Received 23 June 2011
2,3-Dimethoxy-10-oxostrychnidinium 2-(2,4,6-trinitroanilino)benzoate monohydrate: a 1:1 proton-transfer salt of brucine with o-picraminobenzoic acid
In the structure of the title 1:1 proton-transfer compound of brucine with 2-(2,4,6-trinitroanilino)benzoic acid, C23H27N2O4+·C13H7N4O8-·H2O, the brucinium cations form classic undulating ribbon substructures through overlapping head-to-tail interactions, while the anions and the three related partial solvent water molecules (having occupancies of 0.73, 0.17 and 0.10) occupy the interstitial regions of the structure. The cations are linked to the anions directly through N-HOCOO- hydrogen bonds and indirectly by the three water molecules, which form similar conjoint cyclic bridging units [graph set R24(8)] through O-HOC=O and O-HOCOO- hydrogen bonds, giving a two-dimensional layered structure. Within the anion, intramolecular N-HOCOO- and N-HOnitro hydrogen bonds result in the benzoate and picrate rings being rotated slightly out of coplanarity [inter-ring dihedral angle = 32.50 (14)°]. This work provides another example of the molecular selectivity of brucine in forming stable crystal structures, and also represents the first reported structure of any form of the guest compound 2-(2,4,6-trinitroanilino)benzoic acid.
Although brucine has been used largely for the resolution of certain chiral compounds (Wilen, 1972), it has proven utility in the formation of crystalline adducts and salts with achiral carboxylic acids. In particular, the benzoic acid analogues have provided a number of brucinium salt structures, many of which are solvated, e.g. benzoic acid (a trihydrate) (Bialonska & Ciunik, 2006b), 3-nitrobenzoic acid (methanol monosolvate) (Oshikawa et al., 2002), 4-nitrobenzoic acid (isomorphous dihydrate and methanol monosolvate) (Bialonska & Ciunik, 2007), 4-hydroxybenzoic acid (isopropyl alcohol monosolvate) (Sada et al., 1998), 3,5-dinitrobenzoic acid (trihydrate, methanol monosolvate and disolvate) (Bialonska & Ciunik, 2006a) and the anhydrous example with 5-nitrosalicylic acid (Smith, Wermuth, Healy & White, 2006). Three 1:1 salts are also known, viz. with 5-nitrophthalic acid (a dihydrate) (Smith et al., 2005), isophthalic acid (a trihydrate) (Smith, Wermuth, Young & White, 2006) and 4,5-dichlorophthalic acid (anhydrous) (Smith et al., 2007a). However, with these acids, formation is certainly a hit-or-miss process, the selectivity being dependent upon guest molecule compatibility with the interstitial cavities in the brucinium cation substructures which are present in a large number of brucine adduct and brucinium proton-transfer compounds (Gould & Walkinshaw, 1984; Dijksma et al., 1998; Oshikawa et al., 2002; Bialonska & Ciunik, 2004; Smith, Wermuth, Healy & White, 2006). In these substructures, the brucine species form undulating ribbons comprising overlapping head-to-tail molecules, this host structure then accomodating the compatible guest molecule or molecules and interacting with them through hydrogen-bonding associations. This phenomenon accounts for the presence in many of the structures of various polar solvent molecules. It has also been noted that the two-molecule brucine repeat period will be ca 12.5 Å (the cell dimension) in the direction of a 21 screw axis, of which there is a high incidence among the small number of space groups into which brucine and its compounds and adducts fall (Smith, Wermuth, Healy & White, 2006).
The isomeric picraminobenzoic acids [2-, 3- and 4-(2,4,6-trinitroanilino)benzoic acid] were first synthesized by the reaction of the corresponding monoaminobenzoic acid with picryl chloride in 1911 (Crocker & Matthews, 1911). We have synthesized these three compounds using picrylsulfonic acid rather than picryl chloride, reporting the crystal structure of the para isomer (Smith et al., 2007b). However, the uncompromising crystal morphology of the ortho and meta isomers precluded the structure determinations of these. The 1:1 stoichiometric reaction of 2-(2,4,6-trinitroanilino)benzoic acid with brucine in aqueous ethanol gave good crystals of the orange-red hydrated title salt, (I), and the structure is reported here. No suitable crystals resulted from the reactions of brucine with the meta and para isomers.
In (I), protonation has occurred, as expected, at N19 of the brucine cage (Fig. 1), the absolute configuration of the seven chiral centres of the brucinium cation being invoked (Peerdeman, 1956). These cations form the previously described undulating ribbon host substructures, which have a dimeric repeat period in (I) of 12.4407 (3) Å along the direction of propagation [a 21 screw axis, the a cell dimension] (Fig. 2). This value for the dimeric repeat in (I) is consistent with those for similarly structured brucine compounds (Gould & Walkinshaw, 1984; Smith, Wermuth, Healy & White, 2006). There is a molecule offset of ca 120° in the repeat unit of (I).
