities allows to design a node of the network and consequently the topology of the final supramolecular assembly. In this context, archetypal examples of building blocks we have investigated are the pseudotetrahedral shaped coordination complexes [M(C2O4)4]4(M ) Zr(IV), U(IV), C2O42- ) oxalate) that display four potential oxalate linkers. This geometry is in favor of obtaining high-dimensional architectures and is efficient at spacing the associated units. In a previous work, the association of [M(C2O4)4]4- building units with M2+ metal ions led us to the genesis of original supramolecular nanoporous architectures in which the 3D heterometallic frameworks are developed through the linkages between the oxygen atoms of the [M(C2O4)4]4units and the Mb2+ metal ions. 27,36,37 Considering that these oxygen atoms may also act as efficient H-bond acceptors, we have explored the possibility to develop hydride architectures by assembling this anionic coordination complex with appropriate organic cations through charge-assisted H-bonds. Within this latter approach, we have recently reported a series of architectures constructed by combination of [Zr(C2O4)4]4- with different cationic N-H donors able to act as bridges.43 Depending on the features of the organic modules (geometry, dimensions) and the nature of the donor groups, assembling patterns ranging from 1D to 3D have been observed. Taking into account these observations, we have considered the association of [Zr(C2O4)4]4- with {DabcoH2}2+ (Dabco ) 1,4-diazabicyclo[2. 2. 2]octane) with the aim of generating an open framework. We have observed that under selective experimental conditions, the assembling processes of the [Zr(C2O4)4]4- anion with the [DabcoH2]2+ dication singularly differ leading exclusively to either compounds {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O (1), {(DabcoH2)2[Zr(C2O4)4]}. 3H2O (2), {(DabcoH2)2[Zr(C2O4)4]} · 5.5H2O (3) or {K2(DabcoH2)2[Zr2(C2O4)7]} · 6H2O (4). For all four compounds, selective synthetic
10.1021/cg700971n CCC: $40.75 2008 American Chemical Society Published on Web 05/13/2008
Modular Assembling of [Zr(C2O4)4]4- and [DabcoH2]2+
Crystal Growth & Design, Vol. 8, No. 6, 2008 1871
Table 1. Crystallographic Data and Structural Refinement Parameters for Compounds 1-4
empirical formula formula mass cryst syst space group T (K) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Dc (g cm-3) Z µ (mm-1) no. of reflns measured no. of unique reflns/Rint no. of reflns with I > nσ(I) NV R Rw GOF ∆Fmax/∆Fmin (e Å-3) Flack param
1
2
3
4
C14H27K2N2O22.5Zr1 752.78 monoclinic P21 180 10.225(5) 19.053(5) 13.953(5) 90 99.509(5) 90 2681.1(2) 1.83 4 0.822 23 433 11152/0.04 8964 (n ) 3) 684 0.0432 0.0540 1.029 +1.23/-0.95 0.29(4)
C20H34N4O19Zr1 725.73 monoclinic P21/c 105 9.995(1) 13.635(1) 20.192 (1) 90 103.156(5) 90 2679.7(2) 1.799 4 0.510 25 361 7186/0.09 3152 (n ) 2.1) 312 0.0985 0.0985 1.110 +2.46/-1.51
C20H39N4O21.5Zr1 770.76 monoclinic P21/n 180 12.066(5) 14.581(5) 16.927(5) 90 93.872(5) 90 2971.2(2) 1.72 4 0.471 25 586 7934/0.02 6289 (n ) 3) 424 0.0274 0.0316 1.050 +0.66/-0.44
C26H40K2N4O34Zr2 1213.25 monoclinic P21/c 180 13.391(1) 10.388(1) 14.712(1) 90 90.875(9) 90 2046.3(3) 1.97 2 0.835 20 496 3837/0.04 2963 (n ) 1.5) 292 0.0390 0.0450 1.099 +1.09/-0.79
processes involving crucial stoichiometry and kinetic parameters have been developed. Compound 1 exhibits an open 3D framework able to release solvates without collapsing. The resulting guest-free host then becomes accessible for the reversible sorption of small molecules. Herein we report the syntheses, a detailed structural discussion of the four compounds as well as TGA, XRPD, and preliminary solvent and gas sorption studies of the porous compound 1. Results and Discussion Syntheses and Characterization. From our previous studies it appeared that a N-H donor unit with a single H atom available for bonding was best suited to develop roomy networks. The selected [DabcoH2]2+ dication disposes of two N-H units positioned in such a way that they permit to bridge two [Zr(C2O4)4]4- units. Moreover, its thickness to length (HN · · · NH segment) ratio should avoid network catenation (i.e., interpenetration). The 1:2 stoichiometric reaction of K4[Zr(C2O4)4] and Dabco in an acidic (pH ∼2) aqueous solution affords {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O, 1, as colorless crystals. A similar reaction in a 1:4 stoichiometry results in the formation of colorless crystals of {(DabcoH2)2[Zr(C2O4)4]} · 3H2O, 2, or {(DabcoH2)2[Zr(C2O4)4]} · 5.5H2O, 3. The selective formation of 2 or 3 depends on the final step of the crystallization procedure, i.e., vapors diffusion of methanol for 2 and slow evaporation under aerobic conditions for 3. Finally and with the initial aim to further substitute the potassium cations in {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O by [DabcoH2]2+ dications, the reaction of 1 and Dabco in a 1:2 molar ratio in an acidic (pH ∼2) aqueous solution leads to colorless crystals of {K2(DabcoH2)2[Zr2(C2O4)7]} · 6H2O, 4. The crucial role of the kinetic parameter in these experimental processes must be pointed out. Crystals of 1 containing the [Zr(C2O4)4]4- building block have to be kept in H2O for one week to evolve and form crystals of 4, which contain the bimetallic [Zr2(C2O4)7]6- moiety. The kinetics of the crystallization seems to be the determinant factor in the selective synthesis of 2 or 3. The reason for the incorporation of the potassium cation only in compounds 1 and 4 is not clear for
the moment; however, on the basis of the procedures used, the molar ratio between K4[Zr(C2O4)4] · 5H2O and {DabcoH2}2+ can be mentioned as a main factor. The four different compounds thus obtained were characterized by single crystal X-ray diffraction, infrared spectra and chemical analyses. Although IR spectroscopy is usually an appropriate technique in order to reveal the nature of interactions between C2O42- and its surroundings, through hydrogen or coordination bonds,44 it cannot be used as a diagnostic tool to unambiguously distinguish between 1-4 because of the high similarity of their infrared spectral patterns. Crystal Structures. All the compounds 1-4 have been characterized by single crystal X-ray diffraction; crystallographic data and structure-refinement parameters are given in the Experimental Section and Table 1. In the following description, the oxygen atoms of the oxalate ligands coordinated to zirconium (in the [Zr(C2O4)4]4- and [Zr2(C2O4)7]6- moieties) are referred to as “internal” (or “inner”) whereas the remaining ones are referred to as “external” (or “outer”) oxygen atoms. Compound {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O, 1, crystallizes in the P21 space group. The asymmetric unit of 1 consists of two enantiomeric [Zr(C2O4)4]4- anions, two {DabcoH2}2+ cations, four potassium cations, and 13 water molecules all in general positions (see the Supporting Information). The two nonequivalent Zr(IV) ions contain four oxalate ligands in their coordination spheres. Each of the four non equivalent potassium cations (K1-4) are linked to four external oxygen atoms of two oxalato groups from two equivalent [Zr(C2O4)4]4- moieties (Zr1 for K1-2 and Zr2 for K3-4) and to two internal oxygen atoms of two different oxalate ligands from the same [Zr(C2O4)4]4- moiety (Zr2 for K1-2 and Zr1 for K3-4) with bond lengths ranging from 2.692 to 2.863 Å (Figure 1). Each [Zr(C2O4)4]4- is connected to six K+ cations, whereas each potassium is linked to three zirconium units developing a 3D network. The resulting scaffold frames apertures generating channels where both the [DabcoH2]2+ units and the water molecules are located (Figure 1). In the final architecture, the roles of the two {DabcoH2}2+ modules is singularly different. The first {DabcoH2}2+ unit (with N1 and N2 as nitrogen atoms) is linked to the coordination polymer formed by the [Zr(C2O4)4]
1872 Crystal Growth & Design, Vol. 8, No. 6, 2008
Figure 1. {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O, 1: (top) detail of the interconnection between the {Zr(C2O4)4}4- and K+ ions; (middle) view of the framework along the a-axis showing the channels with their guest molecules (H2O and {DabcoH2}2+); (bottom) view revealing the perpendicular arrangement of the two type of {DabcoH2}2+ units in the channels (H2O not shown).
