Redox Noninnocence of the Bridge in Copper(II) Salophen and Bis(oxamato) Complexes

Two square-planar copper­(II) complexes of 1,2-bis­(2-hydroxy-3,5-di-tert-butylbenzimino)-4,5-bis­(dimethylamino)­benzene (1) and N-[4,5-bis­(dimethylamino)-2-(oxalylamino)­benzene]­oxamate (22–) were prepared. The crystal structures of the proligands H2L1 and Et2H2L2, as well as the corresponding complexes, are reported. The proligands each display a one-electron-oxidation wave, which is assigned to oxidation of the bis­(dimethylamino)­benzene moiety into a π radical. Complexes 1 and 22– exhibit reversible one-electron-oxidation waves in their cyclic voltammograms (E1/21 = 0.14 and E1/22 = 0.31 V for 1 and E1/21 = −0.47 V vs Fc+/Fc for 22–). The first process corresponds to oxidation of the bis­(dimethylamino)­benzene central ring into a π radical, while the second process for 1 is ascribed to oxidation of the π radical into an α-diiminoquinone. The one-electron-oxidized species 1+ and 2– exhibit intense visible–near-IR absorptions, which are diagnostic of π radicals. They display a triplet signal in their electron paramagnetic resonance spectra, which stem from magnetic coupling between the ligand-radical spin and the copper­(II) spin. The zero-field-splitting parameters are larger for 2– than 1+ because of greater delocalization of the spin density onto the coordinated amidato N atoms. Density functional theory calculations support a π-radical nature of the one-electron-oxidized complexes, as well as S = 1 ground spin states. The electrogenerated 12+ comprises a closed-shell diiminoquinone ligand coordinated to a copper­(II) metal center. Both 1 and 2 catalyze the aerobic oxidation of benzyl alcohol, albeit with different yields.