Intermediate, ethylidene species

Analysis of the tunneling spectra of the hydrocarbons formed by exposing the samples to H2 ( Fig. 14 ) showed two different species, one a formate like ion, the other an ethylidene ( CHCH3 ) species. The formate ion is not thought to be an active intermediate in hydrocarbon synthesis, but the ethylidene species may well be a catalytic intermediate. Kroeker, Kaska and Hansma were then able to suggest a reaction pathway for the hydrogenation of CO on a supported rhodium catalyst consistent with the formation of ethylidene as a catalytic intermediate. 

In contrast, it is straightforward to rationalize the observed stereochemical retention if degenerate propylene metathesis occurs via a metal-ethylidene species (Scheme 10.17). In this case, the intermediate metallacycles are a,a disubstituted. Although the P-position is unsubstituted, the experiments with 2-butene (Scheme 10.13) have established that a,a -substituents prefer to be oriented in a cis-configuration. This preference explains why there is any stereoselectivity in this degenerate process and also correctly predicts the stereochemistry of the propylene-tfs that was generated. 

If cyclopentene would react pair-wise with 2-pentene, only one product would form, namely 2,7-decadiene, and a similar result for cyclodimers etc. of cyclopentene. If somehow, the alkylidene species would be transferred one by one, we would obtain a mixture of 2,7-nonadiene, 2,7-decadiene, and 2,7-undecadiene in a 1 2 1 ratio. The latter turned out to be the case, which led the authors to propose the participation of metal-carbene (metal alkylidene) intermediates [6], Via these intermediates the alkylidene parts of the alkenes are transferred one by one to an alkene. The mechanism is depicted in Figure 16.4. In the first step the reaction of two alkylidene precursors (ethylidene -bottom- and propylidene -top) with cyclopentene is shown. In the second step the orientation of the next 2-pentene determines whether nonadiene, decadiene or undecadiene is formed. It is clear that this leads to a statistical mixture, all rates being exactly equal, which need not be the case. Sometimes the results are indeed not the statistical mixture as some combinations of metal carbene complex and reacting alkene may be preferred, but it is still believed that a metal-carbene mechanism is involved. Deuterium labelling of alkenes by Gmbbs instead of differently substituted alkenes led to the same result as the experiments with the use of 2-pentene [7],... 

It is the di-cr species on Pt(lll) which is converted at higher temperature into ethylidyne. This conversion had also been investigated by infrared-visible sum-frequency generation (SFG) by Cremer et al. (371), a welcome first application of this new spectroscopic technique to hydrocarbon adsorption chemistry. They observed an absorption characteristic of an intermediate with a nCH3 band at 2957 cm-1 and suggested that this arises from an ethylidene M2CHCH3 (or possibly ethyl) species with its C-CH3 axis at an angle to the surface. It is very clear experimentally, as discussed in Part I, that ethylidyne on Pt(lll) is formed from di-cr-ethene and not directly from its 77-bonded isomer. 

Pt(lll) system by STM (393, 394) are responsible for the stability of these intermediates. Recent SFG spectra (359, 371, 372) indicate that one such intermediate could be ethylidene, CHCH3, although some authors have preferred a CHCH2 species bonded to the surface through each carbon atom (386). It is not clear how the latter can be described as di-cr tri-cr would seem to be more appropriate. This and the ethylidene intermediates could occur in sequence (Section X.D). 

Di-cr-ethene has also been separately detected by spectroscopy but found to play a different mechanistic role related to its stronger bonding to the metal surface. This, not the 77-species, has been shown to be the precursor in the formation of ethylidyne by surface dehydrogenation on Pt(lll) and probably on other facets with triangular metal sites. Ethylidene is now the favored intermediate in this reaction (359,371,372,427), which can formally be represented as... 

Figure 14. Tunneling spectra of a sample with finely dispersed Rh particles on alumina, exposed to a saturation coverage of CO, and heated to various temperatures in a high pressure atmosphere of Ht. The CO is hydrogenated on the surface. Analysis of the resultant spectra using isotopic substitution indicates that an intermediate species, ethylidene di-rhodium, is formed (1).

It is noteworthy that both experimental and theoretical studies suggest that only one a C—M bond can be formed and that C=M bonds do not exist species such as ethylidene (5) and ethylidyne (8) therefore require respectively two and three metal atoms this is in keeping with the principle of maximum utilisation of surface free valencies. aa-Diadsorbed species such as ethylidene have frequently been postulated as intermediates in reaction of alkanes (see e.g. Chapter 6). Group theoretic arguments indicated that a C—M double bond if formed would have a and n components. 

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