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.