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 [Cu₂O ₂(DBED)₂]²⁺ (DBED = N¹,N ²-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)Cu^I(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 Cu ^II 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.