The monoanion and the three associated partial solvent water molecules [O1W (site-occupancy factor = 0.73), O2W (site-occupancy factor = 0.17) and O3W (site-occupancy factor = 0.10)] occupy the interstitial volumes between the brucine substructures and are hydrogen bonded to them. The brucinium cations form an N+-HO hydrogen bond with a carboxylate O-atom acceptor of the anion, while the water linkages are unusual, the three partial molecules forming a set of similar conjoint cyclic associations [graph set R42(8); see Bernstein et al. (1995) for graph-set notation] involving two O-atom acceptor atoms (brucinium carbonyl atom O25 and carboxylate atom O3A of the anion) (Table 1) (see Fig. 2), giving a two-dimensional structure which forms layers down the c cell direction (Fig. 3). Within the anion, intramolecular N-HOCOO- and O-HOnitro hydrogen bonds result in moderate rotation of the benzoate and picrate ring systems out of coplanarity [inter-ring dihedral angle = 32.50 (14)°]. The ortho-carboxylate group of the benzoate ring is rotated slightly out of the plane of the benzene ring [C1A-C2A-C22A-O3A = 159.4 (3)°], while the two ortho-related nitro groups are similarly non-coplanar with the picrate ring [C11A-C21A-N21A-O22A = 151.7 (3)° and C11A-C61A-N61A-O61A = -165.2 (3)°]. The less sterically compromised para-nitro group is essentially coplanar with the picrate ring [C31A-C41A-N41A-O42A = -177.8 (3)°]. One of the O atoms of the ortho-related nitro group at C21A is involved, not unexpectedly, in some short intramolecular nonbonded interactions [O21AC1A = 2.852 (4) Å and O21AN1A = 2.892 (4) Å].
The structure presented here provides another example of the molecular selectivity of brucine in forming stable complexes and is also the first reported structure of any form of the guest compound 2-(2,4,6-trinitroanilino)benzoic acid.
| || Figure 1 |
The molecular configuration and atom-numbering scheme for the brucinium cation, the o-picraminobenzoate anion and the partial solvent water molecules (O1W-O3W) in (I). Displacement ellipsoids are drawn at the 50% probability level. Inter-species hydrogen bonds are shown as dashed lines.
| || Figure 2 |
The cation-anion-water hydrogen-bonding environment in (I), showing the head-to-tail overlap of the brucinium cations which are part of the substructure extending along a. Hydrogen bonds are shown as dashed lines and non-associative H atoms have been omitted. [For symmetry code (i), see Table 1.]
Compound (I) was synthesized by heating together brucine tetrahydrate (1 mmol) and 2-(2,4,6-trinitroanilino)benzoic acid (o-picraminobenzoic acid) (1 mmol) in ethanol-water (1:1 v/v, 50 ml) under reflux for 10 min. After concentration to ca 30 ml, partial room-temperature evaporation from the hot-filtered solution gave short orange-red prisms of (I) (m.p. 475 K).
A nonstandard orthorhombic axial setting was chosen for a better comparison with previous similar brucine structures. C-bound H atoms were included at calculated positions, with C-H = 0.93 (aromatic and sp2) or 0.96-0.98 Å (aliphatic), and treated as riding, with Uiso(H) = 1.2Ueq(C). The H atom of the brucinium N+-H group was located in a difference Fourier synthesis and its positional and isotropic displacement parameters were allowed to ride in the refinement [Uiso(H) = 1.2Ueq(N)]. The occupancies of the three partial solvent water molecules were determined as 0.73 (O1W), 0.17 (O2W) and 0.10 (O3W) from peak heights, and the O atoms of the two minor-occupancy components were refined isotropically. All three partial water molecules were found to be associated with the same two O-atom acceptors, and the H atoms on these were derived geometrically and also allowed to ride in the refinement [Uiso(H) = 1.2Ueq(O)]. The known absolute configuration of the parent strychnidin-10-one molecule (Peerdeman, 1956) was invoked and Friedel pairs were averaged for data used in the final cycles of refinement.
Data collection: CrysAlis PRO (Oxford Diffraction, 2010); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) within WinGX (Farrugia, 1999); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: FG3225 ). Services for accessing these data are described at the back of the journal.
The authors acknowledge financial support from the Australian Research Grants Committee and from the Faculty of Science and Technology, Queensland University of Technology.
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