Reactive Intermediate Chemistry

REFERENCES 203

147.A. Rajca, J. Wongsriratanakul, and S. Rajca, Science 2001, 294, 1503.

148.K. Itoh, Pure Appl. Chem. 1978, 50, 1251.

149.Y. Teki, T. Takui, M. Kitano, and K. Itoh, Chem. Phys. Lett. 1987, 142, 181.

150.T. Matsumoto, T. Ishida, N. Koga, and H. Iwamura, J. Am. Chem. Soc. 1992, 114, 9952.

151.T. Matsumoto, N. Koga, and H. Iwamura, J. Am. Chem. Soc. 1992, 114, 5448.

152.M. Murata and H. Iwamura, J. Am. Chem. Soc. 1991, 113, 5547.

153.C. Ling, M. Minato, P. M. Lahti, and H. vanWilligen, J. Am. Chem. Soc. 1992, 114, 9959.

154.W. P. Chisholm, H. L. Yu, R. Murugesan, S. I. Weissman, E. F. Hilinski, and J. A. Berson, J. Am. Chem. Soc. 1983, 105, 4419. This paper reports experiments of this kind in the formation of the TMM biradical 14 from the diazene 13 (see Scheme 5.2).

155.A.-T. Wu, Y.-J. Chen, and W.-S. Chung, J. Am. Chem. Soc. 2003. Submitted for publication. I thank these authors for an advance copy of their paper.

156.D. F. Evans, J. Chem. Soc. 1959, 2003.

157.D. H. Live and S. I. Chan, Anal. Chem. 1970, 42, 791.

158.K. R. Stickley, T. D. Selby, and S. C. Blackstock, J. Org. Chem. 1997, 62, 448.

159.T. D. Selby and S. C. Blackstock, J. Am. Chem. Soc. 1999, 121, 7152.

160.R. J. Bushby, in Magnetic Properties of Organic Materials, P. M. Lahti, Ed., Marcel Dekker, New York, 1999, 179ff. A review.

206 ORGANIC RADICAL IONS

Figure 6.1. The Jovian moon Io; deep ultraviolet (UV) photolysis of its methane atmosphere proceeds with electron ejection, generating the molecular ion of methane (see color insert). NASA JPL Galileo program image from Voyager 1, http://www.jpl.nasa.gov/galileo/io/ vgrio1.html

to natural occurrence as stable minerals. Some radical ions were observed as colored transients in the nineteenth century; their true nature was not recognized until the twentieth century, long after the discovery of trivalent carbon by Gomberg.1 Even in the twenty-first century, the coverage of radical ions in elementary, or even advanced organic chemistry texts, leaves much to be desired. The following vignettes illustrate the range of radical ion chemistry.

The Jovian moon, Io, shows an orange hue (Fig. 6.1), which may be due to longchain alkane radical cations. The atmosphere of Io consists mostly of methane; deep UV photolysis proceeds with electron ejection; thus, the molecular ion of methane was perhaps the earliest organic radical cation, generated by solar irradiation aeons ago.

On the planet Earth, the most important photoreaction occurs in green plants or in green or purple organisms. Their photochemical reaction centers contain a ‘‘special pair’’ of chlorins (cf. the purple bacterium Rhodobacter sphaeroides, Fig. 6.2).2 Solar photons cause electron transfer and generate a radical ion pair. Within two picoseconds, the negative charge is transferred to a second chlorin, and from it to a quinone.3–5

A remarkable example of a persistent radical anion is the semiprecious stone, lapis lazuli, known and appreciated as a pigment since ancient times. The species imparting the blue hue is trisulfur radical anion, S3 , accompanied by variable fractions of S2 , which introduces a green tint; the sulfur radical anions are incarcerated

Figure 6.2. (a) Photosynthetic reaction center of Rhodopseudomonas viridis Reprinted from the Protein Data Bank, H. M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T. N. Bhat, H. Weissig, I. N. Shindyalov, P. E. Bourne, Nucleic Acids Res. 2000, 1, 235 (http:// www.pdb.org/) PDB ID: IDXR, C. R. D. Lancaster, M. Bibikova, P. Sabatino, D. Oesterhelt, H. Michel, J. Biol. Chem. 2000, 275, 39364.2 (b) arrangement of the essential components in the purple bacterium Rh. sphaeroides (see color insert). [Adapted from Ref. 5.]

inside the small-pore zeolite sodalite, which immobilizes and protects these reactive species.6,7

As for radical ion chemistry in the laboratory, in 1835 Laurent treated benzil (in his view the ‘‘radical’’ benzoyl) with potassium—the resulting spontaneous ignition8 did not encourage further experiments. Some 50 years later, Beckmann and Paul9 reacted aromatic ketones and diketones with sodium. Because the colored solutions and the solid products were sensitive to air and moisture, they worked in a hydrogen atmosphere (Fig. 6.3). Twenty years later, Schlenk et al.10,11 recognized the colored products as a new class of trivalent-carbon compounds. They proposed the term ‘‘metal ketyls’’; the suffix ‘‘yl’’ was meant to indicate the ‘‘radical nature’’ of these substances. This view was finally confirmed by conductivity and magnetic susceptibility measurements in the 1930s12,13 and electron spin resonance (ESR) since the 1950s.

