Organic Radical Ions

BY A. G. DAVIES AND G. GESCHEIDT

1 Introduction

This report covers the two years 1994 and 1995, with a few references taken from earlier years and from 1996 with the aim of improving continuity.

We note that an increasing number of papers are referring to the use of both ESR and ENDOR spectroscopy, and to the simultaneous application of ESR spectroscopy together with another technique such as UV-VIS spectroscopy or electrochemical measurements. More work is also being directed to radical trianions. There are a number of interesting studies of those factors, particularly steric strain, which can affect electron distribution and the ordering of energy levels in conjugated π-systems, and a lot of attention is being paid to counterion interactions. The interest in radical ions derived from the fullerenes has not slackened, and an increasing number of papers are concerned with the development of devices for molecular electronics.

2 Bibliography

There is a very timely review of the use of ENDO R spectroscopy in the study of radical ions in solid matrices. Topics in Current Chemistry have commenced a new series on the topic of electron transfer, and ESR studies are covered in articles on 'Radical ions: where organic chemistry meets material science' and on 'Photophysical and photochemical properties of fullerenes'.

3 General Technique

A simple experimental setup has been described permitting the simultaneous measurement of the ESR and optical absorption spectra of the same sample. It involves a diode array UV-VIS spectrometer which is coupled to an optical ESR cavity by quartz fibre optics. A small (0.2 ml) robust electrochemical cell capable of rapid-scan voltametry allows simultaneous electrochemical and ESR (SEESR) measurements to be carried out in solvents of high or low dielectric constant over a wide range of temperature, and similar measurements can be carried out using a cylindrical rotating cavity.

The pulse-mode product-yield-detected ESR (PYESR) technique measures spin-adduct yield as a function of the delay period for the on/ off time of a microwave pulse, which causes a reduction in the yield from radicals generated by a UV laser. The PYESR response curves, thus obtained, can be reproduced by numerical calculation to determine the radical-pair dynamics.

4 Calculations of Hyperfine Coupling Constants

Obviously the isotropic coupling constants map the electron distribution of radicals. They are inter alia indicative for the singly occupied orbital. Thus almost all papers reporting ESR data also include theoretical calculations. The methods utilised range from Hückel to high level ab initio, including perturbation treatment, but semiempirical calculations prevail. The agreement between theoretical and experimental values however often varies significantly. Recently, density functional theory (DFT) has been used to calculate the Fermi contact interactions of small radicals. The results of these calculations look very promising, and it seems very likely that OFT calculations will become an efficient tool for the calculation of hyperfine data. The good applicability of DFT is due to an improved inclusion of electron correlation compared to semiempirical and less sophisticated ab initio treatment. The major advantage versus sophisticated ab initio methods is a considerably shorter CPU time. Thus it should be possible also to investigate more extended molecules with DFT.

5 Radical Cations

5.1 Reviews

Radical cations of small molecules generated by γ-irradiation in freon matrices and investigated by ESR/ENOOR have been reviewed by Gerson and silicon radical cations by Bock. The isomerisations of radical cations have been compiled by Gebicki and Cope-type rearrangements of hexa-1,5-diene and semibullvalene radical cations by Williams.

5.2 Experimental Methods

The techniques for generating radical cations in fluid and rigid solution are well established. The common methods include oxidation with acids (e.g., trifluoroacetic acid (TFA), H2SO4), Lewis acids (e.g., AlCl3, SbCl3, SbCl5), metal salts (e.g., Pb(OAc)4) or differently substituted triarylaminium radical cation salts. For time-resolved studies strong electron acceptors like chloranil (with narrow ESR signals which do not greatly distort the spectrum of the desired compound) are applied. Predominately the radical cations of small strained molecules with high ionisation potentials (up to ca. 12 eV) are generated by γ-irradiation in freon matrices at low temperatures. Often radical cations can be formed in zeolites either directly or by additional radiolysis.

Phenyl iodine bis(trifluoroacetate) and 4–tolylthallium(III) bis (trifluoroacetate) were shown to serve as efficient oxidants and 1,1,1,3,3,3–hexafluoropropan-2–ol is a solvent with a highly stabilising ability for radical cations and allows them to be studied even at room temperature; a whole variety of oxidising agents can be utilised. Application of 2,3–dichloro-5,6–dicyanobenzoquinone (DDQ) under acid conditions thermally leads to radical cations of substrates with oxidation potentials below ca. 1.6 V (vs. SCE). Irradiation with UV light allows the observation of ESR spectra of radical cations derived from compounds with significantly higher oxidation potentials, e.g. benzocrown ethers. The presence of an acid quenches the radical anion of DDQ which then presumably decomposes to ESR-silent products.

