A Supramolecular Ladderlike Structure Formed by the Auto-Assembly

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A Supramolecular Ladderlike Structure Formed by the Auto-Assembly of Benzene-1,3,5-triphosphonic Acid Gary B. Hix,*,† Vincent Caignaert,‡ Jean-Michel Rueff,‡ Loı¨c Le Pluart,§ John E. Warren,⊥ and Paul-Alain Jaffre`s*,# School of Biomedical and Natural Sciences, Nottingham Trent UniVersity, Nottingham NG11 8NS, United Kingdom, UMR CNRS 6507 & 6508, UniVersite´ de Caen, ENSICAEN, BouleVard Mare´ chal Juin, 14050 Caen cedex, France, CCLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, United Kingdom, and UMR CNRS 6521, Faculte´ des Sciences et Techniques, UniVersite´ de Bretagne Occidentale, 6 aVenue Le Gorgeu, 29238 Brest, France

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 208-211

ReceiVed NoVember 17, 2006; ReVised Manuscript ReceiVed January 9, 2007

ABSTRACT: Benzene-1,3,5-triphosphonic acid 1, which is the phosphonic acid analogue of trimesic acid, forms a columnar packing in the solid state that is characterized by strong hydrogen bonds (contact distances O‚‚‚O ranging from 2.55 to 2.60 Å) and π-stacking (d(Ar‚‚‚Ar) ) 3.89 and 3.69 Å) involving the aromatic rings, thus designing an organic supramolecular ladder structure by auto-assembly. Introduction The generation of 2D or 3D hydrogen-bonding supramolecular materials has been widely investigated.1,2 In the majority of cases, carboxylic acids,3 amides,4 sulfonic acids,5 or urea6 have been selected to generate the supramolecular arrangement. These functional groups can be either used alone (self-assembling) or associated with a partner (e.g., amine, guanidine, etc.) to produce co-assembly architectures. One strategy to produce multidimensional supramolecular materials is based on the use of polyfunctional precursors. Benzene-1,3,5-tricarboxylic A (also called trimesic acid), which illustrates this strategy, has been indeed frequently used as a building block for the design of supramolecular materials.7 Trimesic acid A, which is a rigid planar molecule, self-assembles into a hydrogen-bonded sheet composed of repeating hexagonal arrays.8 This compound has also been employed to design metalorganic frameworks (MOFs) on the basis of the reactivity of the carboxylic acid function with metal salt.9 Several porous MOF, synthesized from polyfunctional organic precursors, have been reported over the last years.10 There are several similarities between the chemistry of the carboxylic acid and the phosphonic acid functional groups in the sense that both can be used to design materials on the basis of either hydrogen-bonding networks or MOFs. Hybrid organic-inorganic materials based on the reactivity of phosphonic acid and a metal salt have been widely studied over the last 15 years.11 Porosity12 or ionic conductivity13 are a few examples of the properties associated with metal-phosphonate materials. The nature of the metallic salt has also a great influence on the architecture adopted by these hybrid materials. For instance, several microporus materials have been reported using aluminum 14 or zinc salt as inorganic precursors. The phosphonic acid function has also been used in the generation of 2D or 3D hydrogen-bonding architectures.15 Nevertheless, the complexity of the resulting hydrogen bond networks has certainly limited its use as a building block. On the other hand, materials synthesized from phosphonic acid function allow characterization by solid-state 31P NMR spectroscopy.16 Another characteristic of hydrogen-bonding assemblies involving phosphonic acid function is the presence of strong hydrogen bonds (contact distances d(O‚‚‚O) < 2.50 Å). This feature is of interest in the design of robust materials. In our laboratory, we have studied the crystal structure of polyphosphonic molecules, such as compounds B (Scheme 1), to investigate the * To whom correspondence should be addressed. E-mail: gary.hix@ ntu.ac.uk (G.B.H.); [email protected] (P.A.J.). † Nottingham Trent University. ‡ CRISMAT UMR CNRS 6508, Universite ´ de Caen. § LCMT, UMR CNRS 6507, Universite ´ de Caen. ⊥ CCLRC Daresbury Laboratory. # CEMCA, UMR CNRS 6521, Universite ´ de Bretagne Occidentale.

Scheme 1. Representation of Three Organic Synthons, Precursors of Either Supramolecular Architectures or Hybrid Materials: Trimesic Acid (A); Benzene-1,3,5-tris(methylenephosphonic Acid) (B); Benzene-1,3,5-trisphosphonic Acid (1)

