Chromium hexacarbonyl, reaction with arenes

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. 

Reaction of 1,4-benzodioxin with chromium hexacarbonyl gives a 41% yield of the > -arene chromium complex (62). Reaction of this complex with n-butyllithium forms a yellow solution of the aryllithium. Reaction of this with a range of electrophiles gives a series of 5-substituted derivatives in modest yield, which are readily decomplexed by iodine to form the useful 5-substituted 1,4-benzodioxins (63) <89TL5519>. 

The reactions of the phosphabenzene system [124] confirm these conclusions. Phosphabenzenes have low basicity towards hard acids. They are not protonated by CF3CO2H nor alkylated by trialkyloxonium salts. However, soft acids attack at phosphorus. For instance, 2,4,6-triphenyl-phosphabenzene forms compounds 4 with the hexacarbonyl derivatives of Cr, W and Mo in which the phosphorus coordinates to the metal, possibly with metal-P back-donation. The complexes 4 rearrange photochemically or thermally affording the 67i-heteroarene complexes 5. Although 2,4,6-triphenyl-pyridine is protonated on nitrogen, it undergoes complex formation with chromium hexacarbonyl exclusively on the phenyl moieties yielding the ri -arene complexes 6 [125]. 

Treatment of chromium(III) chloride with phenylmagnesium bromide under an atmosphere of carbon monoxide provided a route to chromium hexacarbonyl (Job-Cassels reaction). When this reaction was carried out under nitrogen it was possible to isolate organochromium complexes, but these are now known to be rj -arene compounds, formed via unstable chromium phenyls (see p. 314). 

Both arene and cyclopentadienyl 7i-complexes can undergo redistribution reactions. These reactions involve the exchange of ligands between two species possessing the same or different metal. For example, the reaction of dibenzenechromium with chromium hexacarbonyl provides a method for the preparation of benzenechromium tricarbonyl (2-6). A second example of a... 

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