Metal-alkylidynes

W.L. Gladfelter, University of Minnesota What is the possibility of forming carbon-carbon bonds using two metal alkylidyne fragments ... 

F.G.A. Stone With increasing frequency, as our work in this area develops, we are observing reactions in which the metal alkylidyne fragments link to form C-C bonds Please refer to compounds (39) and (40), and the relevant references to these species. 

A corresponding Lewis-like picture can be developed for triply bonded metal alkylidynes (H MCH), such as the duodectet-rule-conforming examples... 

Figure 4.16 Optimized structures of transition-metal alkylidynes H MCH (M = W-Ir).

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

Since the Schrock carbyne is active in the transalkylidynation reaction, the possibility of ring-opening polymerisation of cycloalkynes by acetylene transalkylidynation catalysts (metal alkylidyne complexes) has been evaluated [151]. Unfortunately, cyclooctyne is one of the few relatively stable strained cyclic acetylenes, but it is not strained enough to react selectively with the catalyst and yield a polymer according to the scheme... 

Bocarsly and Mayr first reported the luminescence properties of transition metal alkylidyne see Alkylidyne) complexes [XW(CO)2(L2)(CPh)] (X = hahdes, L2 = TMEDA, (py)2 or dppe). The complexes exhibited broad and stmctureless emission bands at ca. 625-640mn, which was attributable to an excited state of MLCT (dxy(W) - 7r (W=C)) character. With an extension of the alkylidyne unit, the complexes [XW(CO)2 (L2) (C-C6H4-/ -(C=C-C6H4-/ -) -N=C)] (L2 = TMEDA, dppe n = 0, 1) containing an isocyanide moiety have been shown to possess rich luminescence properties. Perturbation of the absorption and emission properties of the complexes by coordination of the isocyanide moiety to metal-containing fragments such as/ac-ReCl(CO)3, cw-PdCb, trans-Vdh, and trans-Vth was also studied. 

Metal alkylidyne fragments are frequently invoked as intermediates in the transformation of hydrocarbons on metal surfaces. These species are usually formulated as triply bridging alkylidynes however, terminal surface alkylidynes may be considered as reactive surface intermediates (30). Evidence for metal carbyne intermediates on Pt—Co bimetallic surfaces was found in a study of the isomerization and hydrogenolysis of alkanes (3]). 

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)... 

The metathesis of alkynes by metal alkylidyne complexes is now a well-established process. The reaction is useful for the catalytic metathesis of alkynes as well as for the synthesis of new metal alkylidyne complexes. The general electronic and steric factors favoring the metathesis reaction were discussed in recent reviews by Schrock (6,7). 

In several reactions reported in the literature metal alkylidynes may have been involved, even though there is currently no direct evidence available. 

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. 

Alkynes insert into both metal-alkylidene and metal-alkylidyne bonds, as shown in reactions (s) and (t), respectively ... 

FIGURE 13. Dimetallacyclopropene complexes obtained in the addition of gold(I) fragments to metal alkylidyne complexes... 

Scheme 37. Metal-alkylidyne/phosphaalkyne metathesis (R = C6H40Me-4 R = Bu, L = P(OMe)3).

Scheme 38. Proposed metal-alkylidyne/carbon disulfide cycloaddition step (L = CO, PPh3).

The metal-alkylidyne complex TpW( = CMe)(CO)2 reacts with Cp2Mo2(CO)4 or Mo(CO)3(NCMe)3 to afford the trimetal complexes Cp2TpMo2W(/t3-CMe)(CO)6 and Tp2MoW2(ia-CMe)2(ia-CO)2(CO)4, respectively (Scheme 75)." In the latter complex, one of the terminal carbonyl ligands of each tungsten centre is f/ -bound to the central molybdenum atom. 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

Related C-H activations by 1,2-additions across metal-ligand bonds have also been reported with unusual group 4 alkylidyne complexes. - As shown in Equation 6.58, a titanium(IV) trimethylsilylmethyl benzylidene complex eliminates tetramethylsilane to generate a Ti(IV) alkylidyne complex. This complex then reacts with benzene to add the arene C-H bond across the metal-alkylidyne unit to form an aryl benzylidene complex. 

Conjugated Complexes and Polymers Derived from Metal-Alkylidyne Building Blocks... 

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