Reductive nitrosylation

The one-electron electrochemical reduction of NP (57) is a reversible process in aqueous solution, provided the measurements are performed at pH > 8 (—0.123 V vs. NHE) (57a,57b). Different chemical reductants such as sodium in liquid ammonia, tetrahydroborate, ascorbic acid, quinol, dithionite, superoxide or thiolates are also known to generate the [Fen(CN)5NO]3 ion (48,57). However, care must be taken in the products analysis, because the negative redox potentials of some of these reductants allow for further nitrosyl reduction (57a). Also, the reduced product is unstable toward cyanide... 

Nitric oxide displays a wonderful diversity of ligand reactions. It is important to distinguish two classes of these reactions reductive nitrosation and reductive nitrosylation. Reductive nitrosation refers to the addition of NO to a bound amide ligand with concomitant reduction of the metal center as in... 

Good linear plots of the pseudo-first order rate constant for the formation of Cyt from Cyt as a function of [OH ] have been found, supporting the above mechanism. Although no evidence for the N-bound nitrous acid intermediate complex was found, the /jqh values derived from the slopes, together with the redox potentials for nitrosyl reduction in the heme compounds are in fair agreement with the general behavior observed for the electrophilic reactions of other nitrosyl complexes, including NP (see below) (51). 

The reaction of 9(1 l)-dehydro steroids with nitrosyl fluoride was studied by Grantz and Rosenthal in pursuit of an alternate source for the important 9a-fluoro-11-oxygenated steroids. As expected, reaction at the more hindered 9(1 l)-double bond proceeds more slowly than at either the 4- or 5-double bonds. After 10 days at 3°, 3 -acetoxy-5a-pregn-9(l l)-en-20-one (50) affords a 45% yield of the 9a-fluoro-ll-nitrimine (51). Other 9a-fluoro-ll-nitrimines were prepared and it was found that the presence of additional keto groups, particularly the 3-keto group gives rise to side products with a concomitant reduction in yield of the desired 9a-fluoro-ll-nitrimines. In the case of the A" -3-ketones the yield is reduced to 10 %. The steric hindrance... 

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

Complexes of manganese with the 1,1-dithiolates have been restricted to the (PrSN)3[Mn(i-mnt)3] complex (112) and nitrosyl or carbonyl derivatives, e.g., [7i--CpMn(NO)(S2C==X)] (X = C(CN)C02Et, C(CN)C0NH2, NCN, C(CN)2, or (HNO2) (110, 204, 205). These complexes undergo electrochemical, one-electron oxidations and reductions to afford the neutral or dianionic species. 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

Sometimes a company will build a larger plant because it expects the reduced prices will open new markets. This, of course, would not be done without some strong indications that the new markets would develop. In 1969, Hercules announced that nitrosyl chloride, which was then sold in cylinders at 5- 8 per pound, would be sold in bulk at 2- 3 per pound. It was also stated that when the sales exceeded 1,000,000 lb/yr the price was expected to drop below 0.50 per pound. They had built a plant that could produce 1, 000,000 lb/yr but only had a market of 200,000 lb/yr. Their marketing experts estimated that the reduction in price could cause a five-fold increase in the demand in only three years.5... 

Coordination of NO to the divalent tetrasulfonated phthalocyanine complex [Co(TSPc)]4 results in a complex formally represented as [(NO )Coin(TSPc)]4 kf= 142M-1s-1, KA 3.0 x 105 M-1). When adsorbed to a glassy carbon electrode, [Co(TSPc)]4- catalyzes the oxidation and reduction of NO with catalytic currents detectable even at nanomolar concentrations. Electrochemistry of the same complex in surfactant films has also been studied.905 Bent nitrosyl complexes of the paramagnetic trivalent tropocoronand complex Co(NO)(TC) ((189), R = NO) have also been reported.849... 

Water soluble iron porphyrins [Fem(TPPS)(H20) ]3-330 and [Fem(TMPy)(H20)2]5+ 331 332 (TPPS = maso-tetrakis(/ -sulfonatophenyl)porphyrin, TMPyP = / /e.vo-tetrakis(7V-methyl-4-pyridi-nium)porphyrin331 or maso-tetrakis (A -methyl-2-pyridinium)porphyrin332 dications) act as effective electrocatalysts for the reduction of nitrite to ammonia in aqueous electrolytes (Equation (64) Ei/2= 0.103 V vs. SCE at pH 7), with NH2OH or N20 also appearing as products depending on the reaction conditions. Nitric oxide then ligates to the iron(III) porphyrin to form a nitrosyl complex [Fen(P)(NO+)] (P = porphyrin) as intermediate. 

Some nitrosyl-Mo1 complexes of the form [Mo(Tp )(NO)Cl(py-R)] (where py-R is a substituted pyridine) also undergo moderate NIR electrochromism on reversible reduction to the Mo° state. In these complexes reduction of the metal center results in appearance of a Mo° —> py(7r ) MLCT... 

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