SU-22 and SU-23: Layered Germanates Built from 4-Coordinated Ge7 Clusters Exhibiting Structural Variations on the 44 Topology Lei Shi,†,‡ Charlotte Bonneau,*,†,‡ Yafeng Li,† Junliang Sun,†,‡ Huijuan Yue,†,‡ and Xiaodong Zou*,†,‡
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3695–3699
Structural Chemistry, Stockholm UniVersity, SE-106 91 Stockholm, Sweden, and Berzelii Centre EXSELENT on Porous Materials, Stockholm UniVersity, SE-106 91 Stockholm, Sweden ReceiVed March 21, 2008; ReVised Manuscript ReceiVed July 2, 2008
ABSTRACT: Two novel layered germanates, denoted as SU-22 and SU-23, have been synthesized hydro(solvo)thermally by using diethylenetriamine (DETA) and 1,7-diaminoheptane (DAHep) as structure directing agents (SDAs), respectively. Their structures were determined by single-crystal X-ray diffraction. Both structures are built from the same Ge7 clusters which are 4-coordinated following a 44 topology with different modes of linkage. SU-23 is the first example of a different mode leading to the formation of 10-rings, instead of 8- and 12-rings observed in the ASU-20 compounds and SU-22. A third type of linkage is also proposed. The study of the hydrogen bonding interaction pattern is developed to provide insight into the formation of these different modes in relation with the size and geometry of the SDAs, as they have a significant effect on the rotation and arrangements of Ge7 clusters. SU-22, [C4N3H15]1.5[Ge7O14X3] · H2O (X ) OH or F), monoclinic, space group C2/c, a ) 16.0583(8), b ) 16.4484(10), c ) 17.7788(8) Å, β ) 98.151(6)°, R1 ) 0.0430, for 4248 reflections with I < 2σ(I). SU-23, [C7N2H19][C7N2H20][Ge7O14X3][GeO2]0.2 · 3H2O, monoclinic, space group P21/n, a ) 13.036(3), b ) 9.4726(19), c ) 30.814(6) Å, β ) 100.03(3)°, R1 ) 0.0349 for 5741 reflections with I < 2σ(I). Introduction The design and synthesis of open-framework oxide materials with ever-increasing pore size and decreasing framework density have been interesting and challenging research fields in chemistry, due to the important applications of these materials in adsorption, separation, and catalysis in industry.1-3 It is noted that open-framework germanates have recently attracted great attention because these materials exhibit a rich structural diversity. The main advantage of incorporating germanium into silicate structures is that the Ge-O-Ge angles can be much smaller (≈ 130°) than the Si-O-Si angles (≈ 145°) and are favorable to form framework structures with 3- and 4-rings.4-6 Germanium can be four (tetrahedral), five (trigonal bipyramidal), and six (octahedral) coordinated to oxygen to form well-defined cluster building units of Ge-O coordination polyhedra, in contrast to the 4-coordinated silicon. So far, a great number of novel germanates with two-dimensional (2D) and threedimensional (3D) structures have been reported, and their structures are built from clusters such as Ge7O12X7 (Ge7),7-12 Ge9O14X12 (Ge9),13-16 Ge9O15X1017,18 (Ge9), and Ge10O16X12 (Ge10)19,20 (X ) O, OH, F). These clusters are much larger than the building units found in other systems and can be used to build open-framework structures with extra-large pores and very low framework densities. For example, 2D ASU-19 and ASU20 are built by 5- and 4-coordinated Ge7 clusters,12 respectively. Both the ASU-127 with 3D interconnected 8-, 10-, and 16-ring channels and ASU-169 (also SU-1211) with interconnected 8-, 12-, and 24-ring channels are formed by 5-coordinated Ge7 clusters. We have successfully obtained a novel 3D germanate structure SU-M19 based on Ge10 clusters forming 30-ring channels and pores extending to the mesoporous range ( 2σ(I)] R indices (all data) largest diff. peak and hole
In the absence of HF acid, only amorphous material is obtained for both gel compositions. Structure Determination. Single-crystal X-ray diffraction data were collected at room temperature on an Xcalibur3 diffractometer equipped with a CCD camera and Mo KR radiation (λ ) 0.71073 Å) from an enhanced optical X-ray tube for SU-22 and on a STOE IPDS diffractometer equipped with an image plate system, using graphitemonochromatized Mo KR radiation from a rotating anode for SU-23. Numerical absorption correction was applied, with a linear absorption coefficient of 9.024 for SU-22 and 5.774 mm-1 for SU-23. For SU22, a total of 14 487 reflections, of which 4248 are unique (Rint ) 0.0513), were collected in the region 3.69° < θ < 25.35°. For SU-23, a total of 20 209 reflections, of which 5741 are unique (Rint ) 0.0510), were collected in the region 3.59° < θ < 24.02°. The structures were solved by direct methods using the SHELX-97 software package.23 All the Ge and O atoms were refined anisotropically. The terminal atoms in both SU-22 and SU-23 were first refined as oxygen belonging to hydroxyl groups. Since the terminal oxygen coordinated to the Ge(O,F)6 