An intermediate (C) that is observed in both phenol hydroxylation and catechol oxidation with the side-on peroxide species [Cu2O 2(DBED)2]2+ (DBED = N1,N 2-di-tert-butylethane-1,2-diamine) is identified as a copper(II) semiquinone species (+) through independent synthesis and characterization. The reaction of the redox-active 3,5-di-tert-butylquinone ligand with [(DBED)CuI(MeCN)]+ yields a copper(II) semiquinone + complex with a singlet ground state and an intense purple chromophore (ε580 ∼ 3500 M-1 cm -1). All other copper(II) semiquinone complexes characterized to date are paramagnetic and weakly colored (ε800 ∼ 500 M -1 cm-1). Antiferromagnetic coupling between the Cu II center and the semiquinone radical in + is characterized by paramagnetic 1H NMR and SQUID magnetometry. Comparative X-ray crystal structures along with density functional theory calculations correlate the geometric structures of copper(II) semiquinone complexes with their magnetic and optical properties. The unique observable properties of + originate from an increase in the overlap of the Cu 3d and semiquinone π orbitals resulting from a large rhombic distortion in the structure with a twist of 51°, attributable to the large isotropic demands of the tert-butyl substituents of the DBED ligand. Independent characterization of + allows the spectroscopic yields of intermediate C to be quantified in this intriguing hydroxylation reaction.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry