Metal-carbyne complex

As with carbene complexes, metal carbynes display a range of reactivity with electrophiles and nucleophiles. Molecular orbital calculations show that even cationic Fischer carbyne complexes are polarized as M, +=C A neutral Fischer- and Schrock carbyne complexes have an even greater negative charge on Ccarbyne.93 If all reactions between carbyne complexes and other species were charge-controlled, we would predict that nucleophiles would always attack at the metal and electrophiles at Ccarbyne. As we should expect by now, the picture is more complicated in practice. 

H. Fi.scher, C. Troll, and J. Schleu, in Transition Metal Carbyne Complexes, (F. R. Kreissl ed.), p.79. Kluwer Academic Publishers, Dodrecht, 1993. 

The importance of transition metal carbene complexes (compounds with formal M=C bonds) and of transition metal carbyne complexes (compounds with formal M=C bonds) is now well appreciated. Carbene complexes are involved in olefin metathesis (7) and have many applications in organic synthesis (2), while carbyne complexes have similar relevance to... 

The chemistry of transition metal-carbyne complexes is rather less developed than the chemistry of carbene complexes. This is almost certainly because reactions which form new carbyne complexes are relatively rare when compared with those forming metal carbenes. The few theoretical studies of carbyne complexes which are available indicate that close parallels exist between the bonding in carbene and carbyne compounds. These parallels also extend to chemical reactivity, and studies of Group 8 complexes again prove instructive. 

It is interesting to note that the decrease in metal electron density that accompanies the change from five- to six-coordinate geometry does not have a detectable effect on the metal-carbene carbon bond length in these complexes. The metal-carbyne carbon bond in several osmium carbyne... 

Transition metal carbyne complexes are still relatively uncommon as only a few synthetic approaches to these compounds has proved generally applicable. In addition to making the initial characterization (723), the Fischer group has made the largest contribution to carbyne complex chemistry, with some 200 mononuclear complexes of Group 6 and 7 metals having been prepared. 

There are, broadly speaking, three general routes to transition metal carbyne complexes ... 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

Scheme 53 Bicyclization of diynes with group 6 metal-carbyne complexes.

Scheme 54 Cyclization of alkynes with metal-carbyne complexes containing Cr or W.

The same dichotomy of bonding models is also found for carbyne complexes that have a formal triple bond M=CR. There are metal-carbyne bonds that belong to the donor-acceptor type (the Fischer car-... 

Table 7.10 Calculated BDEs De (Do) (kcal/mol) of transition metal carbyne complexes.3...

Following the synthesis of metal carbyne complexes, the first metalladiyne derivative was prepared by treatment of W =C(OEt)C=CPh (CO>5 with BX3 (X = C1, Br, I) (pentane, -45°C) to give rranj-W(=CC=CPh)(X)(CO)4 (334 Scheme 77) in good yields (30-60%). Subsequent reactions with NHMea give W sCCH=CPh(NMc2) (X)(CO)4 by addition to the C=C triple bond, the structure of which indicates a contribution from the vinylidene resonance form. ... 

Mechanistic studies revealed that alkyne metathesis and ring-opening metathesis polymerization of cycloalkynes proceed via metal carbyne complexes,217 218 which is also supported by theoretical studies.219 The polymerization of PhC=CMe with NbCIs or TaCIs yields a polymer that degrades to oligomers as a result of secondary metathesis reaction. A stable polymer, however, may be synthesized with TaCIs and Ph4Sn as a cocatalyst.220... 

Nucleophilic attack at a carbocationic carbon, similar to that observed in metal -carbynes, has been observed from CNBu additions to the //2-t/2-ace-tylide in the complex Fe2(CO)6(C=CPh)PPh2 (427). The ligand dipole, indicated from the X-ray structural determination of the product 96, makes it susceptible to additions of primary amines, generating the carbene 97. 

Alkynes can also undergo total metathesis, with cleavage of all three C=C bonds, catalysed by metal carbyne complexes at room temperature and proceeding through met-allacyclobutadiene intermediates as indicated by the framework in equation 43-6. 

Reactions of this type proceed via metal carbyne complexes (equation 4). The most direct evidence is that such complexes (Mt = Mo, W, Re) can act as initiators for the... 

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>). 

I hope that I have been able to demonstrate, with this small selection of our newest research results, what a variety of reaction possibilities the chemistry of transition metal carbene complexes display. In the following I review an area whose development we have made most recently our special task, namely that of transition metal-carbyne complexes. 

In the realm of all-carbon ligands in the formation of transition-metal complexes, the naked carbon atom holds a special position. Based on the geometry of metal-carbon interaction, these compounds can be divided into four classes terminal carbide (I), 1,3-dimetallaallene (II), C-metalated carbyne (III), and carbido cluster (IV) ... 

It is interesting that, to promote the ring-opening metathesis polymerisation of cycloolefins, metal carbyne complexes can also be used in such a case, the carbyne complex is rearranged to form the actual metal carbene complex [scheme (9)] capable of initiating the polymerisation [95] ... 

It is worth realising in this connection that the chain carrying the carbene can also be formed by a direct reaction of the monomer with the metal carbyne complex, in spite of the fact that complete scission of the triple bond in this complex does not take place [96] ... 

From a mechanistic point of view (Scheme 3), a transition metal carbyne undergoes a [2+2] cydoaddition with an alkyne to form a metallacydobutadiene as an intermediate, or possibly as a transition state. The four-membered ring then cycloreverts, expelling a di-substituted alkyne with regeneration of a catalytically active carbyne complex. In a subsequent... 

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