Metal arene complex bonding

The possibility of coordination of a two-electron ligand, in addition to arene, to the ruthenium or osmium atom provides a route to mixed metal or cluster compounds. Cocondensation of arene with ruthenium or osmium vapors has recently allowed access to new types of arene metal complexes and clusters. In addition, arene ruthenium and osmium appear to be useful and specific catalyst precursors, apart from classic hydrogenation, for carbon-hydrogen bond activation and activation of alkynes such compounds may become valuable reagents for organic syntheses. 

The basic qualitative features of the bonding in ferrocene are well understood, and will serve to illustrate the basic principles for all (t7-C H )M bonding. The discussion of bonding does not depend critically on whether the preferred rotational orientation of the rings (see Fig. 16-30) in an (i7-C5H5)2M compound is staggered (Did) or eclipsed (DSh) in any event, the barriers to ring rotation in all types of arene-metal complex are very low, ca. 10-20 kJ mol-1. 

There is continued interest in the application of arene-metal complexes in synthesis. Styrenes, for example, become activated by complexation with the tricarbonylchromium moiety towards attack by nucleophilic species. The resulting stabilized carbanions can be alkylated by various electrophiles, and by choosing the appropriate nucleophile and electrophile two new carbon-carbon bonds can be produced (Scheme 26). ... 

Reactions that generate carbon-carbon bonds via the addition of a carbanion to an arene metal complex or to a hydro-arene complex are likely to join the ranks of those which are sufficiently powerful that a whole synthetic route can be built around their use. Thus Semmelhack and co-workers report a synthesis of acorenone B (84) in which the key step is the conversion of (82) to (83). ... 

This result represents the first example of a synthetic cycle for arene borylation facihtated by an f-element. The authors investigated the reaction mechanism and proposed a concerted direct B—H attack on an aromatic C—H bond, which resulted in the formation of the B—C bond and the release of one molecule of H2. This type of reactivity resembles a-bond metathesis if boron is considered a metal center. Albeit uranium was not directly involved in the borylation of the arene C—H bond, the formation of the diuranium arene inverse-sandwich complex plays an important role. The arene, i.e., benzene or naphthalene, is partially reduced upon coordination to uranium,which makes it more susceptible to attack by an electrophile such as borane. Therefore, although uranium is not direcdy involved in the C—H bond activation step, this example illustrates that f-elements can render arenes reactive in nonmetal-mediated transformations by forming activated arene metal complexes. 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

Longer Hg-7r interactions are observed in the /> ra-/-butylcalix[4]arene mercury complex 162. The mercury atom forms primary bonds with the two sulfur atoms and engages in weaker secondary interactions with two arene rings of the calixarene whose centroids sit at 3.07-3.11 A from the metal center.201... 

Fig. 2. Ligand substitution as a prodrug strategy for metallochem-otherapeutics (a) general scheme of prodrug activation by ligand substitution hydrolysis of a metal—halide bond is a typical activation pathway of metal-based anticancer drugs, as exemplified by the activation of cisplatin (b) and a ruthenium—arene complex (c).

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