Synthesis
of Electron-Rich Uranium(IV) Complexes Supported by Tridentate Schiff
Base Ligands and Their Multi-Electron Redox Chemistry
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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 [UX<sub>2</sub>(<sup>Me</sup>naphtquinolen)<sub>2</sub>] (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-<sup>Me</sup>naphtquinolen
formed through the intramolecular reductive coupling of the imino
groups of each <sup>Me</sup>naphtquinolen unit. The solid state structure
of the [U(μ-bis-<sup>Me</sup>naphtquinolen)]<sub>2</sub> 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)(<sup>Me</sup>naphtquinolen)<sub>2</sub>], <b>3</b>, and [UO<sub>2</sub>(<sup>Me</sup>naphtquinolen)<sub>2</sub>], <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