One‐step multi‐electron transfer mechanism of polynuclear copper complexes for molecular conversions

Abstract

AbstractThe oxidative coupling reaction of 2,6‐dimethylphenol may result in either a desired polymeric substance (i.e. the polyphenylene ether, PPE) or the undesired “dimeric” species diphenoquinone, DPQ. The relative amounts of each product depend on the experimental conditions and the used catalytic system.Usually copper amine compounds are used as a catalyst for the oxidative coupling reactions. They have the advantage of easy access and produce high yields of high molecular PPE; however, other metal coordination compounds, like those of Mn, may also be used as catalysts.The present paper focuses on mechanistic studies with various copper (aliphatic and aromatic) amine compounds as catalysts. Owing to the steric constraints of the amine ligands, dinuclear Cu(II) compounds, with small bridging anionic ligands, are easily formed. Such species are believed to be the catalyst precursors. Upon addition of a base (1:1 on copper) and excess phenol, phenolate ligands coordinate as bridging ligands to copper. After a two‐electron transfer reaction, the resulting phenoxonium ligand, which is a rather poor ligand, remains attached to the Cu(I), probably coordinating via its aromatic ring. Nucleophilic attack by a phenol to the phenoxonium ion at the 4‐position is likey to be most important to the coupling reaction. In the beginning of the reaction the undesired side product DPQ is also formed via a C–C coupling reaction. With copper(II) compounds containing imidazole‐type chelating ligands, good activity was obtained; in the case of pyrazole‐based and bridging S‐donor chelating ligands, that no or very weak activity was found.In a study of the mechanism of the propagation reaction the rate‐determining reaction was thought to be probably a one‐step, two‐electron transfer, during which the two Cu(II) ions in the dinuclear complex oxidize the phenolate to phenoxonium. After the phenoxium ion is formed the bonding with the (then) Cu(I) species is weakened and the reactions with phenolic end groups can take place. The effect of the amine ligands appears to be both steric and electronic. With certain ligands the reoxidationof the reduced catalyst is not possible.