Electroreduction of some pyridine carboxamides on carbon electrodes in aqueous solutions

Abstract

The electroreductions of the NAD' model compounds nicotinamide (I), N,-methyl nicotinamide (11), "-methyl nicotinamide (111) and isonicotinamide (IV) on carbon electrodes have been studied in aqueous media in the pH range 0-12 by linear-sweep cyclic voltammetry (Scheme 1, I-IV). Logarithmic analyses of the reduction peaks were performed by computing the convolution of the current with time as a function of the potential. On the basis of the experimental results it was concluded that the irreversibility of the electron transfers increased when a glassy carbon electrode was used, and this irreversibility being more marked when a plastic formed carbon electrode was employed. The reduction processes occurred with more difficulty on carbon electrodes than on mercury electrodes. Both the reduction and the reoxidation (when occurred) processes changed with respect to those observed on mercury electrodes, being irreversible electron transfers the rate-determining steps in most cases. Thus, for compounds I, I1 and 111 at pH < 2 the reductions occurred by the uptake of two electrons and two H+ ions, and the rate determining step was found to be the first one-electron transfer, for I and 111, and the irreversible second electron transfer, preceded by the uptake of an I%+ ion, for 11. At pH > 3 the processes consisted of electrodimerization reactions, preceded by the protonation of the heterocyclic nitrogen in cases I and IU. The second electron transfer of the electroreduction of IV always appeared irreversible, in contrast with that found for mercury electrodes.

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

“…Our suggested sequential PT–ET–PT–ET sequence (Schemes and , routes I , II , V , and VI ) to form PyH 2 from Py is strongly supported by the fact that an analogous process has been observed for the conversion of the Py-related species nicotinamide, , acridine, , and 3,6-diaminoacridine (proflavine) to their related dihydropyridine species. We point out that we propose the formation of 1,2-dihydropyridine as the kinetic product because protonation of the PyH 0 ’s C 2 carbon is more facile than protonation at the C 4 position, analogous to protonation of nicotinamide where the related 1,2-dihydropyridine is formed .…”

“…Our calculated positive reduction potential for PyH2 +· of E 0 calc= 0.11 V vs. SCE indicates that PyH2 +· reduction is facile and consequently that 1e -transfer (from PyH 0 or via a photoexcited electron) to PyH2 +· to form PyH2 is realized on p-GaP and in MacDonnell and coworkers' homogeneous Ru(II)/ascorbate photochemical system. 115 Our suggested sequential PT-ET-PT-ET sequence (Schemes 2 and 4, route I-II-V-VI) to form PyH2 from Py is strongly supported by the fact that an analogous process has been observed for the conversion of the Py-related species nicotinamide, 44,116 acridine, 45,117 and 3,6-diaminoacridine (proflavine) 46 to their related dihydropyridine species. We note that we propose the formation of 1,2-dihydropyridine as the kinetic product 40 because protonation of the PyH 0 's C2 carbon is more facile than protonation at the 4 position, 108 analogous to protonation of nicotinamide where the related 1,2-dihydropyridine is formed.…”

“…The hydricity of PyH 2 was also calculated according to Muckerman et al's approach; 128 we obtained 41.5 kcal/mol (<43 kcal/mol of HCOO − ), which supports that HT from PyH 2 to CO 2 is thermodynamically favorable. 131 We note that although cyclic voltammetry shows that the oxidation of PyH 2 -related dihydronicotinamide by ET−PT−ET−PT is irreversible and indicates that it is a poor electron transfer catalyst, 115 this does not preclude dihydronicotinamide or dihydropyridines in general from being competent hydride transfer catalysts.…”