Arene-metal compounds

Co-condensation of transition metal atoms with arenes such as benzene and toluene is well known to yield bis-arene-metal compounds. However, in many cases the yields based on the metal atoms are less than 40%. Evidence that competing reactions such as carbon-hydrogen activation can occur is provided by the isolation of non-metal-containing products such as biaryl derivatives (2JL). ... 

Table 10. Bond enthalpy contributions, D(M-ArH) kJ mol, in bis(arene) metal compounds...

Ionization Energy Data (eV) and Tentative Assignments for Open-Shell Arene-Metal Compounds 1... 

Apart from bis(benzene)chromium and other bis(arene) metal compounds, a huge number of mono(arene) transition metal complexes were also prepared in the golden 1950s . In 1957, Fischer and Ofele reported the synthesis of (C6H6)Cr(CO)3, the first representative of the family of arene metal carbonyls [64], The original preparative procedure (a sealed-tube reaction of Cr(CO)6 and Cr(C6H6)2 at 220°C) was soon replaced by a more convenient synthesis, based... 

G. Wilkinson, and F. A. Cotton, Cyclopentadienyl and Arene Metal Compounds, Progr. Inorg. Chem. 1, 1-124 (1959). 

As a historical note, the very first arene metal compounds were made as long ago as 1919 by Franz Hein, but were then believed to be polyphenyl compounds of chromium, e.g., G QHs) This is now known to be (16-XLIV). 

Fig. la, b. Interaction diagrams for sandwich molecules (a) a bis-rj-arene metal compound, (b) a mixed ring sandwich compound, M(t)-CsHs)(t)-C7H7)... 

The sonochemical formation of arene-metal compounds appeared in the literature in 1957, and a number of examples were found (Fig. 9). Benzoquinoline-sodium was prepared in 45 min at room temperature instead of a 2-day reflux in ether. 

It is clear from the preceding section that the field of tethered arene-metal complexes is dominated by ruthenium and by arene-phosphines as ligands. In part, this situation has arisen because of the current surge of interest in the catalytic properties of ruthenium complexes in organic synthesis.85,86 Moreover, the tethered arene complexes are usually air-stable, crystalline solids with a well-defined, half-sandwich molecular geometry that, in principle, can lock the configuration at the metal centre. These compounds should, therefore, be ideal both for the study of the stereospecificity of reactions at the metal centre and for stereospecific catalysis. 

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

However, this simple model does not explain the (kinetically) enhanced reactivity of arenes relative to alkanes, considering the higher (Dc-h) bond energy of arenes, i.e. Dc-h, benzene — 111 lccal mol-1 vs. Dc-h,alkane 95 lccal mol-1. Many more examples of metal-activated cleavage of C-H bonds are known for aromatic compounds than for alkanes [37]. To account for this difference, arene/metal coordination is proposed [38], although experimental evidence for such intermediate complexes and their structural features is lacking. 

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. 

---