Concomitant Electron Transfer Research Articles

ChemInform Abstract: Tandem C—H Activation/Arylation Catalyzed by Low‐Valent Iron Complexes with Bisiminopyridine Ligands.

AbstractThe reaction is proposed to proceed through a metal‐based inner‐sphere C‐H activation involving concomitant electron transfer from the ligand and the substrate.

Pentachlorophenol radical cations generated on Fe(III)-montmorillonite initiate octachlorodibenzo-p-dioxin formation in clays: density functional theory and fourier transform infrared studies.

Octachlorodibenzodioxin (OCDD) forms spontaneously from pentachlorophenol (PCP) on the surfaces of Fe(III)-saturated smectite clay. (1) Here, we used in situ Fourier transform infrared (FTIR) methods and quantum mechanical calculations to determine the mechanism by which this reaction is initiated. As the clay was dehydrated, vibrational spectra showed new peaks that grew and then reversibly disappeared as the clay rehydrated. First-principle density functional theory calculations of hydrated Fe/PCP clusters reproduced these transient FTIR peaks when inner-sphere complexation and concomitant electron transfer produced Fe(II) and PCP radical cations. Thus, our experimental (FTIR) and theoretical (quantum mechanical) results mutually support the hypothesis that OCDD formation on Fe-smectite surfaces is initiated by the reversible formation of metastable PCP radical cations via single-electron transfer from PCP to Fe(III). The negatively charged clay surface apparently selects for this reaction mechanism by stabilizing PCP radical cations.

Unprecedented Matrix-Induced Reduction of Flavins Observed Under FAB and MALDI Conditions

Flavin-derived compounds such as riboflavin, riboflavin 5-phosphate (flavin mononucleotide, FMN) and flavin-adenine dinucleotide (FAD), under matrix-assisted laser desorption ionization (MALDI) and fast atom bombardment (FAB) conditions produce unprecedented reduced species that can be attributed to [M+2H]+. and [M+3H]+ in addition to the anticipated [M+H]+ ions. On the other hand, electrospray ionization (ESI) conditions generate only the expected [M+H]+ as the sole molecular-related ion. A protonation from the matrix accompanied by a concomitant electron transfer, rather than the transfer of hydrogen atom(s), [H. of CH groups], is proposed to explain the observed reduction process. The reduction site expands to N(1)=C(10a)-C(4a)=N(5), a 1,4-diaza-1,3-butadiene system, in the isoalloxazine ring, resembling the behavior of prosthetic groups of aerobic dehydrogenases. Interestingly, CID-MS / MS of most abundant molecular-related ions generated by the FAB and ESI modes, namely [M+2H]+. for FAB and [M+H]+ for ESI, produce the identical even-electron fragment ion of isoalloxazine moiety, indicating a radical loss vs. the evenelectron molecule loss from the individual precursor ions. The observed phenomena are compared with those previously reported for FABMS, thermospray mass spectrometry (TSPMS) and ESIMS. Keywords: Matrix-Induced Reduction, Flavins, thermospray, prosthetic groups, aerobic dehydrogenases

Reduction in Desorption Mass Spectrometry: Multiple Protonation on Flavins without Charge Increment.

Three flavin-containing compounds, namely, riboflavin, riboflavin 5′-phosphate (flavin mononucleotide, FMN) and flavin-adenine dinucleotide (FAD, Scheme 1), were studied using matrix-assisted laser desorption ionization (MALDI), fast atom bombardment (FAB) and electrospray ionization (ESI) mass spectrometry. It was found that the isoalloxazine ring (flavin) was reduced in the positive-ion MALDI and FAB ionization processes producing unusual ions [M+2]+· and [M+3]+ in addition to the anticipated [M+1]+, whereas ESI generated only [M+H]+ as the molecular-related ion. The reduction mechanism is hypothesized as the protonation from the matrix with a concomitant electron transfer,*1 rather than the transfer of hydrogen atom(s), H··ESI, in which matrix is not used, did not produce [M+2]+· nor [M+3]+ more than the natural isotope abundance of 13C in the sample molecules. The reduction site expands to N(1)=C(10a)-C(4a)=N(5), a1,4-diaza-1,3-butadiene system, in the isoalloxazine ring, resembling the behavior of prosthetic groups of aerobic dehydrogenases.

