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, CH3, or CHR'NH2; R' = H, CH3, or CH2SH) 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(mu-RCOO)(12)(mu(3)-O)(4)(mu(3)-OH)(4), in which the neighboring Np atoms are connected by RCOO- syn-syn bridges and the triangular faces of the N-p6 octahedron are capped with mu(3)-O2-/mu(3)-OH-. The structure information of N-p6(mu-RCOO)(12)(mu(3)-O)(4)(mu(3)-OH)(4) in aqueous solution was extracted from the extended X-ray absorption fine structure data: Np-O2- = 2.22-2.23 angstrom (coordination number N = 1.9-2.2), Np-O(RCOO-) and Np-OH- = 2.42-2.43 angstrom (N = 5.6-6.7 in total), Np center dot center dot center dot C(RCOO-) = 3.43 angstrom (N = 3.3-3.9), Np center dot center dot center dot Np(neighbor) = 3.80-3.82 angstrom (N = 3.6-4.0), and Np center dot center dot center dot Np(terminal) = 5.39-5.41 angstrom (N = 1.0-1.2). For the simpler carboxylates, the gross stability constants of Np-6(mu-RCOO)(12)(mu(3)-O)(4)(mu(3)-OH)(4) and related monomers, Np(RCOO)(OH)(2)(+), were determined from the UV-vis-NIR titration data: when R = H, log beta(6,12,-12) = 42.7 +/- 1.2 and log beta(1,1,-2) = 2.51 +/- 0.05 at I = 0.62 M and 295 K; when R = CH3, log beta(6,12,-12) = 52.0 +/- 0.7 and log beta(1,1,-2) = 3.86 +/- 0.03 at I = 0.66 M and 295 K.