Preparation of Arene Complexes

Fischer and coworkers developed the most important and general method for the preparation of arene complexes. It involves the action of aluminum and its chloride on transition metal compounds in the presence of arene  

The cation may subsequently be reduced by strongly reducing agents such as the dithionite ion in alkaline solution  

This method may be utilized for the preparation of most arene sandwich compounds as well as for many other arene complexes. However, it cannot be used for manganese arene derivatives. 

Cationic molybdenum, tungsten, chromium, and vanadium compounds disproportionate to give sandwich complexes M(arene)2  

In alkaline solution, the cation reduces [V(arene)2] and therefore the disproportionation reaction proceeds according to the equation 

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Related complexes are obtained with Pd, where one face of a benzene ring is shared by two Pd atoms . Mono-f/ -benzene ligands on a pentacobalt cluster also form from the aluminum halide-aluminum metal conditions . The preparation of -arene complexes of the lanthanides and actinides has been more diflScult, presumably because of the proclivity of these metals to form ionic bonds. Reactions between hexamethylbenzene and metal halides in the presence of AlClj-Al or AlCl-Zn reagents in the isolation of a mononuclear Sm complex, in addition to di- and trinuclear U complexes. ... 

Scheme 2 shows the preparation of carbene complexes starting from [L M-ER3]" = [MeCp(CO)2Mn-SiMePlt2] and [(7r-arene)(CO)2Cr-SnPh3] [12,13]. By a related approach, a cationic cyclopropenylidene complex was prepared by reaction of Cp(CO)2FeSiMe3 with l-chloro-2,3-diphenylcyclopropenylium tetrafluoroborate [14],... 

A second synthetic procedure for preparing silylated arene complexes, based on the metalation of a preformed rj6-arene complex, followed by treatment with an organochlorosilane, has been used only in one case so far namely, bis(benzene)chromium (47) ... 

Displacement of a coordinated arene by a pendant arene (Scheme lb) has been used to prepare tethered arene complexes in which the auxiliary ligand is a carbene. 

Exchange of one arene ligand for another is potentially a powerful method for preparing of arene-metal complexes. With bis(jj -arene)metal complexes, only a few examples are known with no systematic development. Equilibrium of bis(>/ -benzene)Cr(0) with biphenyl at 115°C in the presence of AICI3 allows isolation of bis( -biphenyl)Cr in 20% yield ... 

A serendipitous preparation of arene-metal complexes is the reaction of phenylmag-nesium bromide with chromium trichloride ... 

The first of the above methods is less widely used for preparing mono-> -arene complexes than the bis- y -arene complexes, e.g., in the reaction of chlorotetra-carbonyl-Mn(I) with arenes and AlClj at 60-80°C to give 40-76% yields of -arene-Mn(I) tricarbonyl cations . Several -arene complexes of early transition elements are also prepared by this method , e.g., the chloro-bridged dimer ... 

The synthesis of stable tt-arene complexes of the transition metals has progressed substantially owing to metal atom chemistry. Particularly rewarding has been work on the production of bis(arene)metal(0) complexes of the early transition metals e.g., V, Cr, Mo and W yield bis(benzene)V(0), Cr(0), Mo(0) and W(0) complexes ". In addition, metal-atom reactions have yielded new arene-substituted compounds of V, Cr, Mo and W that cannot be prepared by classical methods . Hitherto unknown bis(arene)-metal(O) complexes can be prepared for Ti and Nb . The metal atom method is the most versatile (and often the only method) for the preparation of such complexes. Table 1 summarizes the bis(arene)metal(0) complexes prepared to date. 

Table 2 summarizes the mixed arene-arene and arene-ligand complexes prepared. w-Arene complexes of polymers are also listed. Iron and Ru with benzene or toluene yield t-arene complexes that can be isolated at low temperature. The (CgHg)2Ru species is stable to 0°C, whereas the CgHjCHj-Fe analog is stable to — 50°C. Thus, these compounds can be isolated for use as intermediates in other reactions, e.g., the addition of cyclopentadiene to the (benzene) Fe complex, where on warming hydrogen transfers to benzene to yield ( -cyclopentadienyl) ( -cyclohexa-dienyl)Fe ... 

Condensation of metal vapors with various 6e ligands (cf. preparation of diene complexes, Chapter 8) allows the synthesis of many complexes which would be difficult or impossible to obtain by other methods owing to the reactivity of substituents. The following bisarene complexes were obtained in this manner [Cr(arene)2], where arene = PhX (X = F, Cl, CF3, COOMe, Me, /-Pr) or (X = F, Cl, CF3,... 

Arene complexes are utilized for the preparation of metallic layers which are deposited on various surfaces (See Table 2.31). They are also used for the preparation of highly pure metals. This is especially true for chromium group compounds. Pyrolytic decomposition of arene complexes found application for deposition of chromium on magnesium oxide in order to prepare the catalyst for dehydration of isopropyl alcohol. 

Chromium was discovered by Louis Nicolas Vauquelin in France in 1797. There was litde use of chromium in organic chemistry until the mid-twentieth century. Much of this work was initiated by Prof. E.O. Fischer in Munich, Germany. His discovery of the preparation of arene (Cr(C03)) complexes lead to him sharing the Nobel Prize in Chemistry in 1973 with Prof. Geoffrey Wilkinson of the United Kingdom. 

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