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

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Pratik Verma, John Weir, Liviu Mirica, and T. Daniel P. Stack*

Department of Chemistry, Stanford University, Stanford, California 94305, United States

Inorg. Chem., 2011, 50 (20), pp 9816–9825

DOI: 10.1021/ic200958g

Publication Date (Web): June 22, 2011

E-mail: stack@stanford.edu.

Section:

Inorganic Chemicals and Reactions

Abstract

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)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.