Dioxygen uptake and catalytic activity of some monomeric copper(II) compounds containing an N,S donor ligand. The X‐ray structure of bis[5‐methyl‐4‐(ethylthiomethyl)imidazole]bis(tetrafluoroborato)copper(II)

Abstract

Abstract The synthesis and characterization of the copper tetrafluoroborate compound containing the bidentate N,S ligand 5‐methyl‐4‐(ethylthiomethyl)imidazole (memi), [Cu(memi) 2 (BF 4 ) 2 ], is described. The activities of this compound and of [Cu(memi) 2 (H 2 O) 2 ](NO 3 ) 2 as oxidation catalysts have been determined in the reaction of 2,6‐dimethylphenol with molecular dioxygen to give polyphenylene oxide (PPO) and diphenoquinone (DPQ). For optimum catalysis, an additional base is required in order to dehydronate the phenol. Triethylamine proved to be most useful as a base. Initial reaction rates encountered are between 1 × 10 −5 and 6 × 10 −5 M O 2 /s and ca. 50% of the phenol was converted after a reaction time of one hour. Less than 10% phenol was converted into DPQ. Both compounds have comparable reactivities. A single crystal of [Cu(memi) 2 (BF 4 ) 2 ] was used in an X‐ray structure determination: monoclinic space group C 2/ c, a = 13.844(4) Å, b = 11.251(1) Å, c = 15.753(4) Å, β = 111.39 (2)°, Z = 4 and T = 293 K. The structure was solved using heavy‐atom techniques and refined by least‐squares methods to a residual R value of 0.050. The copper ion is in a centrosymmetric distorted octahedral environment with all chemically identical donor atoms in trans positions. The tetrafluoroborate anions are coordinated to the copper ion with relatively short distances, i.e. 2.55 Å; Cu‐N and Cu‐S distances are 1.93 and 2.41 Å.