Arene coordination

Scheme 7 Intramolecular arene coordination in bis(imino)pyridine iron complex 10...

Although deprotonation at the benzylic position of arenes coordinated to ruthenium and chromium was reported,27 in the case of the coordinated oxo-ri5-dienyl unit, nucleophilic attack at one terminus of the complexed r 5-dienyl ligand, rather than deprotonation, was expected.28 The reason for the successful deprotonation (even at relatively hindered isopropyl sites) is, according to the authors, the cationic nature of the Cp M fragment. In addition, the transition state for the deprotonation might involve stabilization by the metal (Scheme 3.15). 

Reactions of cobalt and nickel atoms with toluene and other arenes yield condensates in which the metal is in a very reactive state (105). However, none of the products of reaction of these condensates with other ligands has contained the arene coordinated to the metal. It seems possible that the condensates contain ditoluenecobalt and ditoluene-nickel, but in these complexes (unlike the chromium or iron complexes) the two arene rings are probably not symmetrically bonded to the metals. 

A series of cobalt carbonyl complexes of polyphosphazenes have been prepared via arene coordination sites. Examples are shown as 3.65 and 3.66.112 These are synthesized via the reactions of (NPC12) with the sodium salt of the appropriate metal-arene terminated alcohol. Mixed-substituent polymers with trifluoroethoxy or phenoxy cosubstituents have also been prepared. 

As a consequence of the chelate effect, the arene in a tethered arene might be more effectively anchored to a metal centre, perhaps resulting in a stabilization of r 6-arene coordination for a greater variety of metals and oxidation states than is available for non-tethered arenes. Conversely, the lability of a coordinated, tethered arene might be used to protect potentially catalytic sites at a metal centre. Both effects could be modulated by variation in the length and nature of the tether, the nature of the other donor atom, and by the introduction of substituents on the aromatic ring. 

Arenes are inert to nucleophilic attack and normally undergo electrophilic substitution. However, arenes coordinate to Cr(CO)6 to form the i/fi-arenechromium tricarbonyl complex 79, and facile nucleophilic attack on the arene generates the anionic jy5-cyclohexadienyl complex 80, from which substituted arene 81, or cyclohexadiene is obtained by oxidative decomplexation. In this reaction, strongly... 

Aniline complexes are unique in [Os] chemistry in that //2-arene coordination competes with // -nitrogen coordination. However, rj1 -coordination becomes favored when the nitrogen is substituted. For example, the N-ethyl aniline complex 77 is isolated solely in its ring-bound form (Table 11). Without Lewis acid promotion, 77 will undergo regioselective addition of... 

The donor/acceptor properties and the electronic coupling interactions determine the redistribution of electron density between the aromatic donor and the electron acceptor upon complexation. Significant changes in structure and reactivity of the coordinated arene can be rationalized in terms of spectral and thermodynamic properties within the framework of the CT formalism. This section is devoted to a consideration of the structural effects of arene coordination (in terms of donor/acceptor bond distance and type of bonding, distortion of arene planarity, expansion of the aromatic ring, and re-bond localization). 

Localization of re-bonds (also termed double-bond fixation ) is an important structural feature frequently observed upon arene coordination to a metal center. To rationalize this effect, the formation of covalent (a) bonds between the metal and particular carbon atoms of the arene ring is commonly invoked. However, this cannot explain all the unusual bond distances observed and is not generally applicable to the analogous findings with organic acceptors. Analysis based on the CT concept allows a comparative treatment of all types of donor/acceptor complexes, and predicts a close relationship between the degree of bond localization and the donor/acceptor strengths of the complexed partners. 

Nucleophilic addition to unsaturated ligands, such as olefins, acetylenes, and arenes, coordinated to various metal centers is a useful strategy in organic synthesis (98). The ion-pair interaction of organometallic cations and anions is facile and can successfully lead to the nucleophilic activation of ethylene, acetylene, and benzene (99), for example,... 

Whilst metallocenes have an enormously rich chemistry, in the case of arene coordination it is the half-sandwich ( piano-stool ) class of complex which finds the widest utility in organic synthesis, in particular within the chemistry of Cr(CO)3(ti6-arene) complexes. These are generally prepared directly from the thermal reaction of Cr(CO)6 and arene in a high-boiling solvent (e.g. Bu20, 2-picoline). For less robust arenes, the labile complexes Cr(L)3(CO)3 (L = NH3, THF, MeCN, py) may be employed under milder conditions (Figure 7.38). 

Figure 7.43 Dihapto arene coordination synthetic implications L = NH3 Tf = S02CF3...

Arenes add to metals much more readily than alkanes, due to their ability to form comparatively stable rf-arene complexes prior to C—H activation. Arene coordination is favored by Cp compared to Cp, and by Rh1 compared to Ir1. The equilibrium... 

Hexafluoro-2-butyne undergoes 1,4 addition by reaction with Rh(dik)(diolefin) (dik = acac, dpm, dbm diolefin = cod, norbornadiene), with formation of a C—C bond between the alkyne and the methyne carbon atom of the /3-diketonate. Additional trimer-ization of hexafluoro-2-butyne to hexakis(trifluoromethyl)benzene and /j -arene coordination of the latter to rhodium affords 103 (equation 73). 

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