Formation of soluble hexanuclear neptunium(IV) nanoclusters in aqueous solution: growth termination of actinide(IV) hydrous oxides by carboxylates

Complexation of Np(IV) with several carboxylates (RCOO(-); R = H, CH(3), or CHR'NH(2); R' = H, CH(3), or CH(2)SH) in moderately acidic aqueous solutions was studied by using UV-vis-NIR and X-ray absorption spectroscopy. As the pH increased, all investigated carboxylates initiated formation of water-soluble hexanuclear complexes, Np(6)(μ-RCOO)(12)(μ(3)-O)(4)(μ(3)-OH)(4), in which the neighboring Np atoms are connected by RCOO(-)syn-syn bridges and the triangular faces of the Np(6) octahedron are capped with μ(3)-O(2-)/μ(3)-OH(-). The structure information of Np(6)(μ-RCOO)(12)(μ(3)-O)(4)(μ(3)-OH)(4) in aqueous solution was extracted from the extended X-ray absorption fine structure data: Np-O(2-) = 2.22-2.23 ? (coordination number N = 1.9-2.2), Np-O(RCOO(-)) and Np-OH(-) = 2.42-2.43 ? (N = 5.6-6.7 in total), Np···C(RCOO(-)) = 3.43 ? (N = 3.3-3.9), Np···Np(neighbor) = 3.80-3.82 ? (N = 3.6-4.0), and Np···Np(terminal) = 5.39-5.41 ? (N = 1.0-1.2). For the simpler carboxylates, the gross stability constants of Np(6)(μ-RCOO)(12)(μ(3)-O)(4)(μ(3)-OH)(4) and related monomers, Np(RCOO)(OH)(2)(+), were determined from the UV-vis-NIR titration data: when R = H, log β(6,12,-12) = 42.7 ± 1.2 and log β(1,1,-2) = 2.51 ± 0.05 at I = 0.62 M and 295 K; when R = CH(3), log β(6,12,-12) = 52.0 ± 0.7 and log β(1,1,-2) = 3.86 ± 0.03 at I = 0.66 M and 295 K.