Hydrothermal Synthesis of Uranyl Squarates and Squarate-Oxalates: Hydrolysis Trends and in Situ Oxalate Formation

We report the synthesis of two uranyl squarates and two mixed-ligand uranyl squarate−oxalates from aqueous solutions under hydrothermal conditions. These products exhibit a range of uranyl building units from squarates with monomers in (UO2)2(C4O4)5·6NH4·4H2O (1; a = 16.731(17) Å, b = 7.280(8) Å, c = 15.872(16) Å, β = 113.294(16)°, monoclinic, P21/c) and chains in (UO2)2(OH)2(H2O)2(C4O4) (2; a = 12.909(5) Å, b = 8.400(3) Å, c = 10.322(4) Å, β = 100.056(7)°, monoclinic, C2/c) to two squarate−oxalate polymorphs with dimers in (UO2)2(OH)(C4O4)(C2O4)·NH4·H2O (3; a = 9.0601(7) Å, b = 15.7299(12) Å, c = 10.5108(8) Å, β = 106.394(1)°, monoclinic, P21/n; and 4; a = 8.4469(6) Å, b = 7.7589(5) Å, c = 10.5257(7) Å, β = 105.696(1)°, monoclinic, P21/m). The dominance at low pH of monomeric species and the increasing occurrence of oligomeric species with increasing pH suggests that uranyl hydrolysis, mUO22+ + nH2O ⇌ [(UO2)m(OH)n]2m−n + nH+, has a significant role in the identity of the inorganic building unit. Additional factors that influence product assembly include in situ hydrolysis of squaric acid to oxalic acid, dynamic metal to ligand concentration, and additional binding modes resulting from the introduction of oxalate anions. These points and the effects of uranyl hydrolysis with changing pH are discussed in the context of the compounds presented herein.