Nitrosyl group electrochemical reduction

Electrochemical reduction of the nitrosyl group is possible, which ultimately yields ammonia thus, 4-electron reduction of an aqueous solution of [Fe(CN)5(NO)]2- gives [Fe(CN)5(NH20H)]3- (260), and reaction of [Cr(N0)(H20)5] or [Ru(NO)(NH3)s]3+ with chromous ion gives Cr2+(aq) -I- NH3 or [Ru(NH3)]e], respectively (261). Electrochemical reduction of [Ru(NO)(trpy)(bpy)] + (262) has been shown also to yield ammonia. Nitrido complexes can be also isolated in the reduction of [RuClslNO)] ". With SnCl2-HCl or aqueous formaldehyde, for example, [Ru2NCl8(H20)2] is formed (263). 

A nitrite-sensitive material has been developed by Fabre et al. with a poly(iV-methylpyrrole) film incorporating a metal-substituted heteropolyanion [(H20)Fe XWn039]" (X = P, n = 4, or X = Si, n = 5) as a doping anion [38C1-382]. Such a film was electrochemically stable and exhibited an efficient elec-trocatalytic activity vis-a-vis the nitrite reduction. In contrast, poor results were obtained when PPy was used as the immobilization matrix [383, 384]. The key step of this electrocatalytic process was the formation of an iron-nitrosyl complex generated from the replacement of H2O initially coordinated to the iron center by an NO group, the reduction of which led to the catalytic conversion of NO2 into ammonium ions [385, 386]. The measured catalytic currents were linear with the nitrite concentration over the range 1 X 10 to 3 X 10 M [382]. Furthermore, anions such as NOJ,... 

Extensive studies of the reductions of nitro substituents in the cobalt(II) cage complex, [Co(sar)] ", have been reported. These groups do not mediate electron transfer in the reactions of the metal center. The complexes [Ru(Hedta)(NO )] and [Fe(Hedta)(NO )] are catalysts in the electrochemical reduction of NOf, and the mechanism of the reaction has been investigated. Whether the product is mainly NjO or NH/ can be determined by appropriate choice of catalyst and pH. The chromium(I) complex, [Cr(N0)(H20)5] ", is resistant to oxidation. Reactions with the powerful oxidants lO and BrO are thought to be inner-sphere, with attack on the nitrosyl nitrogen atom to give nitrite and chromium(III) as products in a net two-electron reaction. ... 

As briefly alluded to, there are different classes of redox-active ligands in addition to the above mentioned ones. For example, we have seen in Chapter 5, Section 8, that azo-groups (in particular, 2-(phenylazo)pyr-imidine) are able to undergo two separate one-electron reduction processes. Conjugated polynitriles (mnt, tcne, tcnq) also constitute an important class of redox-active molecules and the electrochemical behaviour of their metal complexes has been reviewed.107 The same holds as far as alkyldithiocarbamates (Rdtc) and their metal complexes are concerned,108 or nitrosyl complexes in their possible NO+[NO fNO redox sequence.109 Thus, we would like to conclude the present Chapter by discussing a few less known redox non-innocent ligands. 

Important early electrochemical characterizations of nitrosyl adducts of porphyrin complexes were performed by the Kadish group. Voltammetric studies of the nitrosyl adduct of Fe (TPP) demonstrated an electrochemically reversible reduction which was substantially positive to that of the parent Fe (TPP). The product of the reduction, a nitroxyl adduct formulated as Fe (TPP)(NO) , could be generated in situ and its absorbance spectra observed reduction of the dinitro-syl, Fe (TPP)(NO)2, who s formation is characterized by a positive 1/2 shift. Table 4.2, also forms the same product. Nevertheless, attempts to prepare the reduced nitrosyl product via bulk electrolysis were unsuccessful. Up to ten reducing equivalents were passed through a sample solution electrolytically, but the major species in solution remained the ferrous nitrosyl, i.e., the nitroxyl apparently decomposed via some unspecified reaction to regenerate the more stable nitrosyl. 

One important characteristic of cobalt porphyrins is their ability to bind or react with small molecules, such as NO [27, 67, 70, 91, 93, 100], CO [36, 114, 115], O2 [314-320], or CO2 [321], and several studies have focused on the chemical and/or electrochemical reactivity of (P)Co toward these small molecules. The interaction of cobalt porphyrins with NO and the electrochemical properties of the resulting cobalt-nitrosyl porphyrins have been investigated by several research groups [7]. (TPP)Co(NO) exhibits two oxidations and three reductions at a microelectrode in CH2CI2 [90]. The NO group remains coordinated after electrooxidation and the initial electron abstraction from (TPP)Co(NO) was proposed to involve the porphyrin jr-ring system. Other electrode reactions were accompanied by a dissociation of NO from the compound and the site of electron transfer could not be determined. 

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