Formation of Soluble Hexanuclear Neptunium(IV) Nano-Clusters in Aqueous Solution: Growth Termination of Actinide(IV) Hydrous Oxides by Carboxylates

Complexation of NpIV with several carboxylates (RCOO–; R = H, CH3, CHR’NH2; R’ = H, CH3, CH2SH) in moderately acidic aqueous solutions were studied by using UV-vis-NIR and X-ray
absorption spectroscopy. As pH increased, all investigated carboxylates initiated formation of watersoluble hexanuclear complexes, Np6(μ-RCOO)12(μ3-O)4(μ3-OH)4, where the neighboring Np atoms are connected by RCOO– syn-syn bridges and the triangular faces of the Np6 octahedron are capped with μ3-O2–/μ3-OH–. The structure information of Np6(μ-RCOO)12(μ3-O)4(μ3-OH)4 was extracted from the EXAFS data: Np–O2– = 2.22-2.23 Å (coordination number, N = 1.9-2.2), Np–O(RCOO–) and Np–OH– = 2.42-2.43 Å (N = 5.6-6.7 in total), Np···C(RCOO–) = 3.43 Å (N = 3.3-3.9), Np···Np(neighbor) = 3.80-3.82 Å (N = 3.6-4.0), Np···Np(terminal) = 5.39-5.41 Å (N = 1.0-1.2). For the simpler carboxylates, the gross stability constants of Np6(μ-RCOO)12(μ3-O)4(μ3-OH)4 and related monomers, Np(RCOO)(OH)2 +, were determined from the UV-vis-NIR titration data: R = H, log β6,12,–12 = 42.7 ± 1.2, log β1,1,–2 = 2.51 ± 0.05 at I = 0.62 M and 295 K; R = CH3, log β6,12,–12 = 52.0 ± 0.7, log β1,1,–2 = 3.86 ± 0.03 at I = 0.66 M and 295 K.