nature of the hydrogen bonds with the aim of designing two- or three-dimensional hydrogen-bonding polymers. As expected, we identified strong hydrogen bonds and even some hydrogen transfer17 between two phosphonic acid functions to generate charged-assisted hydrogen bonds. The formation of this kind of strong hydrogen bonds (O‚‚‚O contact distances < 2.50 Å) has been postulated to explain the singular conformation of molecule B in the solid state, for which the three methylenephosphonic arms are located on same side of the benzene ring to generate a dimer. This result, which demonstrated the richness but also the complexity of the supramolecular chemistry associated with phosphonic acid function, leads us to study the auto-assembly of molecule 1 (benzene-1,3,5triphosphonic acid), which is more rigid than molecule B because of the absence of the methylene bridge. Surprisingly, the crystal structure of compound 1 has not been reported previously, which is almost certainly due to the difficulties in growing single crystals.18 Nevertheless, several recent studies have reported the co-assembly of molecule 1 with different partners (4-(dimethylamino)pyridine;19 bipyridine20). Compound 1 has also been used to generate hybrid organic-inorganic materials according to a hydrothermal synthesis involving its reaction with copper salts and bipyridine derivatives.21 Compound 1 is therefore an interesting intermediate to design both 3D supramolecular hydrogen-bonding architectures and organicinorganic hybrid frameworks. In this paper, we report the crystal structure of 1,3,5-benzenetriphosphonic acid 1, which is the phosphonic acid analogue of trimesic acid A. Compound 1 was synthesized according to the literature20 by hydrolysis of hexaethyl 1,3,5-benzenetriphosphonic ester with concentrated HCl. Recrystallization in water leads to isolation of a microcrystalline powder. Very small single crystals (150 × 30.25 µm) have been selected from this powder and proved to be suitable for diffraction on Station 9.8 at Daresbury SRS.22,23 The crystal

10.1021/cg060813k CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007


Crystal Growth & Design, Vol. 7, No. 2, 2007 209 Table 1. Hydrogen Bond Distances and Angles for Benzene-1,3,5-triphosphonic Acid 1 (Å and deg) entry

bond D-H‚‚‚Aa

1 2 3 4 5 6

O(3)-H(1A)‚‚‚O(1)#1 O(2)-H(1B)‚‚‚O(4)#2 O(7)-H(2A)‚‚‚O(1)#3 O(8)-H(2B)‚‚‚O(9)#4 O(5)-H(3A)‚‚‚O(4)#5 O(6)-H(3B)‚‚‚O(9)#6

d(D-H) d(H‚‚‚A) d(D‚‚‚A) 0.84 0.84 0.84 0.84 0.84 0.84

1.76 1.75 1.76 1.77 1.73 1.76

2.594(3) 2.574(3) 2.591(3) 2.599(3) 2.550(3) 2.598(3)

angle (DHA°) 170.0(3) 168.3(3) 170.9(3) 168.0(3) 165.8(3) 176.2(3)

a Symmetry transformations used to generate equivalent atoms: #1 -x + 1,-y + 1,-z + 1; #2 -x, -y, -z + 1; #3 x, y - 1, z; #4 -x + 1, -y, -z + 2; #5 -x, -y - 1, -z; #6 x, y, z - 1.

Figure 1. ORTEP drawing (50% probability ellipsoids) of the asymmetric unit of benzene-1,3,5-triphosphonic acid 1.

structure of compound 1 (Figure 1) is representative of the full sample in view of the comparison of the simulated (from singlecrystal data) and experimental XRD powder pattern (see the Supporting Information). The structure has been solved with SHELXS and refined using SHELXL.24 The hydrogen atoms were placed geometrically. Compound 1 crystallized in the triclinic P1h space group, and the cell parameters are a ) 7.631(2) Å, b ) 9.3667(13) Å, c ) 9.5372(13) Å, R ) 110.347(2)°, β ) 102.058(3)°, γ ) 109.887(3)°. As expected, each phosphorus atom has a tetrahedral environment. One of the P-OH bonds for each phosphorus atom is almost orthogonal to the benzene ring (O5, O3, and O8; dihedral angle from 79.0 to 85.7°), and noticeably, these P-OH bonds are all located one the same side of the benzene ring. The two other bonds (PdO and P-OH) for each phosphorus atom form a dihedral angle with the benzene ring, from 37.8 to 42.5° for the PdO bonds and from 15.83 to 19.54° for the second P-OH bond. The location of the phosphorus atom in the center of a tetrahedral is the main difference between benzene-trisphosphonic acid 1 and trimesic acid A. This difference has a strong consequence on the packing. In the case of trimesic acid, the coplanarity of the carboxylic acid functional groups with the benzene ring and the formation of pairs of carboxylic acid functional groups lead, basically, to the formation of a 2D networks. The 3D nature of the packing of trimesic acid arises from the interpenetration of 2D supramolecular structures. With the benzene-triphosphonic acid, hydrogen bonding generates a 3D arrangement. The molecules stack in an ABABAB manner, where A and B are related by an inversion center located midway between the rings.