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and K by its two NH moieties forming H-bonds with two external oxygen atoms from two different oxalate ligands of two Zr2 units (N1-H11 · · · O24, N2-H21 · · · O27; Table 2) The second {DabcoH2}2+ unit (with N3 and N4) has no interactions with the framework but is H-bonded to two uncoordinated water molecules (N3-H31 · · · O39, N4-H41 · · · O34). It may be noticed that the organic cations do not directly act on the dimensionality of the structure since compound 1 has already a 3D structure based on the Zr(oxalate)K network; nevertheless, the first {DabcoH2}2+ can be seen as a pillar contributing to the topology of the framework. The diamond-like shaped channels are running through the structure along the a axis with apertures of approximately 10 Å × 13 Å containing the lattice held solvent molecules. The potential free volume accessible for small molecules (H2O) determined by PLATON45 calculations is 450 Å3 per unit cell volume (2681 Å3), which represents almost 17% of void per unit volume of 1. Compound 2, {(DabcoH2)2[Zr(C2O4)4]} · 3H2O crystallizes in the P21/c space group. The asymmetric unit of 2 (see Supporting Information) consists of one [Zr(C2O4)4]4- anion, two {DabcoH2}2+ cations and four water molecules (including two with an occupancy factor of 0.58/0.42) all in general positions. Differing from 1, the anionic and cationic modules are interconnected via hydrogen bonds in compound 2 developing a 1D network (Figure 2). Thus, one {DabcoH2}2+ is bridging two [Zr(C2O4)4]4- units with its two N-H moieties (N1 and N2) bound to two (one outer and one inner respectively) oxygen atomsoftwooxalateligands(N1-H11 · · · O16andN2-H21 · · · O2). In comparison, the second {DabcoH2}2+ unit adopts a terminal mode. Its two N-H moieties (N3 and N4) are linked to two oxygen atoms belonging to an uncoordinated H2O molecule and to an oxalate (outer oxygen) respectively (N4-H41 · · · O18 and N3-H31 · · · O4). Because of the involvement of a H2O molecule, the second {DabcoH2}2+ unit does not participate in the dimensionality of the network. As a result, each zirconium unit is connected to two [Zr(C2O4)4]4- by two bridging [DabcoH2]2+ units developing a zigzag chain running along the b direction (Figure 2). The intrachain Zr · · · Zr distances (13.19(1) Å) are much longer than the interchain ones (9.995(1)Å) as the adjacent chains tend to overlap in a shifted stacking mode. Compound {(DabcoH2)2[Zr(C2O4)4]} · 5.5H2O, 3, crystallizes in the P21/n space group. The asymmetric unit of 3 (see the Supporting Information) consists of one [Zr(C2O4)4]4-- anion, two {DabcoH2}2+ cations and six water molecules (including one with an occupancy factor of 0. 