Intramolecular dissociative electron transfer

Abstract

Dissociative electron transfers (ET) are reactions in which the ET is associated with the cleavage of a sigma bond. Although a rather satisfactory amount of information is currently available on the intermolecular and heterogeneous dissociative ET reactions, less is known for the corresponding intramolecular processes, despite the relevance of these reactions in both chemistry and biochemistry. This tutorial review focuses on the most recent developments in this area, with particular emphasis on the reactions occurring in well-defined Donor-Spacer-Acceptor molecular systems. The goal is to provide the reader with the essential background to understand and possibly predict the feasibility and rates of these reactions, as well as to stimulate the application of the intramolecular dissociative ET concepts and related issues to still unexplored molecular systems.

Double‐Layer Correction for Electron‐Transfer Kinetics at Glassy Carbon and Mercury Electrodes in N,N‐Dimethylformamide

“…Comparison between the apparent and double-layer corrected couples of inner reorganization energy terms points to differences of 1.6 -2.6 kcal mol À1 , which shows that the rate of the electrode process could be somewhat underestimated if apparent k het data are used. Finally, we note that both sets of DG 0,i = values confirm that the reduction proceeds by a stepwise DET mechanism involving formation of a rather loose radical anion species: unlike stepwise DET involving p* radical anions, for which DG 0,i = is usually on the order of 20 -30% of DG 0 = [37,38], the DG 0,i = of the reduction of Ph 3 CSPh is in the range 64 -72% (including both electrodes and apparent and doublelayer corrected values).…”

“…By studying several classes of DET type acceptors [37,38], however, we noticed that for compounds displaying reduction peaks below À 2 V mercury appears to behave as an inert electrode with no detectable evidence of adsorption phenomena. Thus, we used mercury in the study of the heterogeneous reduction of a series of dialkyl peroxides in DMF/0.1 M tetrabutylammonium perchlorate (TBAP) [45].…”

“…Unlike fast ET processes, for which solvent reorganization is the major term, concerted DETs are endowed by very large inner reorganization energies, amounting to as much as 1/4 of the bond dissociation energy of the cleaving bond [39], and thus large DG 0 = values. Both outer-sphere ETs and concerted DETs obey a DG = -DG8 Marcus-like quadratic relationship and thus the dependence of log k on DG8 is controlled by DG 0 = , as the curvature of the DG = -DG8 parabolic function is 1/ 8DG 0 = [38]; the same is true for the log k -E relationship, as DG8 is a linear function of E. The E dependence of log k is thus considerably smaller for a concerted DET than an outer-sphere ET and this implies that the actual reduction (or oxidation: hereafter, we shall focus on the more common case of reductions) process occurs at an appreciable rate only at very large activation overpotentials. As a matter of fact, the voltammetric peak potential (E p ) can be more negative than the standard potential (E8) by more than one volt [36 -38].…”

“…Similar problems may affect the reduction of compounds undergoing a stepwise DET but proceeding via formation of a loose radical-anion species [37,38]. In these processes, the singly-occupied molecular orbital (SOMO) has a pronounced s* character and thus radical anion formation implies significant stretch of the cleaving bond.…”

Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2

“…The direction of the process depends on both the substrate nature and external factors, e.g., temperature or potential. 13 The exist ence of the radical anion, whose lifetime is longer than the period of bond vibration (~10 -13 s), is a necessary but not sufficient pre requisite for the stepwise mechanism to occur. In addition to two opposite mechanisms, there are boundary situations due to which distinctions between the concerted and stepwise mechanisms become not so unambiguous.…”