Alkylidynes reactivity

A.J. Carty, Guelph-Waterloo Centre What are the frequencies of the metal-carbon (v(M=C)) stretching frequencies in the tungsten alkylidyne compounds One might expect these to be quite high in view of the x-ray data showing short M=C bonds and evidence of multiple bond reactivity. 

The difference in reactivity of metal clusters and metal surfaces has also been well illustrated in these iridium-based systems [205]. A lack of reactivity of alkyli-dyne species on Ir4/y-Al203 with H2 is observed meanwhile, the chemisorption of H2 is not hindered. This behavior contrasts with that of metallic surfaces, which allow the reaction between alkylidyne species and H 2. It is inferred that over metallic clusters the reaction of H2 with alkyklidyne is not allowed because of the lack of available adjacent metal sites, which are necessary for the formation of the intermediates [205]. 

At the same time, Filrstner used tungsten alkyUdene complex 150 developed by Schrock for ring-closing alkyne metathesis. He compared the reactivities of tungsten alkylidyne complex 150 and Mo(CO)6-p-ClC6H40H (Table 6.4) and showed that both catalysts work well, although a higher reaction temperature is required in the case of Mo(CO)6-p-chlorophenol. 

At the same time, Fiirstner and others used Schrock s tungsten alkylidyne complex 120 for ring-closing alkyne metathesis. They compared 120 with the Mo(CO)6-/>-ClC6H40H system (Table 4) in reactivity and found that... 

The organic reactivity of the alkylidyne complexes has been extensively studied especially with regard to alkene and alkyne metathesis reactions.353,356... 

It is satisfactory to note, for example, that the quoted temperatures for particular reactions involving C2 or C4 species are usually closely similar within the typical estimated uncertainties of 30 K. This implies that the reactivity on a particular metal depends principally on the functional group attaching the hydrocarbon species to the surface, as in M3CCH2R for the alkylidynes or MCH2R for the surface alkyls (R = alkyl). In the case of alkylidyne decomposition, it should be noted that different reactions are involved for the C2 and C4 species. 

It is apparent that Pt is rather generally the least reactive of the group VIII (IUPAC 8-10) metals, as its reaction-completion temperatures are substantially the highest. Only the temperature of alkylidyne formation is an exception to this generalization, where Pt is not notably different from the other metals. 

It has already been seen that the reactivity of alkylidene and alkylidyne complexes may be dictated by the nature of both the metal and the carbon substitutents. These principles apply equally to alkene coordination (Figure 6.8). Two metal-ligand fragments serve to illustrate these principles in the activation of alkenes to electrophilic and nucleophilic... 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

The reactivity of alkylidyne compounds has also been widely studied, particularly by Seyferth and co-workers who have carried out a great variety of reactions with cobalt complexes (303-312). Other groups... 

Interconversion of alkyl, alkylidene, alkylidyne, and carbide moieties has now been demonstrated in a variety of cluster systems. This work provides perhaps the most complete picture of the fundamental reactivity of the prototypical organometallic cluster. 

There are a few examples of coordination of an alkylidyne as a bridging ligand between two metals. Most of these concern aUcylidynes with r-donor substituents, such as OMe or NMe2. The reactivity is similar to that of the /u.3-CX ligand. One unusual reaction is the rearrangement of HRu3(/u- ) -COMe)(CO)io to HRu3(/u - )2-0=CMe)(CO)io under photolytic conditions (equation 23). 

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]). 

The tungsten alkylidyne complexes 141 are reactive toward borane reagents Treatment with BH3 affords the dinuclear complexes 142 [Eq. (128)], and reaction with borabicyclononane gives complex 143 [Eq. (129)]. Complex 143 contains both fragments of the hydroboration reagent attached to the carbyne carbon. 

The generation and interconversion of hydrocarbon fragments on metal surfaces is an important aspect of transition metal catalysis. In an effort to model and understand these transformations, much attention has been focused on the synthesis and reactivity of organic species coordinated at polynuclear transition metal centers. Organodiruthenium complexes have provided a particularly rich area of study. The availability of a variety of organometallic derivatives of the bis(T) -cyclopentadienyl)diruthenium carbonyl system has allowed extensive examination of the reactivity of bridging alkylidene, alkylidyne, and ethenylidene ligands. 

The aesthetically pleasing silica-supported alkyl, alkylidene, alkylidyne Re complex =SiORe[=C u][=CH Bu][CH2 Bu] (Scheme 3, Structures 15-17), prepared from the molecular precursor, has also been found to be highly reactive for the metathesis of propene [13]. Moreover, the evolution of roughly one equivalent of a 1 3 mixture of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene is consistent with a cross-metathesis of the neopentylidene ligand of 1 and propene. The turnover obtained also exceeds those obtained with classical heterogeneous catalysts such as W03/Si02 used industrially for decades (Lummus process). 

The reactivity of the alkylidyne-bridged binuclear species [(RCH2)2Ta]2(/i2-CR)2 ((22) R = SiMe3, (Scheme 4) towards a variety of 2,6-disubstituted phenols and mono- and di-substituted 1-napthyl phenols has been examined.66,67 For example, reaction with 2 eq ArOH = 2,6-But2-CgH3OH yielded the asymmetric 1,1-disubstituted species (RCH2)2Ta(/i2-CR)2Ta(OAr)2 (37). Silica-supported (22) catalyzed the hydrogenation of benzene and substituted benzenes.68... 

Treatment of (22) with CbH = carbazole (38) in hydrocarbon solvents gave [Cb2M]2(/x2-CR)2 ((39) Scheme 4). The reactivity of this species towards hydrogenation (M = Ta) and to organic isocyanides was studied.69,70 Reaction of (39) with 1 eq EtC=CEt produced the di-metallacyclic compound (40) by alkyne insertion into one of the bridging alkylidyne groups.71,72... 

Studies concerning the behavior of the alkylidyne groups supported by the trinuclear [Cp Ti(0)]3 unit as a molecular model for the interactions of hydrocarbons with metal-oxide surfaces have revealed unprecedented chemical reactivity. Firstly, reactions where the Ti303 core is maintained without participation in the processes have been observed. 

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