Uranium(III) Coordination Chemistry and Oxidation in a Flexible Small-Cavity Macrocycle

U­(III) complexes of the conformationally
flexible, small-cavity macrocycle trans-calix­[2]­benzene­[2]­pyrrolide
(L)2–, [U­(L)­X] (X = O-2,6-tBu2C6H3, N­(SiMe3)2), have
been synthesized from [U­(L)­BH4] and structurally characterized.
These complexes show binding of the U­(III) center in the bis­(arene)
pocket of the macrocycle, which flexes to accommodate the increase
in the steric bulk of X, resulting in long U–X bonds to the
ancillary ligands. Oxidation to the cationic U­(IV) complex [U­(L)­X]­[B­(C6F5)4] (X = BH4) results in
ligand rearrangement to bind the smaller, harder cation in the bis­(pyrrolide)
pocket, in a conformation that has not been previously observed for
(L)2–, with X located between the two ligand arene
rings.