Unexpected Actinyl Cation-Directed Structural Variation in Neptunyl(VI) A-Type Tri-lacunary Heteropolyoxotungstate Complexes

A-type tri-lacunary heteropolyoxotungstate anions (e.g., [PW9O34](9-), [AsW9O34](9-), [SiW9O34](10-), and [GeW9O34](10-)) are multidentate oxygen donor ligands that readily form sandwich complexes with actinyl cations ({UO2}(2+), {NpO2}(+), {NpO2}(2+), and {PuO2}(2+)) in near-neutral/slightly alkaline aqueous solutions. Two or three actinyl cations are sandwiched between two tri-lacunary anions, with additional cations (Na+, K+, or NH4+) also often held within the cluster. Studies thus far have indicated that it is these additional +1 cations, rather than the specific actinyl cation, that direct the structural variation in the complexes formed. We now report the structural characterization of the neptunyl(VI) cluster complex (NH4)(13)[Na(NpO2)(2)(A-alpha-PW9O34)(2)] 12H(2)O. The anion in this complex, [Na(NpO2)(2)(PW9O34)(2)](13-), contains one Na+ cation and two {NpO2}(2+) cations held between two [PW9O34](9-) anions, with an additional partial occupancy NH4+ or {NpO2}(2+) cation also present. In the analogous uranium(VI) system, under similar reaction conditions that include an excess of NH4Cl in the parent solution, it was previously shown that [(NH4)(2)((UO2)-O-VI)(2)(A-PW9O34)(2)](12-) is the dominant species in both solution and the crystallized salt. Spectroscopic studies provide further proof of differences in the observed chemistry for the {NpO2}(2+)/[PW9O34](9-) and {UO2}(2+)/[PW9O34](9-) systems, both in solution and in solid state complexes crystallized from comparable salt solutions. This work reveals that varying the actinide element (Np vs U) can indeed measurably impact structure and complex stability in the cluster chemistry of actinyl(VI) cations with A-type tri-lacunary heteropolyoxotungstate anions.