Crystal structures of non-centrosymmetric fluorouranylates Na₃[UO₂F₅] and CaRb₄[UO₂F₄]₃⋅3H₂O

The structure of Na 3 [UO 2 F 5 ] (I) and CaRb 4 [UO 2 F 4 ] 3 ⋅3H 2 O (II) crystals was studied using X-ray diffraction analysis. The uranium-containing structural units are pentagonal bipyramids [UO 2 F 5 ] 3− , which are isolated from each other in I and linked by bridging fluorine atoms into trinuclear complexes of the composition [(UO 2 ) 3 F 12 ] 6− in II. The latter one fills the existing gap in the crystal chemistry of uranyl fluorides and oxofluorides by representing the first example of a trinuclear complex, while di-, tetra- and pentanuclear complexes have been observed before. The first compound has been known for more than half a century, although here we present the first report on its crystal structure. The crystal chemical formulae of the uranium-containing complexes in I and II are AM 1 5 and A 3 M 2 3 M 1 9 , where A = UO 2 2+ , М 2 and М 1 = F − . In both structures, the uranyl-containing complexes are linked into 3D frameworks by R –F and R –O coordination bonds with outer-sphere R cations ( R = Na, Rb or Ca). As both compounds I and II turned out to be non-centrosymmetric their ability to generate the second harmonic was theoretically predicted based on parameters of Voronoi–Dirichlet polyhedra. Experimental verification of the obtained values is of undoubted interest.