Iron vinylidenes porphyrins

Recently synthesized iron porphyrin-carbene complexes (136) may be considered as carbon analogues of the oxo-iron(IV) porphyrin (HRP-II and CAT-II) species.476 The one-electron oxidation of [Fe(TPP)(C=CR2)] (R = aryl) by CuCl2 affords the stable complex [Fe(TPP)(C=CR2)] Cl (TPP = wieso-tetra-p-tolyl porphyrin and R = p-ClC6H4) which has a visible spectrum very similar to that of CAT-I. X-Ray analysis of this complex has revealed a structure (137) in which the vinylidene moiety has inserted between the iron atom and one pyrrole ring of the porphyrin and it is suggested that this may constitute a model, i.e. structure (138), for the active site of CAT-I as an alternative to the Fe117 cation radical model.477... 

Chevrier, B., R. Weiss, M.C. Lange, J.C. Chottard, and D. Mansuy (1981). An iron(lll)-porphyrin complex with a vinylidene group inserted into an iron-nitrogen bond Relevance of the structure of the active oxygen complex of catalase. J. Am. Chem. Soc. 103, 2899-2901. 

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-... 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

The photochemistry of several of the iron porphyrin halocarbene complexes Fe(TPP)(=CXY) (CXY = CCF, CBr2, CCIF. CCl(CN) and the vinylidene complex Fe(TPP)=C=CAr2 (Ar = p-C6Fl4Cl) has been studied in degassed benzene... 

Unusual iron-porphyrin vinylidene complexes were obtained from DDT [l,l-bis(4-chlorophenyl)-2,2,2-tricMoroethane] and Fe(tpp) [tpp = meso-tetraphenylporphinato (2-)] in the presence ofa reducing agent [10a, 264]. The derived N,N -vinylene-bridged porphyrin reacts with metal carbonyls [Fe3(CO)i2, Ru3(CO)i2] to break one or both N—C bonds with insertion of the vinylidene into an M—N bond. While the iron complex was formed in 90% yield, the reaction with Ru3(CO)i2 afforded three products, the vinylidene being formed in only 40% yield [265]. 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

Scheme 5. Synthesis and reactions of iron-porphyrin vinylidene complexes.

Olmstead, M.M., R.-J. Cheng, and A.L. Balch (1982). X-ray crystallographic characterization of an iron porphyrin with a vinylidene carbene inserted into an iron-nitrogen bond. Inorg. Chem. 21,4143 148. 

These derivatives (Type B) are at least formally prepared by the insertion of a fragment into a M-N bond to yield a new M-X-N unit. Such species have been suggested as possible intermediates in the insertion of an oxygen atom into a G-H bond by cytochrome P-450. Several of these derivatives have formally had a carbene fragment inserted into a M-N bond. Such derivatives include a Ni derivative and a cobalt(III) species that has undergone two such insertion reactions. Other species represent the formal reaction of a vinylidene with iron(III) (two different crystalline forms). This iron derivative has an intermediate-spin state. Other complexes result from the insertion of a nitrene or an oxene This last derivative can also be considered to be a porphyrin N-oxide derivative and the structure of a free base species of a porphyrin N-oxide has also been reported". Appropriate stereochemical parameters for the members of this class are found in Table IX. 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

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