Increasing Complexity in the Uranyl Ion–Kemp’s Triacid System: From One- and Two-Dimensional Polymers to Uranyl–Copper(II) Dodeca- and Hexadecanuclear Species

Kemp’s
triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic
acid, LH3) has recently been shown to be a versatile ligand
for the uranyl
ion, with the presence of additional nickel­(II) cations allowing isolation
of nanotubular and cage species in particular. The high sensitivity
of the uranyl–LH3 system to variations in the experimental
conditions was the incentive for the present investigation of different
solvents as organic components in solvo-hydrothermal synthesis methods
and of different additional species (metal cations, 2,2′-bipyridine).
Reaction of LH3 with uranyl ions under purely hydrothermal
conditions gives [UO2(LH2)2(H2O)2]·2H2O (1), an
unremarkable mononuclear complex. In the presence of manganese nitrate
and in water–THF or water–methanol, respectively, the
complexes [Hbipy]­[UO2(L)]­·0.5H2O­·0.25THF
(2) and [(UO2)3(MeL)2­(OH)2(H2O)]­·8MeOH (3) were obtained; 2 is a two-dimensional (2D) species,
while 3 is one-dimensional (1D) and contains the monoester
derivative of LH3, formed in situ. Another 2D compound,
[UO2Tb­{(L)2H}­(H2O)2] (4), crystallizes in the presence of terbium­(III) nitrate.
The three complexes [(UO2)8{(L)6H2}­(H2O)6]­·H2O (5), [(UO2)8{(L)6H2}­(H2O)6]­·3H2O (6), and [Cu2(C2O4)­(bipy)2(THF)2]­[(UO2)8{(L)6H}­(H2O)6]2­·4H2O­·7THF (7), which were obtained in the presence of copper­(II) or nickel­(II)
cations, all contain homometallic octanuclear cage-like species analogous
to that previously reported. The most interesting complexes in this
series, [(UO2)8Cu4(L)8­(H2O)16]­·9H2O (8) and [(UO2)10Cu6­(L)10(OH)2­(H2O)7] (9), were obtained together in water–THF and in the
presence of copper­(II) cations. Compound 8 is a dodecanuclear
metallacycle comprising four (UO2)2Cu trinuclear
subunits, in which the central copper atom is bound to two uranyl
oxo groups, arranged in helical geometry. Compound 9 is
a large, hexadecanuclear cage-like species devoid of any crystallographic
symmetry. In both 8 and 9, uranyl ions are
topologically sufficient for the formation of the cyclic or cage molecules,
and the hydrated copper ions are located inside. The curved shape
of the three-pronged Kemp’s tricarboxylate ligand appears well
suited to the formation of closed species (nanotubes, rings, and cages),
which are in all cases coated on the outside by the hydrophobic parts
of the ligands.