Uranyl Ion Complexes with 1,1′-Biphenyl-2,2′,6,6′-tetracarboxylic Acid: Structural and Spectroscopic Studies of One- to Three-Dimensional Assemblies

1,1′-Biphenyl-2,2′,6,6′-tetracarboxylic
acid (H4L) was reacted with uranyl nitrate, either alone
or in the presence of additional metal cations (Ni2+, Cu2+, Dy3+) under (solvo)-hydrothermal conditions,
giving six complexes which were characterized by their crystal structure
and, in all but one case, their emission spectrum in the solid state.
[Ni­(bipy)3]­[UO2(H2L)­(H2O)]2(NO3)2·3H2O
(1) crystallizes as a one-dimensional (1D), ribbon-like
coordination polymer, while the homometallic complex [(UO2)2(L)­(H2O)3]·H2O·CH3CN (2) and the heterometallic complexes [UO2Cu­(L)­(H2O)2]·H2O (3), [UO2Cu­(L)­(H2O)]·H2O (4), and [(UO2)5Cu4(HL)6(bipy)4]·2H2O (5) display two-dimensional (2D) arrangements. Lastly, the uranyl–lanthanide
heterometallic complex [(UO2)8Dy­(HL)6(H2O)8]­(I)·8H2O (6) crystallizes as a three-dimensional (3D) framework. Although these
assemblies adopt different topologies, the {42.6} linear
motif found in 1 is discernible in the structures of 2, 5, and 6, in which the higher
dimensionality arises from further bridging of these subunits by uranyl
(2), copper (5), or both uranyl and dysprosium
(6) cations. The tetracarboxylic/ate ligands have their
two aromatic rings nearly perpendicular to one another. No two of
them adopt the same coordination mode in this series (except in the
similar complexes 3 and 4), but chelation
involving one carboxylate group from each ring is nearly ubiquitous,
and the ensuing position of the cation favors the formation of planar
architectures. The emission spectra of complexes 2–5 measured in the solid state show the usual uranyl vibronic
fine structure, although with significant differences in the emission
intensity, while complete quenching of the luminescence is observed
in 1.