Arenes, electronic interactions

The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst environment. Instead, the Fe center can influence a catalytic asymmetric process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. This interaction is often comparable to the stabilization of a-ferrocenylcarbocations 3 (see Sect. 1) making use of the electron-donating character of the Cp2Fe moiety, but can also be reversed by the formation of feirocenium systems thereby increasing the acidity of a directly attached Lewis acid. Alternative applications in asymmetric catalysis, for which the interaction of the Fe center and the catalytic center is less distinct, have recently been summarized in excellent extensive reviews and are outside the scope of this chapter [48, 49], Moreover, related complexes in which one Cp ring has been replaced with an ri -arene ligand, and which have, for example, been utilized as catalysts for nitrate or nitrite reduction in water [50], are not covered in this chapter. 

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-... 

Electronic Interactions Between Polycyclic Arenes in Cyclophanes... 

Figure 2. Effect of orientation on the electronic interactions of arene excimers (a), amine-arene exciplexes (b), and charge-transfer complexes (c).

The classic example for the tt-tt electron interaction between polycyclic arenes is the pyrene excimer (11). Upon UV excitation of a 10 5 M pyrene solution, the structured fluorescence of monomeric pyrene molecules is mainly observed. The increase of the concentration to 10 3 M diminishes the monomeric fluorescence, and a new broad and completely structureless excimer band appears, which is red-shifted by 5000-6000 cm-1. This phenomenon can be explained through potential curves of the electronic ground state and the excited singlet state (I, 12). The spectroscopic shift between the fluorescence of the excimer and the monomer depends on the depth of the potential well in the excited state that is, the red shift is proportional to the binding energy of the excimer. 

Photodimerization of stilbene in van der Waals nanocapsules has been studied [16]. para-Hexanoylcalix[4]arene nanocapsules (Figure 5.3) were used as hosts to carry out photodimerization of cis- and frans-stUbene to syn-tetraphenylcyclobutane. Singlecrystal X-ray diffraction studies were performed to define precisely the location of encapsulated stilbenes inside the capsule. It was shown that ds-stilbene stacks as Jt-Jt dimers located at the center of the capsule, whereas trans-stilbene does not form such a dimer. A possible configuration of two stilbene molecules in hydrophobic nanocapsules based on amphiphilic para-hexanoylcalix[4]arene is presented in Figure 5.4. Because the molecules are shifted with respect to each other, they cannot yield a good n-n stack, although local stabilization is possible. The authors suggested that the entire structure is a compromise between K-electron interaction of a trans-stilbene molecule with the host and n-n interactions of the two trans-stilbene molecules. 

In the solid state, the photophysical properties of C o/p-tBu-calix[8]arene reveal a triplet maximum at 780 nm, matching that of Ceo/y-CD under similar conditions. As a consequence of the electronic interactions between Cso and / - Bu-calix[8]arene, the lifetime at 1.7 ps is, however, substantially shortened relative to triplet lifetimes in solutions. Despite this notable impact, the short-lived triplet is efficiently deactivated in the presence of molecular oxygen, thus, photosensitizing the formation of singlet oxygen, O2 ( Ag). 

The bonding in the arene complex [(T) -C6H6)Ru]2(ii-H)3 + has also been considered in terms of (1) an Ru = Ru triple bond and (2) a situation involving 3-center-2-electron interactions [100,101]. Of these, the latter was concluded to... 

Fullerenes are also known to form the inclusion complexes with calixarenes. In the case of the water-soluble inclusion complex of Qo and calixarene (cationic homoox-acalix[3]arene), substantial interaction between fullerenes and the calixarene was observed in the ground state absorption spectrum [76]. Increase in the absorption intensity around 400-500 nm of C q in calixarene can be attributed to the charge-transfer complex formation due to the n -electron system of calixarene. Strong interaction between the calixarene and fullerenes was also observed in the excited states. The triplet absorption peak of 60 in the calixarene appeared at 545 nm, which is largely blueshifted compared to that of pristine Cgg- The triplet lifetime is as short as 50 ns. The substantial interaction between calixarene and included Qo was also observed in the singlet excited state as a large blueshift of the fluorescence peak. 

---