Capturing hydrolysis products in the solid state: effects of pH on uranyl squarates under ambient conditions.

Two uranyl squarates, (UO(2))(6)(C(4)O(4))(3)(OH)(6)O(2)·9H(2)O·4NH(4) (1; a = 16.6897(7) Å, cubic, I23) and (UO(2))(C(4)O(4))(OH)(2)·2NH(4) (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 [(UO(2))(3)O(OH)(3)] trimers (5 < pH < 8) in 1 and ultimately [(UO(2))(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, mUO(2)(2+) + nH(2)O ↔ [(UO(2))(m)(OH)(n)](2m - n) + nH(+). Structural systematics, physical properties, and a discussion of species selectivity by squarate anions are presented.