Arene EDA complexes

Carbocations as electron acceptors in aromatic EDA complexes 192 Bis(arene)iron(II) complexes with arene and ferrocene donors 198 Carbonylmetallate anions as electron donors in charge-transfer salts 204 Aromatic EDA complexes with osmium tetroxide 219... 

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

The formation of the monocationic intermediate (ArH)2Fe+ attendant upon the charge-transfer excitation of either the ferrocene or methylan-thracene EDA complex (7a and 7b) is responsible for the photo-induced de-ligation of bis(arene)iron(II), as described in (6). Thus, transient electrochemical studies (Karpinski and Kochi, 1992a,b) show that the catalytic de-ligation of (ArH)2Fe+ proceeds rapidly via a (two-step) electron-transfer chain or ETC process (8). 

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-... 

The recent time-resolved spectroscopic studies described above (Sections 2 and 3) identify the charge-transfer excitation (/n cr) of aromatic EDA complexes with various types of acceptors (A) to their ion-radical pairs [ArH+-,A ] (Mataga, 1984 Hilinski et al., 1984 Jones, 1988). Such electronic transitions in weak EDA complexes, like those of the halogen acceptors, are mainly associated with the excited states, such as in (32), since the variations in the ground state are minor owing to formation constants K that are not strongly dependent on the arene donor (Briegleb, 1961, pp. 106 ff.). 

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...

In a first successful approach, Kochi, Renzepis, and co-workers [41] chose EDA complexes of 9-cyanoanthracene (14) and tetracyanoethene (TCNE, 15) since their charge transfer (CT) absorption bands are well separated from the absorption bands of the monomers. Excitation with a 25 ps laser pulse produced two transient absorption bands near 460 and 750 nm, which decayed simultaneously within ca. 60 ps. As was shown in the chloranil-enolether system 9—10, cf. Fig. 6), the transients can be identified with the arene radical cation (14a+ ) and the olefin radical anion (/5- ), respectively (Scheme 5). 

According to Mulliken [9], arenes are classified as electronic donors (D) in measure with their degree of electron-rich character, as evaluated by their ionization potential (IP, gas phase) or oxidation potential (E°ox, solution) [12]. The intermolecular interaction of the arene donor (D) and electronic acceptor (A) spontaneously leads to the electron donor/ acceptor or EDA complex, i.e. 

Kochi and co-workers have recently identified and characterized the weak charge transfer complexes between tropylium ion and a series of substituted arenes in acetonitrile solution [74], Photoexcitation of these electron donor acceptor (EDA) complexes leads to an electron transfer from the arene donors to the tropylium ion in accord with Mulliken s theory [75]. Time resolved spectroscopic observation of the arene radical cations (formation within the 30 ps laser pulse) has confirmed their intermediacy. The subsequent decay of the photogenerated radical cation and the concomitant regeneration of the ground state EDA complex occurs with a rate constant, kBET > 4 x 1010 s 1 (Scheme 11). This fast back electron transfer... 

The linear free enogy relationship obsoved for arene donors relates the activation barrier AG for aromatic substitution directly to the CT transiticxi oiergy Aver of the EDA complex. Since Aver pertains to the energetics of the photoionizadons in equations (27) and (28), the correlation suggests that these arene contact ion pairs ate reasonable approximatims to the transition states for both mercuration and thallation, e.g. Scheme 6. 

Most organometallic EDA complexes of arenes with titanium tetrachloride [116] in solution also follow the general reaction scheme in Eq. 15 in that no net chemical reaction is observed upon charge-transfer irradiation due to rapid back electron transfer (A et 10 ° s ). For example, the transient absorption spectrum of bro-moanthracene (BrAnt) cation radical generated by 532-nm laser excitation of the [BrAnt, TiCU] complex in cyclohexane (see Figure 7) decays completely to the spectral baseline within about 1 ns (see inset) due to back electron transfer [116], (Eq. 18) ... 

Charge-Transfer Activated Reactions of Organometallic EDA Complexes Arene fragmentation via TiCU complexes... 

During these reactions, transient charge-transfer absorption is observed this is ascribed to the EDA complexes of the arene donors with the metal trifluoroacetate [113], (Eq. 26) ... 

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