CRYSTAL GROWTH & DESIGN
Robust Supramolecular Heterosynthons in Chiral Ammonium Carboxylate Salts
2008 VOL. 8, NO. 4 1106–1109
Andreas Lemmerer,*,† Susan A. Bourne,† and Manuel A. Fernandes‡ Centre for Supramolecular Research, Department of Chemistry, UniVersity of Cape Town, Rondebosch 7701, South Africa, and Molecular Sciences Institute, School of Chemistry, UniVersity of the Witwatersrand, Johannesburg 2050, South Africa ReceiVed October 16, 2007; ReVised Manuscript ReceiVed January 28, 2008
ABSTRACT: The ammonium carboxylate salts (m-iodobenzoate) · ((R)-1-phenylethylammonium) (1), (m-iodobenzoate) · ((S)-1-phenyl-
ethylammonium) (2), and (m-iodobenzoate) · ((()-1-phenylethylammonium) (3) self-assemble to form two types of one-dimensional hydrogen bonded columns, depending on the homo- or heterochirality of the cation: 1 and 2 form noncentrosymmetric hydrogen bonded columns having a 2-fold screw axis, whereas 3 forms centrosymmetric hydrogen bonded columns having an inversion center. The rapidly growing discipline of crystal engineering is predicated on the link between molecular structure and crystal structure. To simplify the complexity of the link, the concept of the “supramolecular synthon” as a structural link has been introduced and defined as a “specific pattern of intermolecular interactions associated with a specific type of molecular array in the solid state”.1 Significant among the intermolecular interactions is hydrogen bonding. Ideally, one would want to identify synthons that are favored in a family of molecules or be able to predict how the synthon will respond to structural interference of the molecular structure.2 This “synthon robustness” or lack of structural interference is more common among ionic supramolecular hydrogenbonded synthons than in synthons observed in neutral organic molecules.2 Although it is to be remembered that all portions of the molecule including the hydrocarbon residues have supramolecular functionality,3 it is the identified supramolecular synthon that is primary among them. A subgroup of the supramolecular synthon is the “supramolecular heterosynthon”, which is defined as a supramolecular synthon between unlike but complementary functional groups.4 A robust, ionic, supramolecular heterosynthon exists for (R-NH3+) · (R′-COO-) ammonium carboxylate salts, where charge assisted N-H · · · O hydrogen bonds exist between the two complimentary functional groups. The preparation of ammonium carboxylate salts is synthetically simple and the vast number of possible organic molecules with carboxylic and amine functional groups make this heterosynthon potentially limitless. This makes it an ideal system to study the structural influence of the molecular structure on the supramolecular crystal structure. The work by Kinbara et al.5 used many structurally different chiral primary amines and achiral carboxylic acids to classify the commonly occurring supramolecular assemblies found in ammonium carboxylate salts in the formation of conglomerates. They identified two hydrogen-bonded columnar networks, one where the ammonium cations and carboxylate anions are related by a center of inversion (i-column) and a second one where the ammonium cations and carboxylate anions are related by a 2-fold screw axis (21-column). In Kinbara’s work, an investigation of 28 ammonium carboxylate salts, they observed that the 21-column is the characteristic synthon for salts of chiral monoammonium cations with achiral monocarboxylate anions. Of the 28 ammonium carboxylate salts, 17 consisted of a racemic ammonium cation and achiral carboxylate anion. For the racemate salts formed by a racemic mixture of the chiral ammonium cation and achiral carboxylate anion, there are two different ways of packing the columns: either * Corresponding author. E-mail:
[email protected]. Fax: 27-21685-4580. Tel: 27-21-650-2562. † University of Cape Town. ‡ University of the Witwatersrand.
Table 1. Geometrical Parameters for N-H · · · O Hydrogen Bonds in 1–3 compd
d(D-H) (Å)
N1-H1A · · · O1 N1-H1B · · · O2i a N1-H1C · · · O1ii a
0.91 0.91 0.91
N1-H1A · · · O1 N1-H1B · · · O2iii a N1-H1C · · · O1iva
0.91 0.91 0.91
N1-H1A · · · O1 N1-H1B · · · O2iv a N1-H1C · · · O1v a
0.91 0.91 0.91
d(H · · · A) (Å)
d(D · · · A) (Å)
2σ(I)], R1obs ) 0.0254, wRall ) 0.0600. Anal. Calcd for C15H16I1N1O2: C, 48.80; H, 4.37; N, 3.79. Found: C, 48.78; H, 4.44; N, 3.86. Melting point (determined from DSC): Tonset ) 170.6°C. (b) Crystal data of 2: chemical formula C15H16I1N1O2, formula weight 369.19, orthorhombic, spacegroup P212121, a ) 6.1866(4) Å, b )
Communications 7.095(2) Å, c ) 33.1580(4) Å, V ) 735.07(7) Å3, Z ) 4, Fcalcd ) 1.685 Mg m-3, T ) 173 K, µ ) 2.198 mm-1 (face-indexed absorption corrections), 13631 reflections measured, 3440 unique reflections, 3284 observed reflections [I > 2σ(I)], R1obs ) 0.0323, wRall ) 0.0891. Anal. Calcd for C15H16I1N1O2: C, 48.80; H, 4.37; N, 3.79. Found: C, 48.70; H, 4.45; N, 4.19. Melting point (determined from DSC): Tonset ) 170.1°C. (c) Crystal data of 3: chemical formula C15H16I1N1O2, formula weight 369.19, triclinic, spacegroup P1j, a ) 6.2004(4) Å, b ) 7.1899(3) Å, c ) 16.7399(10) Å, R ) 89.278(3)°, β ) 80.259(2), γ ) 88.070(3)°, V ) 735.07(7) Å3, Z ) 2, Fcalcd ) 1.668 Mg m-3, T ) 173 K, µ ) 1.668 mm-1 (face-indexed absorption corrections), 20657 reflections measured, 3440 unique reflections, 3230 observed reflections [I > 2σ(I)], R1obs ) 0.0247, wRall ) 0.0600. Anal. Calcd for C15H16I1N1O2: C, 48.80; H, 4.37; N, 3.79. Found: C, 48.76; H, 4.42; N, 3.35. Melting point (determined from DSC): Tonset ) 153.3°C.
Crystal Growth & Design, Vol. 8, No. 4, 2008 1109
(9) (10)
(11) (12)
(d) Thermal analysis traces (TGA and DSC) and powder XRD plots of 1, 2, and 3 are given as Supporting Information. Etter, M. C.; MacDonald, J. C. Acta Crystallogr., Sect. B 1990, 46, 256. In Kinbara’s paper,5 the use of the term “21-column” is synonymous with R34(10) hydrogen bonded rings and the term “i-column” is synonymous with alternating R24(8) and R44(12) hydrogen-bonded rings. Fabian, L.; Kalman, A. Acta Crystallogr., Sect. B 1999, 55, 1099– 1108. The hydrogen bond distances are very similar between structure 1 and ZUSMOV,5 being 2.805(3), 2.735(3), and 2.696(3) Å; and 2.787(8), 2.744(6), and 2.697(8) Å for the same respective N · · · O pairs.
CG701020S
