Coordination of Uranyl to the Redox-Active Calix[4]pyrrole Ligand

Reaction of [Li(THF)]₄[L] (L = Me₈-calix[4]pyrrole]) with 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ results in formation of the oxidized calix[4]pyrrole product, [Li(THF)]₂[L^Δ] (1), concomitant with formation of reduced uranium oxide byproducts. Complex 1 can also be generated by reaction of [Li(THF)]₄[L] with 1 equiv of I₂. We hypothesize that formation of 1 proceeds via formation of a highly oxidizing cis-uranyl intermediate, [Li]₂[cis-U^VIO₂(calix[4]pyrrole)]. To test this hypothesis, we explored the reaction of 1 with either 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ or 1 equiv of [U^VIO₂(OTf)₂(THF)₃], which affords the isostructural uranyl complexes, [Li(THF)][U^VIO₂(L^Δ)Cl(THF)] (2) and [Li(THF)][U^VIO₂(L^Δ)(OTf)(THF)] (3), respectively. In the solid state, 2 and 3 feature unprecedented uranyl-η⁵-pyrrole interactions, making them rare examples of uranyl organometallic complexes. In addition, 2 and 3 exhibit some of the smallest O-U-O angles reported to date (2: 162.0(7) and 162.7(7)°; 3: 164.5(5)°). Importantly, the O-U-O bending observed in these complexes suggests that the oxidation of [Li(THF)]₄[L] does indeed occur via an unobserved cis-uranyl intermediate.