Effect of the substituent on the thermal stability of spiroborate esters of curcumin

Abstract

Spiroborate esters of curcumin (SBC) are garnering great attention due to their various applications in analytical and medical fields, and hence, their thermal stability studies have great importance. One of the main factors that affect the thermal stability is the nature of a substituent. So in this work, an attempt has been made to identify the effect of substituents on the thermal stability of SBC. Toward this purpose, four different SBC compounds were synthesized from oxalic acid, lactic acid, citric acid and malic acid. The thermal stability of these materials was studied over the range from room temperature to 750 °C under nitrogen atmosphere using thermogravimetry. The study revealed that the four complexes displayed more stability than curcumin by degrading only at 200 °C, compared to curcumin which displays an onset of degradation at 94 °C. Another observation was that the curcumin component degrades after the removal of the acid ligand that was attached to it through boron. A detailed kinetic analysis of the ligand decomposition stage for all these complexes was carried out. Consequently, a comprehensive analysis of the degradation of these complexes was performed and a solid-state reaction model which corresponds to the thermal degradation was proposed. The apparent activation energy (Ea) and other kinetic parameters were estimated using the model-free Friedman, Kissinger–Akahira–Sunose and Flyn–Wall–Ozawa methods. The influence of substituent on the thermal stability of spiroborate esters of curcumin is discussed. The validity of experimental data is also checked by comparing it with theoretical data.