Arene Complexes by

There is a wider general interest in understanding the oxidation of cysteine thiolates in proteins since they are involved in redox-sensing reactions [99], Therefore, such oxidation reactions of thiols induced by Ru coordination may also play a more general role in the pharmacological activity of Ru-arene complexes by coupling Ru coordinative binding to redox processes both outside and inside cells. 

The most common method of formation of t/ -arene-metal complexes is by direct coordination of the arene to the metal, usually by replacement of one or more ligands from the coordination sphere. This section begins with the reactions involving bis-t -arene complexes, followed by the methods leading to mono-/y -arene complexes. The examples illustrate the scope, and the list is not comprehensive . Formation of new arene complexes by exchange of non-arene ligands is not included . ... 

Chirality [17] can be introduced into organometallic arene complexes by modifications such as asymmetric substitution of the arene, introduction of chiral substituents into the ligands, the asymmetric coordination environment of the metal ion, or conformational arrangements of the ligands. Chirality in arene complexes can be classified depending on the components that generate the asymmetry in the molecule (for examples, see Figure 3.3). 

The first complex is the 18e species, [(Ti -indane)IrL2] formed by hydrogenation of the C=C bond by the IrH2 group, and the second is indenyl)IrHL2], formed by oxidative addition of an indane C—H bond, P elimination, then loss of H2 from the metal and oxidative addition of an indane C—H bond. Substitution only of the arene complex by CO is possible because loss of arene is easier than loss of the Cp-like n -indenyl (see Section 5.7). 

The information available so far clearly demonstrates that arenes complexed by Cr(CO)3 are suitable candidates for probes for 7t-stacking effects. Solvated and 7t-stacked structures show different IR band positions, and progressive interconversion of the two forms occurs as the tendency to 7t-stack is increased. This behaviour is more similar to the effects of interconversion of associated and dissociated forms in pH measurements (Section 7.5.2 and Fig. 7.9a) or empty and occupied crowns in metal ion measurement (Section 7.5.3), than to the shifting ofbands that occurs when different solvents are mixed (Section 7.5.1 and Fig. 7.7a). 

The nucleophilic substitution of the nitro group in nitro-arene complexes works almost as well as that of Cl" and such substitutions were achieved by Chowdhurry et al. with O, S, and N nucleophiles and with stabilized carbanions [97,98] Eq. (28) and Table 8. 

Recently, it was shown that the attack of CN on [FeCp(C6H5Cl)]+ PFortho-position. In the intermediate cyclohexadienyl complex, the CN group migrates to the ipso-carbon, whereas Cl is displaced. The monosubstituted benzonitrile complex is subjected to a second ortho-CN- attack but hydride is not removed spontaneously to give back an arene complex (Scheme XIX). Removal of the hydride is achieved by oxidation using DDQ (2,3-dichloro-... 

As was suggested in the preceding discussion, most of the arene complexes isolated by metal-atom techniques are benzene derivatives. However, heterocyclic ligands are also known to act as 5- or 6-electron donors in transition-metal 7r-complexes (79), and it has proved possible to isolate heterocyclic complexes via the metal-atom route. Bis(2,6-di-methylpyridine)Cr(O) was prepared by cocondensation of Cr atoms with the ligand at 77 K (79). The red-brown product was isolated in only 2% yield the stoichiometry was confirmed by mass spectrometry, and the structure determined by X-ray crystal-structure analysis, which supported a sandwich formulation. 

Using an electron-gun source, tungsten atoms were reacted with benzene, toluene, or mesitylene at 77 K, to form the expected (arene)2W complex (42) in a yield of 30%, compared with the —2% yield from the previously published, bis(benzene)W synthesis (32). These arene complexes are reversibly protonated, to give the appropriate [(T7-arene)2WH] species. By using the same technique, the analogous, niobium complexes were isolated (43). 

CeDs solution (Scheme 7). Both t -arene complexes were also determined by the X-ray diffraction and showed no reaction to hydrogen and olefins. Therefore, it was considered that the formation of the t -arene complexes was a deactivation pathway in the catalytic hydrogenation. 

Several other versions of these catalysts have been developed. Arene complexes of monotosyl-l,2-diphenylethylenediamine ruthenium chloride give good results with a,(3-ynones.55 The active catalysts are generated by KOH. These catalysts also function by hydrogen transfer, with isopropanol serving as the hydrogen source. Entries 6 to 8 in Scheme 5.3 are examples. 

The scopes outlined above limit the purpose of this chapter, which will not cover (1) the transition metal complexation by protonated or functionalized forms of calix[4]arenes (2) the metalation of calix[4]arenes using non-transition metals and (3) chemical curiosities derived from the metalation of calix[4]arenes (some recent reviews cover these areas very well).1 In addition, the authors have been particularly careful to report only those compounds which have a well-established synthesis and a full spectroscopic and structural characterization. 

Furthermore, ir-arene complexes of transition metals are seldom formed by the direct reaction of benzene with metal complexes. More usually, the syntheses require the formation of (often unstable) metal aryl complexes and these are then converted to ir-arene complexes. The analogous formation of w-adsorbed benzene at a metal surface via the initial formation of ff-adsorbcd phenyl, merits more consideration than it has yet been given. It is to be hoped that the recognition and study of structure-sensitive reactions will allow more exact definition of the sites responsible for catalytic activity at metal surfaces. The reactions of benzene, using suitably labeled materials, may prove to be useful probes for such studies. 

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