Nitrosyl complexes reactivity

Principles of structure, bonding and reactivity for metal nitrosyl complexes. J. H. Enemark and R. D. Feltham, Coord. Chem. Rev., 1974,13, 339-406 (126). 

The low reactivity of both Cyt111 and Cyt11 toward NO can be attributed to occupation of the heme iron axial coordination sites by an imidazole nitrogen and by a methionine sulfur of the protein (28). Thus, unlike other heme proteins where one axial site is empty or occupied by H20, formation of the nitrosyl complex not only involves ligand displacement but also significant protein conformational changes which inhibit the reaction with NO. However, the protein does not always inhibit reactivity given that Cat and nNOS are more reactive toward NO than is the model complex Fem(TPPS)(H20)2 (Table II). Conversely, the koS values... 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

Factors That Control the Reactivity of Cobalt(III)-Nitrosyl Complexes in Nitric Oxide Transfer and Dioxygenation Reactions... 

In this paper author reported the reactivity of newly synthesized Co(III)-nitrosyls complexes with superoxide radical to follow nitric oxide dioxygenation. Two new Co(III)-nitrosyl complexes bearing N-tetramethylated cyclam (TMC) ligands, [(12-TMC)-Com(NO)]2+ (1) and [(13-TMC)Coin(NO)]2+ (2), were synthesized via [(TMC)Con(CH3CN)]2+ + NO(g) reactions. Spectroscopic and structural characterization showed that these compounds bind the nitrosyl moiety in a bent end-on fashion. Complexes 1 and 2 reacted with K02/2.2.2-ciyptand to produce [(12-TMC)Con(N02)]+ (3) and [(13-TMC)Con(N02)]+ (4), respectively these possess 0,0 -chelated nitrito ligands. 

With the exception of B = OH-, which relates in fact to an acid-base reaction, the other nucleophiles are potential reductants. After forming the reversible adducts [Eq. (5)], redox reactions are usually operative, leading to the reduction of nitrosyl and oxidation of the nucleophile in Eq. (6). Nevertheless, we will consider first the reaction with B = OH- for the sake of simplicity, and also because it allows for some generalizations to be made on the factors that influence the electrophilic reactivities of different nitrosyl complexes (51). We continue with new results for some N-binding nucleophiles (62,67), which throw light on the mecanisms of N20/N2 production and release from the iron centers. A description of the state of the art studies on the reactions with thiolate reactants as nucleophiles will be presented later. 

A. Reactivity of Nitrosyl Complexes with OH-The reaction scheme can be described as ... 

In spite of the abundant work on this type of reactivity, no rate constants for the addition reactions had been obtained, with the exception of the [M(CN)5NO]2 ions (M = Fe,Ru,Os) (55,68), until the recently published kinetic measurements for a representative set of nitrosyl complexes MX5NO (M = mainly ruthenium) (51). Table III... 

In this reaction the nitrosyl ligand bends, undergoing a 2 e reduction, rather than form a 20 e complex. This reaction can also serve to activate the nitrosyl ligand. Whereas the linear nitrosyl may be unreactive if vNO is sufficiently low, the bent nitrosyl is reactive to electrophiles. 

Reactions have been known to occur at the nitrosyl group in the nitroprusside ion, [Fe(CN)3-NO]2-,8 for some considerable time recently similar behaviour has been observed in other systems. Different types of reactivity are exhibited depending on the nature of the nitrosyl complex and the mode of coordination of the NO ligand. For general reviews of this topic see references 9, 11 and 14. 

There are only two examples of this reaction, and it is probably limited to a small group of reactive nitrosyl complexes of early transition metals (equations 84 and 85). 

The pH dependence of the rate of formation of a nitrosyl complex shows that nitrous acid is the reactive intermediate in the reaction when the pH is in the range of 2-8. The catalysts are not deactivated during repeat cycles between their oxidized and reduced states. The catalyzed reduction appears to depend on the ability of the multiply reduced heteropolyanions to deliver electrons to the NO group bound to the iron center. 

Although this catalytic reaction appeared to be of synthetic interest, it has since then neither been applied in synthesis nor further developed. This might be attributed in part to problems with reproducibility and catalyst stability under the reaction conditions, although the Hieber complex was used in a stoichiometric manner for the preparation of a variety of 7i-allyl-Fe complexes. These latter compounds served as starting materials for a plethora of subsequent reactions [34]. The results obtained by Nakanishi and coworkers on the stability and reactivity of n-allyl-Fe-nitrosyl complexes proved such intermediates to be reactive towards a variety of nucleophiles however, the Fe complexes formed upon nucleophilic substitution were catalytically inactive. Hence, in order to maintain the catalytic activity, the formation of intermediate 7i-allyl-Fe complexes had to be circumvented. About 3 years ago we started our research in this field and envisioned the use of a monodentate ligand to be a suitable way to stabilize the proposed catalytically active G-allyl complex. The replacement of one CO by a non-volatile basic ligand was thought to prevent the formation of the catalytically inactive 7t-allyl-Fe complex (Scheme 7.21). 

Tfouni E, Krieger M, McGarvey BR, Franco DW. Structure, chemical and photochemical reactivity and biological activity of some ruthenium amine nitrosyl complexes. Coord Chem Rev 2003 236 57-69. 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

The normal classification of material by oxidation state is inappropriate for nitrosyl complexes because the oxidation state concept is very much a formalism for them. Instead we shall use the generally accepted [M(NO)x] + classification in which x is the number of coordinated NO groups and n the number of metal d electrons, the latter being calculated on the basis that NO+ is the coordinated moiety. As will be apparent, osmium complexes within each such category do in fact show considerable similarities of structure and reactivity, and also with their ruthenium analogues. Osmium is unusual in forming an [M(NO)]5 type of complex. 

As in mononuclear nitrosyl complexes, it is convenient to separate the reactivity into two sections (1) reactions directly involving change at the NO, and (2) reactions of the metals that are enhanced by the presence of the nitrosyl ligand. The reaction of overwhelming importance that occurs with cluster coordinated nitric oxide is deoxygenation. Depending on the conditions this can ultimately sdeld NH, NH2, NCO, or simply N atoms coordinated to the cluster. 

In this section the reactivity of metal nitrosyl complexes is discussed and related to the NO coordination mode. A considerable difference between the chemistry of nitric oxide and carbon monoxide complexes has already been noted. The reactivity of nitric oxide coordinated to transition metal centers, and of nitrosyl clusters, were thoroughly reviewed in 1979 by McCleverty (7) and in 1985 by Gladfelter 11), respectively therefore only a summary is presented here. Nucleophilic reactions of linear nitrosyl groups will not be considered. 

Nitrosyl complexes are also oxidized by NO gas itself, and complexes that are reactive to O2 also undergo this reaction thus, [Co(dmgh)2(NO)(MeOH)] reacts in the presence of pyridine to give [Co(dmgh)2(N02)py] 200). It has been suggested that the reaction occurs in a similar fashion to the first steps of oxidation with molecular oxygen, that is, via the intermediate (38), which can react further with free NO to give N2O and coordinated NO2 . 

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