Organic Radical Ions

BY A. G. DAVIES AND G. GESCHEIDT

1 Introduction

This report covers the two years from January 1992 to December 1993 inclusive, with some references from 1991 which were not included in Volume 13A. The references were obtained from our personal reading, CAS Online and Current Contents searches.

There is an increasing recognition of the importance of electron-transfer reactions in chemistry and biochemistry, and the interest in the initial radical ions in such reactions and in their subsequent transformations is leading to an increasing need for understanding the ESR spectra. The methods for the generation of radical ions are well established, but increasing use is now being made of photochemically assisted reactions.

2 Bibliography

The basic principles and practice of the observation of the ESR spectra of organic radical anions and cations in fluid solution have been reviewed, and a more thorough account of the structure and reactivity of organic radical cations (115 pages, 427 references) covers methods of generation, methods of observation, reactions, and unusual structures.

A book on Radical Ionic Systems: Properties in Condensed Phases, edited by Lund and Shiotani, includes reviews on electronic structure, spectroscopy, and photochemistry of organic radical cations, ESR studies on radical cations of saturated hydrocarbons, and of cycloalkanes and saturated heterocycles, of deuterium labelling studies of cation radicals, of radical cations of aliphatic ethers, of studies of radical cations by time-resolved magnetic resonance, and of ion pairs in liquids, together with other chapters on the study of radical ions by techniques other than ESR.

3 Radical Cations

3.1 Experimental methods

There is little new to report on new techniques for generating radical cations in fluid solution, but it is worth emphasising again that the species which is observed is not necessarily the same as the initial substrate. The ESR technique pinpoints the solute with the lowest ionisation energy, which can give a long lived radical cation, and even very small amounts of impurities or of transformation products which may result from the often strongly acid conditions, can be potentially misleading. This is particularly so when radical cations are generated with a Lewis acid such as aluminium chloride in dichloromethane, and Friedel Crafts reactions can occur with the solvent.

Thus (Me5C6)6Sn2, (Me4HC6)6Sn2, Mes6M2 (Mes = mesityl, M = Ge or Sn), and Mes4Ge with aluminium chloride in dichloromethane, and (Me5C6)2CH2, (Me5C6)(Me4HC6)CH2, and (Me4HC6)zCH2 + CH2O in trifluoroacetic acid (TFAH) all show the same spectrum of the 1,2,3,4,5,6,7,8-octamethylanthracene radical cation 1•+, the structure of which has been confirmed by X-ray crystallography; the species obtained from hexa- and pentamethyl benzene had previously been misidentified as the 1-methylene-2,3,4,4,5,6-hexa-methylcyclohexa-2,5-diene radical cation 2•+. Similarly a series of 1-aryl3-methyl-3-p-anisyl triazenes all show the same spectrum of the dimethoxy-N,N-dimethyldihydrophenazine radical cation.

The part that laboratory lighting may play in inducing photoxidation in TFAH solvent has often been overlooked. These reactions are usually regarded as involving electron transfer from the substrate to its conjugate acid (equation 1), but Eberson has proposed that the key to the effectiveness of TFAH is that electron transfer occurs from photoexcited ArH to TFAH dimer (equation 2), back electron transfer being avoided by synchronous proton transfer within the dimer.

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[FORMULA NOT REPRODUCIBLE IN ASCII] (2)

The current techniques for generating radical cations in fluid solution are limited to substrates with ionization energies below about 9.5eV and which can survive the acid conditions that are usually involved. Most simple classes of compounds which can be handled by these techniques have now been investigated to some degree, and studies are focusing on compounds with special structural features such as steric strain, bridged rings, or heterocyclic groups. On the other hand, a lot of attention is being paid to the generation of radical cations in frozen freons by γ-irradiation, which makes it possible to extend studies to substrates which may be sensitive to acids, and particularly those with higher ionisation energies (ca. 12 eV), including saturated hydrocarbons.

A number of research groups are also studying the ESR spectra of radical cations in zeolites, often generated again by radiolysis, but sometimes by exploiting the intrinsic acidity of the zeolite. They provide very convenient microreactors which retain their rigidity and dimensions over a much wider range of temperature than can be covered by rare gas matrices or the freons, and permit studies to be carried out up to and above room temperature. The polarity of the medium can be varied by changing the SiO2/Al2O3 ratio, the reaction volume can be controlled by regulating the size of the cavity, and intra- and inter-molecular reactions can be studied by varying the loading of the substrate in the zeolite. On the theoretical front, Bally has given a critical account of the scope and limitations of the various MO methods that can be used for calculating the spectroscopic properties of radical cations.

3.2 π-Systems

3.2.1 Hydrocarbons

Tetramethylethene ionises spontaneously in an HZSM-5 (SiO2:Al2O3 40.7) or H-mordenite (SiO2:Al2O3 8.9) zeolite, or on γ- irradiation in ZSM-5 or silicalite S-115 to give the radical cation Me2C = CMe2•+ with a(4Me) 1.72 mT. Its mobility within the cavity can be interpreted by quantitative simulation of the spectra. Decay occurs by formation of the π-dimer (Me2C = CMe2)2•+, with a(8Me) 0.83 mT, then deprotonation. 1- and 2-pentene in halocarbon matrices between 77 K and 145 K similarly show spectra typical for localised π-bond ionisation if account is taken of the conformation imposed by the matrix.

The spectrum of the radical cation of benzvalene 3 has been observed by γ-irradiation in a CF3CCl3 matrix. At 115 K it shows hyperfine coupling by the two olefinic protons (0.835 mT), by the inplane adjacent (H) protons (0.158 mT), and the γ-protons (2.70 mT) which lie in a W-orientation with respect to the p-orbitals. On heating or irradiation with visible light, the benzene radical cation is formed.

If a variety of pentadienes, or hexa-1,5-diene are adsorbed onto Hmordenite, the dominant spectrum which is observed is that of the radical...