Hexafluorobenzene-benzene complexes

Figure 1.7 X-ray costal structure of the benzene-hexafluorobenzene 1 1 complex, measured at 30 l< in the lowest-temperature modification [17bj.

There is no experimental evidence for 7r-complexation of arenes to gold centers in the condensed phase. Quantum-chemical calculations were carried out on various levels of theory for 1 1 and 2 1 complexes of benzene and substituted benzenes with Au+ in the gas phase. For all model systems investigated, it has been predicted that an -coordination to a single carbon atom is the ground state of the cation (Cy-symmetry for [(C6H6)Au]+ and 6 -symmetry for [(C6H6)2Au] ). Similar results were obtained for hexafluorobenzene, for which a coordina-... 

F.A. Baiocchi et al., Molecular beam studies of hexafluorobenzene, trifluorobenzene, and benzene complexes of hydrogen fluoride. The rotational spectrum of benzene-hydrogen fluoride. J. Phys. Chem. 87, 2079-2084 (1983)... 

The second kind of binary systems contain benzene (B), hexafluorobenzene (HFB), toluene (TL), and cyclohexane (CH) HFB/B (for five different temperatures), HFB/TL, HFB/CH, B/TL, and B/CH. In contrast to the first group, these systems have small deviations from idealitybut some of them have more complex thermodynamic behaviors such as double azeotropy. The information about these mixtures is listed in Table 2. 

The experimental data for the hexafluorobenzene-benzene system in Table 10.2-4 and Fig. 10.2-11 show a rarely encountered degree of complexity in low-pressure... 

Frontera and Deyii [38] warn that although it is true that the interaction energies of benzene with cations and hexafluorobenzene with anions are similar, it is not possible to generalize that the interaction energies calculated for the anion-n complexes are comparable to those obtained for the cation-7T complexes. These authors also indicate that the same is apphcable to Kim s conclusion stating that the largest contribution in anion-n complexes are electrostatic and induction, because as we have seen before, these contributions sharply depend of the Qzz and a values of the aromatic system. 

A specific interaction in the vapour phase between hexafluorobenzene and benzene was reported by Dantzler and Knobler. Recently, Belousova and Verkhova pointed out that interaction virial coefficients for benzene -t-n-hexane have an unusual temperature dependence and are inconsistent with Bia s for benzene with other alkanes. They suggest that this abnormal behaviour is the result of association. Vapour-phase amine complexes with methanol have been studied by i.r. spectroscopy and p, V, r-methods. Equilibrium constants were calculated from the volumetric data under the assumption that the vapours are ideal. 

The condensation of metal vapors with hexafluorobenzene and benzene gives unstable arene complexes of V, Cr, Mn, Fe, Co, Ni, and Pd, for instance, [Cr(C6H6)(C5F6)]. The condensation of vapors of chromium or iron with the mixture of vapors of benzene and PF3 or hexafluorobenzene and PF3 leads to the formation, with low yields, of the derivatives [Cr(C6F6)(PF3)3], [Cr(PhH)(PF3)3], and [Fe(PhH)(PF3)2]. Heteroleptic compounds of chromium, for example, [ (CgF ) (Pp3)3], are stable in contrast to [Cr(C6F6)2] which is unstable and explosive. The condensation method was also utilized for the preparation of hexahapto pyridine complexes of chromium. 

Marx and Lees studied quantum yields, O, for the photochemical reaction of (Cp)It(CO)2 with in hexafluorobenzene at 20°C and found that O shows a saturation effect with increasing and is independent of [CO]. They concluded that the rate-controlling process is not CO dissociation and suggest that it is an T to T slippage, which then allows complexation of benzene prior to oxidative addition. Drolet and Lees proposed a similar slippage mechanism for the photolysis of (Cp)Rh(CO)2 in the presence of PPhj in decalin at lOX. Their conclusion is based on the observation that the quantum yield for CO replacement is first-order in [PPhj] and unaffected by 9xl(T M CO. 

The benzene dimer has two nearly isoenergetic structures (sHpped-parallel and T-shaped) [9,11]. Dispersion stabiUzes the slipped-parallel benzene dimer strongly, while the electrostatic interaction is repulsive in the slipped-parallel dimer. However, the electrostatic interaction stabihzes the T-shaped benzene dimer, while the stabilization due to dispersion is small in the T-shaped dimer [9]. On the other hand, both dispersion and electrostatic interactions stabihze the stacked hexafluorobenzene-benzene complex, producing a large attractive force between hexafluorobenzene and benzene. 

A photochemical route can also be used to effect the oxidative addition of a carbon-fluorine bond to rhodium(I). Such a conversion has been achieved by the photolysis of (>/ -cp )Rh(C2H4)(PMe3) in the presence of hexafluorobenzene, where the initial step involves displacement of C2H4 to give a / -complexed CeFe, followed by the photochemical oxidative addition of an aromatic C-F bond (Scheme 7.6). A similar tj -complexed aromatic moiety has been observed in the pulsed laser flash photolysis of Cr(CO)6 in benzene ... 

Patrick CR, Prosser GS (1960) A molecular complex of benzene and hexafluorobenzene. Nature 187 1021... 

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