A homologous series of [4n + 21annulenes has been reduced with alkali metals to yield (4n)a dianions and stable (4n + 2)n tetraanions. The structural proof of the highly charged products is based on quenching experiments (addition of electrophiles), an analysis of the 'H and I3C NMR spectra, the kinetics of the reduction, i.e., the appearance and disappearance of the respective NMR and ESR spectra, as well as polarographic evidence. A careful control of the electron-transfer processes is necessary to obtain and spectroscopically characterize the intermediate dianions. The n charge distribution within the anions, which can be evaluated by experimental I3C shift data and simple MO models, exhibits characteristic variations along the homologous series. A systematic change of experimental conditions (counterions, solvents, temperature) was performed in order to describe the ion-pairing phenomena. The most obvious spectroscopic consequences of the reduction processes are, clearly, the dramatic ring current effects on 'H chemical shifts. While the (4n)n dianions appear as paratropic systems, the tetraanions can be convincingly characterized as diatropic a-bond delocalized annulene species. Proton resonances, which have been corrected for the specific influences of molecular geometry, a charge, and ion pairing, provide a reliable experimental measure of ring current effects. As the latter are known to reflect the n-bonding situation, the novel dianions and tetraanions appear as useful model compounds for structural considerations, particularly in the study of ring-size influence.

and William C. Fultz for the X-ray structure determinations, to Frederic Davidson for analysis of the NMR spectra of 18 and 19, and to Dr. D. A. Dixon for the PRDDO calculations. We thank Edward R. Holler, Jr., and Carlos Vazquez for their expert technical assistance.Supplementary Material Available: Crystal structure analysis and tables of atomic coordinates, bond distances and bond angles, thermal parameters, and structure amplitudes for compounds 12b, 17, 26, and 27 (25 pages). Ordering information is given on any current masthead page.

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