Benzenoid Type Complexes

Although benezene and benzene type complexes are much less common than those with cyclopentadienyl ligands, a number of such complexes are known for chromium and ruthenium. The bis benzene complex (// -C6H6)2Cr is photochemi- 

Photolysis of the ruthenium arene complexes RuCl2( / -arene)L (L = tertiary phosphine) in the presence of an arene results in partial replacement of the arene by the second one/ Of all the arenes studied, the p-cymene in RuCl2(p-cymene)(PBu3) was the most easily displaced. Arenes with electron-withdrawing substituents failed to displace any other arene. 

The photochemical displacement of the arene is also observed for the complexes ( / -cp)(7 -arene)M (M = Fe, Ru). This reaction occurs from the photoactive a E ligand field excited state. A linear correlation exists between log (0/(l-0)) and o-p, the Hammett parameter for a series of complexes with chloro-and methyl-substituted arenes. The data indicate that a small amount of negative charge builds up at the arene in the transition state for the reaction that results in arene dissociation in these systems. The temperature dependence for the photodissociation of the arene ligand indicates that the metal-arene bond is almost broken in the a E excited state. When the analogous pentamethylcyclopentadienyl complexes (7 -cp )( / -arene)M (M = Fe, Ru) are photolyzed, the quantum yields for arene release are lower than those found for the unsubstituted compounds 

For the complexes (7 -cp)(coumarin)M (M = Fe, Ru), photolysis in acetonitrile solvent results in formation of the acetonitrile complex ( / -cp)Ru(MeCN)J and free coumarin  

Free coumarin dye is emissive, but complexation of the (7 -cp)Ru moiety to the coumarin chromophore inhibits reorganization of the dye to its polar form in the intramolecular charge transfer (ICT) excited state, thereby resulting in nonemissive complexes in which the MLCT or d-d excited states are lowest in energy. Similarly, the multiple substitution of (ri -cp )Ru on rubrene can also impart new structural, photophysical, and chemical properties to the rubrene molecule.  

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On the other hand, ferrocene forms charge-transfer complexes with nitrobenzenes [966], tetracyanoethylene (TCNE) [96, 96c, 96d] and 2,3-dichloro-5,6-dicyanoquinone. Mdssbauer spectroscopy indicates that the ferricenium ion is not present in the 1 1 TCNE ferrocene complex, and, as may also be the case with the nitrobenzene complexes, it is suggested that the complex is of benzenoid type with the TCNE placed above a r-CsHs ring, 4.4 [96c]. In this context it is noteworthy that the high efficiency of ferrocene in quenching triplet states in excited photosensitizers, e.g., benzophenone, may be due to a charge-transfer mechanism [96e]. 

In this initial section the reactivities of the major types of azole aromatic rings are briefly considered in comparison with those which would be expected on the basis of electronic theory, and the reactions of these heteroaromatic systems are compared among themselves and with similar reactions of aliphatic and benzenoid compounds. Later in this chapter all the reactions are reconsidered in more detail. It is postulated that the reactions of azoles can only be rationalized and understood with reference to the complex tautomeric and acid-base equilibria shown by these systems. Tautomeric equilibria are discussed in Chapter 4.01. Acid-base equilibria are considered in Section 4.02.1.3 of the present chapter. 

The protonation studies are of interest in another connection. If protonation of metallocenes can be considered to be a simple form of electrophilic attack, it is possible that other types of electrophilic substitution reactions may proceed through initial coordination of the electrophile with the central metal atom (14, 93). The mechanism of acylation of metallocenes may therefore be more complex than might be expected by analogy to similar reactions of benzenoid compounds. Clearly more studies are needed along these lines, better to define specific metal effects on the properties and reactions of these remarkable compounds. 

The complexes of heterocyclic compounds with acceptors of the type mentioned, like similar complexes of benzenoid hydrocarbons, yield a linear plot of energies of the first charge-transfer bands against HOMO energies of the donors.50 Table V presents experimental and theoretical data for complexes of pyridine-like heterocycles with... 

Data presented in Table IV make it clear that the correlations are sufficiently close. There is apparent similarity between the behavior of the heterocyclic compounds and benzenoid hydrocarbons which supports the claims50,00 that these complexes are of the tt—it type. 

Nitric acid oxidation of coal, with or without oxygen, leads to the formation of complex mixtures of the benzenoid or aromatic type of organic acida Nitric acid oxidation of carbohydrates yfelds oxalic acid. Nitric acid of 50-76 per cent concentration at temperatures of 20-40 C oxidizes tetra-hydrofuran to good yields of succinic acid. Oxidation with nitric acid of a secondary aliphatic alcohol or an aliphatic ketone results in fistion of the aliphatic chain at the point of attachment of the functional group accom-... 

Iron, Ruthenium, and Osmium.—MO-INDO calculations have been carried out 353 foj. cations of the type [CpFe( y-arene)]+. The e.s.r. spectra of the 17-electron dications [CpFe( -arene)] + and 19-electron neutral complexes of the type [CpFe( -naphthalene)] have been discussed, and the Fe Mossbauer spectra of salts [CpFe( y-arene)]+ [PFg]" have been analysed. Ligand-exchange reactions of Cp Fe with pyrene in the presence of AlaCle-Al have given various mixed-sandwich complexes in which a (CpFe)+ residue is attached to one or both of the benzenoid rings of pyrene itself and of its di- and tetra-hydro deriva-... 

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