EDA complex

Apparently the complex formation with a 71-acceptor is suitable for characterization of the donor ability of the entire -system of the monomers. Simultaneously, it can be derived that the EDA-complex formation is only insignificantly influenced by steric effects. Because the above named variation in structure does not disturb the planarity of the center of the monomer double bond, the interaction of the 71-systems from both donor and coplanar acceptor cannot be limited by steric effects. 

The methyl substitution at a-position leads to an increase of the reactivity of styrene during polymerization as well as EDA-complex formation. However, the methyl substitution in p-position achieves an opposite effect. The strengthened complex formation connected with a further increase of the HOMO is faced with a drastically decreased polymerization rate. This can be explained by the well known steric effect of group hindrance around the p-C-atom under attack 72), as well as the polarity switch in the vinyl double bond. The p-C-atom in the p-methyl styrene possesses a... 

During the cationic homopolymerization, orbital effects as well as charge effects are essential in contrast to the EDA complex formation where apparently orbital effects dominate. The polymerizations are also aided by appearence of negative partial charges at the p-C-atom. 

The validity of this statement is confirmed by the rates of IC1 additions (see Table 12). Because for these additions the formation of a cationic intermediate by direct attack of the electrophile on the double bond is rate determining, their order of rates is comparable to those of polymerizations. It is therefore understandable that the polymerization rates correlate much better with the reactivities of the monomers during an electrophilic addition of cationogenic agents (such as IC1) than with the relatively unspecific EDA complex formation. 

In electron donor-acceptor (EDA) complexes, there is always a donor molecule and an acceptor. The donor may donate an unshared pair (an n donor) or a pair of electrons in a ti orbital of a double bond or aromatic system (a it donor). One test for the presence of an EDA complex is the electronic spectrum. These complexes generally exhibit a spectrum (called a charge-transfer spectrum) that is not the same as the sum of the spectra of the two individual molecules. Because the first excited state of the complex is relatively close in energy to the ground state, there is usually a... 

Complexes in Which the Acceptor Is a Metal Ion and the Donor is an Alkene or an Aromatic Ring. Note that n donors do not give EDA complexes with metal ions but form covalent bonds instead. Many metal ions form complexes, which are often stable solids, with alkenes, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture... 

These have often been called charge-transfer complexes, but this term implies that the bonding involves charge transfer, which is not always the case, so that the more neutral name EDA complex is preferable. See Ref. 75. 

For examples of EDA complexes that do not show charge-transfer spectra, see Bentley, M.D. Dewar, M.J.S. Tetrahedron Lett., 1967, 5043. 

Detection of an Intermediate. In many cases, an intermediate cannot be isolated but can be detected by IR, NMR, or other spectra. The detection by Raman spectra of NOj was regarded as strong evidence that this is an intermediate in the nitration of benzene (see 11-2). Free radical and triplet intermediates can often be detected by ESR and by CIDNP (see Chapter 5). Free radicals [as well as radical ions and EDA complexes] can also be detected by a method that does not rely on spectra. In this method, a doublebond compound is added to the reaction mixture, and its fate traced. One possible result is cis-trans conversion. For example, cis-stilbene is isomerized to the trans isomer in the presence of RS- radicals, by this mechanism ... 

It has been shown that in certain cases (e.g., Me4Sn + I2) the reactants in an Se2 reaction, when mixed, give rise to an immediate charge-transfer spectrum (p. 102), showing that an electron donor-acceptor (EDA) complex has been formed. In these cases it is likely that the EDA complex is an intermediate in the reaction. 

This association has its counterpart that was also variously described as an encounter complex, a nonbonded electron donor-acceptor (EDA) complex, a precursor complex, and a contact charge-transfer complex.10 For electrically charged species such as anion/cation pairs (which are relevant to ion-pair annihilation), the pre-equilibrium association results in contact ion pairs (CIP)7 (equation 3)... 

Since the intensity of the charge-transfer absorption is directly related to the concentration of the EDA complex or contact ion pair in equations (4) and (5), respectively, it can be used as an analytical tool to quantify complex formation in equations (2) and (3). According to the commonly utilized Benesi-Hildeb-rand treatment,16 the formation constants are quantitatively evaluated from the graphical plot of the CT absorbance change (Acr) as the donor is progressively added to a solution of the acceptor (or vice versa) (equation 6)... 

A relatively strong organization of an electron donor by an acceptor is typically indicated by experimental values of KEUA or KC f> > 10 M-1. For intermediate values of the formation constant, i.e., 1 < KE A < 10 m, the donor/acceptor organization is considered to be weak.17 Finally, at the limit of very weak donor/acceptor organizations with KEDA 1, the lifetime of the EDA complex can be on the order of a molecular collision these are referred to as contact charge-transfer complexes.18... 