5) all in general positions. Similar to compound 2, compound 3 consists in a hydrogenbonded network involving the [Zr(C2O4)4]4- and {DabcoH2}2+ units. The main difference between the 2 and 3 lies in the respective involvement of the two {DabcoH2}2+ modules. In compound 2, the zirconium units are interconnected only by one of the two organic cations. In comparison, for 3 the two act as a bridge between the anionic units (Figure 3). Their two nitrogen atoms (N1 and N2 of the first unit and N3 and N4 of the second one) are linked by hydrogen bonds to two oxygen atoms (one inner and one outer respectively) of two oxalate moieties from two [Zr(C2O4)4]4- units (i.e., N1-H11 · · · O13, N2-H21 · · · O16, N3-H31 · · · O1, and N4-H41 · · · O4). Here, both the {DabcoH2}2+ units contribute to the dimensionality of the supramolecular network. Consequently, each Zr unit is connected to four [Zr(C2O4)4]4- by four bridging [DabcoH2]2+ cations developing 2D zigzag layers running along the [110] direction (Figure 3). Compound {K2(DabcoH2)2[Zr2(C2O4)7]} · 6H2O, 4, crystallizes in the P21/c space group. The asymmetric unit of 4 (see
Modular Assembling of [Zr(C2O4)4]4- and [DabcoH2]2+
Crystal Growth & Design, Vol. 8, No. 6, 2008 1873
Table 2. Selected Hydrogen Bonds atoms D-H- A
dist. D-H (Å)
dist. H-A (Å)
dist. D-A (Å)
angle D-H-A (deg)
2.765(6) 2.754(5) 2.648(5) 2.676(5)
171.0 171.0 168.0 172.0
2.64(2) 2.73(2) 2.78(2) 2.725(2)
148.0 156.0 150.0 171.0
2.919(2) 2.894(2) 2.808(2) 2.910(2) 2.934(2) 2.877(2) 2.898(2) 2.796(2) 2.801(2) 2.805(2) 2.769(2) 2.805(2) 2.873(2)
167.0 177.0 176.0 156.0 164.0 175.0 148.0 173.0 144.0 165.0 150.0 128.0 148.0
2.872(5) 2.745(2) 2.824(5) 2.825(2) 2.835(2) 2.719(5)
157.0 130.0 144.0 151.0 146.0 164.0
a
Compound 1 N1sH11sO24 N2sH21sO27i N3sH31sO39 N4sH41sO34ii
0.8600 0.8600 0.8600 0.8600
1.9100 1.9000 1.8000 1.8200 Compound 2b
i
N1sH11sO16 N2sH21sO2ii N3sH31sO4iii N4sH41sO18ii
0.8600 0.8600 0.8600 0.8600
1.8800 1.9200 1.9900 1.8700 Compound 3c
O17sH1sO15 O17sH2sO7i O18sH3sO11ii O18sH4sO8iii O20sH5sO5i O20sH6sO19 O21sH7sO2iv O21sH8sO12 N1sH11sO13 N2sH21sO16v N3sH31sO1ii N4sH41sO3i N4sH41sO4i
0.9100 0.9200 0.8700 0.8600 0.8800 0.8800 0.8900 0.8700 0.8600 0.8600 0.8600 0.8600 0.8600
2.0200 1.9800 1.9400 2.1000 2.0800 2.0000 2.1000 1.9300 2.0600 1.9600 1.9900 2.1900 2.1100 Compound 4d
i
O16sH161sO10 N1sH1sO8ii N2sH2sO17iii O15sH152sO17 O17sH171sO12iv O17sH172sO16v
0.8300 0.8800 0.8900 0.8400 0.8300 0.8200
2.0900 2.1000 2.0500 2.0600 2.1100 1.9200
a Symmetry transformations for compound 1: (i) 1 + x, y, z; (ii) -1 + x, y, z. b Symmetry transformations for compound 2: (i) -1 + x, y, z; (ii) 1 x, 0.5 + y, 1.5 - z; (iii) 1 - x, -y, 1 - z. c Symmetry transformations for compound 3: (i) 0.5 + x, 1.5 - y, -0.5 + z; (ii) 1 - x, 1 - y, 1 - z; (iii) 1 + x, y, z; (iv) 0.5 - x, -0.5 + y, 0.5 - z; (v) -0.5 + x, 1.5 - y, -0.5 + z. d Symmetry transformations for compound 4: (i) x, 1.5 - y, 0.5 + z; (ii) 2 - x, 1 - y, 1 - z; (iii) 1 - x, -0.5 + y, 0.5 - z; (iv) 1 - x, 2 - y, 1 - z; (v) 1 - x, 0.5 + y, 1.5 - z.
