Iron vinylidenes bonding

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. 

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. 

In some cases, vinylidene complexes undergo [2-t-2] reactions that are characteristic of Fischer and Schrock carbene complexes. However, these [2+2] reactions involving vinylidene complexes can result from nucleophilic addition at the central carbon, rather than a concerted [2+2] process. For example, the reaction of an imine with the iron-vinylidene complex in Equation 13.28 leads to formation the product of a [2+2] reaction between the carbon-nitrogen double bond and the carbon-carbon double bond. ° - This reaction is believed to occur by nucleophilic attack of the nitrogen at the central carbon, followed by ring closure at the p-carbon, instead of a concerted [2+2] process. 

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... 

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]. 

Whereas osmium dithiocarboxylato complexes are very rare, some dithiocar-boxylato iron and ruthenium complexes have been synthesized by substituting into the metal-halogen complex with lithium dithiocarboxylate [77] and by inserting CS2 into the carbon-metal bond [78,79]. The reaction of a vinylidene complex 32 with CS2 and NaOMe has also been reported to undergo a facile loss of HCl, followed by insertion of CS2 to give the dithiocarboxylato complex 33 (Scheme 7) [80]. 

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]. 

This first volume of the Scripts in Inorganic and Organo-metallic Chemistry addresses graduate students in the fields of coordination compounds and organic synthesis. It covers the chemistry and structural aspects of iron-carbon compounds with an iron-carbon double bond. The first part deals with the carbene moiety, the second with vinylidene ligands. 

---