Fischer-Tropsch carbenes

A variety of attempts has been made to model the single steps of the Fischer Tropsch reaction on a molecular level. Naturally, the question of the catalytic activity of intermediate carbene and carbyne complexes has been pursued [4],... 

The mechanisms proposed over the last 50 years for the Fischer-Tropsch synthesis, principally on the basis of studies using heterogeneous catalyst systems, may be divided into three main classes (a) metal-carbide mechanisms (b) hydroxyl carbene, =CH(OH), condensation mechanisms and (c) CO insertion mechanisms. 

If the primary carbene proposed as an intermediate in the Fischer-Tropsch synthesis may be viewed as nucleophilic, then an alternative mode of reaction could be attack at the carbonyl carbon of an adjacent carbonyl group to give a C2 unit of type 36 ... 

The existence of such a growth step is consistent with the high proportion of C2 products found in the Ir4(CO)12/NaCl-2AlCl3 system (59). Furthermore, in systems where dimerization is less favored, hydrogenation of the primary carbene fragment could explain the considerable amounts of methane formed in many heterogeneous Fischer-Tropsch systems. 

First law of thermodynamics, 24 645-648 First limiting amino acid, 2 601 First-order irreversible chemical kinetics, 25 286-287, 292-293 First-principle approach, in particle size measurement, 13 153 First sale doctrine, 7 793 Fischer, Emil, 16 768 Fischer carbene reaction, 24 35-36 Fischer esterification, 10 499 Fischer formula, 4 697 Fischer-Indole synthesis, 9 288 Fischer lock and key hypothesis, 24 38 Fischer-Tropsch (FT) synthesis, 6 791, 827 12 431... 

In the case of ethane, this mechanism cannot occur since the resulting metal-ethyl intermediate does not display any alkyl group in the P-position. Consequently, with tantalum hydride(s), 3, which cleave ethane, another process must take place, involving only one carbon atom at a time. Among various reasonable possibilities, we assume a carbene deinsertion from a tantalum-ethyl species because the reverse step is known in organometallic chemistry (Scheme 3.4) [22]. Note that this reverse step has been postulated as the key step in Fischer-Tropsch synthesis [23]. 

A new mechanism to interpret alkene formation in Fischer-Tropsch synthesis has been presented 499-501 There is a general agreement that hydrocarbon formation proceeds according to the modified carbene mechanism. Specifically, CO decomposes to form surface carbide and then undergoes hydrogenation to form surface methine (=CH), methylene (=CH2), methyl and, finally, methane. Linear hydrocarbons are formed in a stepwise polymerization of methylene species. When chain growth is terminated by p-hydride elimination [Eq. (3.61)], 1-alkenes may be formed,502 which is also called the alkyl mechanism ... 

The bridging caxbenes of the bimetallic complex, which parallel the surface carbenes of the Fischer-Tropsch catalyst, are involved in C—C bond formation. 

Unusual bridging (//-cyclopropyIidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis248. The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(/(-Co)(//-C,H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and mww-vinylidene complexes [CpCoFe](//-CO)(//-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent /z-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

The rapid development of the chemistry of transition metal complexes containing terminal carbene (A) or carbyne (B) ligands (7) has been followed more recently by much research centered on bridged methylene compounds (C) (2). The importance of /t-methylidyne complexes, whether in recently established binuclear examples (D), the well-known trinuclear derivatives (E), or the unusual complexes (F), has also become apparent. All are based on one-carbon (C,) fragments, and considerable interest is centered on their possible significance as models for intermediates in surface-catalyzed reactions between carbon monoxide and hydrogen (Fischer -Tropsch reactions) and related processes. These topics have been extensively ... 

Complexes of type [LyM (CH2)IIX ], where n > 1, have been shown to be useful precursors for hetero- and homobimetallic n(a.,a>) alkanediyl complexes, [LjcM(CH2)bM L, ] (where ML, is not necessarily the same as M Lj,). Such hydrocarbon-bridged binuclear compounds have been proposed as models for intermediates in the Fischer-Tropsch reaction (18,19) and other significant catalytic processes (20-23). Some [LyM (CH2)BX ] complexes are precursors to cyclic carbene complexes (Section III), whereas others have been shown to have synthetic utility in organic chemistry (24). 

