Arene donor

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). 

X-ray structural analyses reveal that the jt-bonding of dihalogens, halocar-bons and halides to arene donors and acceptors are characterized mostly by over-the-rim coordination in which the dihalogen acceptor generally follows the position of highest electron density on the aromatic donor, and the arrangement of halide donor mostly follows the LUMO shape of the aromatic acceptor. 

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

Productive bimolecular reactions of the ion radicals in the contact ion pair can effectively compete with the back electron transfer if either the cation radical or the anion radical undergoes a rapid reaction with an additive that is present during electron-transfer activation. For example, the [D, A] complex of an arene donor with nitrosonium cation exists in the equilibrium with a low steady-state concentration of the radical pair, which persists indefinitely. However, the introduction of oxygen rapidly oxidizes even small amounts of nitric oxide to compete with back electron transfer and thus successfully effects aromatic nitration80 (Scheme 16). 

Photoactivation of the bis(arene)iron(II) complexes with ferrocene and arene donors by the selective irradiation of the charge-transfer absorption bands as in (6) uniformly results in the de-ligation of the acceptor moiety... 

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.). 

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... 

Fig. 10 Variations of (A) the formation constant and (B) the N-O stretching frequencies in NO+ complexes with the ionization potential of the arene donor (as...

Fig. 11 Degree of charge transfer in NO+ complexes with the variation in the ionization potential of the arene donors, as evaluated by (34).

The mechanistic conundrum presented by such a dichotomy between electron-transfer and electrophilic processes can only be rigorously resolved by the experimental proof of whether the cation radical (or the electrophilic adduct) is, or is not, the vital reactive intermediate. However, in a thermal (adiabatic) reaction between arene donors and the nitrosonium cation, such reactive intermediates cannot be formed in sufficient concentrations to be observed directly by conventional experimental methods since their rates of follow-up reactions must perforce always be faster than their rates of formation, except when they are formed in a reversible equilibrium like the... 

Fig. 18 Energy diagram for the first step of an electrophilic substitution reaction illustrating the crossing of DA and D +A" configurations. The effect of a substituent that stabilizes the D + A configuration (e.g. by improving the arene donor ability) is indicated by the dotted line. The diagram illustrates the correlation between AAv, the difference in excitation energies for the perturbed and unperturbed systems, and AE, the difference in activation energy for the two systems. (Avoided crossing deleted for clarity)...

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. 

The energy difference AGET° between the vibronically equilibrated reactant and product states can be considered as the difference between the IP of the donor and the EA of the acceptor (in the gas phase) or between the corresponding electrochemical potentials (in solution). For a set of structurally related arene donors in the same solvent, a linear (Mulliken) correlation is usually observed experimentally between the donor strength and the CT energy (due to the relatively small changes in A), i.e. ... 

CT transition energy of the high-energy (hvH) and with sterically hindered donors) on the arene donor low-energy (hvi.) bands with the oxidation potential strength. Data from ref. [28]. 

Arene donor E act VvsSCE KcTt hvCT, eV ecTi m cm-1 Arene donor E oxr V vs SCE Kct> M-1 hvcr> eV ecTi M rcm r... 

Arene donor (ArH) Absorption band, X (nm) Arene donor (ArH) Absorption band, X (nm) ... 

Effects of the Donor/Acceptor Interaction on the ET Dynamics of Arene Donors... 

Fig. 9. (A) Free-energy dependence of the second- the formation product (/

If fcf > feBET- the overall transformation can occur rapidly despite unfavorable driving forces for the electron transfer itself. Only follow-up reactions with high kf can compete with back electron transfer. Different kinds of such unimolecular processes can drive the equilibria toward the final product. A representative example is the mesolytic cleavage of the C-Sn bond in the radical cation resulting from the oxidation of benzylstannane by photoexcited 9,10-dicyanoanthracene (DCA). This is followed by the addition of the benzyl radical and the tributyltin cation to the reduced acceptor DCA [59]. In the arene/nitrosonium system, [ArH, NO+] complexes can exist in solution in equilibrium with a low steady-state concentration of the ion-radical pair. However, the facile deprotonation or fragmentation of the arene cation radical in the case of bifunctional donors such as octamethyl(diphenyl)methane and bicumene can result in an effective (ET) transformation of the arene donor [28, 59]. Another pathway involves collapse of the contact ion pair [D+, A- ] by rapid formation of a bond between the cation radical and anion radical (which effectively competes with the back electron transfer), as illustrated by the examples in Chart 5 [59]. 

The same features are observed in the osmylation of arene donors. Thus, osmium tetra-oxide spontaneously forms complexes with arenes, and the systematic spectral shift in the CT bands parallels the decrease in the arene IP [59]. The same osmylated adducts are obtained thermally on leaving mixtures to stand in the dark or upon irradiation of the CT bands at low temperature. Time-resolved spectroscopy establishes that irradiation of the CT band of the anthracene/osmium tetraoxide complex leads directly to the radical-ion pair ANT+, 0s04, which then collapses to the osmium adduct (with a rate constant fc 109 s 1) in competition with back ET [59]. 

C. Charge-Transfer Structures of Bis(arene)iron(ll) Acceptors with Ferrocene and Arene Donors... 

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