A new polymorph of bis(2-aminopyridinium) fumarate–fumaric acid (1/1) from powder X-ray diffraction

Abstract

A new polymorph of bis(2-aminopyridinium) fumarate-fumaric acid (1/1), 2C₅H₇N₂⁺∙C₄H₂O₄²⁻-·C₄H₄O₄, was obtained and its crystal structure determined by powder X-ray diffraction. The new polymorph (form II) crystallizes in the triclinic system (space group P-1), while the previous reported polymorph [form I; Ballabh, Trivedi, Dastidar & Suresh (2002). CrystEngComm, 4, 135-142; Büyükgüngör, Odabaşoğlu, Albayrak & Lönnecke (2004). Acta Cryst. C60, o470-o472] is monoclinic (space group P2₁/c). In both forms I and II, the asymmetric unit consists of one 2-aminopyridinium cation, half a fumaric acid molecule and half a fumarate dianion. The fumarate dianion is involved in hydrogen bonding with two neighbouring 2-aminopyridinium cations to form a hydrogen-bonded trimer in both forms. In form II, the hydrogen-bonded trimers are interlinked across centres of inversion via pairs of N-H∙∙∙O hydrogen bonds, whereas such trimers are joined via single N-H∙∙∙O hydrogen bonds in form I, leading to different packing modes for forms I and II. The results demonstrate the relevance and application of the powder diffraction method in the study of polymorphism of organic molecular materials.

Accurate unrestrained DDM refinement of crystal structures from highly distorted and low-resolution powder diffraction data

“…In addition, the DDM refinement reveals that the molecular conformation of both the fumarate ion and fumaric acid in (II) is essentially nonplanar, with the angles between the planes through C atoms and CO 2 groups being 8.4 (6) and 15.7 7, respectively. The respective angles 3.8 (4) and 6.7 (4) resulting from the restrained Rietveld refinement by Dong et al (2013) were notably underestimated due to the planarity restraints imposed, presumably, by analogy with the singlecrystalline monoclinic polymorph of (II) where these angles did not exceed 5 . Evidently, the planarity restraints were excessive as the conformation of fumarate and fumaric acid molecules is flexible and the above discussed angles may well exceed 20 depending on the packing and hydrogen bonding (Sathya et al, 2013).…”

“…In Fig. 6 and Table 3, the interatomic distances in (II) resulting from the unrestrained DDM refinement are compared with those obtained from an accurate single-crystal refinement of its monoclinic polymorph (Bü yü kgü ngö r et al, 2004) and a restrained Rietveld refinement on the same XRPD data with 77 geometric restraints per 56 variable parameters (Dong et al, 2013). Whereas the restraints were aimed, obviously, at improving the respective geometric parameters, the average deviation of distances from the singlecrystal data appeared to be half as large for the unrestrained DDM compared with the restrained Rietveld refinement.…”

“…Herein we show that even for such imperfect data an accurate unconstrained structure refinement is possible using ISSN 2052-5206 # 2016 International Union of Crystallography the derivative difference method (Solovyov, 2004) with properly chosen weighting schemes thanks to the high resolution and extent of the synchrotron diffraction pattern. For lower-resolution but less distorted laboratory XRPD data of 2-aminopyridinium fumarate-fumaric acid (II), whose crystal structure was recently refined applying 77 geometric restraints by Dong et al (2013), an unrestrained DDM refinement using the same pattern is shown to provide two times narrower dispersion of interatomic distances, as follows from their comparison with high-quality single-crystal data obtained by Bü yü kgü ngö r et al 2004for a monoclinic polymorph of (II).…”

“…The XRPD data for (II) from Dong et al (2013) were collected on a laboratory diffractometer (9 kW rotating anode, Cu K 1 , Ge monochromated; D/teX Ultra onedimensional detector; collection time 6 h) using a finely ground powder sample loaded in a rotated capillary ( Table 1). The powder averaging in this case was good enough, while the data resolution 1.34 Å was too low to expect an accurate refinement.…”

Vibrational Spectroscopic and Computational Studies on Bis(2-aminopyridinium)fumarate - Fumaric Acid (1:1) Complex

“…This motif is one of the 24 most frequently observed bimolecular cyclic hydrogenbonded motifs in organic crystal structures. In 2013 Dong et al [3] found the new polymorph of 2APF-F, which crystallized in triclinic system in centrosymmetric space group. In both polymorphs asymmetric units consist of one 2-aminopirydinium cation, half a fumaric acid molecule and half a fumarate dianion.…”