Oxidative coupling polymerization of 2,6-dimethylphenol catalyzed by macrocyclic tetracopper(II) complexes Jian Gao, Joseph H. Reibenspies, Arthur E. Martell * Department of Chemistry, Texas A and M University, P.O. Box 300012, College Station, TX 77842-3012, USA Received 30 January 2002; accepted 15 March 2002 Abstract The macrocyclic ligand of 3,6,9,16,19,22-hexaazatricyclo[22.2.2.2 11,14 ]-triaconta-1(26),11(12),13,24,27,29-hexaene(L, BP) was synthesized by [2 /2] condensation of p -phthaldehyde and diethylenetriamine. This ligand was used to synthesize dicopper(II) complex [BPCu 2 (II)](ClO 4 ) 4 (1) and the bis-m-X bridged tetracopper(II) complexes [BP 2 Cu 4 (II)(m-X) 2 ] (X /Ph à /COO (2), CO 3 2 (3) and CH 3 O (4)). Complex 3 crystallizes in the triclinic space group P 1, with cell constants a /9.9965(10), b /13.5887(14), c / 14.0262(7) A ˚ , a /90.317(2), b /96.902(2), g /92.691(2)8, V /1889.3(3) A ˚ 3 and Z /2. Complexes 3 and 4 proved to be effective homogeneous catalysts for oxidative coupling of 2,6-dimethylphenol (DMP) to polyphenylene ether (PPE) by dioxygen. These complexes transform DMP to PPE under mild conditions of atmospheric pressure of O 2 and 298 K. The effectiveness of the catalytic reaction depends upon the type of the m-X ligand and the concentration of the auxiliary NaOEt. Crystallographic and mechanistic investigation indicated that the catalytic center is the ‘dimer-to-dimer’ tetranuclear structure, and a novel mechanism for this polymerization reaction was proposed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polymerization; Copper complexes; Macrocyclic complexes; Tetranuclear complexes 1. Introduction Polyphenylene ether (PPE) is a valuable plastic material due to chemical stability, high glass transition temperature and self-extinguishing properties. Oxidative coupling polymerization of 2,6-dimethylphenol (DMP) by the copper(I) /pyridine complex with dioxygen was first found by Hay et al. in 1959 [1]. Since then many dicopper(II) complexes with nitrogen containing ligand have been reported in the investigation of this O 2 related coupling reaction [2 /5]. Previous studies indicated that the phenol-bridged dicopper structure is the key inter- mediate catalyst for this coupling reaction [6]. More recent work proposed that mono-copper(II) complexes with small N-containing ligands (such as pridyl, imida- zol and their derivatives) are effective catalysts [7]. However, macrocyclic ligand chelated copper(II) com- plex system has not been reported on this reaction until now. For macrocyclic complexes, the metal ions are stabi- lized by the N-donor atoms, thus facilitating the mechanistic investigation. Previous work has found that macrocyclic dicopper(II) complexes are effective models of dioxygen metabolism processes such as haemocyanin, tyrosinase and catalase-like reactions [8 /10]. Dicopper(II) complexes of BMXD, BPBDH and BP (Scheme 1) were found to be very stable and their m-peroxo complexes are promoters of oxidation reactions by activation of either O 2 [11] or H 2 O 2 [12,13]. We found for the first time that BP coordinated tetracopper(II) complexes are effective catalysts in the synthesis of PPE. The overall auto-oxidation process of DMP to PPE is shown in Scheme 2. The sole by-product of this reaction is 3,3?,5,5?-tetra-methyl-4,4?-diphenoquinone (DPQ) [14]. The whole reaction process can be monitored by the observation of O 2 consumption and by monitoring the absorption spectra of DPQ. Although three different reaction pathways: radical, anion and phenoxoium * Corresponding author. Tel.: /1-407-845 5055; fax: /1-409-845 4719 E-mail address: martell@mail.chem.tamu.edu (A.E. Martell). Inorganica Chimica Acta 338 (2002) 157 /164 www.elsevier.com/locate/ica 0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0020-1693(02)00912-X