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1. INTRODUCTION

p-Conjugated non-Kekule´ molecules,1 do not conform to the standard rules of valence. For that reason, they have fascinated organic chemists as strange entities that lie on the borderline of existence. Attempts to synthesize such species can be traced back more than a century, and by now, scores of examples are known.

Workers in the field have been mindful of the potential for utility of these species in new reactions for organic synthesis and in new structures applicable to the design of molecular electronic devices. Such developments have in fact emerged, and progress in that area has been reviewed many times.2–9 We must relinquish the opportunity to provide another review here, both because of lack of space and because the objectives and findings of that work are at some remove from our designated subject of ‘‘reactive intermediates.’’ Rather, we focus our attention on the challenging problems that non-Kekule´ molecules themselves present in experiment and theory. These include their synthesis, their structures and spin states, and their chemical reactions. Such investigations make possible significant tests of theory, which will be a recurrent theme. Even so limited, the subject is too large for complete treatment here. Accordingly, we concentrate on recent developments and on the background out of which they grew. Although many references to individual papers are given, the literature is so voluminous that when substantial reviews on a particular subject are readily available, the review may be cited here rather than the individual articles.

2. HISTORICAL BACKGROUND

A number of authors in the the field of non-Kekule´ compounds have given summaries of one or another aspect of the historical record.10–15 However, it may be help-

ful to relate briefly some of the main occurrences, events that in retrospect can be seen to mark the emerging (and sometimes temporally overlapping) eras in the field.

2.1. The Schlenk–Brauns Hydrocarbons

Earliest among these was the synthesis by Schlenk and Brauns16,17 of the bis(triarylmethyls) 1 and 2, the first non-Kekule´ compounds.

They were prepared by adapting Gomberg’s synthesis of triarylmethyls (e.g., triphenylmethyl),18 that is, the reduction of triarylmethyl halides in inert atmosphere. Biradicals 1 and 2 persist under a carbon dioxide atmosphere in fluid solution at room temperature. Superficially, this might seem a not very significant extension of Gomberg’s work, but as Schlenk and Brauns recognized, their biradicals differed in an important way from the triarylmethyls. The Gomberg monoradicals are molecular fragments and by definition cannot achieve a full Kekule´ structure. The Schlenk–Brauns biradicals, on the other hand, formally have enough atoms to satisfy the rules of valence. Despite that capability, they exist with one missing bond, a fact that clearly signifies the appearance of a new kind of matter. This work was done in 1915, well before the advent of quantum chemistry, and at that time, understanding and predicting the properties to be expected of these new species could at best be superficial.

2.2. Hund’s Rule

The second phase began after quantum mechanical thinking had become estab-

lished, even if mostly among physicists. Eugen Mu¨ller, in consultation with Friedrich Hund, was the first chemist to examine the paramagnetism of biradicals.19,20 By

magnetic susceptibility measurements with a Guoy balance, he showed that compound 1 was indeed paramagnetic, presumably because it exists as a triplet state. The latter finding was consistent with a molecular version of Hund’s rule, which was a most gratifying result for the adherents of the new quantum mechanics.

A seminal paper of Longuet-Higgins in 195021 gave powerful impetus to the idea that the state of highest multiplicity should predominate in a p–conjugated non-Kekule´ molecule. He showed by simple Hu¨ckel calculations that the four non-Kekule´ molecules m-quinodimethane (m-xylylene, MQDM) 3, trimethylenemethane (TMM) 4, tetramethyleneethane (TME) 5, and triangulene 6, all had a pair of degenerate frontier orbitals occupied by only two electrons. Relying on Hund’s rule, he predicted that all of these species should have triplet ground states. Hypothetical full-valence Kekule´ isomers of these are shown as structures 7–10.

The arrival of the third phase of non-Kekule´ chemistry now awaited two necessary developments. First, the Guoy balance technique was of limited sensitivity and yielded no structural information about the source of the paramagnetism; thus, a most desirable development would be the appearance of a new and independent

Another series of non-Kekule´ compounds whose parent is 1,8-dimethylene- naphthalene (19)39–46 dates from the same era. Extensive reviews13,47 of that series

are given elsewhere and will not be repeated here.

A non-Kekule´ molecule conceptually formed by fusion of two TMM units and also predicted48,49 to have a triplet ground state is 2,4-dimethylenecyclo-

butane diyl (20), which ultimately was prepared by two independent syntheses.50–52 Matrix ESR spectroscopy and gas-phase photodetachment photoelectron spectroscopy (PES)53 (see Section 4.1.4) eventually agreed that the ground state is triplet.

Also dating from that period was the synthesis of another of the LonguetHiggins molecules, tetramethyleneethane (5), which Dowd54 reported in 1970 and assigned a triplet ground state in 198655 in view of its linear Curie plot. We discuss this molecule and its derivatives in Section 7.

That all these molecules apparently have triplet ground states might be taken as verification of Longuet-Higgins’s predictions. However, we must keep in mind that the difficulty at this time of actually measuring the singlet–triplet separation forced the spin issue into binary form: Is the ground-state singlet or triplet? Even when the ground state had been assigned, one still did not know anything about how changes in structure affected the size of the gap.

2.4. The Dependence of Spin State Preference on Structure

This question, which was to initiate the fourth phase of our history, became urgent following the recognition that Longuet-Higgins’s indiscriminate prediction of triplet ground states for all the non-Kekule´ molecules might be too general. The first inkling of this insight had come decades earlier in a long-buried paper by Erich

¨ 56,57 ¨

Huckel in 1936, which was really a commentary on Eugen Muller’s work in the same year on the Schlenk–Brauns hydrocarbons 1 and 2 (see above). The idea was almost forgotten in the literature until it was (quietly) resurrected by Baudet in 197158 and finally developed in 1977 by Borden and Davidson59 and by Misurkin and Ovchinnikov60,61 into a powerful new criterion for classifying

non-Kekule´ molecules. Major efforts in several laboratories were launched to test these new ideas, which now may be said to be firmly established.62,63 We return

later (Section 7) to these developments.

