Synthesis, spectral properties and thermal behaviour of zinc(II) acetylsalicylate

The thermal behaviour of zinc(II) acetylsalicylate [Zn(acsa)2(H2O)2] with respect to phase transitions, pyrolysis both in air and inert (N2) atmosphere, and product identification has been investigated. The complex was synthesised by metathesis in hot ethanol solution using aspirin (acetylsalicylic acid) as precursor and characterised via electronic and IR spectral analyses. Optical observations showed that the white salt does not undergo a direct transition from the solid to the liquid phase but rather goes slowly through an intermediate mesophase around 80 °C before melting rapidly to the brick-brown isotropic liquid around 134–136 °C. No liquid crystalline phases are however formed. This result was complemented by that from thermogravimetric (TG) studies in the ca. 25–600 °C range, which showed three main weight-loss phases of 8.0, 50.0 and 14.0% (around 200, 250 and 400 °C) corresponding, respectively, to the elimination of CO2, xanthone and acetic acid. The pyrolysis products, as identified using a combination of instrumental (GC–MS) and wet chemical techniques are: CO2; non-stoichiometric zinc oxide, most likely in the form: Zn1+xO (where 0.0000≤x≤0.0003); and a mixture of organic products resulting from further decomposition, charring and other attendant thermal effects at the relatively high temperatures (ca. 600 °C) involved. Six of the principal organic products were identified and included salsalate and benorylate which are pro-drugs of salicylic acid, a well-known pharmaceutical.