Redox coupling transition metal ions

An intriguing example of those enzymes that use both tyrosine residues and transition metal ions as partners in affecting redox chemistry is the fungal enzyme GO (GO EC 1.1.3.9 68 kD) (60,61,81), which catalyzes the two-electron oxidation of primary alcohols coupled to the two-electron reduction of O2 to H2O2, Eq. (2). 

Much of the early work on electron transfer reactions focused on systems involving transition metal ions. Many redox couples based on the Fe(III)/ Fe(II), Co(III)/Co(II), and Cr(III)/Cr(II) can be prepared in the laboratory by changing the ligands coordinated to the central metal ion. From these studies it is possible to distinguish between two types of reactions which would otherwise be classified as simple. A classic example from the work of Taube [3] is the reaction... 

Example 14.3. The Belousov-Zhabotinsky reaction [22,27-29], The reaction is an oxidation of malonic acid by bromate ion in sulfuric acid, catalyzed by a Ce(III)/Ce(IV) redox couple. Many variations with other organic acids and transition-metal ions are possible [22] (Belousov used citric acid, and manganese, ruthenium, or iron can replace cerium). The color of the solution alternates between clear [Ce(III)] and pale yellow [Ce(IV)], and more dramatically between red and blue if ferroin is added as indicator. 

A polyrotaxane sensitive to transition metal ion 98 was prepared by electropolymerization [205]. The precursor was obtained by complexation of a bipyridine sandwiched between two 2,2 -biEDOT moieties with Cu or Zn in the presence of a Sauvage s phenanthroUne macrocycle incorporating a polyether chain. A reversible ionochromic process was observed. On removal of metal ions by ethylenediamine treatment, the color of the polymer film turns from blue-green to red while the reverse effect was observed upon reintroduction of the metal. Another salient feature is the fact that the matching of the polyrotaxane and couple redox potentials results in a contribution to conductivity. 

Transition metal ions, electronically coupled to the conjugated backbone, allowed further tuning of the polymer s redox properties, which behaved complementary to FEDOT. A-B-type polymers with EDOT and carbazole structural units turned out to be another class of polymers anodically coloring that can successfully be combined with alkylenedioxythiophenes. - ... 

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