Cerium–quinone redox couples put under scrutiny

Homoleptic cerous complexes Ce[N(SiMe(3))(2)](3), [Ce{OSi(OtBu)(3)}(3)](2) and [Ce{OSi(i)Pr(3)}(3)](2) were employed as thermally robust, weakly nucleophilic precursors to assess their reactivity towards 1,4-quinones in non-aqueous solution. The strongly oxidizing quinones 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or tetrachloro-1,4-benzoquinone (Cl(4)BQ) readily form hydroquinolato-bridged ceric complexes of the composition [(Ce(IV)L(3))(2)(μ(2)-O(2)C(6)R(4))]. Less oxidising quinones like 2,5-di-tert-butyl-1,4-benzoquinone (tBu(2)BQ) tend to engage in redox equilibria with the ceric hydroquinolato-bridged form being stable only in the solid state. Even less oxidising quinones such as tetramethyl-1,4-benzoquinone (Me(4)BQ) afford cerous semiquinolates of the type [(Ce(III)L(2)(thf)(2))(μ(2)-O(2)C(6)Me(4))](2). All complexes were characterised by X-ray diffraction, (1)H, (13)C{(1)H} and (29)Si NMR spectroscopy, DRIFT spectroscopy, UV-Vis spectroscopy and CV measurements. The species putatively formed during the electrochemical reduction of [Ce(IV){N(SiMe(3))(2)}(3)](2)(μ(2)-O(2)C(6)H(4)) could be mimicked by chemical reduction with Co(II)Cp(2) yielding [(Ce(III){N(SiMe(3))(2)}(3))(2)(μ(2)-O(2)C(6)H(4))][Co(III)Cp(2)](2).