Structure and characteristics of the ionic forms of heteroarenium salts in organic solvents (review)

Abstract

The results of investigations into the formation of the various ionic forms of heteroarenium salts, their characteristics, and their reactivity are summarized. Salts containing (4n+2)π-electron aromatic cations (n = 0-3), including chalcogenopyrylium (O, S, Se), azinium, benzo-and dibenzoazinium, and chalcogenoazolium (O, S, Se, NR) ions are examined. In this system of compounds the relationships between the structure and the macroscopic characteristics of the heteroarenium salts (ionization constants, association/dissociation constants, distances of closest interionic approach) and the effect of the nature of the ionic forms in the solutions on their reactivity in heterolytic reactions are analyzed.Salt forms and in particular heteroarenium salts occupy an extremely important position in the chemistry of heterocyclic compounds, being of interest both in structural respects and in their high reactivity in nucleophilic reactions, recyclization reactions [1-3], and more recently at the applied level as ionic liquids [4]. It is on the basis of heteroarenium salts that the modern chemistry of stable carbenes -the organic chemistry of the divalent carbon atom -has developed [5][6][7][8][9].In spite of numerous investigations into the structure of the salts, carried out by various physicochemical methods (spectral, polarographic, X-ray crystallographic, etc.) direct information about their properties as salts, i.e., their capacity for dissociation/association and the formation of various ionic forms (free ions, various ionic associates -ion pairs and ion triplets), has remained almost in the dark. Among the characteristics directly describing the state of heteroarenium salts in solutions the most important are clearly the thermodynamic characteristics (which can be regarded as macroscopic in contrast to the microscopic characteristics relating, for example, to the spectral characteristics) -the ionic association (K a )/dissociation (K d ) constants, the distance of closest approach of the ions, and theories about the dependence of these quantities on the structure of the salts. Information on the ionic association of heteroarenium salts is of considerable interest for assessing their reactivity. It is also important for understanding the processes that give rise to the effectiveness of organic electrolytes containing heteroaromatic cations in physical systems (e.g., as luminophores [10]).