Hexamethylbenzene, complex

The hexamethylbenzene complex could be similarly deprotonated [54, 57] but the red complex obtained is then stable and its x-ray crystal structure could be recorded [55], showing a dihedral angle of 32° between the cyclohexadienyl plane and the exocyclic double bond (Fig. 5). This complex can also be cleanly obtained by the reaction of dioxygen with the 19e wostructural complex Fe CpfCgMeg) as shown in the preceding section. 

The composition of the electrical effect in the case of the 1,4-benzo-quinones seems to depend upon the degree of substitution. The association constants and charge transfer absorption energies of the 2-substituted-l,4-benzo-quinone-hexamethylbenzene complexes (sets 15-2 and 154) show values of pj of 55 and 44 respectively. Values of p are set forth in Table XVII. 

Figure 16a shows the progressive bathochromic shift in the CT absorption bands (hvct) obtained from PyN02+ with aromatic donors with increasing donor strength (or decreasing ionization potential). A similar red shift is observed in the CT absorption bands (hvCj) of hexamethylbenzene complexes with various para-substituted JV-nitropyridinium cations (X-PyNO ) as shown in Fig. 16b. Such a trend in the hvct is in accord with the increasing acceptor strength of X-PyNO in the order X = OMe < Me < H < Cl < C02Me < CN.

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... 

An intermediate case occurs in another Ru complex, which is formed (equation 23) by endo protonation of the i -Me4T in the i -hexamethylbenzene complex (r] -Me4T)Ru()] -C6Me6). The C(sp )-S distance (1.91(1)A) in (10) is considerably longer than a normal C(sp )-S bond... 

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 ... 

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. 

Fig. 16. Polarographic data for complex formation as a function of donor concentration (in CHaCls at 25°). Hexamethylbenzene complexes of A, tetracyanoethylene B, tetra-cyanobenzene. Pyrene complexes of C, chloranil D, tetracyanobenzene E, tetracyano-quinondimethide. (Data from ref. 64)...

Several years ago Fischer and ROhrscheid (165) prepared hexamethylbenzene complexes of niobium and tantalum of the type [M3 [Cg (CH3) g] 3C10] by reaction of a mixture of the metal halide, hexamethylbenzene, alum-invim, and aluminum chloride at 130 C these cations could be isolated either as the chlorides or as hexa-fluorophosphates. The same workers (165) presented conductivity data in support of the above-mentioned trimetallic formulations, but x-ray data seem necessary before their structures can be considered certain. The reported preparations of the [M3[Cg (CH3)g] 3Clg] ... 

In the [NO, hexamethylbenzene] complex, the N—O bond length is measured as 1.108 A, increasing from 1.06 A found in the uncomplexed nitrosonium ion (NO ) [53]. The effects of inner-sphere electron donation are also observed in the infrared N—O stretching frequencies. With the uncomplexed nitrosonium ion, N—O stretch is observed at 2272 cm", while the [NO, hexamethylbenzene]+ %-complex exhibits a stretch at 1885 cm" [52], For comparison, nitric oxide (-NO) has an estimated bond length of 1.15 A and an N—O stretch frequency of 1876 cm". These data indicate a significant degree of electron transfer in the x-complex however, the complexes are ESR silent, suggesting the nitric oxide and hexamethylbenzene radical cation are not fuUy formed species. 

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 ... 

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... 

---