Homometallic UO22+ diphosphonates assembled under ambient and hydrothermal conditions

Abstract

Four uranyl methylenediphosphonates have been synthesized from reactions of uranyl nitrate and methylenediphosphonate in water. Three of the compounds, (1) (UO₂)[CH₂(PO₃H)₂](H₂O)·2H₂O, (2) (UO₂)[CH₂(PO₃H)₂](H₂O)·4H₂O, and (3) (UO₂)[O₃PCH₂PO₃H](H₂O)·3H₂O·NH₄, have been synthesized under ambient conditions whereas compound 4, (UO₂)[CH₂(PO₃H)₂](H₂O)·H₂O, was prepared hydrothermally. Each compound is built from the UO₂²⁺ cation coordinated to one water molecule and four oxygen atoms from three diphosphonate ligands to form pentagonal bipyramid geometry. Despite these similarities however, compounds 1–4 exhibit remarkable structural diversity with 1 and 2 forming 1-D structural units, compound 3 adopting 2-D sheets, and compound 4 exhibiting a 3-D structure. Such differences are attributed to variations in the preparation of the materials (reaction temperature and presence of counterions) as well differences in hydration and/or the position of the bound water molecules in each of the structures. The thermal and fluorescent behaviors of the compounds are also presented.