Theoretical exploration of uranyl complexes of a designed polypyrrolic macrocycle: Structure/property effects of hinge size on Pacman-shaped complexes

A polypyrrolic macrocycle with naphthalenyl linkers between the N ₄-donor compartments (L ²) was designed theoretically according to its experimentally-known analogues with phenylenyl (L ¹) and anthracenyl (L ³) linkers. The uranyl and bis(uranyl) complexes formed by this L ² ligand have been examined using scalar-relativistic density functional theory. The calculated structural properties of the mononuclear uranyl-L ² complexes are similar to those of their L ¹ counterparts. The binuclear L ² complexes exhibit a butterfly-like bis(uranyl) core in which a linear uranyl is coordinated in a side-by-side fashion to a cis-uranyl unit. The calculated UO bond orders in the uranyl-L ² complexes indicate partial triple bonding character with the only exceptions being the U-O _endo bonds in the U ₂O ₄ core of the butterfly-shaped binuclear complexes. Overall, the bond orders agree with the trends in the calculated UO stretching vibrational frequencies. Regarding the bis(uranyl) L ¹, L ² and L ³ complexes, the phenylenyl-hinge L ¹ complexes adopt a butterfly-like and a T-shaped isomer in the oxidation state of U(vi), but only a butterfly-like one in the U(v), which differs from that of the naphthalenyl-hinge L ² complexes as well as the lateral twisted structure of the anthracenyl-hinge L ³ complexes. The intramolecular cation-cation interactions are found in the L ¹ and L ² complexes, but are absent in the L ³ complexes. Finally, using model uranyl transfer reactions from the L ¹ complexes, the formation of the mononuclear L ² complexes is calculated to be a slightly endothermic process. This suggests that it should be possible to synthesize the L ² complexes using similar protocols as employed for the L ¹ complexes.