Studies on Aerobic Reactions of Ammonia/3,5-Di-tert-butylcatechol Schiff-Base Condensation Products with Copper, Copper(I), and Copper(II). Strong Copper(II)−Radical Ferromagnetic Exchange and Observations on a Unique N−N Coupling Reaction

Abstract

Stoichiometric quantities of 3,5-di-tert-butylcatechol and aqueous ammonia react in pyridine solution to form 2-amino-4,6-di-tert-butylphenol. Under an atmosphere of dioxygen the aminophenol is oxidized to either the corresponding iminosemiquinone or iminobenzoquinone. In the presence of Cu(II) iminosemiquinone condensation with the aminophenol gives the Cat-N-SQ radical ligand obtained as the Cu(py)2(Cat-N-SQ) complex. Metal and ligand magnetic orbitals are orthogonal and couple ferromagnetically to give a S = 1 molecular spin state at temperatures up to 300 K. In nonpolar solvents the complex undergoes ligand oxidation and disproportionation to give Cu(Cat-N-BQ)2. Crystallographic characterization on crystals obtained as the i-propanol solvate [orthorhombic, C2221, a = 19.548(3) Å, b = 24.536(5) Å, c = 23.655(5) Å, V = 11346(4) Å3, Z = 8, R = 0.068] show that the expected Jahn−Teller distortion appears in the trans Cu−O lengths of the equatorial plane rather than for the axial Cu−N lengths. Reactions carried out with both Cu(I) and Cu metal require metal oxidation to give the Cu(II) products obtained. With metallic Cu this occurs by a reaction with iminoquinone to give bis(iminosemiquinone)copper(II). Further reaction of this product with aminophenol gives Cu(py)2(Cat-N-SQ) by condensation, and, with O2, oxidation gives a coordinated azophenolate ligand in Cu(py)(azophenolate) by a unique N−N bond-forming reaction. Cu(py)(azophenolate) has been characterized crystallographically [monoclinic, P21/n, a = 10.990(2) Å, b = 10.736(3) Å, c = 26.848(4) Å, β = 98.09(1)°, V = 3136(1) Å3, Z = 4, R = 0.048].