Electron transfer donor acceptor complexes

Aromatic substrates are by far the most commonly used substrates in the rapidly expanding area of photoinduced electron transfer [1,2]. This is obviously due to the favourable location of the frontier molecular orbitals in such compounds. The same factor facilitates the formation of electron transfer donor-acceptor (EDA) complexes both in the ground state (these possibly are intermediates in some thermal reactions, e.g. selected electrophilic substitutions), and in the excited state (exciplexes). 

Wynne K, Galli C and Hochstrasser R M 1994 Ultrafast charge transfer in an electron donor-acceptor complex J. Cham. Phys. 100 4796-810... 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

The donor-acceptor formation can be considered by transfer of electrons from the donor to the acceptor. In principle one can assume donor-acceptor interaction from A (donor) to B (acceptor) or alternatively, since B (A) has also occupied (unoccupied) orbitals, the opposite charge transfer, from B to A. Such a view refers to mutual electron transfer and has been commonly estabUshed for the analysis of charge transfer spectra of n-complexes [12]. A classical example for a donor-acceptor complex, 2, involving a cationic phosphorus species has been reported by Parry et al. [13]. It is considered that the triaminophosphines act as donor as well as an acceptor towards the phosphenium cation. While 2 refers to a P-donor, M-donors are in general more common, as for example amines, 3a, pyridines, 3b, or the very nucleophilic dimethylaminopyridine (DMAP) [ 14], 3c. It is even a strong donor towards phosphorus trichloride [15]. 

As we have seen from reaction 4.49 donor-acceptor complexes (Lewis- or 7r-type) occur in a fairly inert medium (such as cyclohexane) via charge transfer between a base (electron donor) and an acid (electron acceptor by its electron deficiency). In a few instances, e.g., in the Bonitz titration29 of the precatalyst diethylalaminium chloride with isoquinoline, the complex constists of an ion-pair ionizate. 

Mulliken [3] presented a classification of electron donor-acceptor complexes based on the extent of intermolecular charge transfer that accompanies complex formation. An outer complex is one in which the intermolecular interaction B- XY is weak and there is little intra- or intermolecular electric charge redistribution, while an inner complex is one in which there is extensive electric charge (electrons or nuclei) redistribution to give [BX] + - -Y . Inner complexes are presumably more strongly bound in general than outer complexes. 

The donor-acceptor complexes [Ir(/r-dmpz)(CO)(PPh2 0(CH2)2R )]2 exhibit photo-induced electron-transfer rate constants of 1012s—1 and charge recombination rates slower than 2 x 10los-1 when R = pyridine and 4-phenylpyridine.534 Further studies on these complexes revealed that recombination reactions were temperature dependent and slower for the deuterated acceptors.535... 

Molecules of this type are often called donor-acceptor complexes or sometimes charge transfer complexes (because charge is transferred from the donor to the acceptor as the nonbonding electron pair of the donor atom is shared with the acceptor atom). In other words, there is a formal transfer of one electron, which is evident in the formal charges on the atoms in the complex. Once formed, however, the bond is simply a covalent bond consisting of a pair of shared electrons, whose origin is irrelevant to the nature of the... 

Decay of donor-acceptor complex through electron transfer to solvent molecules yielding primary radical ion pairs ... 

Having shown that the enol silyl ethers are effective electron donors for the [D, A] complex formation with various electron acceptors, let us now examine the electron-transfer activation (thermal and photochemical) of the donor/ acceptor complexes of tetranitromethane and quinones with enol silyl ethers for nitration and oxidative addition, respectively, via ion radicals as critical reactive intermediates. 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

Indeed, the extent of disproportionation of NO according to equation (89) clearly depends on the donor strength of the aromatic hydrocarbon.240 For example, hexamethylbenzene which is a strong donor (IP = 7.85 V) promotes the ionization of NO to an extent of 80% whereas the weaker donor durene (IP = 8.05 V) affords less than 25% ion-pair formation. Furthermore, the resulting NO+ cation is a powerful electron acceptor (Erea = 1.48 V versus SCE) in contrast to NO (Ered = 0.25 V versus SCE) and thus readily forms donor/acceptor complexes with a variety of aromatic, olefinic and heteroatom-centered donors. Accordingly, the donor/acceptor complexation and electron-transfer activation are the critical steps in various transformations in Chart 8 as described below. 

Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions, 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1... 

Electron transfer, in thermal and photochemical activation of electron donor-acceptor complexes in organic and organometallic reactions, 29,185 Electron-transfer, single, and nucleophilic substitution, 26,1 Electron-transfer, spin trapping and, 31,91 Electron-transfer paradigm for organic reactivity, 35, 193... 

We now return to the thermal electron transfer reaction in eq 20, in which the rate-limiting activation process has been shown to proceed from the electron donor acceptor complex (23), i.e.,... 

Ferraris J, Cowan DO, Walatka V Jr, Perlstein JH (1973) Electron transfer in a new highly conducting donor acceptor complex. J Am Chem Soc 95 948-949... 

Electron Transfer in the Thermal and Photochemical Activation of Electron Donor-Acceptor Complexes in Organic and Organometallic Reactions... 

The counterpart to the photo-induced electron transfer is the corresponding thermal transformation of the electron donor-acceptor complex the barrier to such an adiabatic electron transfer is included in Fig. 18 as T, with the implicit understanding that solvation is an intrinsic part of the activation process (Fukuzumi and Kochi, 1983). When the rate of back electron transfer is diminished (e.g. by a reduced driving force), the dynamics for the contact ion pair must also include diffusive separation to solvent-separated ion pairs and to free D+- and A-- (Masnovi and Kochi, 1985a,b Yabe et al., 1991). 

A theoretical formalism is available for understanding optical charge transfer processes in a variety of chemical systems (mixed-valence ions, donor-acceptor complexes, metal-ligand charge transfer chromophores, etc) where the extent of charge transfer is large and where electronic coupling between the electron donor and acceptor sites is relatively small. 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

Attack on the aromatic ring and formation a n-complex or electron donor-acceptor complex N02 + ArH —> ArH N02. This complex involves high electrostatic and charge-transfer interactions between the n-aromatics and nitroninm ion. 

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