Ion-pair annihilation

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)... 

The explanation for the solvent and salt effect in Scheme 22 lies in the dynamics of the photogenerated ion-radical triad in equation (81). Thus, the ion-pair annihilation is favored in nonpolar solvents such as dichloromethane to afford the alkylation product237 (equation 82). 

Due to the longer lifetime of the arene radical ration in the SSIP, 37 undergoes isomerization to 38 yielding after return electron transfer and collapse of the reactive intermediates the Hetero Diels Alder adducts 40 and 41 in a. ratio of ra. 1 2.5. There is also experimental evidence for some participation of an ion pair annihilation leading directly to 40 and 41. 

V. Nucleophilic and Electron-Transfer Processes in Ion-Pair Annihilation.. 96... 

Electron donors (D) and acceptors (A) constitute reactant pairs that are traditionally considered with more specific connotations in mind, such as nucleophiles and electrophiles in bond formation, reductant and oxidant in electron transfer, bases and acids in adduct production, and anion and cations in ion-pair annihilation (12). In the latter case, the preequilibrium formation of contact ion pairs (CIP), that is,... 

The formulation in Scheme 4 derives from the ion-pair annihilations via the known behavior of 19- and 17-electron carbonylmanganese radicals (67). Accordingly, the initiation by electron transfer is included in the generalized mechanism for Mn-Mn bond formation (Scheme 4). The... 

Such an ion-pair annihilation can be formally considered as the microscopic reverse of the disproportionation in Eq. (30). 

The presence of 1 equivalent of TPP1 OTf- with the chromium anion TpCr(CO)3- as the tetrabutylammonium salt in dichloromethane results in the loss of the carbonyl bands of the anion at 1890 and 1740 cm-1. Their complete replacement by the sharps, band at 2018 cm-1 and the broad E band (1898 and 1838 cm-1) of the 17-electron radical TpCr(CO)3- indicates that the ion-pair annihilation proceeds to completion. Variation of the pyrylium cation, by the replacement of TPP+ with a weaker acceptor such as tri-p-anisylpyrylium triflate (TAP+ OTf-), consistently results in lower conversions of the carbonylmetal anions. For example, the treatment of TpMo(CO)3 with the TAP+ salt leads to a light red solution of TAP (Am 560 nm) (92) and a greatly diminished concentration of TpMo(CO)3- as judged by the reduced carbonyl absorbances in comparison with that obtained from TPP+ at the same concentration. Even with this weaker acceptor cation, however, the strong chromium anionic donor TpCr(CO)3- is completely oxidized by 1 equivalent of TAP+ to form TpCr(CO)3- in essentially quantitative yields. 

Indeed, the 17-electron radicals in Eq. (43) with M = Mo and W can both be readily collected as red and orange crystals merely when the ion-pair annihilation is carried out in acetonitrile solutions. 

NUCLEOPHILIC AND ELECTRON-TRANSFER PROCESSES IN ION-PAIR ANNIHILATION... 

The formation of 6a is particularly diagnostic, since this unique carbon-carbon bonded reductive dimer was demonstrated by Wrighton and coworkers to arise via the transient 19-electron radical ( -cyclohexa-dienyl)Fe(CO)3 by regiospecific coupling at a ligand center (105). Furthermore the 17-electron radical CpMo(CO)3- is the precursor to the accompanying oxidative dimer [CpMo(CO)3]2 (7), as described in the earlier anodic studies of CpMo(CO)3 (106). Accordingly these products (6 and 7) of ion-pair annihilation are referred to hereafter as radical (homo) dimers. 

The singular absence of a nucleophilic adduct from either 3 or 4 suggests that much milder conditions be employed in the ion-pair annihilation. When a THF slurry of the cyclohexadienyliron cation 3 is treated at — 78 °C with CpMo(CO)3-, it gradually dissolves to afford a bright yellow mixture. No evidence of the red intermediate that is readily apparent at higher temperatures (see above) can be discerned. Instead, after the separation of the colorless precipitate of PPN+ PF6 at — 50°C, the IR spectrum of the chilled solution is found to be essentially identical to that of the nucleophilic adducts (5) (see above), that is,... 

The mechanism for ion-pair annihilation in Scheme 11 taken in a more general context predicts the relative amounts of nucleophilic adduct and homodimers to be modulated by the homolytic rate constant, namely, whether kH is small or large. Such a conclusion requires the high selectivity to derive from a strong dependence of the feL—mo bond strength on the structure of the nucleophilic adduct. Thus, a closer consideration of feLmo... 

In Scheme 12, step (i) represents ion-pair annihilation by nucleophilic addition to the methylene terminus of the ligand to yield the initially observed intermediate A, which is transformed by a 1,2-shift of mo to the iron center in step (ii) to generate the fluxional species B and B (step iii)... 

The generalized mechanism for ion-pair annihilation as presented in Scheme 11 involves the rather circuitous route for radical-pair production [involving Eqs. (55) and (56), certainly in comparison with the direct electron-transfer pathway (Scheme 8)]. In other words, why do ion pairs first make a bond and then break it, when the simple electron transfer directly from anion to cation would achieve the same end The question thus arises as to whether electron transfer between Fe(CO)3L+ and CpMo(CO)3 is energetically disfavored. The evaluation of the driving force for the electron transfer process obtains from the separate redox couples, namely,... 

The overall driving forces for electron transfer [— AG = (E + is ,)] for ion-pair annihilation in THF between mo and feL+ (i.e., cations 1, 2, 3, and 4) are AG — 3.2, 3.9, 5.3, and 6.0 kcal/mol, respectively, based on the electrochemical measurements (115). Such driving forces all easily lie within the isoergonic bounds for the facile electron transfer between feL+ and mo-. Moreover, the differences in driving forces are not sufficient to strongly distinguish the cations 1 and 2 from their cyclic analogs 3 and 4 for the consideration of simultaneous nucleophilic addition and electron transfer, as presented in possibility (c) above. 

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