Tailor-Made Zinc Uranyl Diphosphonates from Layered to Framework Structures

Hydrothermal reactions of zinc uranyl acetate and 1-hydroxyethylidenediphosphonic
acid (H4L) with 1,10-phenanthroline (phen), 2,2′-bipyridine
(bipy), 1H-benzo­[d]­imidazole (bi),
or 1-phenyl-1H-imidazole (pi) resulted in the formation
of four new zinc uranyl compounds, namely, [Zn2(phen)2(UO2)2(L)2(H2O)3]·3H2O (ZnUP-1), Zn2(bipy)2(UO2)2(L)2(H2O)2 (ZnUP-2), (Hbi)­[Zn0.5(UO2)2(L)­(H2L)­(H2O)3]·3H2O (ZnUP-3), and (Hpi)­[Zn­(UO2)2(H2O)4(L)­(HL)]·H2O (ZnUP-4). These four structures all comprise
uranyl diphosphonate layers formed by UO7 pentagonal bipyramids
and PO3C tetrahedra. Such layers are further connected
by Zn-centered polyhedra by sharing oxygens from phosphonate groups.
For ZnUP-1 and ZnUP-2, the zinc atoms are
terminally coordinated by phen and bipy molecules, respectively, resulting
in two-dimensional (2-D) hybrid materials. In ZnUP-3 and ZnUP-4, the uranyl phosphonate layers are joined together
by Zn–O polyhedra forming three-dimensional (3-D) frameworks.
The structures of ZnUP-3 and ZnUP-4 contain
large channels along the a-axis with apertures around
3.4 × 13.3 and 4.4 × 12.2 Å2, respectively.
Protonated templates exist in the channels, filling the space and
compensating the negative charge of the anionic frameworks. Photoluminescent
studies reveal that ZnUP-1 and ZnUP-2 exhibit
the characteristic vibronically coupled charge-transfer based UO22+ emission.