Synthesis and Study of the First Zeolitic Uranium Borate

A complex, three-dimensional, open-framework, lead uranyl borate, (H₂O)­Pb₃(UO₂)₃­B₁₄O₂₇, denoted as LUBO, was synthesized via a hydrothermal method. LUBO crystallizes in the hexagonal space group P6₃/m and exhibits a zeolite-like anionic borate framework (B₁₄O₂₇)^12‑. The main structural unit of the framework is a tubule consisting of six-membered B₆O₁₈ rings. Each ring is connected to the successive one by three diborate groups, and these tubules propagate along the c axis. The tubules possess six-membered ring (MR) windows in the axial direction and 8-MR windows on its sides. Interconnection of the parallel tubules, which consist exclusively of BO₄ tetrahedra, is provided by triangular BO₃ fragments perpendicular to the axis of the tubules. The framework has large pores as well as channels with 8-MR windows extending along the [100], [010], and [110] directions that are consistent with the overall hexagonal symmetry of the structure. The lead cations occupy 8-MR windows and form [Pb₃(H₂O)] groups with attached water molecules that are located at the center of the tubules. The method of Voronoi–Dirichlet tessellation reveals that the lone pairs of the lead cations are located outside the tubule. Uranyl cations form UO₈ coordination polyhedra in the shape of a hexagonal bipyramid. The thermal stability and vibrational spectroscopy of LUBO are also delineated in this work.