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-α-PW9O34)2]·12H2O. 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(UVIO2)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.