Oxidation of 2,6-di-t-butylphenol by molecular oxygen : catalysis by cobaltous polyamine chelates

Abstract

The oxidation of 2,6-di-t-butylphenol by molecular oxygen in catalyzed by the (mu)-peroxo dicobalt(III) complexes formed by the ligands tetraethylenepentamine (tetren), 1,4,10,13-tetraaza-7-oxa-tridecane (taotd), 1,4,10,13-tetraaza-7-thiatridecane (tattd) and bis(pyridylmethyl)diethylenetriamine (pydien), as well as by the (mu)-peroxo, (mu)-hydroxo dicobalt(III) complexes formed by the ligands (bis)bipyridine (bipy), triethylenetetramine (trien), tris(aminoethyl)amine (tren), bis(pyridylmethyl)ethylenediamine (pyen), N,N'-bis(2-amino-ethyl)glycine (sdtma) and N,N'-ethylenediaminediacetic acid (sedda). This reaction is shown to be first order with respect to the concentration of both the cobalt-dioxygen complex and substrate. Reaction products are identified as the oxidative coupling product, 3,3',5,5'-tetra-t-butyldiphenoquinone and the partial oxygen insertion product, 2,6-di-t-butylbenzoquinone. Rate constants are given for the total reaction of the phenol as well as for the formation of the benzoquinone and diphenoquinone. Questions concerning the mechanism of reaction are raised, in conflict with previous workers' results which proposed the coordinated oxygen as the reactive intermediate. The oxidation of phenol in the presence of the dioxygen complexes investigated does not occur unless free molecular oxygen is present in solution. On the other hand, it was also found that insertion does not occur unless a dioxygen complex is present. Furthermore, the rate of the reaction varies greatly with the thermodynamic stability of the dioxygen complex. In addition to the oxidation catalysis studies, solution equilibria were investigated for the ligands taotd, tattd, and pyen in order to determine values for K(,O(,2)) and K(,O(,2))(''), the equilibrium constants for the addition of dioxygen to the preformed metal chelates to form the monobridged or dibridged oxygen complexes. To further understand the effects of the heteroatoms oxygen and sulfur in the formation of stable dioxygen complexes, the ligands 1,9-bis(2-pyridyl)-2,8-diaza-5-oxaonane (pydae) and 1,9-bis(2-pyridyl)-2,8-diaza-5-thianonane (spydae) were also studied in this regard. Acid dissociation constants and metal chelate formation constants for Co(II), Ni(II), Cu(II), and Zn(II) with taotd, tattd, spydae, and pydae are also given. Formation constants for the dioxygen adducts of pyen, tattd, taotd, and spydae are given. Pydae does not form a detectable dioxygen complex. The thioether cobaltous complexes form more stable dioxygen adducts than their oxygen analogues but less stable than the analogous amino nitrogen ligands.