Protonation and Free Energy Changes Associated with Formation of QBH2 in Native and Glu-L212 .fwdarw. Gln Mutant Reaction Centers from Rhodobacter sphaeroides

Formation of the quinol QBH2 in Glu-L212-->Gln mutant [EQ(L212)] reaction centers (RCs) from Rhodobacter sphaeroides was investigated by measuring the proton uptake (using dyes), UV absorption changes, and free energy changes associated with the two-electron reduction of QB. The advantage of using the EQ(L212) RCs for these studies is that the individual protonation steps can be kinetically resolved and analyzed; conclusions reached regarding the mechanism of formation of QBH2 are expected to apply also to native RCs. The proton uptake by EQ(L212) RCs was strongly biphasic: the fast phase was essentially concomitant with the second electron transfer to QB- (approximately 1 ms at pH 7.5); the slow phase was approximately 2000-fold slower. The rate constant of the slow phase depended on the redox state of the primary quinone QA; for QA- the rate constant was larger (i.e., 8-fold at pH 6.0) than for QA. The electron and proton transfers to QB- in EQ(L212) RCs were modeled with a two-step scheme as follows: (1) fast, QA-QB- + H+(1)-->QA(QBH)-; (2) slow, QA(QBH)- + H+(2)-->QAQBH2, where reaction 1 involves concomitant electron transfer and proton uptake [Paddock, M. L., McPherson, P. H., Feher, G., & Okamura, M. Y. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6803-6807]. The stoichiometry of the fast proton uptake associated with the two-electron reduction of QB varied from 1.1 to 1.4 H+/2e- at pH 6.5-8.5, consistent with the uptake of H+(1) plus an additional fractional proton uptake due to amino acid residues whose pKa values are shifted by interactions with the charge of (QBH)-. The total steady-state proton uptake stoichiometry was 2.0 H+/2e- at pH < or = 7.5, consistent with the formation of the quinol QBH2 (reactions 1 and 2). At pH 8.5, the steady-state proton uptake was 1.6 +/- 0.1 H+/2e-, which is consistent with an apparent pKa for H+(2) of approximately 8.5 [McPherson, P. H., Okamura, M. Y., & Feher, G. (1993) Biochim. Biophys. Acta 1144, 309-324]. The proton uptake kinetics indicate that Glu-L212 is a component of the proton transfer chain for H+(2) that connects reduced QB (buried in the RC protein) to the aqueous solvent as proposed previously [Paddock, M. L., Rongey, S. H., Feher, G., & Okamura, M. Y. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6602-6606].(ABSTRACT TRUNCATED AT 400 WORDS)

Rechargeable Alkaline Manganese Dioxide Batteries: I . In Situ X‐Ray Diffraction Investigation of the (EMD‐Type) Insertion System

The electrochemical insertion and deinsertion of H+ was investigated by studying its influence on the evolution of the crystallographic structure of of EMD type (electrochemical method deposition) during the discharge and recharge processes of an alkaline battery, using in situ x‐ray diffraction. During the first discharge, proton insertion up to H+ per with concomitant electron transfer causes an expansion of the crystalline cell. The lattice expansion is attributed to the reduction of Mn4+ ions to Mn3+ ions which have a larger ionic radius. Between 0.5 to 0.8 H+ inserted the long‐range order is lost and the presence of a quasi‐amorphous phase which could not be identified is detected. This amorphous phase is reduced to (or ) and to when more than 0.8 H+ is inserted. On recharge, the is oxidized to (or ) leading to poor cycling of the battery. When the discharge is limited to 0.8 H+the battery can be recharged and cycled many times. Still more cycles can be obtained when the discharge is limited to 0.5 H+, corresponding to the range where the structure is retained. In this range, strong hysteresis in the voltage curves indicated that while there is structural reversibility, structural changes arise at different voltages on discharge and recharge.

Photodissociation of cytochrome c oxidase-nitric oxide complexes.

