Uranyl–Organic Frameworks with Polycarboxylates: Unusual Effects of a Coordinating Solvent

Uranyl nitrate was reacted with several
polycarboxylic acids under
solvo-/hydrothermal conditions using N-methyl-2-pyrrolidone
(NMP) as the organic component to give six novel complexes, which
were crystallographically characterized. NMP is coordinated to the
uranyl ion in all cases but one. The complex with terephthalic acid
(H2tph), [UO2(tph)­(NMP)] (1), crystallizes
as a three-dimensional (3D) framework, an unprecedented feature in
uranyl complexes with this ligand. The two complexes obtained with
2,5-thiophenedicarboxylic acid (H2thd), [UO2(thd)­(NMP)] (2 and 3), crystallize as 3D
frameworks having the same formula and topology but different packings.
Two complexes were also obtained with 1,3,5-benzenetriacetic acid
(H3bta), [(UO2)3(bta)2(NMP)3]·0.5H2O (4) and [Hbipy]­[UO2(bta)]·H2O (5). Complex 4, with NMP coordinated, is a 2D assembly, with a sheet thickness
of ∼12 Å arising from the superposition of three sublayers.
Complex 5, obtained in the presence of 2,2′-bipyridine
(bipy), is a 1D polymer with a nanotubular shape subtended by π-stacking
interactions. In the additional presence of nickel­(II) nitrate, nitrilotriacetic
acid (H3nta) gives the complex [(UO2)2Ni­(nta)2(NMP)2]·NMP (6),
which crystallizes as a 3D framework that, like complexes 1 and 3, displays channels containing coordinated or
free NMP molecules. The emission spectra under excitation at 350 nm
were measured for all of the complexes. The usual vibronic fine structure
in the ∼460–600 nm range is apparent for 1–5, while 6 shows only a weak and
featureless band indicative of quenching of the uranyl luminescence
by Ni­(II). The properties of these complexes illustrate the potential
of solvo-/hydrothermal methods, particularly those involving coordinating
organic solvents, for the synthesis of new uranyl–organic species.