Oxidation potentials and autoxidation of copper(I) complexes with a series of [N 2S 2]-macrocycles

Abstract

A combination of sulfur and nitrogen donor atoms is well-known in the active center of copper enzymes and may be essential for the specific properties of biological copper. Little is known, however, about the redox properties of corresponding low-molecular copper complexes. Then macrocyclic ligands I–VI were obtained by high-dilution cyclisation via the corresponding amides and reduction with diborane [1, 2]. ▪ These ligands, while structurally very similar, span a considerable range of cavity sizes which in turn are strongly reflected in the susceptibilities of the corresponding Cu(II) complexes toward autoxidation. Reactions with O 2 are very fast with the twelve-membered macrocycles I–II and with III, and they can only be followed by stopped-flow techniques. Complexes with the 14-macrocycle IV can be studied conveniently with an oxygen electrode. The complexes with the 16-membered macrocycles V–VI show no significant reactivity. The kinetics of autoxidation are relatively simple. Under conditions where the formation of the complexes is essentially complete, the rate is independent of the pH and of the concentrations of buffers and acetonitrile. With the trans-14-macrocycle IV the kinetics are completely described by a second order rate constant: −d[O 2]/dt = k☆[O 2] [CuL +], k = 40 M −1 s −1. For the other ligands, the same rate law is obtained from the initial rates. Stability constants and standard potentials of the CuL 2+/CuL + couples are correlated to the kinetics of autoxidation. As an example, with V the electrode potential is too high to allow the accumulation of H 2O 2 under our experimental conditions (pH = 6–8), in line with the insignificant rate of autoxidation. The correlation of redox potentials with the kinetics of autoxidation will be discussed in detail.