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

Abstract

A polypyrrolic macrocycle with naphthalenyl linkers between the N₄-donor compartments (L²) was designed theoretically according to its experimentally-known analogues with phenylenyl (L¹) and anthracenyl (L³) linkers. The uranyl and bis(uranyl) complexes formed by this L² ligand have been examined using scalar-relativistic density functional theory. The calculated structural properties of the mononuclear uranyl-L² complexes are similar to those of their L¹ counterparts. The binuclear L² 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-L² complexes indicate partial triple bonding character with the only exceptions being the U–O_endo bonds in the U₂O₄ 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) L¹, L² and L³ complexes, the phenylenyl-hinge L¹ 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 L² complexes as well as the lateral twisted structure of the anthracenyl-hinge L³ complexes. The intramolecular cation–cation interactions are found in the L¹ and L² complexes, but are absent in the L³ complexes. Finally, using model uranyl transfer reactions from the L¹ complexes, the formation of the mononuclear L² complexes is calculated to be a slightly endothermic process. This suggests that it should be possible to synthesize the L² complexes using similar protocols as employed for the L¹ complexes.