Figure 2. View of the 1D net developed by {(DabcoH2)2[Zr(C2O4)4]} · 3H2O, 2.
the Supporting Information) consists of half a [Zr2(C2O4)7]6anion, one {DabcoH2}2+ cation, one potassium cation, and three water molecules all in general positions. In the centrosymmetric
anionic unit [Zr2(C2O4)7]6-, the two equivalent zirconium ions are linked by a bridging oxalate (C7 as central carbon, Figure 4). Among the three remaining oxalate ligands surrounding each zirconium, only two act as bridging ligands, the third one being terminal. Each K+ is connected to two external oxygen atoms of one oxalato group from one [Zr2(C2O4)7]6- unit, two internal oxygen atoms of two different oxalates from another [Zr2(C2O4)7]6- unit, and two oxygen atoms (one internal, one external) of the same oxalate from a third anionic unit (Figure 4). Moreover, the coordination sphere of each K+ contains two H2O molecules (O15 and O16) including one in a weak bond mode. As a result, each [Zr2(C2O4)7]6- is linked to six K+ by oxalate bridges while each potassium center is connected to three bis-zirconium units. The structure can be more easily understood by taking into account the [K6Zr2] motif represented in Figure 4 as elemental unit. Thus, the architecture can be seen as a succession of such units in the bc plane leading to a 2D layer. The {DabcoH2}2+ units are located between the layers and are linked only by one N-H group to the framework developed by the Zr and K coordination polymer. This linkage occurs between the N1 atom and an external oxygen atom (O8) from an oxalate ligand while the second ammonium moiety is hydrogen-bonded to an uncoordinated H2O (N2-H2 · · · O17). Both structures 1 and 4 display extended {Zr(oxalate)K} coordination networks. The {DabcoH2}2+ units act mainly as a template agent as they do not directly contribute to the dimensionality of the two compounds even if involved in H-bonding with the framework. The incorporation of the potassium cation, which has a major role in the crystal packing,
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Figure 4. {K2(DabcoH2)2[Zr2(C2O4)7]} · 6H2O, 4: (top) detail of the linkages between the [Zr2(C2O4)7]6+ and K+ units generating 2D coordination networks, (bottom) view highlighting the intercalation of the organic cations between the inorganic layers (the H2O are not shown).
Figure 3. Organization of {(DabcoH2)2[Zr(C2O4)4]} · 5.5H2O, 3: (from top) detail of the hydrogen bond linkages between the anionic and cations units; resulting 2D hydrogen-bonded network (for clarity, the Dabco unit is represented by a pseudobond (yellow) linking its two NH groups, the CH2 are hidden; the H2O solvates are not shown).
is observed only in those two compounds 1 and 4. Although the reason is not clear for the moment, the initial stoichiometry of the building units can be mentioned as a main factor.
In contrast to compounds 1 and 4, the potassium ions are not involved in the genesis of compounds 2 and 3. The two compounds 2 and 3 present very similar formulas, but they differ significantly from a structural point of view. For both the compounds 2 and 3, {DabcoH2}2+ units are engaged as bridges between [Zr(C2O4)4]4- anions and directly contribute to the dimensionality of the structure with networks developed only through charge-assisted hydrogen bonds. The difference between 2 and 3 relies on the fact that in 2 only one of the two organic cations acts as a bridge, which leads to a network of lower dimensionality. It is also worthy to note that in both 2 and 3, only two of the four oxalate ligands of the anionic coordination compound are involved in the interunit interactions while they are all solicited in the 3D coordination network of 1. All those results lead to underline the dramatic importance of the experimental procedure in this system. The formation of a given architecture is dependent not only on the characteristics of the building units but also on the syntheses/crystallization conditions. TGA, XRPD, and Preliminary Sorption Studies of {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O, 1. The thermogravimetric analyses (TGA) of compound 1 reported in Figure 5 indicate a one step mass decrease of ∼14. 5% in the temperature range 30-120 °C corresponding to the loss of six H2O molecules approximately. This behavior reveals that all the water molecules located in the framework of 1 are progressively released under
Modular Assembling of [Zr(C2O4)4]4- and [DabcoH2]2+
Figure 5. Thermogravimetric analyses of 1 as synthesized (black), after MeCN (red), and EtOH (blue) sorption (see text).
