https://doi.org/10.15255/KUI.2004.002
Published: Kem. Ind. 54 (1) (2005) 11–18
Paper reference number: KUI-02/2004
Paper type: Review
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Nitroso-compounds as Monomers and Dimers

V. Šimunić-Mežnarić and H. Vančik

Abstract

The majority of nitroso-compounds are isolated as colourless or white solids although some are obtained as blue or green liquids. When detailed IR spectroscopic studies were made, the existence of two different dimeric forms was found. Thus the structural chemistry of nitroso-compounds may be discussed in terms of three distinct molecular forms: the monomer, the Z-dimer and the E-dimer. That interrelation is illustrated in Scheme 1. By use of a variety of organic solvents, different temperatures, and compound concentrations it is possible to vary proportions of the three molecular forms (table 1). But, some of them do not form dimers under any conditions of concentration and temperature, for example N-substituted 4-nitrosoanilines (scheme 2). Monomer- dimer equilibria of nitroso-compounds in solution have been investigated, with ΔH°, ΔS° and ΔG° data calculated for dissociation of, both, Z- and E-azodioxy dimers to monomers (table 2). Kinetic data, based on TD and 2D-EXSY NMR measurements, have been obtained for the dissociation of 3-methylnitrosobenzene and 3,5-dimethylnitrosobenzene dimers (table 3). A mechanistic and kinetic study was made for E/Z isomerisation of azodioxycyclohexane in acetonitrile at different temperatures (table 5). Kinetic data indicate that monomer molecules dimerize at low temperature under the kinetic control to the Z-dimer. At elevated temperature, the thermodynamically more stable E-dimer is formed. Nitroso-compounds were obtained by several procedures: addition of NOX (nitroso-chloro-addition), nitrosation (nitroso-de-hydrogenation), oxidation of amines, reduction of nitro compounds, reaction of halogen with oxime, catalytic autoxidation of oxime and reaction of organomercurials with NOCl. Monomer-dimer equilibrium of nitroso-compounds was investigated in the solid state under cryogenic photochemical conditions. It was found that nitroso dimers can be UV-photolytically converted to nitroso monomers at 12 K, and reverted to dimers by visible light irradiation or warming above 170 K (scheme 8 and 9). Such a photothermal “chemical switch”, by which it is possible to break the chemical bond between two nitrogen atoms and bind them again, could in principle be used in supramolecular self-assembly system.

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Keywords

nitroso-compounds, monomer-dimer equilibrium, synthesis, selectivity, dimerization, photochromism, thermochromism