Received 18 August 2012
Supramolecular association in proton-transfer adducts containing benzamidinium cations. II. Concomitant polymorphs of the molecular salt of 2,6-dimethoxybenzoic acid with benzamidine1
Two concomitant polymorphs of the molecular salt formed by 2,6-dimethoxybenzoic acid, C9H10O4 (Dmb), with benzamidine, C7H8N2 (benzenecarboximidamide, Benzam) from water solution have been identified. Benzamidinidium 2,6-dimethoxybenzoate, C7H9N2+·C9H9O4- (BenzamH+·Dmb-), was obtained through protonation at the imino N atom of Benzam as a result of proton transfer from the acidic hydroxy group of Dmb. In the monoclinic polymorph, (I) (space group P21/n), the asymmetric unit consists of two Dmb- anions and two monoprotonated BenzamH+ cations. In the orthorhombic polymorph, (II) (space group P212121), one Dmb- anion and one BenzamH+ cation constitute the asymmetric unit. In both polymorphic salts, the amidinium fragments and carboxylate groups are completely delocalized. This delocalization favours the aggregation of the molecular components of these acid-base complexes into nonplanar dimers with an R22(8) graph-set motif via N+-HO- charge-assisted hydrogen bonding. Both the monoclinic and orthorhombic forms exhibit one-dimensional isostructurality, as the crystal structures feature identical hydrogen-bonding motifs consisting of dimers and catemers.
The present study is a continuation of work carried out in our laboratory on the design and synthesis of hydrogen-bonding systems formed by benzamidine with benzoic acid derivatives. The protonated analogue of benzamidine is a multiple hydrogen-bond donor, and its amidinium group exhibits ideal requirements to couple with carboxylate groups in crystal structures. These acid-base complexes are usually arranged in a dimeric motif similar to that found in carboxylic acid dimers via N+-HO- (±)-CAHB (charge-assisted hydrogen bonds with both plus and minus charges on the donor and acceptor atoms, respectively; Gilli & Gilli, 2009), provided that pKa is sufficiently large [pKa = pKa(conjugate acid of the base) - pKa(acid), where the pKa values are for aqueous solutions at 298 K]. It is generally accepted that for large pKa values (i.e. greater than 3), salts of the type B+A- are formed, while with smaller pKa values, BH-A compounds (cocrystals) can be expected, but this parameter seems inappropriate for accurately predicting salt or cocrystal formation in the solid state when pKa is between 0 and 3 (Portalone & Colapietro, 2009; Portalone, 2011a; Delori et al., 2012). This dimeric motif is commonly observed in biological systems between arginine and aspartic and glutamic acids (Saenger, 1984).
In this study, we report the molecular and supramolecular structures of two concomitant polymorphs, i.e. two different polymorphs simultaneously crystallized from the same solvent (Bernstein et al., 1999), of the acid-base complex formed by benzamidine (Benzam) with 2,6-dimethoxybenzoic acid (Dmb). Interestingly, Dmb has two polymorphic modifications, viz. the orthorhombic form (Portalone, 2009) and the tetragonal form (Portalone, 2011b). In the former polymorph, the carboxyl group is twisted away from the plane of the aromatic ring by 56.1 (1)° and the OH group adopts an antiplanar conformation, while in the latter the twist angle is 65.7 (2)° and the OH group is synplanar. The molecular components of the orthorhombic polymorph do not form the conventional R22(8) dimeric units [see Etter et al. (1990), Bernstein et al. (1995) and Motherwell et al. (1999) for graph-set nomenclature of hydrogen bonds] as they do in the tetragonal polymorph, but are associated in the crystal structure as catemers through single O-HO(carbonyl) hydrogen bonds between adjacent molecules. As the observation of polymorphism in multicomponent systems such as cocrystals and molecular salts is scant, although of topical interest given the growing relevance of pharmaceutical cocrystals (Tiekink & Vittal, 2006), we have been attracted by the planned synthesis of polymorphic molecular salts resulting from the combination of two conformationally flexible molecules, such as Dmb and benzamidine. For BenzamH+·Dmb-, since pKa = 7.5, the salt is expected. Indeed, in this proton-transfer compound protonation occurs at the imino N atom attached to Benzam as a result of proton transfer from the acidic hydroxy group of Dmb.