Figure 6. Temperature dependence of the X-ray powder diffractogram recorded for compound 1. From bottom: as synthesized at 25, 50, 80, and 120 °C after H2O release.
mild conditions. All solvent molecules are removed at 120 °C and the guest free compound remains stable up to 200 °C. The slight difference in H2O content between the samples used for TGA and the corresponding crystal structure data can be attributed to a partial release at room temperature. Further information about the behavior of this open framework upon solvent release was provided by the X-ray powder diffractograms (XRPD) recorded at different temperatures; respectively 25, 50, 80, and 120 °C (Figure 6). The evolution of the XRPD pattern during the desorption process (from 25 to 120 °C) chiefly consists of a modulation in the relative intensity of some of the diffraction peaks up to 80 °C with a progressive appearance of new peaks seen for the dehydrated sample at 120 °C. The diffraction patterns recorded in the range 25-120 °C show that the sample remains crystalline; however, broader and weaker signals observed from 80 °C and upward reveal a partial loss of crystallinity upon solvent removal. Upon exposure to moist air, the evacuated framework readsorbs H2O molecules. However, it has to be underlined that the sorption of the water molecules in 1 is a slow process that
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Figure 7. X-ray powder diffractograms recorded for 1 containing (from bottom) CO2, H2O, EtOH, and MeCN as guest molecules.
requires the use of an air humidificator to be speed up but remain slow even with. Besides, the ability of sorption of other small molecules like ethanol and acetonitrile has been investigated to determine if the free space of the porous framework is accessible. The sorption experiments have been realized as follows: under a N2 atmosphere, a crystalline sample of compound 1 was dehydrated by heating at 120 °C and then the guest-free host immersed in the liquid phase of the appropriate distilled and degassed guest-compound. The resulting samples were investigated by XRPD and TGA techniques (Figure 5 and 7). The thermogravimetric analyses of the compounds after sorption of EtOH and MeCN indicated a mass decrease of 6.83 and 7.53%, respectively, in the temperature range 30-120 °C corresponding to the loss of 1.0 and 1.25 mol of guest molecules per mol of degassed 1, respectively. These values are just indicative because of a probable partial release occurring already in air at 298 K. With both ethanol and acetonitrile as guest molecules the XRPD patterns reveal new phases with a full recovery of the crystallinity, which can be clearly assigned to the sorption of the guest molecules in the porous solid (Figure 7). It is worth specifying that reversible structure transformations triggered by guest molecules of the crystal phase in particular are not common in zeolites but have been found in some coordination polymers. As discussed by Kitagawa et al.,9 such transformation is attributed to a combination of variations of the three following features: coordination bonds, hydrogen bonds and other week nonbonding interactions (Van der Waals forces). Still with the aim of exploring the sorption properties of 1, we investigated the sorption ability in regard of CO2 gas. The sorption experiment has been realized as follows: in the TGA instrument and under CO2 atmosphere, a crystalline sample of compound 1 was first dehydrated by heating at 120 °C and then cooled to 298 K and maintained at that temperature afterward; this was followed by a second sorption cycle. The CO2 uptake has been clearly established by recording the mass increase of the degassed sample (Figure 8). The first CO2 sorption passes by a rounded maximum (ca. 44 cm3 g-1) before reaching a plateau value of 34.2 cm3 g-1. Upon heating, the adsorbed CO2 is released but again taken up as temperature is decreased. For this second sorption the saturation is reached smoother and faster (3 h) than for the first sorption. The uptake is 27.3 cm3 g-1, which corresponds to the sorption of 0.71 mol of carbon dioxide per mole of 1. The unusual sorption behavior observed for the
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the migration of the guests into the framework. Such a pore blocking does not apply with CO2. Experimental Section
Figure 8. TGA under CO2 atmosphere (760 mmHg) for 1: (blue) two sorption cycles after initial H2O release, the CO2 uptake is give as sample weight increase (%) and corresponding volume of gas g-1 of guest-free 1; (red) temperature of the sample.
