Inner-sphere outer-sphere reduction of uranyl supported by a redox-active, donor-expanded dipyrrin.

The uranyl(vi) complex UOCl(L) of the redox-active, acyclic diimino-dipyrrin anion, L is reported and its reaction with inner- and outer-sphere reductants studied. Voltammetric, EPR-spectroscopic and X-ray crystallographic studies show that chemical reduction by the outer-sphere reagent CoCp initially reduces the ligand to a dipyrrin radical, and imply that a second equivalent of CoCp reduces the U(vi) centre to form U(v). Cyclic voltammetry indicates that further outer-sphere reduction to form the putative U(iv) trianion only occurs at strongly cathodic potentials.