Polymorph (I) crystallizes in the monoclinic space group P21/n, with two crystallographically independent dimers of monoprotonated benzamidinium cations (BenzamH+) and benzoate anions (Dmb-) in the asymmetric unit (Fig. 1). Polymorph (II) crystallizes in the orthorhombic space group P212121, with one BenzamH+ cation and one Dmb- anion in the asymmetric unit (Fig. 2).
The molecular structures of polymorphs (I) and (II) are very similar. In both polymorphic forms, the BenzamH+ cations are not planar. The dihedral angles between the mean plane of the benzene ring and the amidinium group [20.2 (1) and 14.4 (1)° for polymorph (I), and 23.6 (1)° for polymorph (II)] are close to the values observed in benzamidine [22.7 (1)°; Barker et al., 1996], benzamidinium (2-acetamidobenzoyl)formate [16 (3)°; Joshi et al., 1994] and benzamidinium acetylsalicylate [15.2 (2)°; Kolev et al., 2009]. For both polymorphs this disposition is a consequence of an overcrowding effect, i.e. steric hindrance between the NH2 group and the benzene ring, as indicated by the N-C(ortho) contacts in the range 2.859 (2)-2.893 (2) Å, and prevents conjugation between the N-C-N system and the benzene ring. Indeed, the C-C(amidine) distances are in the range 1.479 (3)-1.481 (3) Å and compare well with the expected Csp2-Csp2 single-bond length of 1.482 (1) Å (Allen et al., 1987). Interestingly, only the nonplanar conformation has been observed in small-molecule crystal structures, whereas in structures containing benzamidinium in the Protein Data Bank (PDB; Berman et al., 2000), the most frequently encountered one is the planar conformation (Li et al., 2009). The C-N bond lengths are in the range 1.305 (2)-1.317 (2) Å, evidencing the delocalization of the electrons and double-bond character compared with the corresponding bond lengths found in benzamidine [1.294 (3) and 1.344 (3) Å; Barker et al., 1996] and benzdiamidine [1.283 (2) and 1.349 (2) Å; Jokic et al., 2001].
The benzene rings in the Dmb- anions of polymorphs (I) and (II) are essentially planar, and the methoxy substituents force the carboxylate groups to be almost orthogonal to the plane of the aromatic fragment [the twist angles between the aromatic ring and the carboxylate group are 88.7 (1) and 72.3 (1)° for polymorph (I), and 78.9 (1)° for polymorph (II)]. In these anions, the bond lengths and angles of the benzene ring are in accord with the corresponding values obtained for both the orthorhombic and tetragonal forms of 2,6-dimethoxybenzoic acid (Portalone, 2009, 2011b) and for 4-methoxybenzamidinium 2,6-dimethoxybenzoate (Portalone, 2012). The C-O distances of the carboxylate group range from 1.246 (2) to 1.255 (2) Å, indicating delocalization of the negative charge.
The molecular components of both polymorphs are joined by two N+-HO- (±)-CAHB hydrogen bonds to form ionic dimers with graph-set motif R22(8). Remarkably, at variance with the well known carboxylic acid dimer R22(8) motif, in both polymorphs the carboxylate-amidinium pairs are not planar, as the dihedral angles for the planes defined by the CN2+ and CO2- atoms are in the range 26.8-29.5°. Nonetheless, the interface is very stable, as indicated by the N+-HO- parameters: N+O- distances in the range 2.785 (2)-2.868 (2) Å) and N+-HO- angles in the range 148 (2)-177 (2)°. This deviation from planarity of carboxylate-amidinium pairs has previously been observed in the crystal structure of 3-amidinium benzoate (Papoutsakis et al., 1999).
Due to the protonated base, supramolecular aggregations in (I) and (II) are dominated by an extensive series of N+-HO- (±)-CAHB hydrogen bonds (Tables 1 and 2). In the supramolecular structures of polymorphs (I) and (II) (Figs. 3 and 4), eight and four hydrogen bonds, respectively, link the molecular components into a one-dimensional structure. As mentioned previously, each subunit, built from the ion pairs of the asymmetric unit, forms R22(8) dimers via the interaction of the N-H and C=O groups. These subunits are then joined into linear chains through hydrogen bonding to adjacent antiparallel dimers. Such a disposition of dimers and catemers has frequently been observed in the solid-state structures of amides (Meléndez & Hamilton, 1998).