first uptake might be ascribed to a structural change on adsorption that modifies the effective porosity of 1.15 Such a framework deformation is supported by the PXRD (Figure 7) recorded for 1 after CO2 sorption (recorded under CO2 atmosphere), which shows a high degree of crystallinity and a diffraction pattern different from those obtained with the other guest molecules investigated. Conclusion Through the strategy of using a preformed coordination compound as anionic building unit likely to be assembled with an organic cation into extended networks by charge-assisted hydrogen bonding, the association of the [Zr(C2O4)4]4- anion and the {DabcoH2}2+ dication has been envisaged for the preparation of a supramolecular nanoporous architecture. While such an open framework has been achieved, we have observed that the assemblage of [Zr(C2O4)4]4- and {DabcoH2}2+ is strongly dependent on the experimental conditions. Three association schemes exhibiting drastically different networks topologies have been characterized ranging from 1D to 3D. A fourth compound was also obtained but containing the anionic moiety [Zr2(C2O4)7]6-, due to the evolution of the initial [Zr(C2O4)4]4- anion during the association process. All those results lead to the point that the assembling scheme of the building units considered here is also dependent on the synthetic conditions and not solely on the characteristics of the starting materials. This may be ascribed directly to the weaker hydrogen bond, which allows an equilibrium between several association options in solution, the crystallization of one of these coexisting species being favored by the experimental conditions (stoichiometry, concentration, or solvent combination, time). Besides the reversible sorption of small molecules like EtOH and MeCN, the open framework 1 was found to adsorb CO2 at ambient temperature and pressure (ca. 30 cm3 of gas g-1 at 298 K), whereas its propensity to take up H2O from air is low. The slow sorption of H2O is ascribed to a blocking of the entrance of the channels by the first H2O taken up by the solid. These H2O molecules form hydrogen bonds with the “free” {DabcoH2}2+ units and among them, which drastically reduce
General. All reagents were purchased from commercial sources and used as received. K4[Zr(C2O4)4] · 5H2O has been prepared according to the reported procedure. 46 Infrared spectra were recorded in the range 4000-400 cm-1 as KBr pellets on a FTIR Perkin-Elmer spectrum GX 2000 spectrometer. Elemental analyses were performed using a PerkinElmer 2400 series II instrument. Crystallographic Studies. Intensity data were collected at low temperature (105 K for 2 and 180 K for 1, 3, and 4) on an Xcalibur Oxford Diffraction or STOE IPDS diffractometer using a graphitemonochromated Mo KR radiation source and equipped with an Oxford Cryosystems cryostream cooler device. Structures were solved by direct methods using SIR9247 and refined by full-matrix least-squares procedures on F using the programs of the PC version of CRYSTALS.48 Atomic scattering factors were taken from the International tables for X-ray crystallography.49 Except for the water molecules of 1 and 4, all non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to carbon atoms were located in a difference map, then repositioned geometrically and refined using a riding model. Only a few hydrogen atoms could be located by Fourier difference on water molecules. Absorption corrections were introduced using the program MULTISCAN. The number of water molecules for each compound was confirmed by chemical analyses and TGA measurements. Pertinent crystallographic data and structural refinement parameters are listed in Table 1. The powder X-ray diffraction pattern was collected on a XPert Pro (θ-θ mode) Panalytical diffractometer with λ(CuKR1, KR2) ) 1.54059, 1.54439 Å. coupled to an Anton Parr oven. All data were collected in 5° < 2θ < 50° range, with 0.02 steps and 10 s of exposure. TGA measurements have been done on a Perkin-Elmer Diamond TG/ DTA instrument. The compounds were heated at 1 °C min-1. {K2(DabcoH2)[Zr(C2O4)4]} · 6.5H2O, 1. K4[Zr(C2O4)4] · 5H2O (300 mg, 0.43 mmol) was slowly added under aerobic conditions