- Solvent and Electrolyte Effects on the Kinetics of Ferrocenium-Ferrocene Self-Exchange: A Reevaluation
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Rate constants, kex, for ferrocenium-ferrocene self-exchange have been evaulated by using the proton NMR line-broadening technique as a function of added electrolyte in acetonitrile, acetone, nitrobenzene, and methanol.Increasing the ionic strength over the range ca. 2 x 1E-4 - 0.25 M by adding tetraethylammonium hexafluorophosphate, tetrafluoroborate, or perchlorate salts yielded monotonic decreases in kex.These rate variations are relatively modest, typically 30 percent or less, and are substantially smaller than are anticipated on the basis of thermodynamic ion-pairing effects.Similar results were also obtained for cobaltocenium- cobaltocene and decamethyl(ferrocenium-ferrocene) self-exchange and for the addition of chloride and bromide anions in the latter case.However, the ferocenium-ferrocene kex values themselves deviate substantially (up to ca. 10-fold), particularly in nitrobenzene, methanol, and also dimethyl sulfoxide, from earlier published values obtained by using the same technique.These disparities are traced to systematic errors in the earlier measurements associated with uncertainties in the paramagnetic line widths.The new solvent-dependent kex data lead to a significant reevaluation of the factors controlling the electron-transfer dynamics for the ferrocenium-ferrocene system.They suggest that the barrier-crossing frequency is limited by donor-acceptor electronic coupling in addition to solvent polarization dynamics.This behavior is briefly compared and contrasted with that for other, more facile, metallocene redox couples.
- Nielson, Roger M.,McManis, George E.,Safford, Lance K.,Weaver, Michael J.
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- Redox behavior of nickel acylate complexes
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The cyclic voltammetry and bulk oxidation with ferrocenium ion and molecular iodine of three nickel complexes were studied. The results are most consistent with both Cp2Fe+ and I2 acting as single-electron oxidants.
- Pinhas, Allan R.,Hershberger, James W.
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- Difference in reactivity of triel halides EX3 towards ferrocene
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A facile route to the Fe2+-arene complex [(C6H6)FeCp][AlBr4] is the reaction of ferrocene with AlBr3 in benzene. The structure of the Fe2+-arene complex [(C6H6)FeCp][AlBr4] · C6H6 was found to be isomorphous with those of [FeCp2][ECl4] · C6H6 (E = Al, Ga). The crystal structures of the [FeCp2][AlCl4] · C6H6 (E = Al, Ga) presented here show structural features which are different from those of solvent-free ferrocenium salts [FeCp2][ECl4] (E = Al, Ga, Fe). However, the cell parameters of solvent-free ferrocenium tetrafluoroborate [FeCp2][BF4] are also quite different from those of solvent-free [FeCp2][ECl4] (E = Al, Ga, Fe). In contrast to the eclipsed conformation in solvent-free [FeCp2][ECl4] (E = Al, Ga, Fe) the cyclopentadiene rings in [FeCp2][BF4] and [FeCp2][ECl4] · C6H6 (E = Al, Ga) are in a staggered conformation.
