Effect of Ion Pairing on Charge Transfer in the Bicyclooctatetraenyl Dianion

Abstract

The rate constants for bond shift (k BS) and intramolecular charge transfer (k CT) in alkali metal salts of bicyclooctatetraenyl dianions (1 2-) in THF-d 8 were determined by dynamic NMR spectroscopy. The values of k BS and k CT increase in the order K < Na ≤ Rb < Cs and K < Rb < Na < Cs ≪ Li, respectively. It is concluded that the K, Rb, and Cs salts react as contact ion pairs (CIPs) while the Li and Na salts react as solvent-separated ion pairs (SSIPs). Increases in k BS and k CT are observed in 1 2-/2K+ on going to more polar solvents (NH3/CH3NH2 and THF-d 8/DMEU). Comparison with kinetic studies of four other dianions has led to the conclusion that 1 2-/2Na+ is the first example of a Na salt of a dianion to undergo charge transfer by SSIPs. This is attributed to the difficulty in forming a Na-bridged CIP transition structure in 1 2-/2Na+ that can compete with an SSIP transition structure.

Bond shift and charge transfer dynamics in methylene- and dimethylsilyl-bridged dicyclooctatetraene dianions

“…8-10 Note that CT is an intramolecular process since k CT does not change with a change in concentration. 8,9, 11 The ∆G ‡ values were derived from the Eyring equation k = κk B T/h exp (Ϫ∆G ‡ /RT), where the transmission coefficient κ is taken as unity and the other quantities have their usual meanings.…”

“…[35][36][37] Some information about the ion-pair structure may be gained from a qualitative relationship between the strength of the cation-anion interaction and the average 13 C chemical shift for the carbons in the dianion ring. 8,10,11 In this simple model, a stronger interaction with the cation is assumed to involve some stabilization of the highest occupied MOs of the anion while the levels of the virtual MOs should be less affected by the interaction. As a result, the average excitation energy of magnetic field-induced mixing of certain filled and virtual MOs will become larger and the paramagnetic moment will decrease according to the Karplus-Pople equation for the paramagnetic contribution to the nuclear shielding.…”

“…We recently postulated that the dipotassium, dirubidium and dicaesium salts of the more nearly planar bicyclooctatetraenyl dianion (COT-COT 2Ϫ ) undergo charge transfer predominantly through CIPs. 11 This conclusion was based on the observation that k CT for these salts decreases [k(Cs + ) > k(Rb + ) > k(K + )] as the degree of tightness of their CIPs increases. 11,32 A completely CIP mechanism seems less likely in 1 2Ϫ and 2 2Ϫ unless the exo K + first converts to an endo K + .…”

“…In addition, the influence of counterion and solvent on the rate constant for CT (k CT ) has been studied. 8,11 Intramolecular two-electron-cation transfer in such species is relatively slow, with k CT in the range of 10 Ϫ2 -10 2 s Ϫ1 at ambient temperature, which is explained by the requirement for flattening the neutral ring. Rate constants have been obtained by 1 H and 13 C dynamic NMR methods.…”

Structure of an Unsymmetrical Isomer from the Attempted Synthesis of 1,8-Dicyclooctatetraenylnaphthalene

“…We have been interested in the “gated” transfer of charge (electrons and counterions) in bridged dicyclooctatetraene dianions. − Reversible charge transfer (CT) in these mixed-valence systems occurs on the NMR time scale as a consequence of a high-energy gating step, i.e., ring flattening and bond length equalization in the neutral cyclooctatetraenyl (COT) ring, that must precede CT.…”