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

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) Å3, 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) Å3, 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 UVI to UIV 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.