Mechanisms underlying the activity of zeolites were investigated with benzene as a probe. When benzene is adsorbed in ZSM-5 which is activated with a-oxygen (by decomposition of N2O) it is first converted to phenol by the insertion of an O atom into a CH bond, and then the phenol is oxidised to its radical cation. Generation of the benzene (or d6-benzene) radical cation in H-ZSM-5 zeolite by irradiation (2.8 eV, 1000-W Hg lamp) at 93 K yields a paramagnetic precursor. The primarily observed ESR signal is ascribed to a complex consisting of two interacting radicals with a distance of ca. 5.36 Å, but it is not possible from the experimental data to specify its structure.

Ion pairing is a well established phenomenon found for radical anions. In contrast, the interaction of radical cations with their environment is only rarely observed and analysed. For AlCl3 oxidation, a close contact with a π-radical cation may lead to partial breaking of the symmetry. Some evidence about the influence of the environment can also be derived from different rearrangement pathways in various freon matrices or zeolites (see below), but at present no explicit evidence about the role of the rigid solvents exists.

5.3 π-Systems

In this section not only classical extended π-systems will be reported. Strained alkanes frequently undergo rearrangement into π-type radical cations, and these are also dealt with here.

The one-electron reduced or oxidised stages of D6h benzene are prototypes for Jahn-Teller distortions due to the doubly degenerate LUMO and HOMO. A recent study of the radical ions of hexafluorobenzene in rigid solutions combined with ab initio and INDO calculations indicates that the radical cation possesses a2B2g (D2h) structure with an elongated ring and coupling constants of –9 G (2 F) and 66 G (4 F) whereas the radical anion has a C2v structure with two out-of-plane fluorines (2B1).

Hexamethyl(Dewar) benzene (1) was deposited on solid dioxygenyl hexafluoroantimonate (O2•+ SbF6-) at 77 K and the sample was slowly warmed up to room temperature. A 13–line ESR spectrum was detected which was tentatively ascribed to 1•+ because the spectral pattern was almost identical to that observed in freon matrices at low temperatures (see ref. 6 in ref. 56). A spectrum with a dominant 13–line pattern could, however, also be recorded after the reaction of 1 with TlIII or HgII trifluoroacetate or DDQ in TFA (with irradiation) or dichloromethaneTFA (for the Tl salt at 213 K). In addition, 1 was reacted with Tim trifluoroacetate in TFA; after the decay of the corresponding spectrum of 1•+, the sample was irradiated and again the 13–line pattern appeared. Thus it was concluded that this ESR signal emerges from the penta methylbenzyl trifluoroacetate radical cation 2•+. The reaction mechanism was investigated in detail.

Three isomers of cyclooctatetraene, C8H8, with a dihydropentalene skeleton were synthesised (1,2-3, 1,4-3, 1,6-3) and their radical cations and subsequent thermal and photochemical conversions were studied by electronic absorption in freon or argon matrices, and ESR spectroscopy in freon matrices at low temperatures as well as theoretical methods at various levels. The shapes of the SOMOs of the radical cations of 1,2-3, 1,4-3 and 1,6-3 were established from their ESR spectra. With this knowledge, the photochemical isomerisation of the 1,4-isomer to 1,5-3 and 1,6-3 and back to 1,4-3 was demonstrated upon irradiation at selected wavelengths.

The radical cation of quadricyclane (4) was established by two methods. Electron-beam irradiation of 4 in Na-ZSM-5 zeolite yielded a spectrum with coupling constants of 9.8 G (4 H), 7.2 G (2 H) and 2.8 G (2 H), distinctly different from the corresponding data for 5•+ (norbornadiene; 7.98, 3.28, and 0.58 G). The application of time resolved ESR led to a spectrum with coupling constants of 5.1 (4 H), 6.6 (2 H), 2.0 G (2 H) when 4 was treated with chloranil and irradiated with a 10 ns laser pulse. The lifetime of 4•+ under these conditions was 1.5 ms at 298 K in acetonitrile. The rearrangement product 5•+ could not be established in this study. The significant differences in the hyperfine data of 4•+ in acetonitrile solution and in the zeolite may be due to interactions between the zeolite and the radical cation (ion pairing).

When housane type molecules (obtained after photolysis of the corresponding azo compounds) 6a–c dissolved in freon matrices at 77 K are irradiated with γ-rays, the subsequent ESR spectra reveal that the 1,4-dimethyl and 5-methyl derivatives form persistent radical cations whereas the monomethyl radical cation 6a•+ cannot be detected. In the latter case only product 7•+ is formed regiospecifically.

