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The Radical Anion and the Dianion of [ 161Annulene J. F. M. Oth," H. Baumann, J.-M. Gilles, and G. Schroder Contribution f r o m the Laboratory f o r Organic Chemistry, Swiss Federal Institute of Technology, 8006 Zurich, Switzerland, and the Institute f o r Organic Chemistry, Karlsruhe University, 7.500 Karlsruhe 1 , West Germany. Received July 19, 1971
In aprotic solvents [16]annulene can be reduced electrochemically or by alkali metals to its radical anion and its dianion. These two species have been studied by different spectroscopic methods and information concerning their structure and their stability has been obtained. The esr spectrum of the radical anion is reported. Its interpretation, based on symmetry and HMO (with variable ,8) considerations, has allowed us to establish unambiguously the structure of this species and to conclude which of the two nonbonding MO's is in fact occupied by the unpaired electron. The structure of the dianion could be deduced from nmr studies. The nmr spectrum of this aromatic species is very informative; the diamagnetism associated with the delocalized 18~-electronsystem is clearly demonstrated by the fact that the resonance signal of the inner protons appears at extremely high field (T 18.17). The nmr spectrum of the dianion is temperature independent, unlike that of the parent [16]annulene molecule and that of the isoelectronic [18]annulene. This difference in dynamic behavior is discussed. Uv-visible absorptionspectra of the two species (R.-and RZ-)are reported; they indicate that both species are most probably planar. The thermal stability of the dianion is found to be very great in contrast to that of the parent [16]annulene molecule. Abstract:
y nmr spectroscopy studies' [ 16]annulene, synthesized by Sondheimer and Gaoni2 and by Schroder and Oth,r has been shown to exist in solution as a dynamic equilibrium between two interconverting configurations,
large enough above - 60" to bring about coalescence of the nmr lines of the two configurations. We were interested in studying how the relative energies, the geometries, and the dynamic behavior of these configurations would be affected by the introduction of one and of two excess electrons into their 7r a K91 systems. In particular the dianion(s) of [ 161annulene "85 a "91 possessing (4n 2 ) 7r electrons may be expected to be quite stable, to show no bond alternation, and to ex[16]-85-annulene [I61-91 - on n u le ne hibit a dynamic behavior (which could cause nmr 75 % 25 % equivalence of the inner and outer protons) much slower each exhibiting single and double bond alternation but than that of the neutral molecules. Such a dynamic differing from the other by the number and sequence of behavior is in fact observed in the case of [18]ann~lene.~ cis and trans double bonds.4 Each of these configuraWe report here the spectroscopic evidence that we tions undergoes conformational mobility (Kxs and ICgl) have obtained for the existence and the stability of the and fast bond shift (Vx6and Vgl). These isodynamic radical anion R . - (uv-visible and esr spectra) and of processes5 are responsible for the fact that the inner and the dianion R2- (uv-visible and nmr spectra) of [16]outer protons in each structure are indistinguishable by annulene. The configuration, the geometry, and the nmr spectroscopy above -80" (at 60 M H Z ) . ~ Furrelative stability of these species are discussed. thermore, the isomerization rates k85-91 and ksl-85 are I. Theoretical Considerations (1) J. F.M. 0 t h and J.-M. Gilles, Tetrahedron Lett., 6259 (1968). (2) F. Sondheimer and Y . Gaoni, J . Amer. Chem. Soc., 83, 4863 The main problems that we had to solve were to (1961); I. C. Calder, Y . Gaoni, and F. Sondheimer, ibid., 90, 4946 establish the configurations of the radical anion and (1968). (3) G. Schroder and J. F. M. Oth, Tetrahedron Lett., 4083 (1966). of the dianion and to find out if these species do or do (4) Each configuration is identified by a characteristic code number not show bond alternation and are or are not planar. which is formed according to the following rule: we designate a cis double bond by the figure 0 and a trans double bond by the figure 1; Answers to these questions could be obtained by comwe then write the sequence of cis and trans double bonds in the form of parison of the experimental facts (the esr spectrum in the smallest binary number compatible with the configuration; this the case of Re-, the nmr and uv-visible spectra in the binary number is then converted to its denary form case of R2-) with the theoretical results based on symmetry considerations and MO computations. A. The Variable /3 Huckel Approximation. As is well known, the successes of Huckel molecular orbital C (HMO) calculations are, to a great extent, due to the fact that they take into account the "topology" of the sequence C T C T T C T T molecular skeleton.8.9 In cases where the symmetry of smallest binary no. 0 1 0 1 1 0 1 1 the correct electronic Hamiltonian is governed only by denary no. 26+24+223+2+1 = 9 i
B
+
Hence, this structure is identified as the [16]-91-annulene. (5) For the definition of an isodynamic process, see: S. L. Altman, Proc. Roy. SOC.,Ser. A , 298, 184 (1967). (6) The exchange mechanism by which all the protons of [16]annulene become equivalent in nmr spectroscopy is explained in ref 3 ([16]-85-annulene) and in ref 1.
Journal of the American Chemical Society
94.10
(7) Y . Gaoni, A. Melera, F. Sondheimer, and R . Wolovsky, Proc. Chem. SOC.,397 (1964); F. Sondheimer, Proc. ROY. SOC., Ser. A , 291,
173 (1967). (8) I
