Alkylidyne metal complexes, reactions

The chemistry of CR fragments ligating metal centers has been a topic of considerable interest since the discovery of the first alkylidyne-metal complexes in E.O. Fischer s Laboratory in 1973 (1). These ligands have been implicated in Fischer-Tropsch reactions (2) and in alkyne metathesis (5). Moreover, the isolation of stable compounds containing carbon-metal triple bonds completed the matrix of bond types represented here ... 

Several types of dimetal product have been obtained by this method. Thus, treatment of equimolar mixtures of the (alkynyl-alkylidyne)metal complexes 4g or 4h and the alkylidyne(carbaborane)metal salts la or lb, with HBF4-Et20 in the combinations shown in Scheme 7, yields the compounds 45 (44). In these products, the icosahedral cage from the precursors 1 adopts the form familiar from protonation reactions described... 

Since this article is concerned with protonation reactions, some mention of earlier related work on the more classical alkylidyne-metal complexes is appropriate. The proton has proved a popular experimental probe for examining the reactivity of alkylidyne-metal complexes in studies attempting to establish the nature of the M=C bonding interaction. Calculations by Kosti6 and Fenske (20) on Fischer-type alkylidynes suggest that the metal-carbon bond is generally polarized —C ", but that the... 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

The metal complexes in Figure 8.20 have been suggested as models for understanding the metal catalysis of alkylidyne chain lengthening and metathesis (Bino et al., 2005). Consistent with this hypothesis, the redox reaction ... 

Metal alkylidyne complexes undergo a variety of oxidation and reduction reactions as well as redox-induced transformations of the alkylidyne ligands. A method for the direct transformation of Fischer-type carbyne complexes into Schrock-type alkylidyne complexes was developed in our laboratory. Bromine oxidation of the /ra/7, -carbyne bromo tetracarbonyl complexes 49 of molybdenum and tungsten in the presence of dimethox-yethane affords the dme-stabilized alkylidyne tribromo metal complexes 50 [Eq. (42)] (81). For alkyl-substituted complexes (R = Me, CH2CMe3)... 

H NMR shows that the alkyne ligand in 242 (R = H) rotates about the metal-alkyne bond axis with an activation barrier of only 50 kJ/mol. The structure of the diphenylacetylene complex 242 was determined by X-ray crystallography. The reaction shown in Eq. (183) was expected to generate alkylidyne alkyne complexes such as W(CPh)CI(PhC2R)(CO)-(PMe3)2. Such species are probably intermediates in the reaction however, a n conflict between the adjacent C=C and M=C triple bond systems... 

The chemistry of metal-carbon triple bonds has developed considerably during the late 1980s. The synthetic basis was broadened, the utility of high-valent metal alkylidynes in metathesis reactions was further developed and refined, and the potential of low-valent carbyne complexes for applications in organic synthesis has become more apparent. The discovery of novel iridium alkylidyne complexes indicates that the full range of metal-carbon triple bonds is not yet known. We can therefore expect that future work in this area of organometallic chemistry will lead to new discoveries with fundamental implications and practical applications. 

Treatment of the alkylidyne(cyclopentadienyl)metal species 4 with halo-acids (HX X = Q, Br, I) has been described in Section I. The carbene- or acyl-ligated metal complexes so formed are stabilized by coordination to the metal of the halide ion intrinsically involved in the protonation reaction, thus highlighting the important difference between employing HX and HBF4 Et20 in the protonations. 

On the other hand, neutral Os alkylidyne 82 undergoes reaction with methanol to give carbene complex 83 (equation 10.56).96 It would appear that 82 undergoes reaction with nucleophilic methanol at 0 first, which is followed by proton transfer to Os. Such reactivity would be consistent with that associated with Fischer carbyne complexes, yet the metal center is more electron-rich than the group 6 metal complex reactant in equation 10.55. 

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