- Scholz, Stefan,Scheibitz, Matthias,Sch?del, Frauke,Bolte, Michael,Wagner, Matthias,Lerner, Hans-Wolfram
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p. 3323 - 3329
(2008/10/09)
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- Solvent Dynamical Effects in Electron Transfer: Comparisons of Self-Exchange Kinetics for Cobaltocenium-Cobaltocene and Related Redox Couples with Theoretical Predictions
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Rate constants, Kex, and activation parameters for the self-exchange of cobaltocenium-cobaltocene Cp2Co+/0, and the decamethyl derivative Cp'2Co+/0, in 13 organic solvents have been evaluated by using the proton NMR line-broadening technique with the objective of probing the influence of solvent dynamics upon the electron-transfer kinetics.Together with some corresponding measurements reported earlier for ferrocenium-ferrocene Cp2Fe+/0, additional measurements for the decamethyl derivative, Cp'2Fe+/0, and with corresponding data for Cp2Co+/0 electrochemical exchange, these results enable a systematic comparative examination to be made of the effects of solvent dielectric relaxation on the barrier-crossing frequency for such simple outer-sphere reactions.For the facile Cp'2Co+/0 couple the solvent dependence of the observed frequency factors, νn(obsd), extracted from the kex values by correcting for the solvent-dependent barrier height, ΔG*, is in approximate accordance with the relative frequency factors νos(calcd), predicted from the continuum model of overdamped solvent relaxation.The subunity (ca. 0.7-0.8) slope of the logarithmic νn(obsd) - ??os(calcd) plot for Cp'2C0+/0 self-exchange is consistent with a recent theoretical prediction of the combined effect of overdamped solvent motion and reactant vibrations (ref 2g).In a given solvent, the sequence of kex values is Cp'2Co+/0 > Cp2Co+/0 ca.Cp'2Fe+/0 > Cp2Fe+/0, with Cp2Co+/0 and Cp2Fe+/0 being about 10- and 100-folds lower, respectively, than Cp'2Co+/0 self-exchange.While these reactivity differences can be traced to variations in donor-acceptor orbital overlap, the solvent dependencies of kex for Cp2Co+/0 and Cp2Fe+/0 electron exchange nevertheless exhibit a strong influence from overdamped solvent relaxation.Marked deviations from the dielectric continuum predictions are seen, however, in several solvents.Thus the barrier-crossing frequencies in propylene carbonate, N-methylformamide, and especially methanol are substantially (4-100-fold) larger than expected from νos(calcd), implicating the presence of surprisingly rapid relaxation modes in these solvents.The solvent-dependent activation parameters also differ significantly from the expectations of conventional theoretical modes.
- Nielson, Roger M.,McManis, George E.,Golovin, Neal M.,Weaver, Michael J.
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p. 3441 - 3450
(2007/10/02)
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- ZUR VERWENDUNG VON FERRICENIUM-KATIONEN ALS SELECTIVEM OXIDATIONSMITTEL IN DER METALLORGANISCHEN CHEMIE
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The use of ferricenium cations X (X = BF4, PF6, SbF6) as one-electron oxidizing agents for organometallic complexes is demonstrated.Sandwich compounds M(C5H5)2 (M = Cr, Co, Ni) and Cr(C6H6)2 are oxidized in nearly quantitative yield to the corr
- Schumann, Hans
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p. 341 - 352
(2007/10/02)
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- ZUR OXIDATIVEN SPALTUNG VON METALL-METALL-BINDUNGEN IN METALLORGANISCHEN VERBINDUNGEN MITTELS FERRICENIUMKATIONEN
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The dinuclear organometallic compounds ((C5H5Fe(CO)2)2, Co2(CO)8 and (C5H5NiCO)2) are oxidized by X (X = BF4, PF6) in the presence of neutral ligands L to form the cationic organometallic complexes X, X and C5H5N
- Schumann, Hans
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p. C34 - C36
(2007/10/02)
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- Magnetic susceptibility study of various ferricenium and iron(III) dicarbollide compounds
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The ground electronic state [2E2g(a1g)2(e2g) 3] of the ferricenium ion has been further characterized by a variable-temperature (40-300°K) magnetic susceptibility study of nine ferricenium and two analogous iron(III) dicarbollide (1,2-B9C2H112-) compounds. The observed temperature independence of the effective magnetic moments for these ferricenium compounds is explicable in terms of either a temperature-dependent, low-symmetry crystal field distortion (external and/or internal) or thermal population of the 2A1g(a1g)1(e2g) 4 state. Solvent and counterion effects on the magnetic properties of the ferricenium ion are appreciable. Copyright 1971 by the American Society.
- Hendrickson, David N.,Sohn,Gray, Harry B.
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p. 1559 - 1563
(2008/10/08)
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