Neutral iron complexes

The potentiometric titration curve of ferric enterobactin, shown in Figure 3, has a sharp inflection after the addition of six equivalents of base. Such a break indicates that the six phenolic oxygens from the three dihydroxybenzoyl groups are displaced by ferric ion in the ferric enterobactin complex. This interpretation is further supported by the absorbance maximum at 490 nm (e 5600), which is very similar to simple tris(catecho-lato)iron(III) complexes (, T). The very low pH at which com-plexatlon of enterobactin occurs, with virtually complete complex formation by pH 6, is a strong indication of a very stable complex. However, the titration is prematurely terminated at pH 3.8 by the precipitation of a purple neutral iron complex (whose composition and structure will be discussed later) which makes it impossible to determine the stability consteint of ferric enterobactin from potentiometric data alone. 

A number of coordinatively unsaturated half-sandwich iron compounds have been synthesized, characterized, and reviewed." " Neutral iron complexes include compounds 57 and 58 which are prepared by reaction of FeCl2 with Li[G5Me4(CH2)2N(C4H8)] in THF/NEts or with Li[C5Me4(CH2)3(0GH2GH2)30Me] in At low tem-... 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Fe(II)-tetrakis-N, N, N, N (2-pytidylmethyl-2-amino-ethyl)amine (Fe-TPEN) and Fe(III)-ttis [N-(2-pyridylmethyl)2-aminoethyl]amine (Fe-TPAA) are novel iron complexes containing lipophilic and neutral ligands. 

A number of binuclear iron complexes have also been isolated (with a neutral base attached to each metal in an axial position). The iron complexes undergo net two-electron redox reactions with dioxygen to yield products containing two identical low-spin Fe(n) metal sites superoxide or peroxide are simultaneously generated. Remarkably, the reaction can be partially reversed by removal of 02 from the system by, for example, flushing with N2 in a mixed aqueous solvent at 0°C. 

Protons are in general indispensable for the dismutation of superoxide (Eq. (4)). Also in the case of its dismutation catalyzed by a metal center, two protons are needed for the dissociation of the product (H2O2) from the metal center (Scheme 9). Therefore, a complex which can accept two protons upon reduction and release them upon oxidation is an excellent candidate for SOD activity. The studies on proton-coupled electron transfer in Fe- and Mn-SODs 48), demonstrated that the active site of MnSOD consists of more than one proton acceptor (Scheme 10). Since the assignment of species involved in proton transfer is extremely difficult in the case of enzymatic systems, relevant investigations on adequate model complexes could be of vast importance. H2dapsox coordinates to Fe(II) in its neutral form, whereas in the case of Fe(III) it coordinates in the dapsox form. Thus, oxidation and reduction of its iron complex is a proton-coupled electron transfer process 46), which as an energetically favorable... 

Reduction of arene-Tr-cyclopentadienyliron(II) cations, which are iso-electronic with the di-ir-cyclopentadienylcobalt cation, with lithium aluminium hydride yields the neutral arenecyclopentadiene-iron complexes (100) ... 

Much of the work on the identification of carboxylic add complexation has been pioneered by Tiffen. He was the first to positively identify an iron-citrate complex in plant xylem.22 Iron-dtrate complexes have since been identified in a number of plants. The complex formed in these plant fluids is anionic, and Tiffen has shown that a number of other metal ions (Cr, Cu, Ni, Mn and Zn) are also present as anionic complexes.23 Although the neutrality of complexes may be considered a prerequisite for metal ion penetration of membranes, this has not been demonstrated with plant roots or with leaf-cell membranes. Involvement of negatively charged dtrate complexes of Ni11 has been confirmed both for nickel uptake and for translocation in plant species.24 Trisoxalatochro-mate(III) anion has been found in the leaf tissues of a plant species.25... 

Mn(diene)(CO>3] complexes are isoelectronic with [Fe(diene)(CO>3] complexes but owing to their anionic character are much more reactive toward electrophiles than the neutral iron analogs. The first examples of these species to be reported were [Mn(cyclohexadiene)(CO>3] complexes prepared via... 

Neutral ri2-alkene-tetracarbonyliron complexes can be prepared from the corresponding alkene and nonacarbonyldiiron via a dissodative mechanism. The organic ligand in the alkene-iron complex is more easily attacked by nucleophiles than the corresponding free alkene due to the acceptor character of the tetracarbonyliron fragment. The reaction principle is demonstrated in Scheme 1.8 [30],... 

Nitrilotriacetic acid (NTA) is a constituent of various domestic and hospital detergents and is a common water contaminant. NTA forms water-soluble chelate complexes with various metal ions, including iron, at neutral pH. Its iron complex, Fe-NTA, is a known potent nephrotoxic agent. The renal toxicity is assumed to be caused by the elevation of serum free-iron concentration following the reduction of Fe-NTA at the luminal side of the proximal tubule, which generates reactive oxygen species and leads to enhancement of lipid peroxidation. 

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