The Role of Amine Sulfates in Hydrothermal Uranium Chemistry

A series of novel uranium sulfates containing organic structure directing cations has been synthesized from amine
sulfate precursors under hydrothermal conditions. The amine sulfates act as a soluble source of the protonated
amines and sulfate ions at low temperature and provide a reaction pathway in which no amine decomposition is
observed. The protonated amines act as both space fillers and hydrogen-bond donors in the three-dimensional
structure. The factors governing the formation of the observed hydrogen-bonding networks were probed through
the use of bond valence sums, which allow the quantification of residual negative charge and determination of the
relative nucleophilicity of each oxide ligand. The hydrogen bonding in these new compounds is dependent upon
two factors. First, the oxide ligands with the highest nucleophilicities are preferential acceptors with respect to their
less nucleophilic counterparts. Second, geometric constraints that result from the formation of multiple hydrogen
bonds from a single ammonium center can dictate the donation to oxides with smaller negative charges. Crystal
data for [N4C6H12][SO4]2·2H2O, a = 7.2651(2) Å, b = 7.3012(2) Å, c = 8.3877(3) Å, α = 90.260(1)°, β =
100.323(1)°, γ = 113.0294(15)°, triclinic, P-1 (No. 2), Z = 1; for [N4C6H22][UO2(H2O)(SO4)2]2·6H2O, a = 6.7318(1)
Å, b = 9.2975(1) Å, c = 13.1457(3) Å, α = 72.3395(6)°, β = 89.1401(7)°, γ = 70.0267(12)°, triclinic, P-1 (No.
2), Z = 1; for [N4C6H22][UO2(SO4)2]2, a = 9.3771(2) Å, b = 12.9523(3) Å, c = 18.9065(6) Å, orthorhombic, Pbca
(No. 61), Z = 4; for [N5C8H28]2[(UO2)5(H2O)5(SO4)10]·H2O, a = 7.76380(5) Å, b = 14.16890(5) Å, c = 56.46930(5)
Å, orthorhombic, Pbnm (No. 62), Z = 4.