Reactivity inner sphere electron transfer

The analogy between electron-transfer via addition/elimination (Eq. 2b,c) or abstraction/elimination (Eq. 2a, c) and classical solvolysis involving closed-shell molecules (nonradicals) is seen by comparing Scheme 1 with Scheme 3, in which XY, the precursor of the ions X and Y , is formally derived from the two radicals X and Y". Analogous to Scheme 1, on the way to the ionic products that result from the interaction between X and Y there are two possibilities if XY denotes a transition state, the reaction (Eq. 3a, a ) is a case of outer-sphere electron transfer. If, however, a covalent bond is formed between X and Y, the path (Eq. 3b, b ) is an example of inner- sphere electron transfer. Obviously, part b of the scheme describes the classical area of S l solvolysis reactions (assuming either X or Y to be equal to C) [9, 10]. If a second reaction partner for C (other than the solvent) is allowed for (the (partial) ions then represent transition states), then Eq. 3b also covers Sn2 reactions. If looked upon from the point of view of radical-radical reactivity, Eqs. 3a and b show well-known reactions radical disproportionation in Eq. 3a,a and combination in Eq. 3b. 

For the unusual reactivity of ferrocenylsilanes toward 5u in THF, affording ketones instead of the expected tertiary alcohols, a mechanism was proposed including the inner-sphere electron transfer from 5u within a reactant complex. The proposition was based on an electrochemical CV examination, which indicated that the outer-sphere process is thermodynamically unfavorable. 

Although chromate(VI) is photochemically inactive in all of its forms in neat aqueous solution, the photochemical oxidation of alcohols by chromate(VI) has been known for more than 80 years and interpreted in terms of photochemical reactivity of the chromate(VI) esters [94], Recent studies have shown, however, that LMCT excitation of CrVI species is quenched not only by inner-sphere but also by outer-sphere electron transfer [23, 87,92,94,95], Moreover, inner-sphere electron transfer in chromate(VI) esters was found to involve two electrons, yielding a CrIV species and appropriate aldehyde or ketone ... 

The photoredox behavior of Cu(II) complexes, similarly to those of Fe(III), is derived mostly from the reactive decay of their LMCT states. Excitation to LMCT excited states can be attainable by direct sunlight, when the ligands coordinated to Cu(II) are good enough electron donors. As a consequence of the reactive decay of the LMCT excited state by inner-sphere electron transfer, the Cu(II) central atom is reduced to Cu(I), whereas the ligand or another available electron donor is oxidized to its radical, for example ... 

In the chemical reaction reductive or oxidative elimination to the product (inner sphere electron transfer) [7] or in electrochemistry generation of the reactive intermediate by electron transfer (outer sphere electron transfer) [7], and follow-up reaction of the intermediate to the product. 

Besides the superoxide dismutation mechanism, the reactivity of metal centers, in particular manganese complexes, toward NO is very much dependent on the possibility for binding a substrate molecule. As it will be shown later, the possibility that MnSOD enzymes and some mimetics can react with NO has been wrongly excluded in the literature, simply based on the known redox potential for the (substrate) free enzymes, mimetics, and NO, respectively. Therefore, the general fact that, upon coordination, redox potentials of both the metal center and a coordinated species are changed should be considered in the case of any inner-sphere electron-transfer process as a possible reaction mechanism. 

The reactivity pattern of the second reaction step observed at pH >3, fe2(obs). is very similar to that observed for the reaction between the [Co(P)(N02 )(H20)] complex and NO under similar conditions. Importantly, the observed rate constant for this reaction does not depend significandy on the NO2 concentration, but it depends linearly on the NO concentration. This finding together with the dissociative nature of the activation parameters determined for this reaction leads to the conclusion that in the presence of NO the [Co(P)(N02 )(H20)] complex must exist in equilibrium with small amounts of the [Co(P)(N02 )(NO)] intermediate (see lower part of Scheme 12). The last intermediate slowly decomposes through an inner-sphere electron-transfer reaction to generate the final products [Co (P) (NO )] and NO2. Since the latter reaction represents the rate-determining step for the second reaction, the proposed mechanism seems to be consistent with the observed NO and N02 concentration dependences, as well as with the activation parameters determined in this study (126). 

An electroactive compound confined in a coated polymer film can act to relay electrons (in a reduction) from the electrode to the solution in two-step processes. First the electron is transferred from the electrode to the catalyst. Then follows the outer-sphere or inner-sphere electron transfer from the catalyst to the substrate. Another mechanism includes the interaction of the catalyst (which is not capable of mediating electrons) with the incoming substrate to alter its chemical reactions or electrochemical reactivities. 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

These kinetics data are consistent with a preequilibrium dissociation of dmf from the molybdenum center to form a reactive five-coordinate species that rapidly reduces the Fe(III) center via an inner sphere (halogen transfer) reaction. Other one-electron atom transfer reactions are known in oxo-molybdenum chemistry (262). An innersphere (atom transfer) mechanism is not a viable model for intramolecular transfer in sulfite oxidase because in the enzyme the Mo and Fe centers are almost certainly held too far apart by the protein framework. Moreover, the 65-type heme center of sulfite oxidase is six-coordinate with axial histidine ligands from the protein and hence cannot participate in atom transfer reactions. 

Both inner- and outer-sphere electron transfer mechanisms will be investigated in this series of experiments. The chapter begins the synthesis of four cobalt(ffl) coordination complexes followed by analysis and reactivity studies. Electronic structure will be investigated using visible spectroscopy and the redox chemistry of two of the complexes will be examined... 

Since electrophilic and charge-transfer nitrations are both initiated via the same EDA complex and finally lead to the same array of nitration products, we infer that they share the intermediate stages in common. The strength of this inference rests on the variety of aromatic substrates (with widely differing reactivities and distinctive products) to establish the mechanistic criteria by which the identity of the two pathways are exhaustively tested. On this basis, electrophilic nitration is operationally equivalent to charge-transfer nitration in which electron-transfer activation is the obligatory first step. The extent to which the reactive triad in (90) is subject to intermolecu-lar interactions in the first interval (a few picoseconds) following electron transfer will, it is hoped, further define the mechanistic nuances of dissociative electron transfer in adiabatic and vertical systems (Shaik, 1991 Andrieux et al., 1992), especially when inner-sphere pathways are considered (Kochi, 1992). 

Major emphasis is placed on the reactions of metal complexes in solution undergoing either inner-sphere ligand substitution or electron transfer to and from the metal center. Such studies relate to the important selective role of metal catalysts in many areas of enzymatic, commercial, and modem synthetic chemistry. Clearly, this field has now matured to the point where basic theoretical considerations, although incomplete, can provide a logical framework for understanding the chemical reactivity of such systems and stimulate the investigation of (1) new and unique reaction pathways, (2) modified reagents, and (3) unorthodox matrices. 

Similarities between [Ru(bpy),]2+ (discussed in Chapter 13) and [Pt,(pop)J4 are apparent. Reactive excited states are produced in each when it is subjected to visible light. The excited state ruthenium cation, [Ru(bpy)3]" +, can catalytically convert water to hydrogen and oxygen. The excited slate platinum anion, [Pt,(pop)J 4-, can catalytically convert secondary alcohols to hydrogen and ketones. An important difference, however, is that the ruthenium excited stale species results from (he transfer of an electron from the metal to a bpy ligand, while in the platinum excited state species the two unpaired electrons are metal centered. As a consequence, platinum reactions can occur by inner sphere mechanisms (an axial coordination site is available), a mode of reaction rot readily available to the 18-clectron ruthenium complex.-03... 

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