Arenes hexamethylbenzene complexes

The same conclusion is not applicable to the NO+ complexes, in which the magnitudes of the formation constants are much more strongly dependent on the ionization potential of the arene donor (see Fig. 10A). Thus the factor of >104 that separates the formation constant of the benzene complex with NO+ from that of the hexamethylbenzene complex corresponds to more than 5 kcal mol-1 of extra stabilization energy in the... 

The increased importance of charge transfer in proceeding up the series of NO+ complexes with the enhanced donor strength of arenes that vary from benzene with IP = 9.23 eV to the electron-rich hexamethylbenzene (IP - 7.85 eV) has its chemical consequences with respect to thermal (adiabatic) electron transfer. Thus the benzene complex with Z = 0.52 is persistent in acetonitrile solution for long periods, provided the solution is protected from adventitious moisture and light. By contrast, the hexamethylbenzene complex with Z = 0.97 slowly liberates nitric oxide under... 

Fig. 12 Typical time-resolved absorption spectrum following the charge-transfer excitation of nitrosonium-EDA complexes with arene (hexamethylbenzene) showing the bleaching of charge-transfer absorption and growth of the donor cation radical...

Mesitylene and hexamethylbenzene cyclohexadienyl ruthenium complexes 236 have also been prepared in good yield by reduction of their corresponding bisarene ruthenium dications 235 with sodium borohydride in water (142,143). Cyclohexadienyl derivative 236c can be easily formed by treatment of bisarene ruthenium(O) complex 197 (arene = hexamethylbenzene) with a solution of hydrochloric acid in acetone. The structure of 236c and the presence of the endo C—H bond have been clearly resolved by infrared and 1H-NMR spectra (144,145) [Eq. (28)]. 

E. O. Fischer was the first to prepare arene complexes of technetium. Arene ligands in these compound are typically six-electron donors. The nature of the Tc-arene bond is probably very similar to the one of the Tc-Cp bond. The [Tc(arene)2] complexes (arene = benzene or hexamethylbenzene) are prepared by the reaction of TcCLt with the appropriate arene in the presence of AICI3/AI. In a typical reaction, TcCLt, AICI3, and aluminum were heated with benzene in a sealed tube to 135 °C for two days. The product is precipitated as a salt using the hexafluorophosphate ion. A variety of other 18-electron compounds can be prepared from this material, particularly through chemical reduction. 

A number of research groups have reported the preparation of a large number of star-shaped molecules and dendrimers containing ferrocenyl or arene cyclopentadienyliron complexes at the core or the peripheries.300-320 A number of these dendrimers were prepared via cyclopentadienyliron-mediated per-alkylation, -benzylation and -allylation reactions of cationic tri-, tetra- and hexamethylbenzene complexes. These dendrimers were multifunctional materials and have been used in the synthesis of branched organic and organometallic polymers. 

Arene exchange can be induced by irradiation, but give few results of preparative significance, Irradiation of -(p-xylene)-) -cyclopentadienyl-Fe(I) in the presence of hexamethylbenzene gives the corresponding hexamethylbenzene complex in high yield ... 

The formation of the inclusion compounds was selective. /3-CD formed 1 1 inclusion compounds with benzene, toluene, and o-xylene chromium tricarbonyl complexes, and not with w-xylene, />-xylene, guaiacol, methyl anthrani-late mesitylene, or hexamethylbenzene chromium tricarbonyl complexes. Whereas a-CD did not form inclusion complexes with any arene chromium complexes, 7-CD formed 1 1 inclusion complexes with all arene chromium... 

The metal arene bond in (arene)FeCp complexes is photolabile [93], and this forms the basis of the use of these complexes as photoinitiators for cationic polymerizations of epoxides [94]. Arene exchange via this route has not been used extensively. On photolysis of (p-xylene)FeCp in CH2CI2 or acetone by a medium pressure Hg-arc or by bright sunlight , the complex undergoes arene exchange with hexamethylbenzene, paracyclophane and thiophene under mild conditions... 

J0 The reaction of [Fe(t) -arene)] and Tl2[ V/o-7,8-C2B9Hii] results in arene displacement and formation of [l-(ij -arene)-c/oso-l,2,3-FeC2B9Hii] where the number of methyl substituents on the arene vary from 1 to 6 (B. Stfbr, M. Bakardjiev, J. Holub, A. Ruzicka, Z. Padelkovd, P. Stepnicka, Inorg. Chem., 2011, 50, 3097). Explain why product yields were lowest for the pentamethylbenzene and hexamethylbenzene complexes. Provide four unique trends (spectroscopic and electrochemical) that were correlated to increasing methyl substitution at the arene. 

Another study by Kochi et al. examined the chemistry of nitrosation [52]. Several [NO, arene]" Jt-complexes were characterized by UV-visible, NMR, and IR spectroscopy. X-ray quality crystals were also obtained at -78°C with mesitylene, hexamethylbenzene, and other arenas. The highly colored nitroso jt-complexes were prepared directly from nitrosonium salts (i.e., NO+ SbCl ) and the aromatic compound. Structural studies revealed the distance between the ring and the electrophile is roughly 1A less than the sum of the van der Waals radii and the N—O bond length is significantly lengthened as a result of strong donor-acceptor interaction. 

In the analogous hexamethylbenzene complex, the steric control dominates, and the preferred attack occurs on the Cp ligand, although rule 1 gives priority to the arene ligand ... 

As indicated above, manganese and cobalt arene Ti-complexes could not be prepared by a direct method however, cyclization of 2-butyne provides a method of preparation of di(hexamethylbenzene) cobalt Ti-complex (2-14). 

The molecular structure of the binuclear hexamethylbenzene complex is illustrated in Fig. 22.14. The arene is bonded in an mode with a rather long... 

Both dimetallic and trimetallic chloro-bridged uranium hexamethylbenzene complexes have been reported and the structure of the first lanthanide arene complex (hmb)Sm(AlCl3)3 described. ... 

Using the above procedure, arene complexes of many of the block transition metals may be prepared, and substituted benzenes such as toluene, mesitylene (Mes) and hexamethylbenzene may be used instead of benzene. Generally, mesitylene and hexametl lbenzene metal complexes are more readily prepared than the unsubstituted benzene analogues. When halogenated aromatics are used, they undergo dehalogenation reactions affording hydrocarbon Ti-arene-metd complexes [25, 26]. 

The additions of other (polycyclic) aromatic donors to solutions of dichlorine, dibromine or diiodine afford similar new bands, which show significant red shifts with increasing strength of the arene donor. For example, the absorption maximum of the dibromine complexes varies from 280 nm (with chlorobenzene) to 369 nm (with hexamethylbenzene) and similar variations of the new absorption maxima are observed with diiodine complexes (Fig. 2). 

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