Capturing Hydrolysis Products in the Solid State: Effects of pH on Uranyl Squarates under Ambient Conditions

Two uranyl squarates, (UO2)6(C4O4)3(OH)6O2·9H2O·4NH4 (1; a = 16.6897(7) Å, cubic, I23) and (UO2)(C4O4)(OH)2·2NH4 (2; a = 8.5151(4), b = 15.6822(8), c = 7.3974, orthorhombic, Pbcm), have been synthesized from ambient aqueous solutions as a function of pH. Oligomerization of the uranyl cation from monomeric pentagonal bipyramids (pH < 5) to [(UO2)3O(OH)3] trimers (5 < pH < 8) in 1 and ultimately [(UO2)(OH)2]n chains (7 < pH < 8) in 2 is observed. This evolution of speciation versus pH is consistent with what has been observed in solution and thus may be represented by the uranyl hydrolysis equilibrium, mUO22+ + nH2O ↔ [(UO2)m(OH)n]2m − n + nH+. Structural systematics, physical properties, and a discussion of species selectivity by squarate anions are presented.