Iron complexes example structures

The composition of I, and possibly its structure, may be deduced by identifying Q. Certain examples from peroxide chemistry will illustrate the scope of the method. The reactions of ferrous(nitriloacetate) and ferrous(ethylenediamine-N,N -diacetate) with hydrogen peroxide are complicated processes.1 A particular scavenger T did indeed divert the reaction at high concentrations of T. The required levels of T were, however, much higher than those that would have been needed to trap the hydroxyl radical, HO. It is thereby ruled out. With this and with spectroscopic evidence, a reactive hypervalent iron complex was suggested as the intermediate. 

In addition to nonheme iron complexes also heme systems are able to catalyze the oxidation of benzene. For example, porphyrin-like phthalocyanine structures were employed to benzene oxidation (see also alkane hydroxylation) [129], Mechanistic investigations of this t3 pe of reactions were carried out amongst others by Nam and coworkers resulting in similar conclusions like in the nonheme case [130], More recently, Sorokin reported a remarkable biological aromatic oxidation, which occurred via formation of benzene oxide and involves an NIH shift. Here, phenol is obtained with a TON of 11 at r.t. with 0.24 mol% of the catalyst. 

In line with these current developments, publishing a book dealing with the most recent achievements in this field is particularly timely. The volume is structured in chapters according to the type of metal complex. In every chapter, a brief introduction on the general chemical properties of the respective class of Fe-complexes will be given. Subsequently, representative examples for different catalytic transformations with a special emphasis on the various reaction manifolds will be presented. This structure implies that the reviews are not comprehensive but are meant to improve the understanding of the catalytic role a certain iron complex plays within the mechanism. 

Perhaps chemists will be able to mimic nature without duplicating the iron-sulfur-molybdenum structure. For example, a zirconium complex with tetramethyl cyclopentadiene can bind dinitrogen in a manner that breaks the NON bond, as shown below. Treatment with hydrogen gas results in formation of small amounts of ammonia. Although the yields are too low to make this a viable commercial process, researchers hope to make the process more efficient through chemical modifications and changes in conditions. 

A further example of pentacoordinate iron complexes is constituted by [FeLCl2] (L = 2,6-diacetylpyridinebis(2,6-diisopropylanil)), the (distorted) square pyramidal structure of which is illustrated in Figure 64.97... 

Aminoboranes have been used as ligands in complexes with transition metals (66) in one instance giving a rare example of two-coordinate, non-/0 transition-metal complexes. The molecular structure of the iron complex Fe[N(Mes)B(Mes)2]2 where Mes = 2,4,6-(CH3)3C5H2 is shown in Figure 1. The less sterically demanding lithium borylamide, LiN(CH3)B(CH3)2, used to prepare mercury and tin complexes, has also been prepared (67). 

Porphyrin. A complex planar structure containing four substituted pyrroles covalently joined in a ring and frequently containing a central metal atom. For example, heme is a porphyrin with a central iron atom. 

The best known example is enterobactin (otherwise called enterochelin), which is produced apparently by all enteric bacteria. It has three 2,3-dihydroxybenzoyl groups attached to a macrocyc-lic lactone derived from three residues of L-serine condensed head-to-tail. The structures of enterobactin and its iron complex are shown in Figure 45, which shows that the iron is bound by six phenolate oxygen atoms in an octahedral environment. Enterobactin has the highest known affinity for Fem, with log K = 52 at pH 7.4.1182 The iron(III) complex can exist as isomeric forms, which may be associated with selectivity in binding to the receptor site. 

Clearly, while porphyrin complexes are obvious candidates for modelling these kinds of biomimetic oxidations, a range of non-heme iron complexes based on macrocyclic and podand ligand have also proved to be successful structural and functional mimics.19 To take one example, Figure 12.13 shows the X-ray structure of the iron (IV) tetramethylcyclam (tmc) oxo complex [Felv(tmc)(0)(MeCN)]2+... 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

Suspected Si H—M interactions were also discussed in connection with the mononuclear complexes HReCp(CO)2(SiPh3) 161), HMnCp(CO)2(SiPh3) 161,162> and HFeCp(CO)2(SiF2Me)2 163>. From an analysis of known or estimated Si — H distances, it was concluded that Si — H interactions were most likely absent in the rhenium and iron complexes. In the case of HMnCp(CO)2(SiPh3), it was originally believed that a true example of Mn—H Si interaction existed162), but a subsequent re-assessment of the problem indicates that the structural evidence is, at best, inconclusive 161.163). 

The main difference between mononuclear complexes containing either a M—H—C or a M—H—Si three-center bond is that most tj2-CH complexes correspond to an earlier stage of the addition reaction than do the 7j2-SiH complexes 7(CMH) coupling constants are usually closer to the values for /(OH), while /(SiMH) values are closer to 2/(SiMH), and the relative lengthening of the C—H distance on 172 coordination is usually smaller than that of coordinated Si—H bonds. For example, in the representative iron complex 21 [the structure of which was determined by neutron diffraction analysis (74)], the coordinated C—H bond... 

The high sensitivity of the g-values of low-spin iron(III) to structural variations and their large anisotropy imply that the prediction of the EPR spectra must be based on highly accurate structures12071. The MM-AOM method for low-spin iron(III) complexes was tested on a number of examples involving bi-, tri- and hexadentate ligands with amine and pyridyl donor sets (Table 10.8). 

Acid dyes include metal-complexed azo structures, where the metals used are cobalt, chromium, and iron.10 Examples are 1 1 and 2 3 chromium complexes and 1 2 cobalt complexes, where the numbers employed represent the ratio of metal atoms to dye molecules. Metal-complexed dyes can be formed inside textile fibers by treating suitably dyed fibers with a solution containing metal ions.11 In this case, the metal-free forms of these azo dyes are known as mordant dyes and contain mainly ortho, ortho -bis-hydroxy or ortho-carboxy, ortho -hydroxy groups (e.g., C.I. Mordant Black 11, Mordant Yellow 8, and Mordant Orange 6). When the metal complexes are formed prior to the dye application process, the resultant dyes are known as... 

Some analogous rathenium- and osmium-bismuth clusters have been found. Examples include Bi2M3(CO)9 and H3BiM3(CO)9 (M = Ru, Os). The stmctures of the hydride compounds have both been determined and they are isostractural with the iron complexes as is Bi2Ru3(CO)9 withBi2Fe3(CO)9. The structure 0fBi2Os3(CO)9, on the other hand, has not been determined and its IR spectrum indicates that it probably has a different structure. A spirocyclic cluster [Ru2(CO)8(/X4-Bi)Ru3(CO)io(/x-Ft)] (39) has been reported. 

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