Crystallization
of the nonsteroidal anti-inflammatory drug (NSAID)
flunixin in acetone and in acetone–hexane produced two conformational
polymorphs, as predicted by hydrogen-bond propensity analyses, which
are similar to three of the polymorphs of the related NSAID clonixin.
The two polymorphs of flunixin display an intramolecular hydrogen
bond between the amine N–H and the carbonyl oxygen of the carboxylic
group. However, the intermolecular hydrogen-bond pattern of Form I
(crystallized from acetone) is characterized by the acid–pyridine
heterosynthon, while Form II (from acetone-hexane) displays the acid–acid
homosynthon. Form I of flunixin is similar to form I of clonixin,
while form II of flunixin is related to forms III and IV of clonixin.
Hirshfeld surface maps were used to show the differences in the interactions
present in the two flunixin polymorphs and the similarities with the
polymorphs of the related compound clonixin. Energy framework calculations
corroborate the similarity between Form I of flunixin and clonixin
and indicate that Form II of flunixin is more closely related to Form
IV of clonixin than to Form III.
Several benzothiophene-based compounds, including 1-benzothiophene-2-carboxylic acid, exhibit a wide variety of pharmacological activities. They have been extensively used to treat various types of diseases with high therapeutic effectiveness. In this contribution, the crystal structure of a new polymorph of 1-benzothiophene-2-carboxylic acid (BTCA) was determined from laboratory X-ray powder diffraction data with DASH, refined by the Rietveld method with TOPAS-Academic, and optimized using DFT-D calculations. The new form of 1-benzothiophene-2-carboxylic acid crystallizes in space group C2/c (No. 15) with a = 14.635(4), b = 5.8543(9), c = 19.347(3) Å, β = 103.95(1)°, V = 1608.8(6) Å3, and Z = 8. The structure is a complex 3D arrangement which can be described in terms of hydrogen-bonded dimers of BTCA molecules, joined by the acid–acid homosynthon, which interact through C–H⋯O hydrogen bonds to produce tapes further connected through head-to-tail π⋯π and edge-to-face C–H⋯π interactions. A comparison with a previously reported triclinic polymorph and with the related 1-benzofuran-2-carboxylic acid (BFCA) is also presented.
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