Carbenes ligands iron porphyrins

The isocyanide ligand, CNR, is formally isolobal with a carbene ligand. Several studies have investigated iron(Ill) porphyrin isocyanide complexes which have the general formula [FeCPorKCNR) ]. However, these studies have mostly been concerned with spin state and spectroscopic properties rather than chemical transformations and will not be discussed in detail here. Crystallographic details are given for two of the complexes. " ... 

Several iron porphyrin carbenes are known1110 that are obtained by the general reaction of the chloro iron(III) complex with RCX3 (X = Cl, Br) in the presence of iron powder or sodium dithionite as in equation (101). The air sensitivity of the product depends on the nature11111112 of R. Formally these complexes could be described as iron(II) species. They are diamagnetic, and can bond an additional axial ligand. The structure of the dichlorocarbene complex [Fe(TPP)(CCl2)(H20)] has been determined by X-ray methods,1113 which showed a short Fe—C distance of 1.83 A. This complex is reactive and, for instance, with primary amines a coordinated... 

The four-coordinate sqnare planar iron(n) porphyrins discussed above are not only of great valne in heme protein model chemistry, but also in chemical applications, since they undergo a wealth of ligand addition reactions. For example it has been shown that TPPFe complexes are active catalysts for important carbon transfer reactions in organic chemistry and are found to catalyze the stereoselective cyclopropanation of aUcenes, olefin formation from diazoalkanes, and the efficient and selective olefination of aldehydes and other carbonyl compounds. The active species in these carbon transfer reactions are presumably iron porphyrin carbene complexes. " It was also found that ferrous hemin anchored to Ti02 thin films reduce organic halides, which can pose serious health problems and are of considerable environmental concern because of their prevalence in groundwater. ... 

A.L. Balch (1981). Reversible migration of an axial carbene ligand into an iron-nitrogen bond of a porphyrin Implications for high oxidation states of heme enzymes and heme catabolism. J. Am. Chem. Soc. 103,4271-4273. 

An Fe oxidation state, apart from those of catalase and peroxidase compounds 1 and II, is attained only in the carbene complexes, RR C—Fe (Por), nitrene complexes,R N—N=Fe (Por), or the dimeric compounds, Fe (Por)=C=Fe (Por), Fe (Por)=N—Fe or) and [Fe (Por)—O—Fe HPor)]". Both carbene and nitrene complexes are diamagnetic, and the former appear to coordinate RNHj, py, Im, ROH, and RS . They lose the axial ligand in the presence of an excess of pyridine, and form Fe"(Por)(py)2. Though the iron oxidation state in these formally Fe (Por) complexes is still ambiguous, their reactivity implies an iron(III) oxidation state. Attempted synthesis of Fe porphyrins by a one-electron oxidation of Fe (TPP)Cl resulted in formation of the corresponding porphyrin n cation radical Fe "(TPP) . The high oxidation state iron porphyrins are of particular interest in relation to the cytochromes P-450, peroxidases and catalases, and Fe" 0(Por)L (L = 1-MeIm, py, Pip) have been spectroscopically characterized (Scheme 18). - "... 

The reversible one-electron oxidation of the vinyhdene complex (TPP)Fe (C = CAr2) induces the carbene ligand to adopt a bridging structure [76,95, 104] giving a complex with an intermediate spin iron(lll) [95]. Further oxidation results in the formation of a NAf -4-vinyhdene-bridged porphyrin [105]. 

The porphyrin ligand can support oxidation states of iron other than II and III. [Fe(I)Por] complexes are obtained by electrochemical or chemical reduction of iron(II) or iron(III) porphyrins. The anionic complexes react with alkyl hahdes to afford alkyl—iron (III) porphyrin complexes. Iron(IV) porphyrins are formally present in the carbene, RR C—Fe(IV)Por p.-carbido, PorFe(IV)—Fe(IV)Por nitrene, RN—Fe(IV)Por and p.-nittido, PorFe(IV)... 

The reductive electrochemistry of iron-carbene porphyrins has been investigated in aprotic solvents [115]. With the vinylidene complex, there is a 2e -I- H reduction of the ligand leading to the formation of the corresponding iron(II) vinyl complex. The energies required to reduce by two electrons the other carbene complexes are quite similar [115]. The dichlorocarbene complex is an exception because the reduction is facihtated by the extreme instability of the one-electron intermediate. Formation of cr-alkyl iron(III) porphyrins has been confirmed by independent synthesis [116]. The a-alkyl iron(III) porphyrins can then be obtained by a one-electron reoxidation re-... 

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