to an acidic aqueous solution (12 mL; pH ∼2) of Dabco (100 mg, 0.87 mmol) with continuous stirring. Vapors diffusion of methanol into the resulting solution, at room temperature, afforded colorless crystals of 1 (Yield: 320 mg, 98%), which were filtered and air-dried. Anal. Calcd for C14H27N2O22.5K2Zr (752.78): C, 22.3; H, 3.6; N, 3.7. Found: C, 22.6; H, 3.3; N, 3.7. IR (cm-1): ν 3440br, 3011w, 2924w, 2805w, 1742m, 1716s, 1688vs, 1397s, 1320w, 1289m, 1269m, 1198w, 1053m, 907m, 858w, 851w, 802m, 793m, 599br, 523m, 490br. Anal. Calcd for 1 after sorption of EtOH (C14H14N2O16K2Zr + 0.8 C2H5OH (672.5)): C, 25.0; H, 2.1; N, 4.2. Found: C, 25.3; H, 2.3; N, 4.0. Anal. Calcd for 1 after sorption of MeCN (C14H14N2O16K2Zr + 1.2 CH3CN (684.95)): C, 24.5; H, 2.1; N, 4.1. Found: C, 24.6; H, 2.0; N, 4.0. {(DabcoH2)2[Zr(C2O4)4]} · 3H2O, 2. K4[Zr(C2O4)4] · 5H2O (172 mg, 0..25 mmol) was slowly added under aerobic conditions to an acidic aqueous solution (14 mL; pH ∼2) of Dabco (114 mg, 1 mmol) with continuous stirring. Slow addition of methanol (12 mL) to the resulting solution, with continuous stirring, led to the precipitation of a white powder which was filtered, air-dried and redissolved in water. Vapors diffusion of methanol into the final solution, at room temperature, afforded colorless crystals of 2 (Yield: 140 mg, 78%), which were filtered and dried in air. Anal. Calcd for C20H34N4O19Zr1 (725. 72): C, 33.1; H, 4.7; N, 7.7. Found: C, 33.3; H, 4.4; N, 7.9. IR (cm-1): ν 3564br, 3010w, 2804w, 1716s, 1686vs, 1651s, 1403s, 1287m, 1268m, 1202w, 1052m, 997w, 906m, 858w, 849w, 801m, 792m, 525w, 482w. {(DabcoH2)2[Zr(C2O4)4]} · 5.5H2O, 3. K4[Zr(C2O4)4] · 5H2O (172 mg, 0.25 mmol) was slowly added under aerobic conditions to an acidic aqueous solution (14 mL; pH ∼2) of Dabco (114 mg, 1 mmol) with continuous stirring. Slow addition of methanol (12 mL) to the resulting solution, with continuous stirring, led to the precipitation of a white powder that was filtered, air-dried, and redissolved in water. Slow evaporation of the final solution, at room temperature, resulted in the formation of colorless crystals of 3 (Yield: 106 mg, 56%), which were filtered and air-dried. Anal. Calcd for C20H39N4O21.5Zr1 (770. 76): C, 31.2; H, 5.1; N, 7.3. Found: C, 31. 3; H, 4.5; N, 7.4. IR (cm-1): ν 3564br cm-1, 3009w, 2803w, 1716s, 1687vs, 1652s, 1406s, 1384s, 1288m, 1268m, 1202w, 1052m, 998w, 906m, 858w, 849w, 802m, 791m, 522w, 489w. {K2(DabcoH2)2[Zr2(C2O4)7]} · 6H2O, 4. Compound 1 (145 mg, 0.19 mmol) was slowly added under aerobic conditions to an acidic aqueous
Modular Assembling of [Zr(C2O4)4]4- and [DabcoH2]2+ solution (10 mL; pH ∼2) of Dabco (44 mg, 0.38 mmol) with continuous stirring. The resulting solution was slowly concentrated till crystals of 1 start to form and then covered as such for 1 week at room temperature. During that interlude, crystals of 1 dissolve and evolve into colorless crystals of 4 (Yield: 78 mg, 68%), which were filtered, and air-dried. Anal. Calcd for C26H40N4O34K2Zr2 (1213.24): C, 25. 7; H, 3.3; N, 4.6. Found C, 25.7; H, 3.3; N, 4.5. IR (cm-1): ν 3424br cm-1, 3048w, 2805w, 1712vs, 1692vs, 1679vs, 1652vs, 1482w, 1403s, 1382s, 1311w, 1273m, 1235w, 1160w, 1054m, 1001w, 979w, 903m, 853w, 811s, 806s, 797s, 527m, 495m, 423w.
Acknowledgment. The authors gratefully acknowledge the CNRS (Centre National de la Recherche Scientifique) for financial support. F.T. thanks FSE for a grant, and T.S.V. was supported by the Centre Franco-Indien pour la promotion de la Recherche Avance´e/Indo-French Centre for the promotion of Advanced Research (Project 3108-3). The authors thank J.-F. Meunier (LCC) for technical assistance in TGA measurements and Dr. P. Guionneau (Bordeaux University) and Dr. L. Vendier (LCC) for technical assistance in XRPD measurements. Supporting Information Available: ORTEP plots with numbering scheme and tables of bond lengths and angles for compounds 1-4, view of the oxalate-bridged network for 1 (PDF). X-ray crystallographic information files in CIF format are available for all compounds (1-4). This material is available free of charge via the Internet at http://pubs. acs. org.
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