Aromaticity is an important concept to understand the stability and physical properties of π-conjugated molecules. Recent studies on pro-aromatic and anti-aromatic molecules revealed their irresistible tendency to become diradicals in the ground state. Diradical character thus becomes another very important concept and it is fundamentally correlated to the physical (optical, electronic and magnetic) properties and chemical reactivity of most of the organic optoelectronic materials. Molecules with distinctive diradical character show unique properties which are very different from those of traditional closed-shell π-conjugated systems, and thus they have many potential applications in organic electronics, spintronics, non-linear optics and energy storage. This critical review first introduces the fundamental electronic structure of Kekule diradicals within the concepts of anti-aromaticity and pro-aromaticity in the context of Huckel aromaticity and diradical character. Then recent research studies on various stable/persistent diradicaloids based on pro-aromatic and anti-aromatic compounds are summarized and discussed with regard to their synthetic chemistry, physical properties, structure-property relationships and potential material applications. A summary and personal perspective is given at the end.

Pro-aromatic and anti-aromatic π-conjugated molecules: an irresistible wish to be diradicals

Electrocatalysis in fuel cells requires as well substances capable of catalyzing the anodic oxidation of fuels as catalysts for the cathodic reduction of oxygen. Several dyestuffs that catalyze oxygen reduction are known, but up to now only one has been described as active in anodic reactions. All these dyestuffs are N4-chelates. Comparative studies have shown that chelates with other types of coordination, in particular N202-, 04-, N2S2- and S4-chelates, are able to catalyze the reduction of oxygen, though they are considerably less active than the N4-compounds. With a given type of coordination, the nature of the central atom has a decisive influence on the catalytic activity of the dyestuff, whereas substitution on the organic skeleton has only a slight effect. Thermal pretreatment of the N4-chelates can considerably increase their stability in electrolytes containing sulfuric acid. All the experimental results point to the conclusion that, with electrocatalysts, as with natural oxygen carriers, the interaction essential for catalysis takes place between the oxygen and the central metal ion. Various assumptions may be made as to the nature of the rate-determining step. The cathodic reduction of oxygen can be regarded as redox catalysis, or it can be considered from the standpoint of molecular orbital theory. The models hitherto suggested for the mechanism of oxygen reduction are tested against the experimental results and a modified model based on MO theory is put forward.

Organic dyestuffs as catalysts for fuel cells

Direct methanol–air fuel cells for road transportation

Publisher Summary The liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p -xylene, acetaldehyde and vinyl acetate from ethylene, and propylene oxide from propylene. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. This chapter describes the subject of metal-catalyzed oxidations of organic compounds in the liquid phase, largely within a mechanistic framework. A better understanding of the catalytic action of metal complexes is essential from the point of view of increasing selectivity and efficiency. Metal-catalyzed oxidations may be conveniently divided into two types—homolytic and heterolytic. The homolytic catalysis involves soluble transition metal salts (homogeneous), such as the acetates or naphthenates of Co, Mn, and Cu, or the metal oxides (heterogeneous). Heterolytic catalysis involves reactions of organic substrates coordinated to transition metals.

Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach

The electro-reduction of oxygen is effectively catalysed by metal chelates of the N4-type. The mechanism of this process has been found to be a modified ‘redox catalysis’. O2 molecules and the products of their reaction, at least up to H2O2, remain strongly co-ordinated to the central metal ion of the chelates XMeII. The potential-determining step, which regenerates the reduced form, is the following: (XMeIII...O2H)++H++ 2e→XMeII+H2O2.

The redox mechanism of the chelate-catalysed oxygen cathode

Metal complexes of phthalocyanine and $alpha;,$beta;,$gamma;,$delta;-tetraphenyl porphyrin as heterogeneous catalysts in oxidative dehydrogenation. Correlation between catalytic activity and redox potential

Organic Polymers. Correlation between Their Structure and Catalytic Activity in Heterogeneous Systems. II. Models for Dehydrogenation Catalysts

A Comparison between the chemical and electrochemical catalysis by tetraphenylporphyrin and phthalocyanine complexes

Heterogeneous and homogeneous catalysis by substituted cobalt tetraphenylporphyrins, and correlations with IR spectra

Organic polymers : Correlation between their structure and catalytic activity in heterogeneous systems. IV. Oxidative dehydrogenation : A comparison between the catalytic activity of an organic polymer and that of some molybdate catalysts.

J. Manassen

Papers

Metal complexes of phthalocyanine and $alpha;,$beta;,$gamma;,$delta;-tetraphenyl porphyrin as heterogeneous catalysts in oxidative dehydrogenation. Correlation between catalytic activity and redox potential

Organic Polymers. Correlation between Their Structure and Catalytic Activity in Heterogeneous Systems. II. Models for Dehydrogenation Catalysts

A Comparison between the chemical and electrochemical catalysis by tetraphenylporphyrin and phthalocyanine complexes

Heterogeneous and homogeneous catalysis by substituted cobalt tetraphenylporphyrins, and correlations with IR spectra

Organic polymers : Correlation between their structure and catalytic activity in heterogeneous systems. IV. Oxidative dehydrogenation : A comparison between the catalytic activity of an organic polymer and that of some molybdate catalysts.