2.5. The Singlet–Triplet Gap Is Not the Whole Story

To this point, it would be fair to say that the dominant question was the comparison of experiment with theory by attempts to designate the ground state and if possible to determine the energy separation between it and the next higher state. To a considerable extent, this line of development was driven by the advances in theory itself, whose predictions were made with ever-increasing confidence, sophistication, and refinement. However, one should not conclude that this is the extent and sole purpose of research in the non-Kekule´ domain. These molecules really

are a new form of matter, and much progress has been made in the exploration of their physical and chemical properties, which differ significantly from those of the familiar monoradicals.

In the following pages of this chapter, brief introductions or literature references to the various experimental techniques are given. Closely related theoretical and computational work is described in Chapter 22 by Borden in this book.64 The interplay of theory and experiment, as well as the mutually supporting roles of preparative ‘‘wet’’ chemistry and instrumental techniques, are emphasized.

The very nature of non-Kekule´ species as reactive intermediates suggests that studies of them under conditions far from those used in conventional investigations of the synthesis and reactions of stable molecules are indispensable. These requirements frequently are met by immobilizing the species in crystalline hosts or randomly oriented matrices, as is described in Chapter 17 by Bally in this book.65 Although some information available from crystal studies usually must be sacrificed in the random matrix technique, the latter is usually far more convenient, and most studies of non-Kekule´ compounds in solids have used it.

2.6. Connecting the Spectroscopy of a Non-Kekule´ Molecule to Its Structure

The generation of a non-Kekule´ compound from an appropriate precursor may (and often does) generate other species as well. Consequently, spectroscopic investigations aimed at characterizing a non-Kekule´ molecule may give misleading information if they detect the side product in addition to or instead of the target compound. Examples of this include an early attempt66 to observe the ultraviolet–visible (UV–vis) absorption and emission spectra of the biradical 2-isopropylidenecyclopentane-1,3-diyl prepared from the corresponding diazene in a low-temperature matrix. Although some spectroscopic features were observed,66 their interpretation was subsequently judged to be questionable, and the transitions were thought to be more probably associated with a side product.40

It is true that in some cases, the spectroscopic data on a reactive intermediate are so persuasive that the connection between structure and spectroscopic features is firm. However, in general this will not be the case, and additional spectroscopic or preparative criteria will have to be provided. So we are faced with the question: How can we connect the information obtained, for example, from observations in matrices or in solution-phase fast kinetic studies, to molecular structure? How do we know that the results of these experiments, using what we hopefully call ‘‘direct’’ methods, really pertain to the species we are trying to characterize? I attempt to deal with this issue in what follows. Since the methods used vary from one class of non-Kekule´ species to another, specific classes are individually discussed, and special techniques are introduced as needed. Electron spin resonance spectroscopy has played such a pervasive role that it will be useful to give first a brief outline of that method.

3. ELECTRON SPIN (OR PARAMAGNETIC) RESONANCE SPECTROSCOPY IN MATRICES

Space limitations of this chapter preclude a thorough treatment of this topic. Read-

ers who wish more information will benefit from the many reviews available else- where.13,23,67–69 We will concentrate here on its practical applications to non-

Kekule´ molecules, its strengths, and its limitations.

The most detailed ESR spectroscopic information on paramagnetic species comes from studies in crystalline hosts, as exemplified in the ground-breaking work of Hutchison and Mangum.70 A more conveniently applied technique involves matrix-immobilized, randomly oriented samples (also called powders). These give broader spectral lines than the crystalline preparations, and it is often more difficult to obtain hyperfine splittings from them. However, for many purposes, the powder spectra are extremely useful, as was soon demonstrated for paramagnetic metal complexes by Burns24 and for the triplet states of organic species containing divalent carbon by the group at Bell Laboratories.22

It is important to note that even without hyperfine data, the powder spectra give valuable information about the carrier of the ESR spectrum. Both the molecular symmetry of the molecule and the effective distance between the unpaired electrons usually can be deduced from the spectra.

3.1. Zero-Field Splitting and the Appearance of ESR Spectra in Immobilizing Media

This topic has been extensively treated elsewhere.13,23,67,68,71–73 For the present purpose, it may suffice to mention just a few of the conclusions that can be drawn from such spectra.

The transitions in the X-band ESR spectra of triplet species occur in two regions. The so-called ms ¼ 1 region represents transitions between energetically adjacent pairs of the three triplet sublevels. These are characterized by two so-called zerofield splitting parameters, D and E. The parameter D is inversely proportional to the cube of the average separation of the electron spins, and E is related to the molecular symmetry. The number of lines depends on the molecular symmetry. If all three magnetic axes of the molecular carrier of the spectrum are distinct, the spectrum in the ms ¼ 1 region will show six major resonances, plus any hyperfine lines that may be visible. If two of the principle axes are equivalent by symmetry, only four lines will be observed. In the latter case, the parameter E has the value of zero.74

Note here that in some instances, even when the molecule has lower symmetry, the value of E can be so small as to be indistinguishable from zero, especially with a randomly oriented sample. In that case again, only four of the expected six lines may be observed.67 With this caution in mind, we can see that a non-zero E value may be interpreted confidently as indicative of a carrier with low symmetry, but the converse, an approximately zero value, could be due to true symmetry, or to an accidental equivalence of two axes. We return to this point in the discussion