Precursor complex electron-transfer mechanism

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

From an organometallic point of view, the o-bonded alkyl and aryl complexes are good precursors in the synthesis of metal-metal bonded derivatives. These latter compounds have been studied in order to determine the potentials for oxidation or reduction, electron transfer rates, and electron transfer mechanisms of metalloporphyrins as a function of solvent, axial ligand coordination, and of the macrocycle. Recently, bimetallic compounds have attracted growing interest due to their potential applications as starting materials for synthesizing polymeric conductors Some aspects of the reactivity for this family of compounds have been studied and will be discussed in this review as well. 

Kochi and co-workers reported the photochemical addition of various stilbenes to chloroanil 53, which is controlled by the charge-transfer (CT) activation of the precursor electron-donor/acceptor (EDA) complex. The [2-1-2]-cycloaddition products 54 were established by an x-ray structure of the trans-oxetane formed selectively in high yields. Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Patern6-BUchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies that involves quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate, which appears on a nanosecond/microsecond time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse k = 10 s ) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

A reaction mechanism for the above reactions was proposed which consists of initial formation of the copper precursor complexes of Fig. 3 (without coordinated phenolate), coordination of phenolate, electron transfer from phenolate to Cu2+ and subsequent reduction to Cu1+ with formation of a phenoxy radical, and reoxidation of Cu1+ to Cu2+ with oxygen. Various copper(II) catalysts having different stereochemistries (octahedral or tetrahedral coordination) due to coordination of amines like pyridine (Py) or acetate (OAc) groups in different ligand sites were observed by NMR and electron paramagnetic resonance techniques. 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

The well-known redox chemistry of provides important insights into the mechanism of the VOHPO4 0.5H2O precursor formation in organic medium. Waters and Littler [24] have shown that most V reductions proceed via a free-radical mechanism where complexation of V to alcohol precedes the one-electron transfer step, i.e. an inner sphere electron transfer. Waters and Littler [24] proposed ternary tetrahedral complex formation between V02, H3O+ and R2CHOH to yield [V(0H)30HCHR2] and observed the following kinetic expression... 

The classical (Grignard-like) mechanism for Barbier reactions involves the primary formation of an Sm-alkyl species via halogen abstraction and subsequent reduction of the alkyl radical formed after the first electron transfer. Be that as it may, the Barbier reaction can be used to construct complex polycyclic target molecules, e.g. the synthesis of tetraquinanes from diquinane precursors by two independent intramolecular cyclization steps (Scheme 24) [83]. 

In these systems, the donor and acceptor diffuse together to give a precursor complex, D A, whose formation is described by the equilibrium constant Kp. Electron transfer, characterized by rate constant eTj occurs within the associated donor-acceptor pair, converting the precursor complex to successor complex D A. Subsequent separation of the oxidized donor (D+) and reduced acceptor (A ) from the successor complex is described by. s- The rate of m/ermolecular electron transfer depends not only on the factors that influence kpj but also on factors affecting the formation of the precursor complex [19]. More quantitatively, as described by Eq. 2, the expression for intermolecular electron transfer has the form of a consecutive reaction mechanism described by an observed rate constant (A obs) consisting of rate constants for diffusion (A ) and the activated electron transfer. 

In 1954, King and Taube published the 1980 Nobel Prize winning work that defined these two different types of electron transfer reactions. In an inner-sphere mechanism, the atoms undergoing redox form bonds to a common atom (or small group of atoms), which then serves as a bridge for electron transfer (ISPC = inner-sphere precursor complex and ket = electron transfer rate constant). 

Electron transfer from the chromium(II) center to the cobalt(III) center within this inner-sphere precursor complex results in a labile cobalt(II) center and inert chromium(III) center. Concurrent with electron transfer, atom transfer (Cl-) also takes place as Cl-remains bound to the inert chromium(III) center to yield [ClCr(OH2)5]2+ and [(H3N)5Co]2+ decomposes to [Co(H20)6]2+. In 1961, Norman Sutin was able to provide direct mechanistic proof of atom transfer in the inner-sphere process by his introduction of the stopped-flow (rapid mixing) techniqe. The transition state in an inner-sphere process is relatively well defined, and because the kinetics are complex, the overall mechanism is also well defined. 

Under favorable conditions, then, the three possible transition states for net inner sphere can differ in composition for the usual interchange substitution mechanism there are (1) precursor formation, [ALXB] (2) electron transfer, [AXB] and (3) successor breakdown, [AXL B]. However, L and L usually are solvent, and the number of solvent molecules in an activated complex cannot be determined kinetically. Therefore, under this normal circumstance all three possible transition states have the same composition and cannot be distinguished by direct kinetic measurements only indirect arguments can be used to determine which of the three possible transition states is operative. 

The observed rate constant for the scheme I mechanism can be written as Kpk, where Kp is the equilibrium constant for formation of the precursor complex, and k is the rate constant for electron transfer within the precursor complex. Because the value of Kp includes not only diffusion of the reactant partners together, but also the free-en-ergy change for the substitution process that creates the bridge, indirect arguments to separate the observed rate constant into its components are rare. 

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