Spontaneous, Chelation-Driven Reduction of the Neptunyl Cation in Aqueous Solution.

Octadentate hydroxypyridinone (HOPO) and catecholamide (CAM) siderophore analogues are known to be efficacious chelators of the actinide cations, and these ligands are also capable of facilitating both activation and reduction of actinyl species. Utilizing X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, as well as cyclic voltammetry measurements, herein, we elucidate chelation-based mechanisms for driving reactivity and initiating redox processes in a family of neptunyl-HOPO and CAM complexes. Based on the selected chelator, the ability to control the oxidation state of neptunium and the speed of reduction and concurrent oxo group activation was demonstrated. Most notably, reduction kinetics for the (NpO2+/)-O-V/Np-IV redox couple upon chelation by the ligands 3,4,3-LI(1,2-HOPO) and 3,4,3-LI(CAM)(2)(1,2-HOPO)(2) was observed to be faster than ever reported, and in fact quicker than we could measure using either X-ray absorption spectroscopy or electrochemical techniques.