In 1866, Berthelot14 obtained a black dipotassium salt from naphthalene; the formula ‘‘C20H8K2’’ reflects an incorrect atomic weight for carbon. Almost 50 years later Schlenk et al.15 assigned the banded spectrum of a blue, transient species obtained from anthracene as a ‘‘monosodium addition product that contains trivalent carbon.’’

The first organic radical cations, derived from p-phenylenediamine, date back to Baeyer’s Strassburg laboratory in 1875.16 Wurster recognized these (Wurster’s)

208 ORGANIC RADICAL IONS

Figure 6.3. Apparatus for the reaction of alkali metals with carbonyl compounds and the separation of the solid adducts. [Adapted from Ref. 9.] A, separatory funnel in which the reaction is carried out; B, cylinder with inert atmosphere; C, perforated porcelain disc with filter paper; D, entry of inert gas (H2 or CO2) from Kipp apparatus; E, wash ether reservoir.

17,18 ¨

salts as oxidation products containing 1 equiv of an acid. Willstatter and

Piccard19,20 formulated the colored salts as molecular aggregates with specific N. . .N bonds, perhaps inspired by the molecular complexes, for example, between aromatic hydrocarbons and nitro compounds21 or quinones,22 which were emerging at the time.

Hantzsch recognized that radical cation ‘‘salts are not molecular complexes, but uniform, monomolecular chemical compounds with an unsaturated nitrogen or sulfur atom, whose unsaturated state would explain the intense color.’’23 A decade later, Weitz confirmed that these species were monomolecular and contained an unpaired electron; he coined the term ‘‘Anionradikale’’ and ‘‘Kationradikale’’: ‘‘Both the salt and the cation have an odd number of electrons because of their radical-like composition.’’ He recognized ‘‘. . . that the single positive charge is distributed between both halves of the cation . . .’’ and considered this ‘‘strange charge distribution’’ to be ‘‘the cause of the deep color.’’24

In the 1930s, Michaelis compared radical cations with trivalent-carbon or divalent-nitrogen intermediates using potentiometric methods. He rationalized their unusual stability as follows: ‘‘The fact that such radicals are capable of existence at all, can be attributed to a particular symmetry of structure resulting in resonance;’’ a

Figure 6.4. Electron spin resonance spectrum of Wurster’s blue ion. [Adapted from Ref. 28.]

number of ‘‘limiting states’’ contribute ‘‘a share to the resonating or mesomeric state;’’ . . . ‘‘letters are atomic kernels, dashes are pairs of electrons, . . . and the dot is a single electron.’’25,26

The advent of ESR opened several classes of moderately stable radical ions to scrutiny, namely, ketyls, semidiones, semiquinones, and those of aromatic systems.27 Wurster’s blue became an early target for an ESR study (Fig. 6.4).28 Its tetramethyl derivative was chosen to study the degenerate electron transfer between an organic radical ion and its parent by nuclear magnetic resonance (NMR) line broadening.29,30

2. RADICAL ION GENERATION

A rich variety of reagents and methods have been applied to generate radical ions. As illustrated above, the first methods were chemical redox reactions. Radical anions have long been generated via reduction by alkali metals. Because of the high reduction potentials of these metals, the method is widely applicable, and the reductions are essentially irreversible.

Radical cations can be generated by many chemical oxidizing reagents, including Brønsted and Lewis acids, the halogens, peroxide anions or radical anions, metal ions or oxides, nitrosonium and dioxygenyl ions, stable aminium radical cations, semiconductor surfaces, and suitable zeolites. In principle, it is possible to choose a reagent with a one-electron redox potential sufficient for oxidation– reduction, and a two-electron potential insufficient for oxidation–reduction of the radical ion.

Certain zeolites, notably H-ZSM-5, spontaneously oxidize a range of substrates with oxidation potentials 1.65 V.31,32 In the nearly cylindrical cavities of H-ZSM-5

The change in free energy ( G) for electron-transfer (ET) reactions is given by an empirical relation (Eq. 11; ET is the excited-state energy, Eox and Ered are the one-electron redox potentials of donor and acceptor, respectively, and e2=ea is a Coulombic term accounting for ion pairing).44 The change in G can be tuned (cf. Eq. 11) by variation of the solvent (polarity) and of the acceptor (reduction potential, excited-state energy).

The redox potential of the acceptor excited state, *Ered (Eq. 12), defines its oxidative strength: PET is limited to substrates with oxidation potentials below

*Ered.

Competing reactions may introduce mechanistic ambiguities: Ketone or quinone triplet states abstract hydrogen atoms, forming neutral radicals. Also, many radical cations are proton donors and radical anions are comparably strong bases. Thus, geminate radical ion pairs may generate neutral radicals by proton transfer.