octahedra had slightly lower atomic displacement parameters than the other terminal oxygen in both structures, they were then assigned as fluorine. The resulting atomic displacement parameters for the framework oxygen and fluorine became then very similar, indicating that fluorine is present in the Ge(O,F)6 octahedra. The presence of F- was also confirmed by energy dispersive spectroscopy performed on a SEM. Germanium atoms were found to be 4-, 5-, and 6-coordinated with oxygen or fluoride atoms. The C and N atoms of the organic molecules were refined isotropically in SU-22 and anisotropically in SU-23 by a full-matrix least-squares technique. In SU-23, an additional partially occupied tetrahedron was linked to the main structure and disordered at two positions ((Ge8A)O4 and (Ge8B)O4) with 9.3(3)% occupancy
Figure 1. (a) Ge7 cluster containing one GeO6 octahedra (red), two GeO5 trigonal bipyramids (yellow), and four GeO4 tetrahedra (green). (b) Abstraction of the cluster to a rigid rectangle with long (L) and short (S) sides. The long side (L) involves two tetrahedra with an octahedron or one pyramid in between, the connecting nodes being intercluster bridging oxygen atoms. The two neighboring tetrahedra define the short side (S). (c) Representation of the Ge7 cluster with the GeO6 octahedron up, shown as a red face. (d) Representation of the Ge7 cluster with the GeO5 trigonal bipyramids up, shown as a yellow face. each. No hydrogen atoms were added. Crystal data and detailed data collection and structure refinement for both compounds are given in Table 1.
Results and Discussion SU-22 and SU-23 belong to a family of compounds built from 4-coordinated Ge7 clusters, which have been encountered in several 2D and 3D germanate compounds. The cluster has a maximum symmetry of mm2 and consists of one GeO6 octahedron, two trigonal bipyramids GeO5, and four tetrahedra GeO4, with one 3-coordinated oxygen atom at its center (Figure 1a). Despite the fact that the cluster displays seven anions available for condensation, the connection in the compounds studied only occurs through the tetrahedra leading to 4-coordinated 44 plane nets. SU-22 belongs to the ASU-20 structure type displaying single layers of Ge7 clusters forming 8- and 12-rings in the ab plane that stack along the c-axis. This series of compounds illustrates remarkably well the structure directing role played by the diamines to induce subtle structural variations. If it remains highly speculative to make direct inferences on which type of organic molecules can produce a certain type of hydrogen bonding interactions, a number of relevant observations can still be made. The different diamines induce distinctive hydrogen bonding patterns particularly influencing the interlayer distance and the stacking pattern (Figure 2). In ASU-20-DACH, the 1,4diaminocyclohexane (DACH) molecules are found on both sides of the 12-rings (Figure 2a), while in ASU-20-DAPe and SU22, no diamines are found at the corresponding location (Figure 2b and 2c). Instead, the diamines (1,5-diaminopentane (DAPe) and diethylenetriamine (DETA), respectively) are found on both sides of the 8-rings more or less aligned with its long diameter. Both DAPe and DETA contain five atoms in their chain, and primary amino groups position themselves similarly between the layers. Unlike in ASU-20-DACH, where none of the amino groups of the same single molecule are involved in hydrogen bonding interactions between clusters within a layer, ASU-20DAPe and SU-22 both show one such molecule on either side of the layer, bridging the same two clusters. The hydrogen bonding pattern generated is equivalent in both compounds, and it becomes clear that the molecules act as a lever between the two Ge7 clusters, which contributes to the rotation of the clusters in respect of each other (Figure 3). The intercluster angle between the two clusters concerned becomes smaller for SU22 (127.6°) compared to ASU-20-DAPe (135.3°). This difference can be rationalized by examining their respective diamine molecules. The substitution of the carbon in the third position of the DAPe molecule by a nitrogen atom has two effects: the C-N bond is slightly shorter than a C-C bond, and more
Layered Germanates from 4-Coordinated Ge7 Clusters
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Figure 2. Different layered structures built by Ge7 clusters showing the location of the diamines in the interlayer viewed in parallel (top row) and perpendicular to (bottom row) the layers. (a) ASU-20-DACH; (b) ASU-20-DAPe; (c) SU-22 (DETA); (d) SU-23 (DAHep). Carbon and nitrogen atoms are represented in black and blue, respectively. The carbon backbone of DAHep in (d) is omitted for clarity, and only terminal amino groups are shown. The water molecules are shown by a red sphere. Dotted lines represent hydrogen bonding interactions.
