Philippe Leduc

Chemical and electrochemical oxidation of aqueous solutions of NADH and model compounds

Abstract Aqueous solutions of NADH and of model compounds such as I -4,dihydro-N1-propyl- and 1–4,dihydro-N1-benzyl-nicotinamide are oxidized on rotating Pt disk electrode at +0.66 ± 0.02 V (N.H.E.). This two-electron wave is pH independent in the range of 7–13. Both macroelectrolysis on Pt at +0.85 V (N.H.E.) and chemical oxidation by [Fe (CN6]3− lead to NAD+ or model compounds. Differences between biochemical, chemical and electrochemical oxidation of NADH are discussed, especially in terms of energetics and mechanisms of reactions.

Structural and spectroscopic characterization of a five-coordinate {Fe(NO)}6 complex derived from an iron complex with carboxamido N and thiolato S donors

Abstract An iron(III) complex, [Fe(N2S2)CI](Et4N)2 (1) with a mixed carboxamido nitrogen and thiolato sulfur donor set derived from (2-mercapto-isobutyryl)-o-phenylene diamine was prepared. The iron(III) is in a square pyramidal ligand environment. EPR, Mossbauer and variable-temperature susceptibility establishes that 1 possesses an S=3/2 ground state. The iron(III) oxidation state is stable over a 1.5 V range (Epc[Fe(N2S2)Cl]3−/2− =−1290 mV and Epa [Fe(N2S2)Cl]2−/1−=+220 mV versus SCE in CH3CN). Complex 1 reacted with a stoichiometric amount of NO yielding an air stable [Fe(N2S2)(NO)](Et4N) complex 2 which belongs, as does the [Fe(S2C4N2)2(NO](Et4N) nitrosyl dithiotene complex 4 to the {Fe(NO)}6 group. Both complexes have a square pyramidal structure with a linear FeNO moiety and a very short FeN(O) bond distance of 1.633 (2) and 1.612 A (4). They differ by the position of the νNO stretching frequency located at 1780 cm−1 for 2 and 1840 cm−1 for 4. Magnetic susceptibility measurements indicated the diamagnetic nature of both complexes. The Mossbauer spectra of complexes 1, 2, 3 ([Fe(S2C4N2)2]2(Na)2), and 4 consist of a doublet with a large quadrupole splitting in the case of 1 and 2 (3.278 and 3.149 mm s−1, respectively, at 293 K). Isomer shifts values of the nitrosylated compounds are lower (−0.171 for 2 vs. +0.177 for 1 at 293 K and +0.053 mm s−1 for 4 vs. +0.331 mm s−1 for 3 at 100 K).

A new manganese-β-heptanitro-porphyrin with extreme redox potentials: spectral, electrochemical and catalytic properties

Abstract Manganese tetra-(2,6-dichlorophenyl)-β-heptanitroporphyrin was prepared by selective heptanitration of Zn(TDCPP=tetra-(2,6-dichlorophenyl)porphyrin), demetallation of the resulting porphyrin and treatment with Mn(OAc)2. It exists as a high-spin Mn(II) complex and it is electrochemically oxidized to the corresponding Mn(III) complex which exhibits the highest redox potential reported so far for an Mn(III) porphyrin (+940mV versus SCE). Moreover it acts as a three-electron reservoir after three successive, reversible reductions at potentials unusually high for such reactions (+43, −220 and −745 mV versus SCE). It is also a good catalyst for alkene epoxidation and alkane hydroxylation by PhIO which constitutes a non-classical behavior for an Mn(II) porphyrin.

Significance of the Zero-Current Potential of the NAD+-NADH system☆

Abstract Since the first electrochemical reduction step of NAD÷ and the electrochemical oxidation step of NADH lie respectively at —0.69 and 0.5–0.9 V (N.H.E.). no direct measure of the formal potential of NAD÷-NADH system may be attained potentiometrically. We examine the significance of the previous potentiometric studies realized in the presence of a mediator and an enzyme. A reliable zer-current potential of NAD÷ and NADH solution is only obtained when small amounts of benzyl-viologen (BV 2÷ ) and xanthine oxidase (XO) are added. The various electrochemical reduction steps of 10 to 1000 μ M BV 2÷ aqueous solutions are studied at pH 9.8 on dropping mercury and rotating platinum disk electrodes. The formal potential of the BV 2÷ -BV − system, equal to —0.360 V (N.H.E.), may be measured on a platinum electrode, while on a mercury electrode a strong adsorption interferes and gives a polarographic prewave. When XO is present, the addition of NADH decreases the BV 2÷ reduction waves: an anodic wave corresponding to BV 2÷ oxidation appears at the same potential as the first BV 2÷ reduction wave. The zero-current potential measured in NAD÷, NADH, BV 2÷ , BV 2÷ and XO solutions is actually fixed by the BV 2÷ -BV÷ system, which equilibrates through chemical oxido-reduction reactions with the NAD÷-NADH system.

A new and efficient biomimetic system for hydrocarbon oxidation by dioxygen using manganese porphyrins, imidazole, and zinc

Epoxidation of alkenes and hydroxylation of alkanes by dioxygen and Zn as a reducing agent, with good yields (up to 50%) based on Zn and rates (up to 200 turnovers of the catalyst per hour), were achieved by using manganese porphyrin catalysts in the presence of 1-methylimidazole and acetic acid.

One-pot dodecanitration of Zn(II) and Ni(II) meso-tetrakis-(2,6-dichlorophenyl) porphyrin, and extreme redox properties of the obtained complexes

Abstract Introduction of 12 nitro substituents on Zn(II) and Ni(II) [ meso -tetra-(2,6-dichlorophenyl)porphyrin=TDCPP] was performed by adapting a recently described method for polynitration of metalloporphyrins, allowing the one-pot synthesis of metallododecanitroporphyrins from commercially available compounds. The corresponding complexes exhibit the highest reduction potentials ever described for such metalloporphyrins, which are shifted by more than +1.6 V when compared with those of Zn and Ni(TDCPP). The Ni (dodecanitroporphyrin) complex underwent four successive, reversible one-electron reductions between +470 and −555 mV (vs. SCE). The one-electron reduction product of the Ni dodecanitroporphyrin is stable for hours at room temperature even in aerobic conditions; its visible and EPR spectra indicate a Ni (porphyrin radical anion) structure.

A new general method for selective β-polynitrationof porphyrins; preparation and redox properties of Zn-porphyrins bearingone through to eight β-nitro substituents and X-ray structure of thefirst Zn β-pernitro porphyrin

Selective β-polynitration of Zn–5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin, Zn(TDCPP), is achieved by controlled titration with the HNO3–CF3SO3H–(CF3S O2)2O system, and affords a full series of Zn porphyrins bearing one through to eight β-nitro groups in high yield and exhibiting a wide range of reduction potentials (from −920 to +155 mV vs. SCE); an X-ray structure of the first reported β-pernitrated Zn porphyrin Zn(TDCPN8P)(EtOH)2·2EtOH confirms the synthetic methodology.