The dissociation of cytochrome c oxidase-nitric oxide complexes was studied by optical spectroscopy at cryogenic temperatures (15 degrees K). With the reduced cytochrome c oxidase-nitric oxide complex, the observations that were reported by Yoshida et al. were confirmed. Photodissociation of the oxidized cytochrome c oxidase-nitric oxide complex did not induce any significant absorbance changes between 350 and 875 nm. With the azide-nitrosyl-cytochrome c oxidase complex, the illumination caused the dissociation of the a2+(3).NO complex to the unligated state a2+(3). Increasing the temperature to 77 degrees K led to the formation of a new complex, probably a3+(3).N3-. The N3(-)-NO-cytochrome c oxidase complex was the only compound for which appreciable photodissociation was achieved by continuous illumination at room temperature (20 degrees C). The effect of illumination was biphasic. In the first phase the a2+(3).NO complex is dissociated and cytochrome a3 oxidized by an electron transfer to CuB. In the second phase nitric oxide, which is still bound to CuB after the first phase, is expelled from the complex by azide, with a concomitant electron transfer from CuB to cytochrome a.

Kinetics of oxidation of hydrazine and of t-butylhydrazine using tris(dimethylglyoximato)nickelate(IV) in the presence of added Cu2+(aq)

The kinetics of the oxidation of hydrazine and t-butylhydrazine using tris(dimethylglyoximato)-nickelate(IV), [Ni(dmg)3]2–, in the presence of added Cu2+(aq), and in the pH range 5.0–7.0 at 35 °C and I= 0.25 mol dm–3in aqueous medium, follow pseudo-first-order and pseudo-zero-order disappearance of the NiIVcomplex, respectively. Results of the Cu2+(aq)-promoted oxidation of hydrazine by [Ni(dmg)3]2–, are consistent with a probable scheme involving pH-dependent equilibrium formation of intermediate adducts between the NiIV and CuII–hydrazine complex species present in the solution and subsequent rate-determining electron transfer(s) to the adduct(s) from the hydrazine species in the presence of H+. Results of the Cu2+(aq)-catalyzed oxidation of t-butylhydrazine are interpreted in terms of a probable mechanism involving a rate-determining decomposition of the 1 : 1 intermediate complex between the CuII and t-butylhydrazine species in the solution, with a concomitant electron transfer. While the oxidation of hydrazine leads to nitrogen, the main products of the t-butylhydrazine oxidation are nitrogen and t-butyl alcohol.

Studies in nickel(IV) Chemistry. Kinetics of the aquacopper(II)‐catalyzed oxidation of phenylhydrazine by tris(dimethylglyoximato)‐nickelate(IV) in aqueous medium

AbstractThe kinetics of electron transfer in the redox system containing phenylhydrazine (S) and tris(dimethylglyoximato)nickelate(IV), in the presence of catalytic amounts of added Cu(II)aq, have been studied in aqueous medium at an ionic strength of 0.25M in the pH range of 6.01–9.06. The kinetics exhibit pseudo‐zero‐order disappearance of Ni(IV) when an excess of [S]0 and small amounts of Cu(II) are present. While the pseudo‐zero‐order rate constants are almost linearly dependent on [S]0 at constant [Cu(II)] and pH tending to become non‐linearly dependent on higher relative [S]0, they are linearly dependent on [Cu(II)] in a 20‐fold range. The pH‐rate profiles with low [S]0 and [Cu(II)] show a monotonic decrease in rates with increasing pH, the rates tending to attain limiting values at higher relative pH. Results are interpreted in terms of a probable mechanism involving the formation of precursor complexes of phenylhydrazine and Cu(II) species in the medium, followed by the rate‐determining breakdown of the precursors with concomitant electron transfer. The hydrolyzed species of Cu(II) reacts more slowly than does the aquacopper(II). Ni(IV) does not appear to have any kinetic role in the redox system and is involved only in rapid product formation steps. The oxidation product of phenylhydrazine is 4‐hydroxyazobenzene.

Studies in Nickel(IV) Chemistry. V. Kinetics of Electron Transfer in Redox Systems Containing Tris(dimethylglyoximato)nickelate(IV), Hydroxylamine, and Catalytic Amounts of Copper(II) in Aqueous Alkaline Media

