Spectroscopic, thermal and single crystal structure investigations of 2-bromotrimesic acid and its trimethyl ester analogue

Abstract

Two analogues of the well investigated trimesic acid viz. the 2-bromobenzene-1,3,5-tricarboxylic acid 1 and their ester trimethyl 2-bromobenzene-1,3,5-tricarboxylate 2, have been synthesised and their X-ray structures were solved. Acid 1 crystallises as 1:1 inclusion compound with water in a layer structure. Like in the solid state structure of trimesic acid, we found strong OH⋅⋅⋅O hydrogen bonds between one of the carboxyl groups and a neighbouring molecule to form a hydrogen bonding motif R22(8). Additionally, a water molecule and a second acid function of 1 are involved in further hydrogen bonding featuring the graph set R44(12) forming what might be called a water inserted dimer. As shown by TG-DSC measurements the water molecule in the 1:1 inclusion compound of 1 is engaged in two strong OH⋅⋅⋅O hydrogen bonds, it escapes at a rather low temperature of 99°C. Bromine monosubstitution at the benzene ring forces the third carboxylic acid out of the mean plane of the molecule, which disturbs the coplanar arrangement of the three COOH moieties. Thus, the typical “chicken-wire” network formation is hindered. In the trimethyl 2-bromobenzene-1,3,5-tricarboxylate (2), the formation of strong OH⋅⋅⋅O hydrogen bonds is disabled by esterification of the acid functions. Nevertheless, the packing of 2 features solvent free molecular layers formed by Br⋅⋅⋅O contacts and connected van der Waals interactions. These layers are linked to each other by inverse bifurcated hydrogen bonds in term weak CH⋅⋅⋅O contacts. The results of the X-ray analysis could be confirmed by infrared spectroscopy.