Pyridinium and its substituted derivatives are effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of carbon dioxide to products such as formic acid, formaldehyde, and methanol. Importantly, high faradaic yields for methanol have been observed in both electrochemical and photoelectrochemical systems at low reaction overpotentials. Herein, we report the detailed mechanism of pyridinium-catalyzed CO(2) reduction to methanol. At metal electrodes, formic acid and formaldehyde were observed to be intermediate products along the pathway to the 6e(-)-reduced product of methanol, with the pyridinium radical playing a role in the reduction of both intermediate products. It has previously been thought that metal-derived multielectron transfer was necessary to achieve highly reduced products such as methanol. Surprisingly, this simple organic molecule is found to be capable of reducing many different chemical species en route to methanol through six sequential electron transfers instead of metal-based multielectron transfer. We show evidence for the mechanism of the reduction proceeding through various coordinative interactions between the pyridinium radical and carbon dioxide, formaldehyde, and related species. This suggests an inner-sphere-type electron transfer from the pyridinium radical to the substrate for various mechanistic steps where the pyridinium radical covalently binds to intermediates and radical species. These mechanistic insights should aid the development of more efficient and selective catalysts for the reduction of carbon dioxide to the desired products.

The chemistry of the electrocatalytic reduction of CO 2 using nitrogen containing heteroaromatics is further explored by the direct comparison of imidazole and pyridine catalyzed CO 2 reduction at illuminated iron pyrite electrodes. The mechanism of imidazole based catalysis of CO 2 reduction is investigated by analyzing the catalytic activity of a series of imidazole derivatives using cyclic voltammetry. While similar product distributions are obtained for both imidazole and pyridine, the imidazole catalyzed reduction of CO 2 likely proceeds via a very different mechanism involving the C2 carbon of the imidazole ring.

The efficient conversion of formate into oxalate could enable the industrial‐scale synthesis of multi‐carbon compounds from CO2 by C−C bond formation. We found conditions for the highly selective catalytic conversion of molten alkali formates into pure solid oxalate salts. Nearly quantitative conversion was accomplished by calcination of sodium formates with sodium hydride. A catalytic mechanism proceeding through a carbonite intermediate, generated upon H2 evolution, was supported by density functional theory calculations, Raman spectroscopy, and the observed changes in the catalytic performance upon changing the nature of the base or the reaction conditions. Whereas the conversion of formate into oxalate by using a hydroxide ion catalyst was previously studied, hydride ion catalysis and the chain reaction mechanism for the conversion involving a carbonite ion intermediate are reported herein for the first time.

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Incorporation of p-propylanisole 1 onto a pentasil zeolite (Na ZSM-5) gives rise to the EPR spectrum of anethole radical cation 2*+; this conversion requires three separate oxidation steps alternating with two deprotonations, a reaction sequence of remarkable complexity for a room temperature reaction, albeit on the internal surfaces of a zeolite.

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Incorporation of diphenyl disulfide (1) into a pentasil zeolite (Na-ZSM5) results in the formation of the extended radical cation, 1 ϩ , as well as thianthrenium radical cation (2 ϩ ) formed by oxidative cyclization. The stabilization of 1 ϩ on the internal surfaces of the zeolite is unprecedented because in solution it is converted spontaneously into 2 ϩ . A mechanism for the conversion of 1 ϩ to 2 ϩ consistent with processes observed on the internal surfaces of the zeolite is proposed.During the past 30 years the structures and catalytic properties of zeolites have been investigated extensively and many chemical reactions on the surface and in the micropores have been studied. 1-6 The generation of π-type radical cations and their stabilization in the channels of pentasil zeolite at ambient temperatures is an attractive application of zeolites. 7,8 The resulting radical cations have long lifetimes and, hence, can be investigated by conventional spectroscopic techniques. The technique may also generate σ-type neutral radicals; for example, oximes were converted to iminoxyls via one-electron oxidation followed by deprotonation. 9,10 Further, selected substrates undergo oxidative dehydrogenation, as exemplified by the generation of anethole radical cation from p-propylanisole. 11 In this communication, we report the utility of zeolites in stabilizing 'extended' radical cations that, in solution, undergo spontaneous intramolecular cyclization reactions. Diaryl disulfide radical cations have not been observed in solution because they rapidly cyclize with dehydrogenation, forming thianthrenium ions. For example, Giordan and Bock observed substituted thianthrenium ions upon oxidation of para-substituted diphenyl disulfides. 12 Incorporation of 1 into the channels of pentasil zeolite (Na-ZSM5) 13 gave rise to an EPR powder pattern characteristic for organic free radicals containing heteroatoms. The observed spectrum is quite complex (Fig. 1, top); its interpretation was aided significantly by comparison with the EPR spectrum of a zeolite sample into which an authentic sample of thianthrene (2) had been sequestered (Fig. 1, bottom). The comparison suggests that introduction of 1 into the zeolite produces two distinct radical species. The EPR powder pattern at high field ( Fig. 1, top) closely matches that observed for 2 ϩ in the zeolite (Fig. 1, bottom). The spectrum shows an orthorhombic powder pattern with g 11 = 2.0136, g 22 = 2.0081 and g 33 = 2.0024; the average of the three principal g values, g avg = 2.0080, is in good agreement with the isotropic g value (g iso = 2.0084) measured for 2 ϩ in solution, 12 particularly when considering possible minor g-factor changes due to host-guest interactions in the zeolite. This feature is ascribed to 2 ϩ . S S • • S S 1 1 •+ 2 •+ ZSM-5A second orthorhombic powder pattern (g 11 = 2.0263, g 22 = 2.0090, g 33 = 2.0022; g avg = 2.0125), characteristic for a free radical (ion) with spin on sulfur, is assigned to the extended radical cation, 1 ϩ , of the precursor, a species not p...

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