Metal-carbyne complexes Schrock

S, F. Vyboishchikov and G, Frenking,/. Am. Chem. Soc., submitted for publication. Structure and Bonding of Low-Valent (Fischer-Type) and High-Valent (Schrock-Type) Transition Metal Carbyne Complexes,... 

Metal carbyne complexes MsCR also have Fischer and Schrock extreme bonding formulations, although the distinction i less marked than for carbenes. In one bonding model, the free carbyne can be considered as doublet for Fischer and quartet for Schrock forms (Fig. A doublet carbene is a 2e donor via its... 

Several routes are known to Schrock s Mo- and W-alkylidene complexes that catalyze the metathesis of alkenes, including those starting from metal-carbyne complexes (see section 3.3). A practical one was discovered starting from Na2Mo04 and produced a large family of catalysts including enantiopure ones ... 

Metal-carbyne complexes M CR are less known than metal carbenes. The carbyne can also be nucleophilic (Schrock type) or electrophilic (Fischer type). Fischer-type metal-carbynes are obtained by reaction of BF3 on a neutral Fischer-type metal-carbene complex, whereas Sehroek earbynes are often obtained by deshydrohalogenation of a Schrock-type metal-carbene eomplex. They catalyze alkyne metathesis and, in particular, give heterocycles with unsaturated substrates. 

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... 

P. Kim and R. J. Angelici, Transition Metal Complexes with Terminal Carbyne Ligands, in Adv. Organomet. Chem., 1987, 27, 51 H. Fischer, P. Hoffmann, F. R. Kreissl, R. R. Schrock, U. Schubert, and K. Weiss, Carbyne Complexes, VCH, Weinheim, Germany, 1988. 

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)... 

Carbyne ligands can also be grouped into two classes Fischer and Schrock carbynes. The metal-carbyne carbon triple bond (Fig. 3) for a Fischer carbyne complex is formed... 

The reactions of Fischer and Schrock carbyne complexes are of interest because they may act as intermediates in chemical synthesis. Typical reactivity of carbyne complexes with nucleophiles (i.e., alkyl lithium reagents and metal(I) alkoxides) are consistent with the electronics and molecular orbital calculations for these types of complexes [Eq. (9) 22]. The nucleophile adds to the carbyne-carbon, resulting in the formation of a carbene complex. The reaction of a Grignard reagent with a carbyne complex is expected to demonstrate similar reactivity. 

Complexes containing metal-carbon triple bonds in which the metal is in a relatively high formal oxidation state are frequently designated alkylidynes. Carbyne complexes are also sometimes classified as Fischer-type or Schrock-type, as in the case of carbene complexes. The distinction between these is discussed in Chapter 10. 

There are several parallels between the chemistries of the carbene and carbyne ligands. The classification of carbyne complexes into two major structural types—Fischer and Schrock—is perhaps the most obvious of these parallels. Complex 64 represents the prototypical Fischer carbyne complex Ccarbyne bonded to a low-valent metal with Ji-accepting CO ligands attached. Structure 65, on the other hand, is a classic example of a Schrock carbyne complex because a high-valent metal is present with electron-donating ligands attached. Atoms attached to Ccaibyne helpful in distinguishing between Fischer and Schrock carbene complexes (i.e., heteroatoms for the former and H and C for the latter), are less important in the case of carbyne complexes. It is convenient to classify carbyne complexes... 

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. 

Reaction of the Mo Schrock carbyne complex in equation 10.58 with HBF4 results in protonation at Ccaibyne, followed by rearrangement to the thermodynamically more stable Mo-H complex. The BF4 ion is such a weakly coordinating ligand that substitution at the metal does not occur.98... 

There are two types of transition metal carbene and carbyne complexes low-valent (so-called Fischer type)i" i" and high-valent (so-called Schrock type). The two classes of compounds are quite different in their chemical behavior. Such different chemical reactivity is sometimes rationalized on the basis that the metal-carbene and metal-carbyne bonds in Fischer-type complexes have donor-acceptor character, whereas the bonding in Schrock-type complexes is more typical for a normal multiple bond. 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

To Fischer carbenes correspond Fischer carbynes that are 18-electron complexes of electrophilic carbyne, and to Schrock carbenes correspond Schrock carbynes that are d° high-oxidation-state early transition-metal complexes of nucleophilic carbyne, often with a less-than-18-electron count. This latter category is important in alkyne metathesis, and all carbyne complexes show a rich chemistry. 

The carbyne ligand CR can be considered either as an LX ligand, or as an X3 ligand. In Fischer carbynes, the carbyne ligand is counted LX whereas in Schrock s carbynes, the carbyne ligand should be viewed as X3. This distinction follows the same logic as for metal-carbene complexes, i.e. is derived from the electronic strncture of the free carbyne ... 

Alkylidene and alkylidyne complexes are also referred to as carbene and carbyne complexes. Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex 2.37, are called Fischer carbenes. In contrast, complex 2.38, wha e the metal ion is in a high oxidation state, is referred to as a Schrock carbene. 

An unusual reaction of W111 with nitriles has been reported by Schrock et al. to provide metal a nitrido tungsten complex [46], Treatment of 98 with acetonitrile or benzonitrile provides the tungsten carbyne 99, along with the nitrido complex 100 (Eq. (37)). Additional studies resulted in the identification of reaction conditions that... 

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