Binuclear uranium(VI) complexes with a "pacman" expanded porphyrin: computational evidence for highly unusual...

On the basis of uranyl complexes reacting with a polypyrrolic ligand (H(4)L), we explored structures and reaction energies of a series of new binuclear uranium(VI) complexes using relativistic density functional theory. Full geometry optimizations on [(UO(2))(2)(L)], in which two uranyl groups were initially placed into the pacman ligand cavity, led to two minimum-energy structures. These complexes with cation-cation interactions (CCI) exhibit unusual coordination modes of uranyls: one is a T-shaped (T) skeleton formed by two linear uranyls {O(exo)=U(2)=O(endo)-->U(1)(=O(exo))(2)}, and another is a butterfly-like (B) unit with one linear uranyl coordinating side-by-side to a second cis-uranyl. The CCI in T was confirmed by the calculated longest distance and lowest stretching vibrational frequency of U(2)=O(endo) among the four U=O bonds. Isomer B is more stable than T, for which experimental tetrameric analogues are known. The formation of B and T complexes from the mononuclear [(UO(2))(H(2)L)(thf)] (M) was found to be endothermic. The further protonation and dehydration of B and T are thermodynamically favorable. As a possible product, we have found a trianglelike binuclear uranium(VI) complex having a O=U=O=U=O unit.