High spin ferric complexes

There are two common features of these two types of hyperfine interactions. First, both 8C and 8pc are inversely dependent on temperature. Second, they both depend on the total electronic spin 5 of the iron atom as shown in Eqs. (1) and (2). The magnitude of the hfs resonances depends on the 5 value of each Hb derivative. For example, met-Hb is a high-spin ferric complex with five unpaired electrons per heme, deoxy-Hb is a high-spin ferrous complex with four unpaired electrons per heme, and both cyanomet- and azidomet-Hb are low-spin ferric complexes, each with one unpaired electron per heme. The hyperfine interactions can shift resonances either upfield or downfield from their diamagnetic counterparts. It should be noted that both HbC>2 and HbCO are low-spin ferrous complexes that are diamagnetic systems (S = 0) and they will not give rise to hyperfine interactions. 

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... 

The oxidation of Fe(III)TMP(wi-chlorobenzoate) with 2 equivalents of mCPBA in toluene has been reported by Groves and Watanabe to give iron(III) porphyrin W-oxide (23, (89) Fig. 9). The reaction proceeded quantitatively only at low concentrations of the heme. The presence of an acid such as benzoic acid drastically depressed the formation of 23. The EPR spectrum, H NMR chemical shifts, and solution magnetic moment (5.4 /Zg) indicated that 23 was a high-spin ferric complex. Inspection of the reaction mechanism indicated that the reaction proceeded via the formation of the acylperoxo-iron(III) precursor 16, similar to the reaction carried out in dichloromethane. 

By contrast, high-spin ferric complexes show only small quadrupole splittings due to the lattice terms, and in the magnetically ordered state the field is basically the Fermi term. Consequently less information is obtainable, with the notable exception of electronic relaxation times which can be deduced from the broadening which often occurs in ferric complexes. 

Resonance Spectra of High-Spin Ferric Complexes... 

In addition, all complexes display a reversible, one-electron reduction at a very negative potential Em —1.70 to -1.90 V vs Fc+/Fc, which is metal centered and nearly invariant with respect to the substitution pattern of the coordinated pheno-lates. It demonstrates the enormous stabilization of the high-spin ferric state by three phenolato ligands. The electrochemistry also nicely shows that unprotected ortho- or para positions of these phenolates lead to irreversible electron-transfer waves on the time scale of a cyclic voltammogram and that methyl substituents are inefficient protecting groups. 

The observation that the isomer shift varies between 0.39 mm s 1 at 4 K and 0.54 mm s-1 at 80 or 298 K only, irrespective of the charge n of the complexes (n = 0, 1,2), immediately implies that all complexes contain an octahedral, high-spin ferric ion (d5, S = ). Furthermore, from the applied-field Mossbauer spectra of [Fem(LBuMet)] (.S t = f), [Fem(LBuMet )]+ (5t = 2), and [Fem(LBuMet )]2+ (5t = ) intrinsic isotropic hyperfine couphng constants, AFe/gNpN, of -21.4, -21.0, -20.8 T, respectively, have been established. These values are also typical of high-spin iron(III). 

The diaqua and aqua (hydroxo) hemin complexes encapsulated in the micelles [20] are found to be high-spin (peff = 5.7 — S.Sps). Their high-spin nature is further confirmed from the ESR spectra at 4.2 K (Fig. 4). The spectra are characteristic of high-spin ferric porphyrins with a large zero-field-split Ai ground state with Mg = 1/2 lying lowest. The spectra are axially symmetric (gf = 2.05, = 6.0) for the diaqua complex, while for the aqua (hydroxo)... 

The active center of the oxidized plant-type ferredoxins contains two iron atoms with almost identical electronic environments at the nuclei. These irons are high-spin ferric (S = 5/2), spin-coupled to give a resultant diamagnetism for the complex. 

In the reduced state, the active center contains one high-spin ferric state spin-coupled to one high ferrous state (S = 2) to give a resultant S = 1/2 complex. 

Figure 3). With ferric ion these ligands form thermodynamically stable, high spin, exchangeable complexes which are virtually specific for Fe111. 

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