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

Reaction of [Li(THF)](4)[L] (L = Me-8-calix[4]-pyrrole]) with 0.5 equiv of [(UO2Cl2)-O-VI(THF)(2)](2) results in formation of the oxidized calix[4]pyrrole product, [Li(THF)](2)[L-Delta] (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 I-2. We hypothesize that formation of 1 proceeds via formation of a highly oxidizing cis-uranyl intermediate, [Li](2) [cis-(UO2)-O-VI (calix[4]pyrrole)]. To test this hypothesis, we explored the reaction of 1 with either 0.5 equiv of [(UO2Cl2)-O-VI(THF)(2)](2) or 1 equiv of [(UO2)-O-VI(OTf)(2)(THF)(3)], which affords the isostructural uranyl complexes, [Li(THF)][(UO2)-O-VI (L-Delta)Cl(THF)] (2) and [Li(THF)][(UO2)-O-VI (L-Delta)(OTf)(2)(THF)] (3), respectively. In the solid state, 2 and 3 feature unprecedented uranyl-eta(5)-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)degrees; 3: 164.5(5)degrees). 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.