ORIGINAL ARTICLE Permethylated b-cyclodextrin/pesticide complexes: X-ray structures and thermogravimetric assessment of kinetic parameters for complex dissociation Dyanne L. Cruickshank • Natalia M. Rougier • Vaughan J. Maurel • Rita H. de Rossi • Elba I. Buja ´n • Susan A. Bourne • Mino R. Caira Received: 7 February 2012 / Accepted: 7 March 2012 / Published online: 13 April 2012 Ó Springer Science+Business Media B.V. 2012 Abstract The X-ray crystal structures of the inclusion complexes formed between three pesticides (two organo- phosphorus insecticides and one chloroacetanilide herbicide) and permethylated b-cyclodextrin (TRIMEB) are reported. The complexes TRIMEB–fenitrothion (1), TRIMEB–fen- thion (2) and TRIMEB–acetochlor (3) are members of a commonly occurring isostructural series. The mode of inclusion of the two organophosphate insecticides is very similar, while the acetochlor molecule, which is structurally quite distinct from the two insecticide molecules, adopts a somewhat different position within the TRIMEB cavity. In addition to the structural elucidation of these complexes, their thermal behaviour was investigated using isothermal and non-isothermal thermogravimetry. The isothermal results showed that the dissociation of the guest molecules from the TRIMEB cavities can best be described by two mechanisms, namely a first-order reaction model and a three-dimensional diffusion model. Both the isothermal and non-isothermal methods allowed the determination of the activation energies of the guest loss process for each complex. Keywords Pesticides  Inclusion complexes  Permethylated b-cyclodextrin  Single crystal X-ray diffraction  Thermal decomposition kinetics Introduction The ability of cyclodextrins (CDs) to form inclusion complexes with various guest molecules has been studied extensively over the last few decades especially within the pharmaceutical industry, as well as the food, cosmetic and biotechnology industries, and in the field of analytical chemistry [1]. In 1985 Szejtli [2] reviewed the subject of CDs and pesticides, stating that within the next decade, there would be a rapid development in the application of CDs in pesticide formulations accompanying the decrease in the price of CDs and the increasing rate of CD pro- duction. Since then the focus has been on studying the interactions between pesticides and CDs in solution as well as in the solid state and evaluating the possible benefits that could be conferred on pesticides upon their complexation with biodegradable host molecules such as CDs. In general, the undesirable physicochemical properties associated with pesticides include their poor aqueous solubility, chemical, thermal and physical instability, high volatility, high soil mobility and poor wettability [3]. The three pesticides selected for this study are the insecticides fenitrothion [O,O-dimethyl O-(3-methyl-4- nitrophenyl) phosphorothioate] and fenthion [O,O-dime- thyl O-[3-methyl-4-(methylthio)phenyl] phosphorothioate], and the herbicide acetochlor [2-chloro-N-(ethoxymethyl)- N-(2-ethyl-6-methylphenyl)acetamide] (Fig. 1). These com- pounds are oils at room temperature and are currently registered for use in several countries [4]. The reduced chemical and thermal stability of these pesticides, as well as the difficulties Electronic supplementary material The online version of this article (doi:10.1007/s10847-012-0145-5) contains supplementary material, which is available to authorized users. D. L. Cruickshank  V. J. Maurel  S. A. Bourne  M. R. Caira (&) Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa e-mail: mino.caira@uct.ac.za N. M. Rougier  R. H. de Rossi  E. I. Buja ´n Instituto de Investigaciones en Fı ´sico Quı ´mica de Co ´rdoba (INFIQC), Departamento de Quı ´mica Orga ´nica, Facultad de Ciencias Quı ´micas, Universidad Nacional de Co ´rdoba, Ciudad Universitaria, X5000HUA Co ´rdoba, Argentina 123 J Incl Phenom Macrocycl Chem (2013) 75:47–56 DOI 10.1007/s10847-012-0145-5