Iron complexes configuration

A model similar to that of the iron complex 31 was proposed for the cobalt species synthesized as a result of co-condensation of cobalt vapors with pyrrole in vacuum. A frozen matrix formed is subsequently warmed to room temperature (89JA3881). An oligomer or a polymer results, in which a- and ir-donor functions are realized simultaneously. The model proposed differs from that for the iron pyrrolyl complex by inclusion of the Co—Co bonds to attain the 18-electron configuration. 

Subsequent carbonylation of the alkyl-iron complexes with carbon monoxide provides the desired chiral iron-acyl complexes, with essentially complete inversion of configuration at... 

Similar effects are observed in the iron complexes of Eqs. (9.13) and (9.14). The charge on the negatively charged ligands dominates the redox potential, and we observe stabilization of the iron(iii) state. The complexes are high-spin in both the oxidation states. The importance of the low-spin configuration (as in our discussion of the cobalt complexes) is seen with the complex ions [Fe(CN)6] and [Fe(CN)6] (Fq. 9.15), both of which are low-spin. 

This is the origin of the various values for self-exchange rate constants. We may now attempt to rationalize some of these in terms of the /-electron configurations of the various oxidation states. Consider the self-exchange rate constants for some iron complexes. 

A typical example of a correlation diagram for Fe is given in Fig. 4.3. It summarizes the isomer shifts for a great variety of iron complexes with oxidation states (1) to (VI) in the order of the respective high-spin, intermediate-spin, and low-spin configurations. The plot of the corresponding values marked by grey, hatched and open bars demonstrates three major trends ... 

The EFF calculations yielded a single Cs-symmetric conformation for each type of ferrichrome analog (Figure 4), both with a A-cis configuration of the hydroxamates about the metal when L-amino acids were used. Taken together with the spectroscopic data, pronounced differences were observed for the conformations of these iron complexes. Inspection of the calculated conformations showed that the backbone amide groups may... 

As already briefly mentioned, the oxygen-atom insertion into Si—H bonds of silanes constitutes a selective method for the chemoselective preparation of silanols, which has been much less studied compared to the CH oxidation. This unique oxyfunctionalization of silanes is also highly stereoselective (equation 35) since, like the CH insertions, it proceeds with complete retention of configuration. A novel application of the SiH insertion process is the synthesis of the unusual iron complex with a silanediol functionality, in which selectively both Si—H bonds of the silicon atom proximate to the iron ligand are oxidized in the silane substrate (equation 36). ... 

Figure 10.6 Stereoselective alkylation of an enolate in the synthesis of captopril. The heavier straight lines are bonds in the plane of the paper while the thin straight lines, are bonds behind the plane of the paper. The shape of the substrate means that an unhindered approach of the tertiary butylthiomethyl bromide (tBuSCH2Br) is only possible from one side of the iron-complex substrate. Consequently, reaction occurs mainly from this side of the complex, which results mainly in a product with an S configuration...

It can be seen from molecular models that two diastereoisomers are possible for the ferric enterobactin complex, A-cis and A-cis. These are not mirror images because of the optical activity of the ligand. The similarity of the roles played by the ferrichromes and enterobactin lent additional speculative interest to the preferred absolute configuration of the iron complex (20). The structural studies of the tris catechol complexes (vide infra) and the spectroscopic properties of the chromic... 

Williams (28) suggested that the band found at about 16 kK in high-spin systems was a charge transfer transition, in which the excited state arises from a configuration in which an electron has been transferred from a porphyrin 71-orbital to a metal -orbital. Such transitions frequently occur in iron complexes. His alternative suggestion that the band could be an intra-metal d—d transition mixed with charge transfer was considered most unlikely on intensity grounds (88). 

To prepare the enantiomerically pure iron acyl complex (R)-(39), a precursor diastereomeric menthoxyaUcyl complex was resolved and then manipulated (Scheme 14). More recently resolution of the chiral-at-metal acyl complexes themselves was achieved, and this has become the basis for a commercial preparation of the iron acyl developed for use as a chiral auxiliary (see below). Cationic iron complex (43) was treated with potassium L-mentholate to produce diastereomeric esters (44) that were not isolated but were reacted with LiBr/MeLi (Scheme 15). After chromatography and recrystallization the enantiomerically pure ironacyl complex (5 )-(39a) was obtained. It was suggested that only one diastereomeric ester can react (with inversion of configuration at iron, as shown) with the methyl nucleophile the unreactive diastereomer suffers from severe steric congestion about the electrophilic CO ligand. 

Organometallic substitution of iron-complexed dioxolenes has been reported. The reaction proceeds with net inversion of configuration, the result of a two-step addition-elimination pathway. Dialkyl-cuprates, higher order cyanocuprates and Grignard reagents have all been employed, and sequential dis-... 

Equations 8.22 and 8.23 illustrate the results of some studies designed to elucidate the stereochemistry involved in S02 insertion into iron complexes. In 8.22, the threo Fe complex undergoes insertion to yield the corresponding erythro S-sulfinate with a high degree of stereospecificity. Unlike CO insertion, retention of configuration occurs at the metal center. Equation 8.23 illustrates an experiment designed to correlate stereochemistry of reactant and insertion product. 

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