Porphyrin Nitrosyls

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

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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). ... 

D. S. Bohle, C. H. Hung, Ligand-Promoted Rapid Nitric Oxide Dissociation from Ferrous Porphyrin Nitrosyls , J. Am. Chem. Soc. 1995,117, 9584-9585. 

The six-coordinate species is spin-admixed (S = 3/2,5/2) while the predominantly five-coordinate species (1-OH) is high spin (S = 5/2). The spin states have significance, it will be shown in interpreting some of the kinetics results. Reversible binding of NO to 1-H20 leads to formation of the linearly bonded diamagnetic porphyrin nitrosyl, (TMPS)Fen(NO + )(H20), (1-NO). The product of reaction of 1-OH with NO has a noticeably different UV/visible spectrum from that of 1-NO, and this was ascribed to the species (TMPS)Fen(NO + )(OH). 

Miranda KM, Bu X, Lorkovic IM, Ford PC. Synthesis and structural characterization of several ruthenium porphyrin nitrosyl complexes. Inorg Chem 1997 36 4838. 

NO reacts with both ferric and ferrous centers in hemoproteins to form the respective iron(II) and iron(III) nitrosyl adducts, whose structural features are similar to those observed for iron (II) and iron(III) porphyrin nitrosyls. These analogies are also reflected in similar chemical reactivity observed for nitrosylated ferri- and ferroproteins and their respective porphyrin models. For example, NO-adducts of Fe(III) undergo reductive nitrosylation in the presence of an excess of NO, and a similar process is commonly observed for synthetic Fe(III) porphyrins. The first step of this reaction involves nucleophilic attack of OH on the nitrosyl ligand coordinated to the iron center, as presented in reaction (13) (33,60) ... 

NO in the unbound form has a very short lifetime in the cell but can be stabilized by the formation of complexes, i.e. metal-porphyrin nitrosyls, dinitrosyl-iron complexes and S-nitrosothiols, which are cmisidered to be its biological transporters. Nitric oxide has a very high affinity for iron contained in the active sites of proteins [74]. 

Choi, I.-K., Y. Liu, D. Feng, K.-J. Paeng, and M.D. Ryan (1991). Electrochemical and spectroscopic studies of iron porphyrin nitrosyls and their reduction products. Irwrg. Chem. 30, 1832-1839. 

Liu, Y. and M.D. Ryan (1994). The electrochemical reduction of iron porphyrin nitrosyls in the presence of weak acids. J. Electroanal. Chem. 368, 209-219. 

Porphyrin-nitrosyl complexes with six other metal ions are also known, and all but one of which has been electrochemically investigated. These are Ru [69, 73, 94-96], Os [5], Rh[97], Cr [98], Mo [99] and Mn [100]. Some nitrosyl metalloporphyrins can be reversibly reduced or oxidized by one or two electrons without loss of the NO ligand and this generally occurs when the electrode reactions involve the 7T-conjugated macrocycle in the case of a metal-centered reduction or oxidation, however, the electron-transfer reactions will most often be accompanied by a loss of the NO ligand, resulting in an irreversible oxidation as shown in Fig. 7 for the case of (TPP)Cr(NO) and (TPP)Mn(NO) in CH2CI2. 

The current example illustrates PVDOS formulation as an effective basis for comparison of experimental and theoretical NIS data for ferrous nitrosyl tetraphe-nylporph3Tin Fe(TPP)(NO), which was done [101] along with other ferrous nitrosyl porphyrins. Such compounds are designed to model heme protein active sites. In particular, the elucidation of the vibrational dynamics of the Fe atom provides a unique opportunity to specifically probe the contribution of Fe to the reaction dynamics. The geometrical structure of Fe(TPP)(NO) is shown in Fig. 5.16. 

The NIS investigation of heme complexes includes various forms of porphyrins (deuteroporphyrin IX, mesoporphyrin IX, protoporphyrin IX, tetraphenylpor-phyrin, octaethylporphyrin, and picket fence porphyrin) and their nitrosyl (NO) and carbonyl (CO) derivatives, and they have been the subject of a review provided by Scheidt et al. [109]. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

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. 

Manganese nitrosyl porphyrins [215] are considered good models for the iron-nitric oxide analogs, which are relatively unstable but very vital to many biological operations. A six-coordinate manganese nitrosyl porphyrin of the form (por)Mn(NO)(L), where por can be TTP (TTP = tetra(4-methylphenyl)porphine) and L = piperidine, methanol, 1-methyhmidazole, has been prepared [216] in moderate yields by the reductive nitrosylation of the (por)MnCl complex with NO in piperidine. The crystal structures of these compounds give indication of a linear Mn-NO bond [215]. 

Table 11. Comparison of the wavenumber of X=Z vibrations ( x=z)> the wavelength of the a-band the chemical shift of the meso-porphyrin protons (6) and the Os /Os -redox potential (Ej ) of various dinitrogen or nitrosyl octaethylporphinatoosmium(II) complexes Os(OEP)(XZ)L [31 a-3 If]...

Amazingly, there is no systematic cis effect of the ligand L trans to carbon monoxide in the carbonylosmium(II) porphyrins Os(OEP)CO(L) [30a-30g], (Table 10). In the nitrosyl derivatives, Os(OEP)NO(L), the anionic ligand Le exerts only a small effect (Table 11) the wavelengths of the a-bands increase in the series [31c] < [3Id] <... 

Very interesting features emerge from the comparison of the carbonyl, dioxygen, and nitrosyl iron porphyrins in Series a, d, and e. (Note that for the tetraarylporphy-rins the 0-bands are given.)... 

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