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

The uranyl complex UOCl() of the redox-active, acyclic dipyrrin-diimine anion [H = 1,9-di--butyl-imine-5-(mesityl)dipyrrin] is reported, and its redox property is explored and compared with that of the previously reported UOCl() [H = 1,9-di--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 UOCl(), the formation of a diamond-shaped, oxo-bridged uranyl(V) dimer, [UO()] is seen, whereas in contrast, for UOCl(), 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 UOCl() only, which subsequently facilitates uranyl(V) dimerization.