Tetracyanoethene, charge-transfer complex

In the case of the reaction of anthracene with tetracyanoethene, there is kinetic evidence for an intermediate that is distinct from the rapidly formed charge transfer complex. The intermediate is proposed to be a tight complex with a geometry favorable for formation of the adduct. The formation of the charge transfer complex is fast and reversible and may also lie on the overall reaction path. 

Flgme 5.43. A depiction of a charge-transfer complex between tetracyanoethene (tetracyanoethylene [(N=C)2C=C(0N)2]), which, in this case, is acting as an electron acceptor (Lewis acid) and benzene (CaHe), which, in this case, is acting as an electron donor (Lewis base). 

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). 

For the charge-transfer spectra of the complexes of some 3-substituted benzo[fo]thiophenes with tetracyanoethene, a linear correlation exists between crp+ substituent constants and the wave number of the first absorption maximum.101... 

First, we note that the charge ordering of the solvent can impose itself on the distribution of products in reaction. Chiappe and Pieraccini [40] report that in their study of electron transfer between Micheler s ketone and tetracyanoethene, they observed that the formation of a radical ion pair to be preferred over formation of a single, neutral complex. Such a preference is only observed for the most highly polar molecular liquids, and is analogous to the spontaneous ionization of metal atoms in fused salts noted above. This represents a novel phenomenon for moderately polar solvents, though its generality is unclear at this time. 

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