Copper-(I) and -(II) complexes of a macrocycle derived from 2:2 condensation of pyridine-2,6-dicarbaldehyde and 4-azaheptane-1,7-diamine; oxygenation of the copper(I) complex

Abstract

A Schiff-base macrocycle was prepared by the 2:2 condensation of pyridine-2,6-dicarbaldehyde and 4-azaheptane-1,7-diamine. It crystallized in the orthorhombic space group Fdd2 with a= 16.984(8), b= 31.322(12), c= 11.026(6)Å and Z= 16. This 20-membered ring-contracted form, with two Schiff-base double bonds, forms a deep red dinuclear copper-(I) complex having two charge-transfer absorptions at 415 and 550 nm in methanol–acetonitrile (4 : 1). The complex forms a deep purple dicopper–dioxygen complex having absorption bands at 390 and 530 nm, with a rate constant at 25.0 °C of 5.6 × 10^–5 s^–1. This is spontaneously converted into a copper(II) complex with a half-life of 240 min. The dinuclear copper(I)–dioxygen complex oxidizes 3,5-di-tert-butylcatechol catalytically to 3,5-di-tert-butyl-1,2-benzoquinone. The dinuclear copper(II) complex of the macrocycle was also found to oxidize this substrate to the same product at a much faster rate.