Electron donor outer-sphere

Experimental tests of the theoretical predictions have involved the electrochemical reduction of alkyl and benzyl halides as well as their reduction by homogeneous electron donors.22,29-31 In the first case, AG° = E - rx r.+x=f where E is the electrode potential and rx r.+x=f is the standard potential of the RX/R + XT couple. In the homogeneous case, AG° = E q — rx r-+xt> where E Q is the standard potential of the outer-sphere electron donor or acceptor couple P/Q, and + stands for a reduction and — for an oxidation. 

This problem has been addressed recently197 with model reactions in which the nucleophile (viz. electron donor) is NO-. It has been found that outer-sphere transition states are easier to compute with NO- than with other electron donors, thus allowing a complete characterization of the competition between the various pathways. 

Traditionally, electron transfer processes in solution and at surfaces have been classified into outer-sphere and inner-sphere mechanisms (1). However, the experimental basis for the quantitative distinction between these mechanisms is not completely clear, especially when electron transfer is not accompanied by either atom or ligand transfer (i.e., the bridged activated complex). We wish to describe how the advantage of using organometals and alkyl radicals as electron donors accrues from the wide structural variations in their donor abilities and steric properties which can be achieved as a result of branching the alkyl moiety at either the a- or g-carbon centers. 

Outer-Sphere Electron Transfer The minimal interpenetration of the coordination spheres of the reactants is inherent in any mechanistic formulation of the outer-sphere process for electron transfer. As such, steric effects provide a basic experimental criterion to establish this mechanism. Therefore we wish to employ the series of structurally related donors possessing the finely graded steric and polar properties described in the foregoing section for the study of both homogeneous and heterogeneous processes for electron transfer. 

Homogeneous Processes with Tris-phenanthroline Metal(III) Oxidants. The rates of electron transfer for the oxidation of these organometal and alkyl radical donors (hereafter designated generically as RM and R, respectively, for convenience) by a series of tris-phenanthroline complexes ML33+ of iron(III), ruthe-nium(III), and osmium(III) will be considered initially, since they have been previously established by Sutin and others as outer-sphere oxidants (5). 

When a reaction is adiabatic, the electron is transferred every time the system crosses the reaction hypersurface. In this case the preexponential factor is determined solely by the dynamics of the inner-and outer-sphere reorganization. Consequently the reaction rate is independent of the strength of the electronic interaction between the reactant and the metal. In particular, the reaction rate should be independent of the nature of the metal, which acts simply as an electron donor and acceptor. Almost by definition adiabatic electron-transfer reactions are expected to be fast. 

We first consider outer sphere transfer (ET) reactions, e.g. D" + A -> D + A, a donor-acceptor electron transfer without significant coupled internal reorganization of the D and A species [27,29,30]. A hallmark of such reactions, which has been long appreciated [27], is that the reactive coordinate is itself a many-body collective solvent variable (and is not the coordinate of the electron itself)- In particular, if R and P stand for the reactant and product, then the reactive coordinate is... 

The second factor is the same as that discussed in Section 3.3 for dissociative electron transfers in which the electron comes from outside the cleaving molecule (i.e., from an electrode or from an outer-sphere electron donor in solution). A decrease in the cleavage barrier is similarly expected. 

Aromatic anion radicals may be used as outer-sphere electron donors, thus giving rise to redox catalysis. The ensuing variations of the electron transfer rate constant with the driving force are shown in Figure 3.2b for... 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

It is important to distinguish between outer-sphere and inner-sphere complexes. In inner-sphere complexes the surface hydroxyl groups act as o-donor ligands which increase the electron density of the coordinated metal ion. Cu(II) bound inner-... 

The Marcus classical free energy of activation is AG , the adiabatic preexponential factor A may be taken from Eyring s Transition State Theory as (kg T /h), and Kel is a dimensionless transmission coefficient (0 < k l < 1) which includes the entire efiFect of electronic interactions between the donor and acceptor, and which becomes crucial at long range. With Kel set to unity the rate expression has only nuclear factors and in particular the inner sphere and outer sphere reorganization energies mentioned in the introduction are dominant parameters controlling AG and hence the rate. It is assumed here that the rate constant may be taken as a unimolecular rate constant, and if needed the associated bimolecular rate constant may be constructed by incorporation of diffusional processes as ... 

Both the electronic couphng matrix element and the outer-sphere component of the nuclear reorientation parameter are thought to vary with donor-acceptor separation and orientation [29, 49]. It has been shown in studies of Os and Ru-ammines bridged by polyproline spacers that the distance dependence of X can be greater than that of [50]. Dielectric continuum models of solvent reorganization predict that Xg will increase with... 

All the evidence gathered so far points to the conclusion that the RX radicals are intermediates in the reductive cleavage of aryl halides by outer sphere electron donors in polar solvents. This was an already established conclusion for the reduction of several aryl halides in the gas phase (Steelhammer and Wentworth, 1969 Wentworth et al., 1967), and for iodobenzene in apolar or weakly polar matrixes from y-irradiation studies with esr detection at low temperatures (Symons, 1981). It might, however, not have been true in the polar media used in direct and indirect electro-... 

In Other classes of organic halides, for example perfluoroalkyl and vinyl halides, the distinction between stepwise and concerted electron-transfer-bond-breaking upon reduction by outer sphere heterogeneous and/or homogeneous electron donors is less unambiguous than in the case of aryl and alkyl halides. As discussed in Section 3, they also present the interest of being active substrates in Sg l reactions. 

Similar questions arise for other polyhaloalkanes such as CCI4, CBr and CCljBr. Their reduction by two bulky coordination compounds (a polyoxo-metallate and a sepulchrate ) reputed to function as outer sphere electron donors have been recently investigated (Eberson and Ekstrom 1988a,b) as well as the reduction of CCI4 by aromatic anion radicals (Eberson et al.,... 

---