Synthesis of Electron-Rich Uranium(IV) Complexes Supported by Tridentate Schiff Base Ligands and Their Multi-Electron Redox Chemistry

The
synthesis, structure, and reactivity of a new complex of U­(IV) with
the tridentate Schiff base ligand Menaphtquinolen are reported. The
reduction of the bis-ligand complexes [UX2(Menaphtquinolen)2] (X = Cl, (1-Cl) ; I (1-I)) with potassium metal affords the U­(IV) complex of the
new tetranionic hexadentate ligand μ-bis-Menaphtquinolen
formed through the intramolecular reductive coupling of the imino
groups of each Menaphtquinolen unit. The solid state structure
of the [U­(μ-bis-Menaphtquinolen)]2 dimer 2 isolated from toluene confirms the presence of a U­(IV) complex
of the reduced ligand. Reactivity studies with molecular oxygen and
9,10-phenanthrenequinone show that complex 2 can act
as a multielectron reducing agent releasing two electrons through
the cleavage of the C–C bond to restore the original imino
function of the ligand. In the resulting U­(IV) and U­(VI) complexes
[U­(9,10-phenanthrenediol)­(Menaphtquinolen)2], 3, and [UO2(Menaphtquinolen)2], 4, the restored tridentate Schiff base allows for
the coordination of the reduced substrate to the metal. Electrochemical
studies of complex 2 show the presence of irreversible
ligand centered reduction processes and of a reversible U­(IV)/U­(III)
couple.