Building Unit and Topological Evolution in the Hydrothermal DABCO−U−F System

Abstract

Compounds NDUF-1 ([C₆H₁₄N₂](UO₂)₂F₆; P2₁/c, a = 6.9797(15) Å, b = 8.3767(15) Å, c = 23.760(5) Å, β = 91.068(4)°, V = 1388.9(5) ų, Z = 4), NDUF-2 ([C₆H₁₄N₂]₂(UO₂)₂F₅UF₇·H₂O), NDUF-3 ((NH₄)₇U₆F₃₁; R3̄, a = 15.4106(8) Å, c = 10.8142(8) Å, V = 2224.1(2) ų, Z = 3), and NDUF-4 ([NH₄]U₃F₁₃) have been synthesized hydrothermally from fixed composition reactant mixtures over variable time periods [DABCO (C₆H₁₂N₂), UO₂(NO₃)₂·6H₂O, HF, and H₂O; 2−14 days]. Observed is a systematic evolution of the structural building units within these materials from the UO₂F₅ pentagonal bipyramid in NDUF-1 and -2 to the UF₈ trigonal prism in NDUF-2 and finally to the UF₉ polyhedron in NDUF-3 and -4 as a function of reaction time. Coupled to this coordination change is a reduction of U^VI to U^IV as well as a breakdown of the organic structure-directing agent from DABCO to NH₄⁺. These processes contribute to a structural transition from layered topologies (NDUF-1) to chain (NDUF-2), back to layered (NDUF-3), and ultimately to framework (NDUF-4) connectivities. The synthesis conditions, crystal structures, and possible transformation mechanisms within this system are presented.