Therefore, the steric interaction between the phosphonic acid function is minimized along the direction orthogonal to the plane defined by the aromatic ring (this direction is slightly different from the a axis). This arrangement certainly allows stabilization of the adjacent molecules by electrostatic interaction between the static charge distributions of each molecules. The interaction between the A and B molecules is enhanced by the formation of O(2)H(1B)‚‚‚O(4) hydrogen bonds; the oxygen atoms are associated with P(1) and P(3), respectively (Table 1, entry 2). In this manner, columns of molecules of 1 are formed (see Figure 2a and Table 1). These columns are arranged such that the rings in one column are at the halfway point between two rings in the next column (Figure 2a). Columns are connected by hydrogen-bond formation (Figure 3) between oxygen atoms associated with the P(3) phosphonic acid groups (Table 1, entry 5), the P(2) phosphonic acid groups (Table 1, entry 4), the P(1) phosphonic acid group (Table 1, entry 1) and finally between oxygen associated with P(2)-P(1) and P(3)-P(2) (Table 1, entries 3 and 6, respectively). By looking parallel to the benzene ring (Figure 2), it can be observed that two distinct distances alternate along an axis normal to the benzene rings. These distances, calculated along a virtual axis orthogonal to the benzene rings (of note, this axis is different from the crystallographic a-axis, as illustrated on the schematic representation of the packing; Figure 2b), which are, respectively, 3.89 and 3.69 Å (Figure 2), are consistent with the presence of π-π interactions between adjacent molecules.25 The bigger distances (3.89 Å) correspond to the stack of the faces of the benzene ring where the 3 P-OH bonds are almost orthogonal to the ring (Figure 2b). On the other hand, the smaller distances between the aromatic rings (3.69 Å) are formed by placing the less-hindered side of the benzene ring face-to-face (Figure 2b). Furthermore, a view along an axis orthogonal to the benzene rings (Figure 4) shows that the columns are indeed formed by π-stacked molecules.

Figure 2. (a) ORTEP representation (50% probability ellipsoids) showing the stacking of the rings and hydrogen bonding in benzene-1,3,5-triphosphonic acid 1. (b) Schematic drawing of the packing of benzene-1,3,5-triphosphonic acid 1 view along a direction parallel to the plane defined by the aromatic ring. Of note, the crystallographic a-axis (purple arrow) is not orthogonal to the benzene ring (symbolized by the flat rectangle). The distances between the benzene rings are calculated along an axis orthogonal to the benzene ring.

210 Crystal Growth & Design, Vol. 7, No. 2, 2007

Communications defined by the aromatic ring. Of note, only a few organic supramolecular ladder structures are reported and noticeably, most of the time they are formed by the co-assembly of two organic partners.29 Further, examples of organic supramolecular ladders resulting from an auto-assembly process are rare.30 In conclusion, the auto assembly of benzene-1,3,5-triphophonic acid is characterized by a columnar packing involving both hydrogen bonding and π-π stacking. This result demonstrates that a right balance between hydrogen bonding and π-π stacking can result in the formation an organic supramolecular ladderlike structure. This is the first illustration of a 1,3,5-trisubstitutedbenzene compound that adopts this kind of packing. This result is also of a great interest in view of the potential of benzene-1,3,5triphosphonic acid as a building block to design both supramolecular networks and hybrid structures (MOFs).


Figure 3. Line representation (Mercury of the packing of molecules 1 viewed along the a-axis showing the hydrogen bonds (dotted blue line) between the columns formed by the stacking of molecule 1.

Acknowledgment. We thank ENSICAEN (BQR program) and CNRS for funding. Supporting Information Available: X-ray crystal structure CIF files, powder X-ray diffraction pattern, and list of bonds parameters. This material is available free of charge via the Internet at http://pubs.acs.org.


Figure 4. (a) Line representation (Mercury 1.4.128) of the packing of three molecules 1 viewed along an axis orthogonal to the plane defined by the aromatic rings (molecules possessing the phosphorus atoms 1, 2, and 3 are, respectively, in front, middle, and behind). (b) Schematic representation of the benzene ring view along the same axis showing the offsets OFF1, OFF2, OFF3, and OFF4 (the benzene rings schematized in blue, green, and orange colors are, respectively, in front, middle, and behind).

This packing producing a supramolecular ladder type structure.26 If the presence of a strong hydrogen bond, which certainly represents the major contribution of the packing energy,27 is common in the packing of molecules possessing phosphonic acid groups, the presence of π-stacking is much less common and worth further discussions. To discuss the π-π interaction in the packing of benzene-1,3,5phosphonic acid 1, it must be remembered that the benzene rings are rigorously parallel to each other (Figure 2a). The second characteristic is visible when looking along an axis orthogonal to the planes defined by the benzene rings (Figure 4). From this view, it clearly appears that the molecules are not exactly superposed. An exact match would certainly lead to an electrostatic repulsion, whereas the presence of an offset can produce an attractive contribution.25 This discard from the exact superposition allows calculating two sets of offset, which depend on the value of the distances separating the aromatic rings. The shorter distance between the aromatic ring (3.69 Å) is associated with the biggest values of offset (Figure 4b), with OFF3 ) 0.77 Å and OFF4 ) 0.22 Å, respectively. Inversely, to the larger distance separating the aromatic ring in the packing (3.89 Å) corresponds the shorter values of the offset (OFF1 and OFF2 are, respectively, 0.16 and 0.32 Å). Of note, this type of π-π stacking has been observed by Clearfield20 in the crystal structure of compound 1 cocrystallized with bipyridine. The presence of dimers characterized by two benzene ring strictly parallel and a distance between these rings ranging from 3.49 to 3.87 Å have been reported. As observed in Figures 2 and 3, an infinite organic supramolecular ladder is formed along a direction orthogonal to the plane

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