Fig. 1 Charge-transfer absorption spectra of enol silyl ethers complexes with re-acceptors. (a) Spectral changes accompanying the incremental additions of cyclohexanone enol silyl ether [2] to chloranil in dichloromethane. Inset Benesi-Hildebrand plot, (b) Charge-transfer absorption spectra of chloranil complexes showing the red shift in the absorption maxima with decreasing IP of the enol silyl ethers, (c) Comparative charge-transfer spectra of EDA complexes of a-tetralone enol silyl ether [6] showing the red shift in the absorption maxima with increasing EAs of the acceptors tetracyanoben-zene (TCNB), 2,6-dichlorobenzoquinone (DCBQ), chloranil (CA), and tetracyanoqui-nodimethane (TCNQ). Reproduced with permission from Ref. 37.

Fig. 2 Mulliken correlation of the ionization potentials (IP) of various enol silyl ethers with the charge-transfer transition energies (/jvct) of their EDA complexes with chloranil. Reproduced with permission from Ref. 36.

Table 1 EDA complex formation of enol silyl ethers with various electron acceptors in dichloromethane.

Indeed, the (200-fs) laser excitation of the EDA complexes of various benz-pinacols with methyl viologen (MV2+) confirms the formation of all the transient species in equation (59). A careful kinetic analysis of the decay rates of pinacol cation radical and reduced methyl viologen leads to the conclusion that the ultrafast C—C bond cleavage (kc c = 1010 to 1011 s- ) of the various pinacol cation radicals competes effectively with the back electron transfer in the reactive ion pair. 

Thermal activation. Cleavage of benzpinacols can also be achieved by thermal reactions, in which powerful electron acceptors such as DDQ ( ed = 0.6 V versus SCE) effectively oxidize electron-rich pinacols. For example, the blue-green color of the EDA complex formed upon mixing of tetraanisylpinacol and DDQ in dichloromethane bleaches within minutes (in... 

Charge-transfer activation of the EDA complex leads to the ion-radical pair, in which the bicumene cation radical undergoes a unimolecular fragmentation (equation 64). 

Photochemical osmylation. The irradiation of the charge-transfer bands (Fig. 13) of the EDA complex of 0s04 with various benzenes, naphthalenes, anthracenes, and phenanthrene yields the same osmylated adducts as obtained in the thermal reactions. For example, irradiation of the purple solution of anthracene and 0s04 in dichloromethane at k > 480 nm yields the same 2 1 adduct (B) together with its syn isomer as the sole products, i.e.,... 

Moreover, the thermal nitration of various aromatic substrates with different X-PyNO cations shows the strong rate dependence on the acceptor strength of X-PyNO and the aromatic donor strength. This identifies the influence of the HOMO-LUMO gap in the EDA complexes (see Chart 3), and thus provides electron-transfer activation as the viable mechanistic basis for the aromatic nitration. Indeed, the graphic summary in Fig. 18 for toluene nitration depicts the isomeric composition of o-, m- and p-nitrotoluene to be singularly invariant over a wide range of substrate selectivities (k/kQ based on the benzene... 

The disproportionation of nitrogen dioxide has been independently confirmed by the direct observation of nitrosonium cation as the EDA complex of hexamethybenzene by spectroscopic means as well as by the spectral comparison with the authentic [HMB, NO+] complex,240 i.e.,... 

Fig. 19 Temperature-dependent interconversion of the hydroquinone ether (MA) cation radical (Amax = 518nm) and its EDA complex with nitrosonium cation (imax = 360 nm) according to equation (86) in the temperature range from +40°C to -78°C (incrementally).

The Wheland intermediate in equation (87) is identified by time-resolved spectroscopy as follows.247 Laser excitation of the EDA complex of NO+ with hexamethylbenzene in dichloromethane immediately generates two transient species as shown in the deconvoluted spectrum in Fig. 20. The absorption band at lmax = 495 nm is readily assigned to the cation radical of... 

However, the Wheland intermediate decays on the early nanosecond time scale to restore the original EDA complex. The observation of the EDA complexes, the ion-radical pair, as well as the Wheland intermediate ArH(NO)+ points to the reaction scheme for thermal nitrosations shown in Scheme 23. 

The electron-transfer formulation in Scheme 23 suggests that the efficiency of nitrosation is the direct result of the competition between deprotonation of the Wheland intermediate versus its breakdown to the original EDA complex via the ion-radical pair, i.e.,... 

The donor-induced disproportionation in equation (91) leads to the EDA complex, i.e., [D, NO+]NO as the (first) directly observable intermediate. The critical role of the nitrosonium EDA complex in the electron-transfer activation in equation (92) is confirmed by the spectroscopic observation of the cation-radical intermediates (i.e., D+ ) as well as by an alternative (low-temperature) photochemical activation with deliberate irradiation of the charge-transfer band252 (equation 95). 

The electron-transfer paradigm in Scheme 1 (equation 8) is subject to direct experimental verification. Thus, the deliberate photoactivation of the preequilibrium EDA complex via irradiation of the charge-transfer absorption band (/ vCT) generates the ion-radical pair, in accord with Mulliken theory (equation 98). 

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

The asterisk identifies an excited ion pair with the same mean separation r as that in the precursor or EDA complex. The ther-... 

We associate such variations in the work term wp with changes in the mean separation r in the EDA complexes. Qualitatively, such changes may be viewed as steric effects which hinder the close approach of the acceptor and the donor. For example, the constancy of wp for the substituted-anthracene donors accords with the minor steric perturbation a substituent is expected to exert... 

---