Abstract Kinetics of electron transfer in the redox system containing hydroxylamine and tris(dimethylglyoximato)nickelate(IV) ([Ni(dmg)3]2−), in the presence of catalytic amounts of added copper(II) ions, have been studied in aqueous medium in the pH range 8.04–11.81. The kinetics exhibit pseudo-zero-order disappearance of [Ni(dmg)3]2− when excess [NH2OH]0 is present, in the presence and absence of added metal ions. While addition of Cu(II) results in significant catalysis, that of Co(II)/Co(III), Fe(II)/Fe(III), Ni(II) or Zn(II) does not. The pseudo-zero-order rate constants (k0) are found to be linearly dependent on [NH2OH]0 and on added [Cu(II)]. The k0 values register a decrease as [OH−] is increased in the pH range 8.04–ca.9.42, attaining a limiting value in the pH range ca. 9.8–ca. 11.81. Results are interpreted in terms of a probable mechanism involving formation of precursor complexes from hydroxylamine and Cu(II) species, followed by rate-determining breakdown of the precursors with concomitant electron transfer. The Cu(II)·aq species reacts several orders of magnitude faster than the Cu(OH)2·aq species. The Ni(IV) does not appear to have any kinetic role in the redox system and is involved only in rapid product formation steps. The stoichiometric ratios (Δ[Ni(IV)]/Δ[NH2OH]) of ca. 1.0 in the lower pH region (7.8–8.6) and ca. 0.5 in the higher pH region (9.1–11.6) are explained in terms of differential product formation.

Electron Transfer between Hexacyanoferrate(III) and Thiosulfate. Kinetic Studies in the trans-Dihydroxotetraoxoosmate(VIII) Oxidation and the trans-Dihydroxotetraoxoosmate(VIII)-catalyzed Hexacyanoferrate(III) Oxidation of Thiosulfate in Aqueous Alkaline Media

Abstract Kinetics of oxidation of thiosulfate to tetrathionate by trans-dihydroxotetraoxoosmate(VIII) (Os(VIII)) and by hexacyanoferrate(III) in the catalytic presence of Os(VIII) have been studied in aqueous alkaline media. The observed pseudo-first-order rate constants for disappearance of Os(VIII) in the Os(VIII)–S2O32− system (kobsd), and those for disappearance of hexacyanoferrate(III) in the Os(VIII)-catalysed Fe(CN)63−–S2O32− system (kcat) conform, respectively, to Eqs. A and B: k obsd=k3KOsK2[S2O32−](1+KOs[OH−]+KOsK2[S2O32−])−1, (A) k cat=k6KOsK2[Os(VIII)][OH−][S2O32−](1+KOsK2[S2O32−])−1, (B) where KOs is the stability constant of trans-dihydroxotetraoxoosmate(VIII) in aqueous alkaline media, K2 and k3 are the formation and decomposition constant of the intermediate complex 1, and k6 refers to the rate constant of electron-transfer from 1 to hexacyanoferrate(III). The values of KOs and K2, as obtained from spectrophotometric titrations and stopped-flow spectrophotometry, respectively, are 19.5±3 dm3 mol−1 and 5.58±0.13; the computed values from kinetic data of KOs, K2, and k3 at 30 °C and μ=0.32 mol dm−3, and that of k6 at 35 °C and μ=0.32 mol dm−3 are 22.5±2 dm3 mol−1, 6.12±0.5, (3.32±0.3)×10−1 dm3 mol−1s−1, and (3.31±0.3)×10−2 dm6 mol−2s−1, respectively. Possible mechanisms for electron-transfer are discussed in terms of an initial equilibrium interaction between Os(VIII) and S2O32− forming an intermediate complex [OsO4(OH)(S2O3)3−] (1) followed by (a) a rate-determining decomposition of 1, with concomitant electron-transfer, in the Os(VIII)–S2O32− system, and (b) a rate-determining outersphere electron-transfer from 1 to hexacyanoferrate(III) in the Os(VIII)-catalyzed Fe(CN)63−–S2O32− system.

Electron transfer between trans‐dihydroxotetraoxoosmate(VIII) and thiosulfate. A kinetic study in aqueous alkaline media

AbstractThe kinetics of oxidation of thiosulfate to tetrathionate by trans‐dihydroxotetraoxoosmate(VIII) in aqueous alkaline media have been studied. The oxidation follows a rate expression where KOs is the formation constant of trans‐dihydroxotetraoxoosmate (VIII), and K2 and k3, respectively, represent the formation constants of the intermediate complex involving Os(VIII) and S2O and its decomposition constant. The KOs, K2, and k3 values have been computed to be (19.5 ± 3) dm3/mol, (6.12 ± 0.5) and (3.32 ± 0.3) × 10−1 dm3/mol s at 303 K, and I = 0.32 mol/dm3, respectively. The rate law is consistent with a mechanism envisaging the equilibrium formation of an intermediate complex involving Os(VIII) and S2O, followed by a rate‐determining decomposition of the complex with concomitant electron transfer.