Oxidation oxochromium complexes

The oxochromium complex shows a very similar electrochemistry it also undergoes two metal centered reversible one-electron redox processes and one reversible oxidation of the ligand. 

The oxidation of alkyl aryl sulfides to sulfoxides with oxochromium(V) complexes is first order in oxidant and in substrate. The better correlation of log k with a+ rather than a and the low magnitude of p+ value (-1.19) were interpreted as evidence for a rate-determining single-electron-transfer mechanism. This was further supported by good correlation in the plots of log k versus oxidation potential/ionization energy. 

One oxochromium(V) complex, CrO(TETMC), containing the trinegative anion of a corrole (279), has been characterized as the solid.1266 It is prepared (Table 102) simply by exposure to air of a solution presumably containing a Cr11 complex. Aerial oxidation of Cr (TPP) produces the oxochromium(IV) complex CrO(TPP) so the corrole ligand apparently facilitates autoxida-tion. The redox behaviour of CrO(TETMC) has been examined by cyclic voltametry.1267... 

Formally, pentavalent neutral metallocorroles have been prepared by Murakami and coworkers.The first of these was the oxomolybdenum(V) corrole derivative 2.179. ° This complex was prepared by heating free-base corrole 2.82 with molybdenum pentachloride in oxygen-free decalin (Scheme 2.1.56). Alternatively, molybdenum hexacarbonyl (Mo(CO)e) could be used as the metal source. In both cases, oxidation to the oxomolybdenum complex 2.179 was believed to occur during workup (involving chromatography on neutral alumina followed by recrystallization). In this way, complex 2.179 was isolated in c. 40% yield. Similar yields of the oxochromium(V) complex 2.180 could be achieved via the reaction of 2.82 with anhydrous chromium(II) chloride in DMF. Here too, spontaneous oxidation during workup was used to afford the formally pentavalent oxo-complex 2.180. 

Recent Applications of Oxochromium-Amine Complexes as Oxidants in Organic Synthesis. A Review"... 

The mechanism of picolinic acid (PA)-catalysed oxidation of formic acid by chro-mium(VI) involves the formation of a Cr(VI)-PA chelate complex in a pre-equilibrium step followed by attack at the Cr(VI) centre by the substrate, leading to a ternary complex within which electron transfer occurs giving a Cr(IV)-PA complex and CO2. The Cr(IV)-PA complex reacts faster with the substrate, ultimately giving rise to a Cr(III)-PA complex. The reaction is catalysed by anionic surfactants and inhibited by cationic surfactants. The oxidation of alcohols by modified oxochromium(VI)-amine reagents has been reviewed. ... 

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