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Volumn 129, Issue 12, 2007, Pages 3683-3697

Continuum of outer- and inner-sphere mechanisms for organic electron transfer. Steric modulation of the precursor complex in paramagnetic (ion-radical) self-exchanges

Author keywords

[No Author keywords available]

Indexed keywords

COMPLEXATION; INFRARED RADIATION; MOLECULAR ORBITALS; ORGANIC COMPOUNDS; POSITIVE IONS; ULTRAVIOLET RADIATION;

EID: 33947689595     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja069149m     Document Type: Article
Times cited : (115)

References (147)
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    • Generally speaking, a separate deligation step will precede the formation of the bridged-activated complex unless the donor or acceptor is coordinatively unsaturated
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    • The vast majority of quantitative electron-transfer studies of inorganic coordination compounds have been carried out with octahedral complexes, especially when compared to those with a lower metal coordination number, such as linear, square planar, square pyramidal, etc. For the necessity of the separate substitution step, see: Kochi, J. K, Powers. J. W. J. Am. Chem. Soc. 1970, 92, 137
    • (a) The vast majority of quantitative electron-transfer studies of inorganic coordination compounds have been carried out with octahedral complexes, especially when compared to those with a lower metal coordination number, such as linear, square planar, square pyramidal, etc. For the necessity of the separate substitution step, see: Kochi, J. K.; Powers. J. W. J. Am. Chem. Soc. 1970, 92, 137.
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    • We anticipate that inorganic electron-transfer reactions with coordinatively unsaturated metal (coordination) donors and acceptors with square planar coordination, etc, will reveal intermolecular charge-transfer bands
    • (b) We anticipate that inorganic electron-transfer reactions with coordinatively unsaturated metal (coordination) donors and acceptors with square planar coordination, etc., will reveal intermolecular charge-transfer bands.
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    • The best organic electron donors and acceptors are generally substitution-stable and contain planar (aromatic and olefinic) redox centers that are sterically favorable for intermolecular π-interactions
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    • 8a is a suitable electronic but a limited (kinetics) model for a precursor complex in intermolecular (diffusive) electron-transfer processes.
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    • Note that the rate constants for electron-transfer self-exchange for most of the ion radicals from Table 1 are available in the literature. 23b,43 However, the reported values were generally obtained with ion radicals prepared in situ in various solvents and with different counterions. Accordingly, to exclude solvent and counterion effects, we have remeasured all the rate constants using consistently pure ion-radical salts with bulky counterions in the same noncoordinating solvent, i.e, the moderately polar dichloromethane
    • 23b,43 However, the reported values were generally obtained with ion radicals prepared in situ in various solvents and with different counterions. Accordingly, to exclude solvent and counterion effects, we have remeasured all the rate constants using consistently pure ion-radical salts with bulky counterions in the same noncoordinating solvent, i.e., the moderately polar dichloromethane.
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    • SE were somewhat less accurate owing to the more complex hfs patterns with overlapping lines (slow exchange limit). Moreover, the relatively slow second-order rate constants of these closed ion radicals prevented their approach to the fast exchange limit (see details in the Experimental Section).
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    • Note that similar NIR absorption bands were observed in other solvents, such as acetonitrile, acetone, THF, etc. The solvent dependence of such systems will be reported later in detail
    • Note that similar NIR absorption bands were observed in other solvents, such as acetonitrile, acetone, THF, etc. The solvent dependence of such systems will be reported later in detail.
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    • The experimental numbers in Table 5 represent the averaged values over a variety of mutual arrangements involving multiple local minima around the basic structure, as discussed in ref 22. The most common example of such a deviation is the lateral shift parallel or perpendicular to the main axis, as well as mutually perpendicular arrangements. Earlier analysis of tetrathiafulvalene dyads showed that several local maxima result from such deviations, and a somewhat similar behavior can be expected for other systems. For example, for TMPD the mutual arrangement atop each other (as in its salt with the C1O4- counterion) is characterized by H DA, 2.1 × 103 cm-1, while the value of HDA, 2.6 × 103 cm-1 pertains to the dyad resulting from shifts by ∼1.8 Å parallel to the main axis. Likewise, the crossed structure produces the rather small coupling of H
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    • It should be noted that, in Mulliken theory, the charge-transfer transitions in intermolecular complexes are primarily considered in terms of the constituent donor/acceptor redox or ionization potentials, whereas, in Hush theory, the intervalence transition in localized or bridged (intramolecular) mixed-valence complexes are primarily related to the reorganization energies. Since the ion-radical associates of interest in this study are intermolecular mixed-valence complexes, we refer to the NIR optical transitions as charge transfer or intervalence interchangeably to combine both features
    • (b) It should be noted that, in Mulliken theory, the charge-transfer transitions in intermolecular complexes are primarily considered in terms of the constituent donor/acceptor redox or ionization potentials, whereas, in Hush theory, the intervalence transition in localized or bridged (intramolecular) mixed-valence complexes are primarily related to the reorganization energies. Since the ion-radical associates of interest in this study are intermolecular mixed-valence complexes, we refer to the NIR optical transitions as charge transfer or intervalence interchangeably to combine both features.
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    • Note that the activation barrier and the calculated electron-transfer rates are affected even by such relatively small values of H DA. For example, in the p-phenylenediamine system with λ, 9500 cm-1, the correction for Hab, 250 cm-1 leads to a 3-fold increase in rate
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    • We believe that, in Type S systems, the electronic coupling between redox centers in organic donor/acceptor dyads is sufficient for adiabatic electron transfer even for the sterically hindered moieties. Compare: Nelsen, S. F, Pladziewicz, J. R. Acc. Chem. Res. 2002, 35, 247
    • (b) We believe that, in Type S systems, the electronic coupling between redox centers in organic donor/acceptor dyads is sufficient for adiabatic electron transfer even for the sterically hindered moieties. Compare: Nelsen, S. F.; Pladziewicz, J. R. Acc. Chem. Res. 2002, 35, 247.
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    • As such, the study of OMB provides the direct link between the earlier separate ESR studies of the self-exchange process, on one hand, and the electronic structure of the paramagnetic dimers, on the other hand.
    • As such, the study of OMB provides the direct link between the earlier separate ESR studies of the self-exchange process, on one hand, and the electronic structure of the paramagnetic dimers, on the other hand.
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    • For other examples of the unusual Type L potential-energy surfaces in intermolecular electron-transfer processes
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    • For example, inorganic donor/acceptor dyads are metallocentric. and electron transfer is largely directed between single atomic centers. In contrast, organic donor/acceptor dyads involve multiple (carbon) centers, and electron transfer is consequently more diffusive. Moreover, ligands usually play important roles in inorganic (coordination) donors and acceptors, whereas the concept is alien to and essentially undefined in organic donors and acceptors
    • For example, inorganic donor/acceptor dyads are metallocentric. and electron transfer is largely directed between single atomic centers. In contrast, organic donor/acceptor dyads involve multiple (carbon) centers, and electron transfer is consequently more diffusive. Moreover, ligands usually play important roles in inorganic (coordination) donors and acceptors, whereas the concept is alien to and essentially undefined in organic donors and acceptors.
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    • Under these circumstances, the molecular or atom-bridged activated complex of Taube can be approximated as either a type M or L system in which the bridged is considered virtual, as in the case of a mixed organic/ inorganic (distorted) redox dyad
    • (a) Under these circumstances, the molecular or atom-bridged activated complex of Taube can be approximated as either a type M or L system in which the bridged is considered virtual, as in the case of a mixed organic/ inorganic (distorted) redox dyad.
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