Metalloporphyrins electron transfer rates

Messer A, Carpenter K, Forzley K, Buchanan J, Yang S, Razskazovskii Y, Cai Z, Sevilla MD (2000) Electron spin resonance study of electron transfer rates in DNA determination of the tunneling constant (1 for single-step excess electron transfer. J Phys Chem B 104 1128-1136 Meunier B (1992) Metalloporphyrins as versatile catalysts for oxidation reactions and oxidative DNA cleavage. Chem Rev 92 1411-1456... 

Electron transfer quenching of the excited metalloporphyrin core in compounds 43 46 was investigated in DMF using benzylviologen as electron acceptor [121], The estimated electron transfer rate constants are similar in the four dendrimers, indicating that small molecules have no difficulties in approaching the porphyrin core. Thus, these porphyrin dendrimers may potentially be employed as catalysts. 

Sun, H. Smirnov, V.V. DiMagno, S.G. Slow electron transfer rates for fluorinated cohalt porphyrins electroiuc and conformational factors modulating metalloporphyrin ET. Inorg. Chem. 2003, 42, 6032-6040. 

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. 

The previous analysis has so far shown that photocurrent responses are directly linked to specifically adsorbed metalloporphyrin at the liquid/liquid boundary. The interfacial organisation of these species is affected by a series of parameters such as the bulk concentration and the Galvani potential difference. In this section, we shall concentrate on the dynamic and mechanistic aspects associated with the photocurrent signal. The information discussed in the previous sections concerning lifetimes of excited states and surface coverage will be relevant to the estimation of the heterogeneous electron-transfer rate constant from the photocurrent data. 

Table 1 contains heterogeneous electron transfer rate data for reduction of these three classes of metalloporphyrins. Clearly, metal-centered reduction of high-spin Mn (TPP) Cl and Fe (TPP) X is slower than that of low-spin Fe (TPP) L2 . In addition, the rate of Fe... 

Secondly, Fig. 5 shows that the polymeric rate constants parallel values of heterogeneous rate constants that have been observed for the electrochemical reactions of solutions of the corresponding dissolved porphyrin monomers. (The slope of the line is 0.5). This re-emphasizes what was said above, that measurements of electron hopping in polymers can give rate constants that are meaningful in the context of the metalloporphyrin s intrinsic electron transfer chemistry. 

Even simple metalloporphyrins such as [(TPP)M]+ where M = Co, Fe and Mn can enable reduction of O2 to H2O by ferrocene derivatives (Fc) used as an electron donor in the presence of HCIO4 in acetonitrile [214, 215]. However, the rate-determining step is the two-electron reduction of O2 to H2O2 (Scheme 15) [214, 125]. Nonetheless, the catalytic effect of [(TPP)M]+ for the reduction of O2 by Fc is remarkable, because the oxidation of ferrocene by O2 hardly occurred in the presence of HCIO4 without [(TPP)M]+ [216]. The rate-determining step for the [(TPP)M]+-catalyzed two-electron reduction of O2 to H2O2 has been shown to be the initial electron transfer from Fc to [(TPP)M]+ as shown in Scheme 15 [215]. The reduced metalloporphyrin, (TPP)M, is rapidly oxidized by acid-catalyzed electron-transfer reduction of O2 by Fc, regenerating [(TPP)M]+ via formation of the putative hy-droperoxo complex (Scheme 15) [215]. 

The rate constants (ket) of electron transfer from Fc to [(TPP)M] agree well with those evaluated in light of the Marcus equations [91] for outer-sphere electron transfer (Eq. 4) [215]. Such agreement clearly demonstrates that electron transfer from Fc to [(TPP)M]+ in Scheme 15 proceeds via an outer-sphere pathway. In contrast to this, the ket value of the acid-catalyzed electron transfer from (TPP)Co to O2 is 10 -fold larger than that expected from an outer-sphere electron transfer [215]. Such huge enhancement of the observed rate relative to that calculated for outer-sphere electron transfer indicates the strong inner-sphere nature of acid-catalyzed electron transfer from (TPP)Co to O2 this should result in formation of the hydroperoxo complex, [(TPP)Co02H]+ (Scheme 15, M = Co). Other metalloporphyrins (M = Fe and Mn) can also act as efficient catalysts of the reduction of... 

Hydrated electrons react with certain water-soluble metalloporphyrin complexes, reducing the porphyrin ligands to pi-radical species. When the metal centers are Zn(II), Pd(II), Ag(II), Cd(II), Cu(II), Sn(IV), and Pb(II), the radical complexes are produced at diffusion-controlled rates and decay with second-order kinetics.188 Fe(III) porphyrins, on the other hand, yield Fe(II) porphyrins.189 Rather different behavior is seen in the reaction of e (aq) with [Ru(bpy)3]3 + here, parallel paths generate the well-known luminescent excited-state [ Ru(bpy)3]2 + and another reduced intermediate, both of which decay to the ground-state [Ru(bpy)3]2+, 190 In a direct demonstration of the chemical mechanism of inner-sphere electron transfer, [Coni(NH3)5L]2+ complexes where L = nitrobenzoate and dinitrobenzoate react with e (aq) to form Co(III)-ligand radical intermediates, which then undergo intramolecular electron transfer to yield Co(II) and L.191... 

Addition of various concentrations of [60]fullerene, for example, to a ZnTPP solution, resulted in an accelerated decay of the 7t-radical anion (ZnTPP "). The observed rate was linearly dependent on the [60]fullerene concentration, which, in turn, has led to the assumption that the ZnTPP tt-radical anion reacts with [60]fullerene. To confirm a probable electron transfer, the formation of the characteristic C60 absorption in the NIR ( ax = 1080 run) was also monitored. The grow-in rate of the C o " absorption at various wavelengths in the 980-1060 nm range was nearly identical to the decay rate of the MP absorption at 650-750 nm. For example, in the case of ZnTPP 7t-radical anion (ZnTPP "), a bimolecular rate constant of (2.5+1.0) x 10 M s was derived from the ZnTPP " decay (720 nm) and (1.4 1.0) x 10 M s from the Ceo formation (970 nm). These two values are in reasonable agreement and confirm unmistakably the electron transfer from the one-electron reduced metalloporphyrin (ZnTPP) to the singlet ground state of the fullerene ... 

Extending the studies on the reductive electron transfer from reduced metalloporphyrin states to [60]fullerene into aqueous media requires employment of a water-soluble form of [60]fullerene. Therefore, a micellar assembly consisting of the fullerene incorporated into Triton X-100 was investigated. The electron transfer does indeed occur across the interface of the micellar assembly. The rates exhibit a considerable slow-down compared to the homogeneous systems but, on the other hand, show a clear dependence on the reduction potential of the water-soluble metalloporphyrin. 

More recently, rate constants were determined for electron transfer from a number of metalloporphyrin radical anions to Cgo and were found to be quite... 

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