3. RADICAL ION DETECTION–OBSERVATION

It is useful to briefly review the methods of observing and studying radical ions, including their strengths and limitations. Mass spectrometry (MS) is the most generally applied technique for radical cations, less frequently for radical anions. Mass spectrometry identifies positive ions, usually generated by electron-impact ionization, by their trajectories through magnetic fields. Although it is a valuable analytical tool,45,46 it provides little information on the structures of radical ions. In special instruments, ions of a particular m/z ratio can be selected and their ion molecule chemistry can be probed.

Photoelectron spectroscopy (PES) is also carried out in the gas phase: photons of known energy ðEhnÞ, for example, the He(I)a line (21.21 eV), ionize a substrate; the kinetic energy (Ekin) of the emitted electrons is measured and the vertical ionization potentials (Iv) derived (Eq. 13). The PES provides information on the energies of occupied molecular orbitals (MOs);47 the highest occupied molecular orbital (HOMO) of the parent reveals the bond(s) likely to be weakened or broken upon ionization. The PES data reflect the geometries of the parent molecule and need not have any bearing on the equilibrium structure of the radical cation.

Optical spectroscopy (OS) is naturally suited for studying (any type of) intermediate; absorption spectra characterize energy differences between occupied and unoccupied or singly occupied orbitals, including the HOMO–LUMO (lowest

212 ORGANIC RADICAL IONS

E

Figure 6.5. Schematic rationalization for lower energy (vis or near-IR) electronic transitions observed for open-shell species with singly occupied bonding (radical cations, left) or antibonding orbitals (radical anions, right).

unoccupied molecular orbital) gap. Emission spectra characterize the radiative decay of excited state species. Time-resolved (TR) spectra allow comparisons of

the decay and rise of consecutive intermediates. The time resolution has evolved from millisecond48,49 to nanosecond50–53 and picosecond resolution54–56 even fem-

tosecond spectroscopy is now being practiced in many laboratories.57 For further discussions on nanosecond, picosecond, and femtosecond spectroscopy, see Chapters 18 by J. C. Scaiano, 19 by E. Hilinski, and 20 by J. E. Baldwin in this volume.

The UV–visible (vis) spectra of many organic radical ions show significant bathochromic shifts relative to their precursors. The open-shell configurations of singly occupied bonding (radical cations) or antibonding orbitals (radical anions) introduce new electronic transitions of lower energies, in the visible or near-IR (cf. Fig. 6.5).58

Although exceedingly useful, the application of OS is not without problems. Optical spectra in condensed media consist of broad bands without identifying features; structural information can be derived by comparison with ‘‘known’’ species, or by high-resolution laser spectroscopy.59 The TR–OS is often limited to wavelengths >300 nm, excluding some prototypes of the most interesting species.

Electron spin resonance can provide detailed information about free radical (ions) in condensed media. Transitions between the electron spin levels are stimulated by radiation at frequencies satisfying the resonance condition:

(h is the Planck constant, g is a parameter characteristic for the radical under scrutiny, m is the Bohr magneton, and H0 is the applied field strength).

The (hyperfine) interaction between magnetic nuclei and unpaired electrons causes characteristic signal patterns; the spacing and relative intensities of the signals

are indicative of the spin density distribution in the intermediate.60,61 Steady-state

ESR has provided detailed insights into structures of radicals and radical ions. Time-resolved laser flash ESR spectroscopy62–67 generates radicals with non-

equilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963,68 are known as chemically induced dynamic electron polarization (CIDEP).69–71 Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively.72

Among NMR methods providing insight into radical ions,73 chemically induced dynamic nuclear polarization (CIDNP) has proved especially useful; it results in

enhanced transient signals, in absorption or emission; CIDNP effects were first reported in 1967;74,75 their application was soon extended to radical ions.76 The

method lends itself to modest time resolution.77,78

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a ‘‘sorting’’ of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical pairs with nanosecond lifetimes.

The quantitative theory79–82 allows one to compute intensity ratios of CIDNP spectra from reaction and relaxation rates and characteristic parameters of the radical pair (initial spin multiplicity, m), the individual radicals (electron g factors, hyperfine coupling constants, a); and the products (spin–spin coupling constants, J). Conversely, the patterns of signal directions and intensities for different nuclei of a reaction product reveal the hyperfine coupling constants of the corresponding nuclei in the radical cation. These results are often unambiguous because NMR chemical shifts clearly establish the identity of the coupled nuclei. Combined with PET as a method of radical ion generation, CIDNP has been the key to elucidating mechanistic details of important reactions, and provided insight into many short-lived radical cations with unusual structures, which had eluded other techniques. As all spectroscopic results, CIDNP results are not immune to misinterpretation.

The claim of direct observation is used occasionally as the ultimate panacea and to support claims of validity and authenticity. Direct observation means nothing more than that some property of an intermediate is measured during its lifetime, that is, the lifetime of the species exceeds the characteristic time scale of the method of observation. The simplest direct observable is absorption or emission of light, transitions to nonor antibonding states and radiative return to the ground state, which have little or no structural information. Problems inherent in relying on direct observation are evident in medieval accounts of unicorns or mermaids or