Figure 3. Detailed geometries of the DAPe and DETA molecules and their influence of the intercluster angle in the same layer.
importantly, the central nitrogen atom is hydrogen bonded to one of the terminal amino groups of the same molecule. As a result, the distance between the nitrogen of the primary amines is 6.41 Å for the DETA molecule, instead of 6.93 Å for DAPe, which leads to the observed reduced intercluster angle for SU22 relative to ASU-20-DAPe. The central nitrogen in DETA, being already involved in interactions with a nitrogen atom of the same molecule, also becomes unavailable for hydrogen bonding with the Ge7 clusters. It is noteworthy that for the three compounds residual water molecules are scattered on both sides of the layers, without interacting particularly with any of the organic molecules. Overall, the intra- and intercluster Ge-O-Ge angles, as well as the bond distances in the ASU-20 series, are within the same range (Table 2). SU-22 contains 1.5 diamine molecules per Ge7 cluster (charge -3) respecting the balance of charges, analogously to ASU-20-DAPe and -DACH. SU-23 constitutes the first example of a layered germanate where Ge7 clusters adopt the same 44 topology found in the ASU-20 series and SU-22 but generate 10-rings instead of 8and 12-rings (Figure 2d). The staggered layer stacking obstructs the 10-ring windows, which avoids the formation of channels.
Figure 4. (a) Three types of arrangements exhibiting the 44 net topology. The sequences refer to the consecutive edges of rectangles within a 4-ring. (b) Geometric relationship between the three arrangements. (c) Possible arrangements resulting from the assignment of a face.
While the ASU-20 series and SU-22 have been obtained using diamines with different shapes but relatively short carbon chains, SU-23 was synthesized using a diamine with a long-carbon chain. It has been shown that for chain lengths of diamines that exceed six carbons the hydrophobic character of the carbon backbone becomes prominent. This regime may manifest itself in two ways in the SU-23 structure. First, it may provide clues regarding the parallel ordering of the DAHep molecules between
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Table 2. Bond Length and Bond Angles of the Average Intra- and Inter- Ge7 Clusters compound a
ASU-19 ASU-20-DACH ASU-20-DAPe SU-22 SU-23
Ge-O(Å)
(Å)
intra
Ge-O-Geinter cluster
1.71...0.2.18 1.73...0.2.03 1.69...0.2.21 1.73...0.2.13 1.72...0.2.07
1.79(9) 1.79(8) 1.78(10) 1.82(21) 1.79(8)
119(4)° 119(4)° 122(4)° 118.9(3)° 119(4)°
126.5...0.139.5° 131.9...0.141.8° 135.3...0.154.2° 127.5...0.140.3° 126.5...0.126.9°
a
ASU-19 is also listed as it is built from the same layer as the ASU-20 series. However, the layers are linked by a bridging GeO2X2 unit to form a slab.