From a supramolecular retrosynthesis perspective, only those synthons that occur repeatedly in crystal structures, namely robust synthons of a particular set of functional groups, are useful in crystal design (Nangia & Desiraju, 1998). Moreover, hydrogen bonds are often formed in a hierarchical fashion (Etter, 1990; Allen et al., 1999; Bis et al., 2007; Shattock et al., 2008) and there is a need to ascertain the prevalence of a particular heterosynthon over another in a competitive environment. From the results reported herein, the crystal structures of polymorphs (I) and (II) are both consistent with the R22(8) supramolecular heterosynthon persistence exhibited by the benzamidinium adducts that have been archived in the Cambridge Structural Database (CSD, Version 5.33, November 2011; Allen, 2002). Consequently, the existence of polymorphism in these molecular salts should be related to the conformational flexibility of the carboxylate and amidinium groups. Polymorphs (I) and (II) therefore represent examples of conformational polymorphism. To the best of our knowledge, the only exception to the complementarity of amidinium and carboxylate groups has been reported for benzamidinium isoorotate (Portalone, 2010).
| || Figure 1 |
The asymmetric unit of polymorph (I), showing the atom-labelling scheme and hydrogen bonding (dashed lines). The asymmetric unit was selected so that the four ions are linked by N+-HO- (±)-CAHB hydrogen bonds. Displacement ellipsoids are drawn at the 50% probability level.
| || Figure 2 |
The asymmetric unit of polymorph (II), showing the atom-labelling scheme and hydrogen bonding (dashed lines). The asymmetric unit was selected so that the two ions are linked by N+-HO- (±)-CAHB hydrogen bonds. Displacement ellipsoids are drawn at the 50% probability level.
| || Figure 3 |
The supramolecular structure of polymorph (I), viewed approximately down a. For the sake of clarity, H atoms not involved in hydrogen bonding have been omitted. Hydrogen bonding is indicated by dashed lines.
| || Figure 4 |
The supramolecular structure of polymorph (II), viewed approximately down c. For the sake of clarity, H atoms not involved in hydrogen bonding have been omitted. Hydrogen bonding is indicated by dashed lines.
Equimolar amounts of benzamidine (Fluka, 95%) and 2,6-dimethoxybenzoic acid (Sigma Aldrich, 99%), dissolved in ethanol (20 ml), were refluxed for 8 h at 323 K. The mixture was cooled to room temperature and the solvent evaporated under vacuum. The product was then recrystallized from water to give, after one week, colourless crystals suitable for X-ray analysis. Careful examination of the batch under a microscope showed crystals in the form of tablets of (I) and small prisms of (II). The majority of the crystals were (II), with a small quantity of (I). Unfortunately, any attempts to produce more crystals of polymorph (I) by repeating the crystallization conditions were unsuccessful. Crystallization of the molecular salt carried out under a wide range of different sets of conditions (different solvents, different molar ratios etc.) led systematically to the orthorhombic polymorph, (II).
In both polymorphs, all H atoms were found in a difference map, but for the final refinements all benzene-bound H atoms were positioned with idealized geometry and included in the calculations as riding on their parent atoms, with C-H = 0.97 Å and Uiso(H) = 1.2Ueq(C). Methyl-bound H atoms were located from idealized local difference electron-density calculations and their C-H distances were allowed to vary during refinement, while being kept equivalent in any one methyl group, with Uiso(H) = 1.5Ueq(C). The positional and displacement parameters of the H atoms of the amidine groups were refined freely, giving N-H distances in the range 0.84 (3)-1.00 (3) Å. In the absence of significant anomalous scattering in this light-atom study of polymorph (II), Friedel pairs were merged.
For both polymorphs, data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006, 2008); data reduction: CrysAlis RED; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).
Supplementary data for this paper are available from the IUCr electronic archives (Reference: UK3051 ). Services for accessing these data are described at the back of the journal.
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