Role of the Meso Substituent in Defining the Reduction of Uranyl Dipyrrin Complexes.

The uranyl complex U VI O 2 Cl( L Mes ) of the redox-active, acyclic dipyrrin-diimine anion L Mes - [H L Mes = 1,9-di- tert -butyl-imine-5-(mesityl)dipyrrin] is reported, and its redox property is explored and compared with that of the previously reported U VI O 2 Cl( L F ) [H L F = 1,9-di- tert -butyl-imine-5-(pentafluorophenyl)dipyrrin] to understand the influence of the meso substituent. Cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory studies show that the alteration from an electron-withdrawing meso substituent to an electron-donating meso substituent on the dipyrrin ligand significantly modifies the stability of the products formed after reduction. For U VI O 2 Cl( L Mes ), the formation of a diamond-shaped, oxo-bridged uranyl(V) dimer, [U V O 2 ( L Mes )] 2 is seen, whereas in contrast, for U VI O 2 Cl( L F ), only ligand reduction occurs. Computational modeling of these reactions shows that while ligand reduction followed by chloride dissociation occurs in both cases, ligand-to-metal electron transfer is favorable for U VI O 2 Cl( L Mes ) only, which subsequently facilitates uranyl(V) dimerization.