Bi(cyclopropylidene) (8) gives, upon γ-radiolysis in CFCl3 and CF2ClCFCl2 matrices, the radical cation 8•+; in CF3CCl3 it immediately rearranges to the tetramethyleneethane radical cation. The symmetry of 8•+ is reduced to D2 (8 is D2h), i.e., the two three-membered rings are twisted out of the plane of the π-system plane. As a consequence one of the methylene protons has a coupling constant of + 22.4 G (4 H) but the other one has a (presumably) negative one of 2.7 G. At temperatures above 100 K the reactivity of 8•+ depends on the constitution of the matrix. In CFCl3 the ESR signal of tetramethyleneethane is detected whereas in CF2ClCFCl2 proton loss occurs.

A rearrangement to the conjugated radical cation of α-terpinene (9) was observed upon adsorption of α-pinene (10) or trans-isolimonene (11) onto activated H-mordenite. When 10 was incorporated into the deuteriated zeolite, no 2H was detected in the rearranged product; thus it was concluded that the reaction takes place at the Lewis-acidic sites.

Analogous behaviour was also established for the reactivity of cyclohexa-1,4-diene (12,) 3,3-dimethylbut-1-ene (13)(or 2,3-dimethylbut-l-ene; 14.) After γ-irradiation in acidic or non acidic zeolites the ESR spectra of the stabilised radical cations 15 and 16, respectively, were recorded. After annealing, the cyclohexadienyl and 1,1,2-trimethylallyl radicals were identified. A plausible explanation for the rearrangement into the stabilised olefinic radical cations is that the excess energy (ca. 12 eV) of the γ-rays is not absorbed by the zeolite but drives the chemical reaction. The subsequent occurrence of the neutral radicals is due to proton transfer.

Depending upon the geometry of the cage, the radical cations of different pagodane-type molecules resemble distinct configurations. The radical cations of [1.1.1.l]pagodane (17) and [2.2.1.l]isopagodane (18) can be regarded as 'extended', and those of[1.1.1.l]isopagodane (19) and [2.2.1.l]pagodane (20) as 'tight' isomers of the 4-centre/3-electron systems embedded in the polycyclic skeleton. These designations were deduced from INDO-calculated coupling constants based on ab initio UHF/3–21G geometries.

In the previous review, we reported on cyclobuta-annelated benzenes. The unusual spin distribution in these radical cations was traced back to strain effects leading to a partial bond localisation and polarisation. This phenomenon was already described in the 1930s and is commonly known as the Mills-Nixon effect. In the last years there has been some discussion about the suitability of this model, but a recent a x-ray crystallographic study has shown that bond fixation in a strained benzene derivative indeed exists. In contrast to the radical anion of binaphthylene (21)with an ESR spectrum which is 25 G broad, the ESR signal assigned to 21•+ has a width of only 12 G and a different line pattern; thus the pairing principle 71 which predicts that the radical anion and radical cation of molecules with delocalised π-systems and an even number of π-centres should have almost identical ESR spectra is not fulfilled. In the case of 21 the radical cation mirrors an 'unexpected' orbital which corresponds to the HOMO 1 (Hückel calculations). The exchange of the two highest occupied molecular orbitals is ascribed to the Mills-Nixon effect.

It is well established that naphthalene, upon oxidation in fluid solution, gives a dimeric radical cation. When the oxidation of alkylated naphthalenes is performed by photolysis in the presence of tetranitromethane, 2,3-dichloro-5,6-dicyanobenzoquinone or Hg11 trifluoroacetate in CH2Cl2 – trifluoroacetic acid, or by thermal oxidation with TlIII trifuoroacetatc, dehydrodimer radical cations are often detected. Such dimerisation reactions can be suppressed when the oxidation is performed in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP). Whereas photooxidation of 1-methoxynaphthalene with tetranitromethane leads to the 4,4'-connected dehydrodimeric binaphthalene in CH2Cl2, the radical cation of the parent compound can be observed in HFP. With the latter method, the radical cation of 1,4-dimethoxynaphthalene was also observed.

The radical anion of dibenzo[a,e]cyclooctatetraene (22) indicated an almost planar geometry; on the other hand 22•+ has the same tub shaped geometry as its neutral precursor. A matching geometry has also been established by x-ray crystallography for the radical cation of tetrakis(bicyclo[2.2.2]octeno )cyclooctatetraene (23).

The 1H coupling constants assigned to the 10 and 11 protons in the radical cation of 5-methylene-5-H-dibenzo[a,d]cycloheptene (24) are not identical but deviate by ca. 1 G. This is tentatively ascribed to the coordination of AlCl4- counterions at the positions carrying the highest positive charge. The radical cations of several alkylated azulenes (25) generated with CH2Cl2/TlIII trifluoroacetate and UV irradiation tend to form 1,1'-biazulenyls if the dimerisation is not hindered sterically by bulky substituents. This reactivity is consistent with the high spin population at the 1,3 positions.