Tale of a Twist: Magnetic and Optical Switching in Copper(II) Semiquinone Complexes

An intermediate (C) that is observed in both phenol hydroxylation and catechol oxidation with the side-on peroxide species [Cu2O2(DBED)2]2+ (DBED = N1,N2-di-tert-butylethane-1,2-diamine) is identified as a copper(II) semiquinone species ([1]+) 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 [1]+ 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 CuII center and the semiquinone radical in [1]+ 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 [1]+ 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 [1]+ allows the spectroscopic yields of intermediate C to be quantified in this intriguing hydroxylation reaction.