Complexation of Protactinium(V) with Chlorides in aqueous solution: Thermodynamic and Structural Insights
Résumé
Protactinium(V) is a singular actinide with unique chemical properties that differentiate it from other pentavalent actinides, particularly due to the absence of the actinyl moiety in its condensed phase compounds. Instead, Pa(V) exhibits a short mono-oxo bond, experimentally observed in oxalic and sulfuric media. However, this mono-oxo bond is not so stable and can vanish upon complexation, which is the case in presence of fluoride [1]. Additionally, in oxalate and sulfate complexes, the Pa-O bond distance is experimentally observed at 1.75 Å, which contradicts theo-retical calculations that predict this distance to be around 1.85 Å [2]. These two phenomena re-main poorly understood and highlight the challenges in studying Pa(V). The study of Pa(V) is further complicated by its remarkable propensity for hydrolysis and polymerization even in very acidic conditions, and the difficulty in isolating significant amounts of this actinide, making it dif-ficult to handle and investigate.
In this study, we focus on the complexation of Pa(V) with chloride ions, using X-ray absorption spectroscopy (XAS) to investigate its chemical environment at different hydrochloric acid con-centrations. Given the weak ligation properties of chlorides, the presence of the mono-oxo bond in these complexes is expected [3]. Thermodynamic studies have already provided the probable chemical forms of these complexes. However no direct experimental data on their structures have been collected until now [4].
From our EXAFS data, two structures of Pa(V)-chloride complexes could be determined at 3M HCl and 12M HCl, corroborating the thermodynamic data. Additionally, the EXAFS spectra ad-justments with structures obtained by theoretical calculations reveal, for the first time, a Pa-O bond distance around 1.83 Å, aligning more closely with theoretical calculations [1,2,3]. This finding supports the presence of an oxo bond in the Pa(V)-chloride system, advancing our un-derstanding of protactinium chemistry. These results not only reconcile experimental observa-tions with theoretical predictions but also provide a basis for future investigations into the coor-dination chemistry of Pa(V).
[1]M. Mendes et al., Inorg. Chem. 49, 9962-9971 (2010).
[2]H. Oher et al., Phys. Chem. Chem. Phys. 25, 10033-10041 (2023).
[3]T. Shaaban et al., Chem. Eur. J. 30, e202304068 (2024).
[4]R. Muxart et al. Compléments au nouveau traité de chimie minérale. (1974).
In this study, we focus on the complexation of Pa(V) with chloride ions, using X-ray absorption spectroscopy (XAS) to investigate its chemical environment at different hydrochloric acid con-centrations. Given the weak ligation properties of chlorides, the presence of the mono-oxo bond in these complexes is expected [3]. Thermodynamic studies have already provided the probable chemical forms of these complexes. However no direct experimental data on their structures have been collected until now [4].
From our EXAFS data, two structures of Pa(V)-chloride complexes could be determined at 3M HCl and 12M HCl, corroborating the thermodynamic data. Additionally, the EXAFS spectra ad-justments with structures obtained by theoretical calculations reveal, for the first time, a Pa-O bond distance around 1.83 Å, aligning more closely with theoretical calculations [1,2,3]. This finding supports the presence of an oxo bond in the Pa(V)-chloride system, advancing our un-derstanding of protactinium chemistry. These results not only reconcile experimental observa-tions with theoretical predictions but also provide a basis for future investigations into the coor-dination chemistry of Pa(V).
[1]M. Mendes et al., Inorg. Chem. 49, 9962-9971 (2010).
[2]H. Oher et al., Phys. Chem. Chem. Phys. 25, 10033-10041 (2023).
[3]T. Shaaban et al., Chem. Eur. J. 30, e202304068 (2024).
[4]R. Muxart et al. Compléments au nouveau traité de chimie minérale. (1974).
