Building unit and topological evolution in the hydrothermal DABCO-U-F system

Compounds NDUF-1 ([C6H14N2](UO2)(2)F-6; P2(1)/c, a = 6.9797(15) Angstrom, b = 8.3767(15) Angstrom, c = 23.760(5) Angstrom, beta = 91.068(4)degrees, V= 1388.9(5) Angstrom (3), Z = 4), NDUF-2 ([C6H14N2](2)(UO2)(2)F5UF7.H2O), NDUF-3 ((NH4)(7)U6F31; R(3) over bar, a = 15.4106(8) Angstrom, c = 10.8142(8) Angstrom, V = 2224.1(2) Angstrom (3), 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). 6H(2)O, 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.