The transition metal chemistry of y-carbenes (14) is of interest because of the possible involvement of such species in Fischer-Tropsch synthesis (15) and alkene metathesis (16,17). However, apart from y-CH2 and one example of y-CHMe, simple hydrocarbon species have, until the work described here, been generally unavailable and the reactivity of y-carbenes is effectively unexplored. An opportunity was therefore presented for such study, in which carbon-carbon bond formation has taken precedence. 

The ability of a p-carbene to react with an unsaturated hydrocarbon and form an enlarged dimetallocycle encourages speculation over their role in such processes as alkene metathesis and Fischer-Tropsch synthesis. In Scheme 6 a possible mechanism for metathesis initiated by a p-carbene is presented, owing much to other workers (T7,22). Reactions of p-carbenes with alkenes are under investigation in our laboratory. Recently Pettit has observed that the p-methylene complex [Fe2(C0)8(p-CH2)] generates propene when subjected to a pressure of ethene and has also suggested the intermediacy of a three-carbon dimetallocycle (23). 

In a recent study, R. Pettit et at. examined the validity of tire Fischer-Tropsch carbide mechanism, the Anderson-Emmett hydroxy carbene mechanism and the Pichlcr-Schulz mediaiiism [174. In a first experiment, the Schulz Flory distribution obtained by CO/H conversion over a cobalt catalyst in the absence and in the presence of CH N] was studied. It was found that addition of CHjN resulted in a signillcant increase of the propagation rate which is in favour of the assumption of methylene as a building block, as predicted by the carbide mechanism. Furthermore, the reaction was carried out using labeled CO (90% CO and 10% CO), H2. and CHjNj in variable ratios. The number of atoms in the propenc fraction was calculated according to the three... 

The formation of ethylene by an alkylidene carbene rearrangement,as observed by Caulton [47], might also be a very significant pathway in Fischer-Tropsch-type reactions under homogeneous conditions. 

The next propagation step involves propylene coordination to the surface carbene with formation of n-butene reaction (3) or isobutene reaction (4). The high selectivity for n-olefin, in our experiments as well as in conventional Fischer-Tropsch catalysts, must be accounted for by a selective coordination and (or) reaction of the olefin (21) according to reaction (3) which is probably due to the electrophilic character of the surface... 

In conclusion, although our results do not rule out the mechanism of carbene insertion into a metal-alkyl bond (2b), the possibility of making C - C bonds in Fischer-Tropsch via a car-bene-olefin mechanism should be considered as an alternative path. Further studies are in progress to decide between both types of mechanisms. 

The nonspecialist reader of this section may come to the conclusion that the area of transition-metal complexes with alkyldiazenido ligands is not very large. This impression is reinforced by the fact that these complexes lose the diazo group relatively easily, as mentioned in the beginning of the section. It is wrong, however, if one considers the fact that diazoalkanes have been used, and still are, extensively in the preparation of carbenes via metastable diazenido intermediates of metal-complex catalysts in laboratory scale syntheses, in the Fischer-Tropsch reaction, and in olefin metathesis (see, e.g., Herrmann, 1978 Doyle 1986a, 1986b and Sects. 8.7 and 8.8 of this book). 

In principle also the possibility should be considered that CO is dissociated, as in Fischer-Tropsch synthesis of hydrocarbons, and thereafter partially hydrogenated. A metal carbene would then produce, upon addition of H2O, a molecule of methanol. However, one would expect that with a metal like Rh, which can dissociate CO, the isotopically labelled atoms from C 0 would be scrambled with C 0 atoms in the methanol product but this has not been found.With metals which dissociate CO even more reluctantly, like Pd and Cu, this mechanism is even less likely than with Rh. This does not however exclude the possibility that higher alcohols can be formed by H2O addition to a carbene-like intermediate. 

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