Philip M. Almond

Hydrothermal synthesis and crystal chemistry of the new strontium uranyl selenites, Sr[(UO2)3(SeO3)2O2]•4H2O and Sr[UO2(SeO3)2]

Abstract The reaction of UO3 with SeO2 in the presence of SrCl2⋅6H2O and Sr(OH)2⋅8H2O in supercritical water at 425 °C for 3 d results in the formation of the new strontium uranyl selenites, Sr[(UO2)3(SeO3)2O2]⋅4H2O (1) and Sr[UO2(SeO3)2] (2). The single crystal X-ray structures of type 1 and type 2 were solved by direct methods and refined by full-matrix least-squares methods. Crystallographic data (193 K): (1), monoclinic, space group C2/m, a = 17.014(2), b = 7.0637(7), and c = 7.1084(7) Å, β = 100.544(2)°, Z = 2, R(F) = 0.0361 for 79 parameters with 1132 reflections with I > 2σ(I), wR2 = 0.0998 for all data; (2), triclinic, space group P1̅ , a = 5.6722(4), b = 6.7627(5), and c = 11.2622(8) Å, α = 104.698(1)°, β = 93.708(1)°, γ = 109.489(1)°, Z = 2, R(F) = 0.0373 for 110 parameters with 1902 reflections with I > 2σ(I), wR2 = 0.0856 for all data. The structure of type 1 contains two-dimensional 2∞ [(UO2)3(SeO3)2O2]2- sheets with the same topology as those found in guilleminite, Ba[(UO2)3(SeO3)2O2]⋅3H2O, and marthozite, Cu[(UO2)3(SeO3)2O2](H2O)8. Sr2+ cations and H2O groups occur between the layers. In contrast, the structure of type 2 contains one-dimensional 1∞[UO2(SeO3)2]2- ribbons with Sr2+ cations residing between them. This compound is isostructural with its Ca2+-containing analog.

An Acyclic Trialkylamine Virtually Planar at Nitrogen. Some Chemical Consequences of Nitrogen Planarity

The synthesis of the exceedingly congested amine tris(1,3-dihydroxy-2-propyl)amine, 9, was achieved in 47- 51% overall yield. The nitrogen atom of 9 is virtually planar; it is 0.082 A out of the plane defined by the three attached carbons. The corresponding out-of-plane measurement is 0.282 A for triisopropylamine and ca. 0.4 A for uncongested trialkylamines. The N-C bonds of 9 are quite short, despite the steric congestion. The conjugate acid of 9 (viz., 9H(+)) is very strong: pK(a) = 3.08 (cf. Et(3)NH(+) pK(a) = 10.7). Comparison with suitable model compounds suggests 9 is less basic than predicted by ca. 1.5 pK(a) units. The structure of 9H(+)Cl(-) was determined by X-ray crystallography. Here too, the nitrogen is severely flattened relative to ordinary ammonium cations. In 9H(+)Cl(-), the proton on the nitrogen of 9H(+) forms three intramolecular hydrogen bonds to hydroxyl groups, i.e., a so-called trifurcated hydrogen bond. The NH...O lengths in 9H(+) are slightly shorter than comparable trifurcated hydrogen bonds. Cyclic voltammetry (CV) on 9 finds E(1/2)(ox) is 0.88 V, which is consistent with the inductive effect of the 1,3-dihydroxy-2-propyl groups attached to nitrogen. It is also observed that the electrochemical oxidation of 9 is reversible on the CV time scale. The (15)N NMR chemical shift of the essentially planar nitrogen atom of 9 is discussed.