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

Abstract

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, (<b>1-Cl</b>) ; I (<b>1-I</b>)) 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 <b>2</b> 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 <b>2</b> 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], <b>3</b>, and [UO₂(^Menaphtquinolen)₂], <b>4</b>, the restored tridentate Schiff base allows for the coordination of the reduced substrate to the metal. Electrochemical studies of complex <b>2</b> show the presence of irreversible ligand centered reduction processes and of a reversible U­(IV)/U­(III) couple