Chromium sandwich compounds

Carbazole will react with 1 or 2 mol of ferrocene in hot decalin in the presence of aluminium-aluminium chloride producing crystalline derivatives in which either one or both" of the benzene rings is linked to iron, 25 and 26, respectively. The sandwich compound 25 was deprotonated to 27 with sodamide in liquid ammonia. A chromium carbonyl complex 28... 

Methyl benzoate, anisole, and diphenyl ether each give sandwich compounds with chromium vapor, although in rather low yield (32, 55, 110). Chromium appears to attack alkyl ethers and this deoxygenation probably competes with complexation with the aromatic oxygen compounds. No simple product has been isolated from chromium atoms and aniline, but bis(7V,7V-dimethylaniline)chromium has been prepared (32). The behavior of molybdenum and tungsten vapors closely resembles that of chromium in reactions with oxygen- and nitrogen-substituted arenes (113). 

Independent of these investigations, Fischer and co-workers reasoned that the compound bis(benzene)chromium (XXV), being isoelectronic with the sandwich compound ferrocene, should be capable of formation and study. In a research program initiated in 1954, Fischer and Hafner (90, 91) discovered that chromium trichloride, reduced by aluminum powder at 150°C. in the presence of benzene and aluminum chloride (Friedel-Crafts-type method), formed a bis (benzene) chromium salt. The latter was readily reduced to the parent substance bis(benzene)chromium (XXV) by alkaline sodium dithionite solution. 

Yet another major advance in arene-metal chemistry was made in 1957 by Fischer and Ofele 95) y who found that a sealed tube reaction involving chromium hexacarbonyl, bis(benzene)chromium, and benzene produced the half-sandwich compound benzene-chromium tricarbonyl (XXVII). 

Figure 7 Structures of some tropylium and arene half-sandwich compounds of chromium and manganese disoussed in Section 12.04.1.2.3.

Another important class of functionalized sandwich compounds that have been extensively used in the construction of hydrogen-bonded aggregates is that of the dicarboxylic acids of bis-benzene chromium and cobalticinium (see Table 3). 

Coordinated polyenes and arenes are a simple, but important, class of ir-donor ligand. Although we considered an rj -arene ligand to be a six-electron donor in Section 1.1, because it contains resonance structures with three double bonds, each of which are two-electron donors, the bonding of such ligands to the metal is more complex. These ligands are more accurately described by a combination of a- and ir-donation from the molecular orbitals of the entire ring. This combination of interactions is illustrated by the molecular orbitals of a sandwich compound, such as bis(benzene)chromium. 

The cyclopentadienyl chromium compound, an open sandwich structure, has a spectrum about as devoid of fine structure as one might expect of a... 

---