Nitrosyl complexes reductive processes

In a related observation, reported by Tanaka et al. (81), the copper(II) complex Cu(tpa)2+ (tpa = tris[(2-pyridyl)methyl] amine) was shown to serve as a catalyst for the electrochemical reduction of nitrite to N20 and traces of NO in aqueous solution. NO and/or a copper nitrosyl complex would appear to be the likely intermediates in this process (81a). 

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. 

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).

We have previously mentioned in Section 4.2 the chemistry developed by Selhnann etal, by using tetradentate or pentadentate S4 ligands. The use of such a ligand in nitrosyl ruthenium chemistry allowed the first conversion of a nitrosyl complex into a ruthenium HNO complex (31) by addition of NaBILi to [Ru(NO)(py S4)]Br. The formation and decomposition of HNO complexes is often invoked in many processes such as combustion of ftiels, oxidation of N2, reduction of HNO2, and so on. ... 

Both the chloroiron and phenyliron octaalkylcorrolates can be oxidized electrochemically by two or three one-electron processes, and reduced by two one-electron processes.One-electron reduction of each produces the simple [XFe Corr], while one-electron oxidation of each produces the [XPe Corr -J+jr-cation radicals. The nitrosyl complex of iron (III) octaethylcorrolate has been formed and both it and its 1-electron oxidized product have been characterized structurally and electrochemically. 

In summary, the substitution of coordinated water by F on [Fe (edta) (H20)] results in considerable reduction of the oxygen sensitivity of the latter, without any limiting effect on the formation of the nitrosyl complex or the reversibility of the NO binding reaction. In view of the fact that the introduced fluoride will not be consumed and will protect the active form of Fe (edta) by in situ formation of the mixed ligand complex, the observed reactivity behavior seems to be very advantageous for practical application in the BioDeNO, process. 

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,... 

It is possible that nitric oxide may be displaced from the metal centre prior to oxidation of the phosphine. Indeed, such a nitrosyl displacement in the complex CrCl(7t-Cp)(NO)2, leading to derivatives of the type CrCl(jr-Cp)NO(L), has been reported.118 However, such behaviour is rare (compared with CO displacements in metal carbonyls) and it seems likely that the above reduction takes place by an alternative process. 

NO adds reversibly to reduced cobalamin, Cbl(II).156 It does not react directly with aquacobalamin(III), (0blni(H2O)), but it does add to Cbl,n(N02 ) and Cblm(NO).175 Acid hydrolysis of the dinitroso species releases nitrite, and binding of nitrite to Cblln(H20) generates Cbln,(. 02 ). This sequence thus affords a nitrite-catalyzed mechanism for NO substitution at Cblln(H20). The reaction of NO with Com porphyrins is quite complex.176 In the first step, NO displaces an axial water ligand to form a weakly bound mono NO complex this mono NO complex reacts with a second molecule of NO to form nitrite and a reduced Co-NO complex. This latter process is called reductive nitrosylation. Manganese(II) porphyrins bind NO very rapidly.177 Stability constants have been measured for the formation of mono and bis NO complexes of Cun(dithiocarbamate)2.157... 

Reductive nitrosylation, on the other hand, can refer to the addition of NO to a metal center Mox with formal reduction of the metal center to yield Mred(NO +), but in the context of ligand reactions reductive nitrosylation refers to the net reactions of NO with metal-bound NO and the ensuing events. Reductive nitrosation of coordinated amines to form nitrosamines occurs through the conjugate base of the amine, and this process has been reported for reactions of NO with [Ni(tacn)2]3 +, 198 with methyl-amine coordinated to a macrocyclic Ni(III) complex,199 with triglycyl complexes of Fe(III), Ni(III), and Cu(III),200 and with Cu(II) macrocyclic complexes.201 Reductive nitrosation of [Ru(NH3)6]3+ produces [Ru(NH3)5N2]2 + with base-catalyzed kinetics the coordinated N2 is produced by hydrolysis after the nitrosation step.170... 

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