Oxidation inner sphere

An inner-sphere oxidation of HN3 by CoOH to N3- is proposed, the azide radicals yielding nitrogen in a bimolecular process. 

HS(CH2)2COOH. Addition of the thiol yields enhanced anodic peak current, but at very high thiol concentrations, the catalytic activity is suppressed, consistent with the inner-sphere oxidation of the thiol. The carboxylic acid group was shown to be important for the reaction in a position 2 or 3 bonds away from the SH group. 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

In spite of the fact that numerous oxidation reactions are known, that lead to a-functionalization of ketones [159,160], in most cases enol radical cations are not involved in these transformations, and rigorous evidence for their formation through selective oxidation of the enol tautomer (Fig. 2, path 2) has only been obtained in a few cases. For example, it could be inferred from kinetic studies that in many cases enols are not intermediates in aqueous oxidation reactions with V(V), Co(III), Ce(IV) and Mn(III) [161-163], whereas in acetic acid Mn(III) was postulated to attack the enol form of ketones [164,165], but not by electron transfer [166]. On the other hand, oxidants as Cr(VI), Tl(III), Hg(II) and Mn(VII) [167] as well as Pb(IV) [168] definitely react with the enol form, but since with these inner-sphere oxidants electron transfer is assumed to occur in a bonded fashion, radical cation intermediates are most likely not implicated. 

M" ) and the protonation equilibrium (97) exists. The latter is not unrealistic considering the results of nitrite oxidation (vide supra). It was also suggested, by way of lack of any direct evidence, that the inner-sphere oxidative behavior of IO4 should also be applicable for [MoiCNlg] ", in which a bridged cyano ligand could interact through substitution with the inner coordination sphere of a labile I(VII). 

The electrochemical properties of a number of isostructural rhe-nium(V) oxo, rhenium(V) imido, osmium(VI) nitrido, of formula [M(E)(X)(Y)(Tp )] (Tp = Tp or Tp, E = O, N-tolyl, N X, Y = hydrocarbyl, halide, triflate), have been described. The reactivity of these complexes as inner-sphere oxidants does not correlate with their peak reduction potentials, whereas the ease of the oxidation of these compounds well parallels their reactivity as oxidants.206... 

On the basis of these observations, the electron-transfer catalysis in the activation of aromatic C—H bonds was proposed to proceed via two possible pathways. Outher-sphere oxidation, which yields [Ir (Cp )Me2(PR3)] " and inner-sphere oxidation via the Cp ligand yielding another type of iridium(IV) radical cation that easily converts to a so-called tucked-in intermediate (Fig. 35). Both intermediates were claimed to be capable of subsequent activation of aromatic C—H bonds. Apparently, the redox and chemical noninnocence of the Cp ligand plays a crucial role in at least part of the observations. 

The one-electron oxidations described briefly above are referred to as non-bondcd (or outer-sphere) electron transfers (Littler, 1971). They differ from bonded (or inner-sphere) oxidations, such as the oxidation of alcohols by Cr(VI), in that a bond between the organic substrate and the metal ion or the complexed metal ion is not formed. Electron transfers of the non-bonded type may be very fast. Although in the equations above it has been convenient to represent the oxidant as the free ion, this cannot be so in solution, in which the ion must be solvated or complexed in some way. Clear cut cases of non-bonded or outer sphere oxidation can be seen in the use of the hexachloroiridate(IV) ion (40) (Littler, 1971) and the 12-tungstocobalt(III) ion (41) (Chester, 1970). In the latter example... 

The substitution of tris(4,7-dihydroxyl-l,10-phenanthroline)iron(II), [Fe(ohp)3], by CN to form [Fe(ohp)2(CN)2] has been examined by SF spectrophotometry in a study of the oxidation of [Fe(ohp)3] by H2O2 (no charges are shown as under the basic conditions of the study oph is probably doubly deproton-ated, but the precise charges of the complexes have not been definitely established ). At pH 13 the substitution rate is found to be independent of [CN ] and characterized by k 295 K) = 8.63 x 10" s in aqueous solution at / = 0.11 mol dm (NaCl), consistent with the rate-determining step being the dissociation of ohp. It is deduced from this that the dissociation of ohp is the rate-limiting step in the inner-sphere oxidation of [Fe(ohp)3] by H2O2. 

Reactions of cobalt complexes of macrocyclic tetramine ligands with various couples (including the highly oxidizing MnCl /, hitherto little studied) have been reported. Inner-sphere and outer-sphere reactions involving, for example, [Ru(NH3)6py] +/ + have been characterized. A free-energy correlation for reactions between [Co(N4)(OH2)2] with inner-sphere oxidants shows the expected slope 9AG /3AG 0.5 down to a limit of AG a 7 kcal mol, which is believed to represent diffusion control. Application of the Marcus equations (1) and (3) to reactions of both types leads to an assessment of the factors con-... 

Recent investigations outline the involvement of this class of polymetallic compounds in reactions of fundamental importance to catalysis, such as inner-sphere oxidation or transmetalation. Given the widespread use of copper(l) and silver(l) salts - and, to a lesser extent, thallium(l) - as additives in palladium- and platinum-based catalytic systems, a systematic study of these reactions is warranted. Additionally, gathering further knowledge on the properties and reactivity of metal-metal-bound species is a critical step toward the rational design of truly bimetallic catalytic cycles. 

Electron transfer between V(IV) and I(V) (as 10J) catalyzed by Ru(VIII) and Os(VIII) and the inner-sphere oxidation of diaqua (nitrilotriacetato)cobalt(II) by 104 have both been studied kinetically. The radiolysis of 10 J and the photolysis of 10" in aqueous solution have also been studied. JV-iodosuccinimide and V(IV) react in a process first order in oxidant and inverse first order in acid [in contrast to the reaction involving V(IV) and... 

Fig. 12.6 Observed red diamonds) and calculated circles) rate constants for reactions of 02 (aq) with electron acceptors. The highlighted complex is the same one as in Fig. 12.5. The electron donors listed by letter and number in the figure ate as follows Co "(sep) " (70, Fe "(edta)(H20)-(a) [59], Ru "(NH3)6 + (80, Fe Cp2+ (b), Ru "(NH3)5isn3+ (20, Ru (NH3)5phen + (T). Rate data for one of these, a, was estimated from data for the reduction of O2 (see ref. [59]). While this estimated value is consistent with an outer-sphere mechanism, experimental data [60] indicate this complex may behave as an inner-sphere oxidant of superoxide (Reprinted from ref. [52] with permission from the American Chemical Society) (Color figure online)...

Iron(ll).— An excellent review of electron-transfer (iimer- and outCT-sphere) reactions involving vanadium(iv) as compared with iron(n) as a reductant for both cationic and anionic species has been published. The rate constants for oxidations of iron(ii) are almost invariably greater than those for vanadium(iv), the difference being smaller when there is evidence for an inner-sphere oxidation. For outer-sphere reactions, on the other hand, the rates may differ by several hundred-fold. 

The inner-sphere oxidation of thiocyanate, thiourea, and alkyl-substituted thioureas have been investigated. The reactions are rapid, the process for SCN being too fast to be followed by the stopped-flow method, although in this case the stoicheiometry of the reaction in excess ligand is identical to that for the thioureas,... 

Removal of an electron in solution can be accomplished in different ways, for example by use of chemical oxidants, electrochemical (anodic) oxidation, or photoinduced electron transfer (PET). However, oxidations involving chemical oxidants should be assessed carefully with respect to inner sphere oxidation mechanisms, which are known for several common metal ion oxidants. ... 

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