Iron nitrosyls coordination

Iron nitrosyls coordination, 28 146, 148 nucleophilic attack, 28 153, 154 Iron oxide, 32 54-55 activation energy, 27 16, 17 in catalytic converter, 24 62 coatings containing, 40 103-105 CO conversion, 28 263 on silver, 27 14-17... 

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... 

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).

In the closely related complex 11, the four-coordinate iron adopts nearly regular tetrahedral geometry with linear iron-nitrosyl groups (51), Fig. 11. 

One of the earliest assignments of an oxidation state of +1 for an iron complex was that of Wilkinson and co-workers85 based on the magnetic susceptibility, IR and absorbtion spectra of [Fe(H20)5N0]2+. The potential for nitric oxide to act as either a two or three electron donor in its coordination to transition metal ions, however, has led to much controversy. For this reason iron nitrosyl complexes are discussed separately in Section 44.1.2. 

A class of tripodal ligands such as (131), synthesized by reaction of 3,5-dimethylaniline and nitrilotriacetic acid with triphenylphosphite, has been reported and its coordination chemistry toward iron nitrosyl acceptors has been investigated. When bonded to a metal ion, they form cavities around vacant coordination sites on metal ions.210... 

There are also other avenues by which -NO may serve a protective role in ischemia-reperfusion phenomena. Under these circumstances, known to include a high rate of production of oxygen free radicals, -NO can react with Oi to divert Oi through ONOO -dependent (and potentially less damaging) oxidative and decomposition pathways. Nitric oxide may also confer protection by reacting with iron to form iron-nitrosyl compounds. By binding free coordination sites of iron, -NO can limit Fenton chemistry and iron-dependent electron transfer reactions (Kanner etal., 1991 Ignarro,... 

Once again, the trans cr-bonding alkyl or aryl ligand exerts a powerful influence on the M-N-O bond angle. Studies on transient intermediates during photolysis of Ru-NO nitrosyls have revealed different modes of binding (and dissociation) of coordinated NO at the ruthenium centers. Metastable NO linkage isomers have been observed for MNO (M = Fe, Ru, Os) and for MNO ° complexes of Ni, as well as for FeNO iron nitrosyl porphyrins [208-212]. 

Hairop TC, Song D, Lippard SJ (2006) Interaction of nitric oxide with tetrathiolato iron (II) complexes relevance to the reaction pathways of iron nitrosyls in sulfur-rich biological coordination environments. J Am Chem Soc 128 3528-3529... 

Attempts to prepare binary iron nitrosyl complexes by treatment of Fe(CO)3Cl with AgPFs or AgBF4 lead to Fe(NO)3(ri -PF6) and Fe(NO)3(T -BF4) rather than salts containing naked [Fe(NO)3] ions the r notation indicates that the anions coordinate through the metal centre through one F atom. 

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,... 

Six-coordinate organoiron porphyrin nitrosyl complexes, Fe(Por)(R)(NO), were prepared from Fe(Por)R (Por = OEP or TPP R = Me, n-Bu, aryl) with NO gas. The NMR chemical shifts were typical of diamagnetic complexes, and the oxidation state of iron was assigned as iron(ll). ... 

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