Figure 5. Model layers of the LLSS arrangement. Ge7 clusters with GeO6 octahedra (a) all up, (b) alternating up and down in rows, and (c) alternating up and down in the rings.
the layers, instead of finding them disordered and entangled as could have been expected from a longer carbon chain. Second, due to the strong hydrophobic interactions from the carbon chains, water molecules become confined to the neighborhood of the terminal amino groups. Indeed, the distance between amino groups of neighboring molecules becomes fixed by hydrogen bonding to 2.6 Å on both sides of one water molecule, making the N-N distance 4.5 Å on the same side of the layer. Regarding the organization of the clusters in the layer, this short N-N distance may contribute to the formation of 10-rings instead of the 12-rings in the ASU-20 series, where the N-N distances between neighboring diamine molecules range from 5.7 to 6.9 Å. The terminal amino groups are located on both sides of the 10-rings and exhibit a well-defined dense network of hydrogen bonding interactions with the clusters (N-O distances: 2.9 Å), which also restrict the degree of tilting that the clusters can experience (Figure 2d). The intercluster Ge-O-Ge angles actually remain almost constant (126.5-126.9°, Table 2). The intracluster bond angles, as well as the bond distances in SU-23, fall in the range observed for the ASU-20 series and SU-22. SU-23 contains two DAHep molecules per Ge7 cluster, as the charge per cluster is always -3, which means that only 3/4 of the diamines are fully protonated. The intracluster Ge-O-Ge angles are consistent from structure to structure, whereas the intercluster Ge-O-Ge angles can vary from 126.5° to 154.2° (Table 2). This shows clearly that the flexibility of the layers resides between the Ge7 clusters regardless of the structure type and emphasizes that a Ge7 cluster can be considered as a rectangular rigid body with long (L) and short (S) sides and corners as 4-coordinated nodes (Figure 1b). There are only three possible ways of connecting the rectangles in their maximum symmetry, to obtain a layer with one kind of 4-coordinated rigid rectangle with a 44 net topology, as shown in Figure 4. We define the three arrangements by the sequence displayed by the consecutive edges of four rectangles resulting in the ring formation: LLLL (and/or SSSS), LSLS, and LLSS (Figure 4a). These three possibilities can be derived from each other by applying a 180° rotation along directions not included in the mirror planes of the rectangle, and such an operation transforms a short (S) edge into a long (L) edge and vice versa (Figure 4b). For each of these linkages, other configurations can be obtained if the rectangles are assigned a face: it results in a lower symmetry of the layer. In the simplest
case where the repeating motif only contains four rectangles, the assignment of a face generates four possible arrangements each, along with their mirror image through the plane they lie in (Figure 4c). The number of possibilities becomes infinite as more rectangles are added to the repeating motif. Experimentally, two arrangements have now been obtained in layered germanates, both with clusters alternating face up and down within a ring: the ASU-20 series and SU-22 corresponding to the sequence LLLL (or SSSS) and SU-23 to LSLS. The third LLSS remains to be discovered. A series of hypothetical isostructural LLSS layers of Ge7 clusters can be constructed with reasonable intercluster Ge-O-Ge angles, variable shifts, and interlayer distances with different up and down cluster configurations. Three such layers are displayed in Figure 5. It is arguable that configurations with all upward clusters may be found (although not yet observed experimentally), as the distance between the terminal oxygen for consecutive upward clusters remains reasonable. Targeting these configurations may be possible provided a thorough understanding of the occurrence of hydrogen bonding interactions between the clusters and the organic molecules. It is noteworthy that the three modes of linkage have been observed in 3D gallophosphate compounds built from hexameric clusters.24-27 Conclusion Two single-layered germanates, denoted SU-22 and SU-23, built from the same Ge7 clusters have been synthesized under the hydro(solvo)thermal conditions. For both compounds, the Ge7 clusters are 4-coordinated and exhibit a 44 net topology with different cluster arrangements, generating major structural differences in their rings. SU-22 has 8- and 12-rings, whereas SU-23 and the proposed model layer both contain 10-rings of a distinct nature. As demonstrated by this study, the Ge7 clusters can lead to numerous layered new compounds. 3D framework structures may also be built from the Ge7 clusters, if a fifth connection can be encouraged. This emphasizes the need for a thorough investigation of the effects of organic molecules on the condensation and degree of connection of the Ge7 clusters, if we are to target specific topologies. Acknowledgment. This project is supported by the Swedish Research Council (VR) and the Swedish Governmental Agency
Layered Germanates from 4-Coordinated Ge7 Clusters
for Innovation Systems (VINNOVA). Lei Shi, Charlotte Bonneau, and Yafeng Li are supported by post doctoral grants from the Wenner-Gren Foundation. Supporting Information Available: Crystallographic information files (CIF) for structures SU-22 and SU-23. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG800304R
