Oxidation phenolate ligands

The H64Y variant of Mb is an example of the former situation in that the tyrosyl side chain coordinates to the heme iron of the oxidized variant. As expected for a variant with an anionic phenolate ligand, the reduction potential of this variant is 40 mV lower than that of the wild-type protein (Table I). Although this change is consistent with stabilization of the oxidized form of the protein, the fact that the tyrosyl ligand is not coordinated in the reduced protein complicates quantitative interpretation of this shift in potential. 

We have utilized somewhat less-effective optional approaches to copolymer purification with attendant catalyst recovery. One of these methods involved the replacement of the f-butyl substituents on the 5-position of the phenolate ligands with poly(isobutylene) (PIB) groups, as illustrated in Fig. 14 [39]. Importantly, this chromium(III) catalyst exhibited nearly identical activity as its 3,5-di-t-butyl analog for the copolymerization of cyclohexene oxide and carbon dioxide. The PIB substituents on the (salen)CrCl catalysts provide high solubility in heptanes once the copolymer is separated from the metal center by a weak acid. 

McBride and Wesselink (1988) studied IR spectra of catechol adsorbed onto the oxide surface and found evidence that the compound was chemically altered, indicating that chemisorption was the dominant mechanism. In addition to catechols, phenols are known to adsorb onto metal oxide surfaces. This adsorption is dependent on the number and position of hydroxy substitutions on the benzene ring. Diphenolic compounds adsorb to a greater extent than monophenolic compounds, suggesting the formation of a bidentate bond with the metal oxide. This bidentate bond is formed when the two phenolic ligands coordinate with one or two surface metal ions (McBride and Wesselink, 1988). 

Another non-heme system made use of hexadentate phenol ligands [98]. However, the catalytically active species was only formed upon ligand oxidation by excess of PhI=NTs. Furthermore, a large excess of the alkene was required (2000 equiv. vs. PhI=NTs). The reaction of cyclooctene and 1-hexene gave yields of about 50%, which represents a significant improvement over the earlier described copper systems [99]. 

Figure 9-30. The oxidation of 9.14 by dioxygen and copper(i) generates a dinuclear copper(n) complex of the phenolic ligand 9.15.

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

The most thoroughly studied class of phenoxyl radical complexes was prepared by oxidation of first-row transition metal complexes of the type of macrocyclic phenolate ligands shown in Figure 4. Studies of those compounds that contain Ga , Sc , and have been particularly useful for differentiating and establishing the properties of coordinated vs. uncoordinated phenoxyl radicals. Due to their closed shell nature, these ions are both spectroscopically silent and redox inactive, thus enabling the properties of phenoxyl radicals in their complexes to be seen unmasked from those of the metal ion. Data for oxidized forms of the complexes [L M] ((l)-(4) are illustrative). These compounds undergo three reversible one-electron oxidations at approximately equally spaced... 

Coordinated phenoxyl radicals in Schiff bases and phenolate ligands have understandably attracted the most attention in the past 10 years because of the reversible redox states that can be achieved [78-82]. The most striking example of this redox chemistry was demonstrated by the recent findings with Ni -salen complexes [77, 78]. Shimazaki et al. [78] studied the electrochemical oxidation of [Ni(l)] (Fig. 3). The Ni -radical valence isomer could be obtained in CH2CI2 and converted into the Ni° -phenolate valence isomer simply by changing the... 

Hemocyanine and tyrosinase are of this type, Hemocyanine is the oxygen carrier in molluscs and arthropods. Two Cu ions are involved in a subunit and bind one O2. A deoxy structure of a lobster hemocyanine from Panulitrus interruptus has been reported [43] for which both the Cu are monovalent. Three His coordinate to each Cu, and two Cu and four His are located in the same plane (Fig. 2-23). The two copper ions are not equivalent and no bridging ligand exist between them. Tyrosinase oxidizes phenols to orthoquinones. O2 is coordinated between two Cu ions as a p-peroxo structure. 

Fig. 18 Various Ruthenium complexes used to induce oxidative PCET, as well as a Rhenium complex featuring an unconjugated phenol ligand, which readily forms the phenoxyl in the presence of base [148]...

Zhang G, ScottBL, Wu R, Silks LAP, Hanson SK. Aerobic oxidation reactions catalyzed by vanadium complexes of bis(phenolate) ligands. Inorg Chem. 2012 51 7354-7361. 

Significant quantities of the diphenoquinone are also produced if the ortho substituents are methoxy groups (36). Phenols with less than two ortho substituents produce branched and colored products from the reactions that occur at the open ortho sites. It is possible to minimize such side reactions in the case of o-cresol oxidation by using a bulky ligand on the copper catalyst to block the open ortho position (38). 

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