Quinone-functionalized Transition Metal Complexes for Multi-electron Transfer. The Electrochemistry of a Bis-Naphthoquinone-Functionalized Schiff Base Ni(II) complex

Ni(II)NQ₂en, a Ni(II) complex with two naphthoquinone groups incorporated into a Schiff-base ligand, undergoes two reversible reductions in which the naphthoquinone (NQ) groups are each reduced by one electron to naphthsemiquinone radical anions (SQ): $$\eqalignno{& {\rm Ni}({\rm II}){\rm NQ}_2 {\rm en} + {\rm e}^ - \mathop \to \limits^{E_1^{\,0} }[{\rm Ni}({\rm II})({\rm SQ}, {\rm NQ}){\rm en}]^ - \cr & [{\rm Ni}({\rm II})({\rm SQ}, {\rm NQ}){\rm en}]^ - + {\rm e}^ - \mathop \to \limits^{E_2^{\,0} } [{\rm Ni}({\rm II}){\rm SQ}_2 {\rm en}]^{2 - } \cr}$$ Analysis of the cyclic and differential pulse voltammetry waves shows that $E_2^0 - E_1^0 = - 36\, {\rm mV}$, a ΔE⁰ that corresponds to two noninteracting redox centers.