Complexes of Benzene and Related Aromatics

Although we have considered compounds of Csl l s as though the ligand is a donor of six electrons, benzene is also donor of six electrons. Therefore, two benzene molecules each donating six electrons 

This compound has been prepared by several means including the following reactions  

Bis(benzene)chromium(0) is easily oxidized, and mixed complexes are obtained by means of substitution reactions. For example, 

Note that in this case, the three carbonyl ligands are staggered relative to the carbon atoms in the benzene ring (as indicated by the dotted vertical lines). Similar compounds have also been prepared containing Mo and W. Methyl-substituted benzenes such as mesitylene (1,3,5-trimethylbenzene), hexamethylbenzene, and other aromatic molecules have been used to prepare complexes with several metals in the zero oxidation state. For example, Mo(CO)6 will react with 1,3,5-C6H3(C]T3)3, 1,3,5-trimethylbenzene, which replaces three carbonyl groups. 

Although the chromium compound is the best known, other metals form similar complexes with benzene and its derivatives. 

---

Methyl-substituted thiophenes bind more strongly to transition metals than the unsubstituted ligand, as illustrated by ligand-exchange reactions of CpRu(77 -Th) complexes. The equilibrium constant measured by NMR increases by a factor of about 6 for each methyl group added on the thiophene " benzene, BT, and DBT also displace T. These observations can be related to the inhibition effect of benzene and other aromatic hydrocarbons on FIDS of BT over solid catalysts, ascribed to a stronger binding of arenes on metal sulfide surfaces as compared to common sulfur... 

Arene(tricarbonyl)chromium complexes, 19 Nickel boride, 197 to trans-alkenes Chromium(II) sulfate, 84 of anhydrides to lactones Tetrachlorotris[bis(l,4-diphenyl-phosphine)butane]diruthenium, 288 of aromatic rings Palladium catalysts, 230 Raney nickel, 265 Sodium borohydride-1,3-Dicyano-benzene, 279 of aryl halides to arenes Palladium on carbon, 230 of benzyl ethers to alcohols Palladium catalysts, 230 of carboxylic acids to aldehydes Vilsmeier reagent, 341 of epoxides to alcohols Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281 of esters to alcohols Sodium borohydride, 278 of imines and related compounds Arene(tricarbonyl)chromium complexes, 19... 

The ability to explain the complex substitution chemistry of benzene and other related aromatic derivatives as the result of benzene s structure signaled the triumph of structural chemistry. When Hermann Kolbe, who hated Kekule, died in 1884, the last real resistance to structural chemistry died with him. In the eloquent words of Brock ... 

Salicylamide complexes (preferably with LiAlH4) have been reported" to be successful aids in the hydrogenation of benzene, toluene, and xylenes to the corresponding naphthenes. Photocyclisations of stilbenes and related compounds in the presence of 7r-acceptors (e.g. tetracyanoethene, chloranil) have led to good yields of the corresponding polycyclic aromatic hydrocarbons. A dihydro-compound such as (14) is formed in the absence of the ir-acceptor, and it can be oxidized to (15). 

Among oxo-metals, osmium tetroxide is a particularly intriguing oxidant since it is known to oxidize various types of alkenes rapidly, but it nonetheless eschews the electron-rich aromatic hydrocarbons like benzene and naphthalene (Criegee et al., 1942 Schroder, 1980). Such selectivities do not obviously derive from differences in the donor properties of the hydrocarbons since the oxidation (ionization) potentials of arenes are actually less than those of alkenes. The similarity in the electronic interactions of arenes and alkenes towards osmium tetroxide relates to the series of electron donor-acceptor (EDA) complexes formed with both types of hydrocarbons (26). Common to both arenes and alkenes is the immediate appearance of similar colours that are diagnostic of charge-transfer absorp-... 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

Tetrachlorohydroquinone (45) is closely related to chloranil and might be expected to show overcrowding effects similar to those found in chloranil by Ueda. The crystal structure of tetrachlorohydroquinone has been investigated by Sakurai (1962) using nuclear quadrupole resonance and X-ray diffraction techniques. The crystal makes use of the molecular centre of symmetry and the asymmetric unit consists of half a molecule. The analysis shows that the C—O bond deviates by 0-9° from the aromatic plane and that the two adjacent C—Cl bonds are also bent out of the benzene plane, in the same direction, through 0-8°. (This is reminiscent of Harding and Wallwork s findings for chloranil in the hexamethylbenzene-chloranil complex.) Sakurai... 

The association of nitrobenzene and nitronaphthalene in non-polar solvents, such as hexane and carbon tetrachloride, forms an exception. Here the association depends in the first place on interaction of the Keesom type with the very large moment (4.20 D). The stronger association of the last-mentioned compound points, however, in addition to an interaction due to complex resonance as observed between nitro compounds and aromatic hydrocarbons in general. It is plausible that when dissolved in benzene this association gives way to a solvation. This interaction between solute and solvent molecules is closely related to the association between like molecules. In benzene etc. no stoichiometric association is observed but, owing to the anisotropy of the polarizability, a more or less parallel... 

---