Iron complex, absorption spectrum structure

McCandlish et al. have isolated a peroxo-iron(III) complex (9) (Fig. 4) in the reaction of Fe(III)TPP(Cl) and KO2 according to Eq. (4) (37). The Soret band of 9 appears at 437 nm with unusually red-shifted a- and -b-bands (565 and 609 nm in DMSO). The EPR spectmm of 9 at 77K showed a relatively narrow, sharp resonance at g = 4.2 and weak resonances at g = 2 and g = 8, typical of rhombic high-spin ferric complexes such as Fe" EDTA (JS). Such a spectrum is not typical of high-spin ferric porphyrin complexes, which usually show resonance at g = 2 and 6, indicative of axial symmetry. An IR band at 806 cm" was observed to shift to 759 cm when K 02 rather than K 02 was used to prepare solutions of 9 these observations suggested the side-on bonding formulation illustrated in Fig. 4. Extended X-ray absorption fine structure (EXAFS) studies of 9 also... 

There are few peroxide adducts of synthetic non-heme iron complexes that are well characterized (Table VI). Perhaps the best known adduct is that derived from Fe (EDTA) under basic conditions. This purple complex has an absorption maximum near 520 nm (e 528 M cm ) (163). These are absorptions characteristics associated with the peroxide-to-Fe" charge-transfer band in oxyHr however, the coordination mode of peroxide in the complex appears to be different from that in oxyFIr. After some debate in the literature, it has been concluded that the peroxide is T --bound on the basis of isotope effects observed in the Raman spectrum of the complex (158, 164). The v(O-O) of the H2 Oi complex is found at 815 cm". When is used, the v(O-O) shifts to 794 cm and appears as a peak of comparable line width. Were the peroxide only ri -bound, two peaks due to the Fe-O -O " and the Fe-" 0- 0 isotopomers would have been expected, as in oxyHr. An ri -peroxo structure is also proposed for [Fe(TPP)02] and has been determined for the corresponding [Mn(TPP)Oi] complex (165). 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

The complexity of the low temperature MCD spectra of the oxidized and reduced trinuclear cluster shows the multiplicity of the predominantly S — Fe charge transfer transitions that contribute to the absorption envelope. While MCD spectroscopy provides a method of resolving the electronic transitions, assignment cannot be attempted without detailed knowledge of the electronic structure. However, the complexity of the low temperature MCD spectra is useful in that it furnishes a discriminating method for determining the type and redox state of protein bound iron-sulfur clusters. Each well characterized type of iron-sulfur cluster, i.e. [2Fe-2S], [3Fe-4S], and [4Fe-4S], has been shown to have a characteristic low temperature MCD spectrum in each paramagnetic redox state (1)... 

Cyclo-octatetraene reacts with iron carbonyls to form complexes with the compositions [Fe(CO)3(C8H8)], [Fe2(CO)6(C8H8)], and [Fe2(CO)7(C8H8)] 152, 168, 180). Nakamura and Hagihara 166) report that the complex [Fe(CO)3(C8H8)] decolorizes bromine in carbon tetrachloride and shows absorption bands in its infrared spectrum at 699, 716, and 720 cm-1 due to cis-double bonds. They suggest structure (XVI) for this complex, i.e., the hydrocarbon retains its tub form in the complex. These results are con-... 

The first step of peroxidase catalysis involves binding of the peroxide, usually H2C>2, to the heme iron atom to produce a ferric hydroperoxide intermediate [Fe(IE)-OOH]. Kinetic data identify an intermediate prior to Compound I whose formation can be saturated at higher peroxide concentrations. This elusive intermediate, labeled Compound 0, was first observed by Back and Van Wart in the reaction of HRP with H2O2 [14]. They reported that it had absorption maxima at 330 and 410 nm and assigned these spectral properties to the ferric hydroperoxide species [Fe(III)-OOH]. They subsequently detected transient intermediates with similar spectra in the reactions of HRP with alkyl and acyl peroxides [15]. However, other studies questioned whether the species with a split Soret absorption detected by Back and Van Wart was actually the ferric hydroperoxide [16-18], Computational prediction of the spectrum expected for Compound 0 supported the structure proposed by Baek and Van Wart for their intermediate, whereas intermediates observed by others with a conventional, unsplit Soret band may be complexes of ferric HRP with undeprotonated H2O2, that is [Fe(III)-HOOH] [19]. Furthermore, computational analysis of the peroxidase catalytic sequence suggests that the formation of Compound 0 is preceded by formation of an intermediate in which the undeprotonated peroxide is coordinated to the heme iron [20], Indeed, formation of the [Fe(III)-HOOH] complex may be required to make the peroxide sufficiently acidic to be deprotonated by the distal histidine residue in the peroxidase active site [21],... 

Because MCD signals can be either positive or negative in sign, considerably more fine structure is seen in MCD spectra than in the corresponding electronic absorption spectra. Furthermore, MCD is a property of the molecular electronic structure of a chromophore, and so the only structural changes that will influence the MCD spectrum are those that modify the electronic structure. Furthermore, the MCD spectrum is relatively insensitive to the environment in which the chromophore is located, whether it is the protein microenvironment for a heme protein center or the solvent for a model complex. Thus, comparisons of the MCD spectra of synthetic heme iron model complexes with those of heme protein active sites are possible and have been shown to be of considerable utility in assigning the coordination structures of the heme protein active sites (I). 

The complexes prepared in this manner are blue or brown and are air- and moisture-sensitive. However, they can be stored for several months under N2. They are diamagnetic and soluble in most organic solvents except petroleum ether and similar hydrocarbons. In the far-IR spectrum, several absorption bands occur in the vicinity of 250-300 cm. These are assigned to V(m cd- The X-ray crystal structure of WCl2(PhN)(PMe3)3 shows a mer arrangement of phosphine and cis chloride ligands, one Irons to the phenylimido function. ... 

---