Uranyl Ion Complexes with all-<i>cis</i>-1,3,5-Cyclohexanetricarboxylate: Unexpected Framework and Nanotubular Assemblies

all-cis-1,3,5-Cyclohexanetricarboxylic acid (LH3)
was reacted with uranyl nitrate under solvo-hydrothermal conditions,
either alone or in the presence of additional metal cations (Na+, K+, Ni2+, Cu2+, or Tb3+), resulting in the crystallization of a series of eight
complexes which were characterized by their crystal structures and
luminescence properties. The six complexes [UO2(H2O)5]­[UO2(L)]2­·2H2O­·3THF (1), [Ni­(bipy)2­(H2O)2]­[UO2(L)]2­·4H2O (2), [Ni­(bipy)3]­[Ni­(bipy)2­(H2O)2]­[UO2(L)]4­·5H2O (3), [Ni­(H2O)6]­[UO2(L)]2­·2H2O (4), [Cu­(H2O)6]­[UO2(L)]2­·2H2O (5), and [Tb­(H2O)8]­[UO2(L)]3­·8H2O (6) all contain the
same {UO2(L)−}∞ anionic
motif, in which the uranyl ion is tris-chelated by three L3– anions to give a two-dimensional assembly with hexagonal {63} topology. The reaction of uranyl nitrate alone with LH3 in water/N-methyl-2-pyrrolidone (NMP) yields
the complex [(UO2)3(L)2­(NMP)2] (7), which crystallizes as a three-dimensional
framework. Finally, in the presence of Na+, K+, or even Kemp’s triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid), the complex
[UO2(LH)] (8) is generated, the structure
of which displays a well-resolved nanotubular species possibly associated
with extremely disordered molecules or counterions of uncertain nature. These nanotubules
have {63} topology and can be seen as resulting from the
folding of the two-dimensional assembly present in the former complexes.
Emission spectra measured in the solid state show the usual vibronic
fine structure, with various degrees of resolution and quenching.