Determination of the Constants of Formationyof Complexes of Iron(III) and Acetohydroxamic Acid

Simple hydroxamic acids (XHA) are hydrophilic organic compoundswith the general formula RCONHOH that can act as O,O donor ligands with high affinities for hard cations such as Fe3+, Np4+ and Pu4+ with which they form 5-membered chelate rings (Muri et al.,2002; Desaraju & Winston,1986; Carrott et al.,2007). They have a wide range of applications in many fields, including as enzyme inhibitors, soil enhancers, fungicides, mutagens, carcinogens, DNA cleavers, drug delivery systems (Ghosh,1997), ion exchange (Vernon,1982) and materials research (Ghosh,1997). Siderophores such as the desferrioxamines contain multiple hydroxamate functionalities and are naturally occurring ligands specifically used by fungi for the sequestration of iron from the environment (Raymond et al.,1984; Renshaw et al.,2002; Monzyk & Crumbliss,1979). The potential role of such hydroxamate siderophores in the mobilisation of actinide ions within the environment has also been considered (John et al.,2001). More recently, XHAs have also been identified as useful reagents for the control of Pu and Np in advanced PUREX and UREX processes proposed for the processing of spent nuclear fuel (Birkett et al.,2005). In such separation processes, U, Np and Pu are separated according to oxidation state specific aqueous/non aqueous solvent extraction in the presence of acetohydroxamic acid (AHA) (Carrott et al.,2007). In this context, Fe(III) is a useful non-radioactive analogue for the interrogation of the complexation behaviour of actinides. It is also well known that hydroxamic acids are susceptible to hydrolysis, particularly in acidic solutions (Ghosh,1997). Whilst there have been many studies of hydroxamic acid hydrolysis and their complexation reactions with metal ions, there have been relatively few studies of the stability of the metal-hydroxamate complexes towards hydrolysis. We have previously reported on the kinetics of the hydrolysis of hydroxamic acids both in free solution and when bound to metal ions and developed a kinetic model describing this process at room temperature (Andrieux et al.,2007; Andrieux et al.,2008; Carrott et al.,2008). However, the applications of hydroxamic acids in biological-related fields requires an understanding of the behaviour of these systems at temperatures of biological interest, for example in vivo (310K) or temperatures at which proteins start to denature (∼ 325K) (Roos,2006). Additionally, the temperature dependencies of the equilibrium constants are useful in gaining a better understanding of the complexation processes and the role of enthalpy and entropy in complex formation. As a precursor to any temperature-dependent kinetic study of Fe(III)-AHA 16