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.