Arene Ligand Exchange

Both rings in dibenzenechromium can be exchanged by toluene and carbon monoxide under pressure, equation (6-58). 

Dibenzenechromium is converted to dibiphenylchromium on similar treatment, and the reaction is irreversible under these conditions. The equilibrium is influenced by the concentration of the reactants and the relative stability of the products. 

Benzene-biphenylchromium cation, however, undergoes a ligand exchange with other aromatic ligands (in vacuo or under nitrogen and in the presence of aluminum chloride) to yield the corresponding diarenechromium complexes, equation (6-59). 

Arene = biphenyl, benzene, mesitylene, tetralin, and p,p -dimethyl biphenyl 

The exchange reaction of an arene ligand in (C6H6)Cr(CO)3 in n-heptane is studied by using C-labeled benzene The rate of the 

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In line with expectations of kinetic inertness for third-row transition metals, little interest has been vested in the development of osmium anticancer drugs, as ligand-exchange rates did not seem favorable on the timescale of cellular processes. Our work, however, shows that the kinetic lability of such complexes can be timed to such extent that anticancer activity comes within range. We have demonstrated how rational chemical design can thus be applied to osmium-arene complexes resulting in specific... 

One of the few useful reactions of ferrocene is the AlCh-catalyzed exchange of one Cp group for an arene ligand (equation 6).39-44... 

The second method involves ligand exchange, usually between (diacetoxyio-do)benzene and a carboxylic acid. The reaction is best performed in mildly heated chlorobenzene under reduced pressure. This simple approach is suitable for the preparation of several [bis(acyloxy)iodo]arenes including dibenzoyloxy derivatives and also 1-adamantylcarboxy derivatives which are formed in high yields (Scheme 5) [24,25]. 

The ligand-exchange reactions are widely used in the syntheses of binuclear metal-carbonyl, r 5-cyclopentadienyl, and r 6-arene complexes [11, vol. 6], Examples of these type of transformations are reactions (3.133)-(3.135) [11, vol. 6] ... 

The yellow CpFe1 (//6-arene) salts (most commonly BF4 or PF6 ) are usually stable up to at least 200 °C, are stable in concentrated sulfuric acid, and are very resistant towards oxidation (until recently, it was believed that they could not be oxidized [23] vide infra). They are not easy to reduce either [23] (vide infra). The chloride salts [CpFe+( f -arene) Cl- are water-soluble they are formed upon hydrolysis following ligand-exchange reactions between ferrocene and the arene in the presence of aluminum chloride [21]. Such aqueous solutions may sometimes be directly used for nucleophilic reactions [22] (vide infra). The BF4- salts are also sometimes quite soluble in water, but the PF6- salts are much less so. Electrophilic reactions that are readily undergone by the free arenes, such as Friedel-Crafts reactions, are no longer possible on the CpFe+( /6-arene) complexes [19, 23]. On the other hand, a range of nucleophilic reactions that are impossible or very difficult to carry out with free arenes become possible under ambient or mild conditions with the CpFe+()/6-arene) complexes (Scheme 2) [16-20]. 

Richard followed the course of the reactions of Cr(CO)6 and Mo(CO)6 with hexamethylborazine by UV-vis spectroscopy but although he observed that the intensity of the absorption maximum of the hexacarbonyls at around 290 nm decreased and a new band at around 350 nm appeared, he was unable to identify the new product. Experiments with other starting materials such as norborna-diene chromium and molybdenum tetracarbonyl or tris(aniline) molybdenum tricarbonyl which readily react with arenes by ligand exchange, also failed. The key to success was to use tris(acetonitrile) chromium tricarbonyl as the precursor, which in dioxan under reduced pressure afforded the desired hexamethylborazine chromium tricarbonyl as a stable crystalline solid in 90% yield. This was the breakthrough and, after we had communicated the synthesis and spectroscopic data of the complex in the January issue 1967 of Angewandte Chemie, Richard finished his work and defended his Ph.D. thesis in June 1967. Six months before, in December 1966,1 defended my Habilitation thesis in front of the faculty and became Privatdozent (lecturer) on the 1st of January 1967. 

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