Carbene and carbyne complexes

In Section 24.7, we introduced carbene and carbyne complexes when we discussed a-hydrogen abstraction. Equations 24.45 and 24.46 exempUfied methods of preparation. Carbenes can also be made by nucleophiUc attack on a carbonyl C atom followed by alkylation (eq. 24.KX)). 

Compounds of the type formed in reactions such as 24.KX) are called Fischer-type carbenes. They possess a low oxidation state metal, a heteroatom (O in this example) and an electrophilic carbene centre (i.e. subject to attack by nucleophiles, e.g. reaction 24.101). Resmiance pair 24.61 gives a IxMiding description for a Fischer-type carbene complex. 

In contrast, Schrock-type carbenes are made by reactions such as 24.45, contain an early tZ-block metal in a high oxidation state, and show nucleophilic character (i.e. susceptible to attack by electrophiles, e.g. reaction 24.102). Resonance pair 24.62 describes a Schrock-type carbene complex. 

The M-Cearbene bonds in both Fischer- and Schrock-type complexes are longer than typical M—Cco(term) bonds, but shorter than typical M—C single bonds, e.g. 

Cr—Cco = 188 pm. This implies some degree of d-p) tt-character as indicated by resonance structures 24.61 and 24.62. The 7r-system can be extended to the heteroatom in the Fischer-type system as shown in diagram 24.63. 

Iron tricarbonyl complexes of 1,3-dienes (e.g. cyclohexa-1,3-diene) play an important role in organic synthesis the complexes are stable under a variety of reaction conditions, and iron carbonyls are inexpensive. The Fe(CO)3 group acts as a protecting group for the diene functionality (e.g. against additions to the C=C bonds), allowing reactions to be carried out on other parts of the organic molecule as illustrated by reaction 24.99. 

The presence of the Fe(CO)3 group also permits reactions with nucleophiles to be carried out at the diene functionality with control of the stereochemistry, the nucleophile being able to attack only on the side of the coordinated diene remote from the metal centre. The organic ligand can be removed in the final step of the reaction.  

Although commercial applications are still future goals, much progress has been made in the design of potential molecular wires. Molecules so far studied have included organic molecules with conjugated alkyne functionalities, porphyrins connected by alkyne units, chains of connected thiophenes, and a number of organometallic complexes. Examples of the latter are shown below  

Anderson (1999) Chemical Communications, p. 2323 - Building molecular wires from the colours of life Conjugated porphyrin oligomers . 

Paul and C. Lapinte (1998) Coordination Chemistry Reviews, vol. 178-180, p. 431 - Organometallic molecular wires and other nanoscale-sized devices. An approach 

Robertson and C.A. McGowan (2003) Chemical Society Reviews, vol. 32, p. 96 - A comparison of potential molecular wires as components for molecular electronics . 

Ward (1996) Chemistry Industry, p. 568 - Current developments in molecular wires . 

The first reported methylene complex of a Group IVA metal was obtained by the reaction of [Zr(PMePh2)a( -C8H5)2] vwth Ph3P=CH2, to yield [Z CHg)-(PMePh2)( 7-C5H8)2].  

In Group VA alkylidenes, one of the major mechanistic problems concerns the existence of the a-H elimination from alkyls. The transformation shown in equation (23), which occurs also for the analogous deuteriated complex) offered 

Paramagnetic carbene complexes of Cr obtained, e.g., by the reaction sequence in equation (26) are stablest with the chelating phosphine, dppe. E.s.r. 

The well known protonation reactions of Mo or W isonitrile adducts to give carbene or carbyne complexes has been extended to alkylation directly, yielding carbyne complexes, equation (27). The alkylation can be accomplished with 

MeSOsF, MegSO, or EtaOBF. No dialkylated products were obtained, and trans to cis isomerization occurred in dichloromethane solution.  

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A variety of attempts has been made to model the single steps of the Fischer Tropsch reaction on a molecular level. Naturally, the question of the catalytic activity of intermediate carbene and carbyne complexes has been pursued [4],... 

Carbene and Carbyne Complexes of Ruthenium, Osmium, and Iridium... 

BONDING MODELS AND REACTIVITY PATTERNS FOR TRANSITION METAL CARBENE AND CARBYNE COMPLEXES... 

The wealth of empirical information collected for transition metal carbene and carbyne complexes may be best interpreted within the framework of sound theoretical models for these compounds. Perhaps the most significant contribution made by the theoretical studies of carbene and carbyne complexes concerns an understanding of the reactivity patterns they display. In this section the relationship between bonding and reactivity is examined, with particular emphasis being given to the ways in which studies of Ru, Os, and Ir compounds have helped unify the bonding models applied to seemingly diverse types of carbene and carbyne complexes. 

In view of the similarities between the bonding models for carbene and carbyne complexes it is not surprising that similar patterns of reactivity should be observed for these compounds. Thus nucleophilic and electrophilic additions to the metal-carbon triple bond are anticipated under appropriate circumstances, and both orbital and electrostatic considerations will be expected to play a role. 

The similarity, then, between carbene and carbyne complex chemistry of Group 8a transition metals, as well as of Group 6a and 7a metals, is apparent. 

Carbyne complex chemistry of osmium and ruthenium is discussed in this section. These studies demonstrate clearly the parallels that exist between the metal-carbon bonds in carbene and carbyne complexes and again emphasize the importance of metal basicity in determining complex reactivity. 

The similarity between the bonding models for transition metal carbene and carbyne complexes was noted in Section II. That the reactivity of the metal-carbon double and triple bonds in isoelectronic carbene and carbyne complexes should be comparable, then, is not surprising. In this section, the familiar relationship between metal-carbon bond reactivity and metal electron density is examined for Ru and Os carbyne complexes. 

Fig. 3.30. Preparation of carbene complexes by sequential [2 + 2] cycloadditions and [2 + 2] cycloreversions of carbene and carbyne complexes to alkenes and alkynes [596-598].

Extension to trimetallic complexes has led to the preparation of similar bridging carbene and carbyne complexes with Pt3,421 Pt2W422 and PtWFe423 metal frameworks. 

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