Evidence for a Nonclassical Structure of a 1,6-Methano[10]annulene: A Cryogenic 13C CPMAS NMR Study of the 11,11-Dimethyl Derivative

Abstract

High-resolution 13C NMR spectra obtained using cryogenic CPMAS methodology at temperatures as low as 10 K suggest that the structure of 11,l l-dimethyl-l,6-methano[l0]annulene is nonclassical.The problem of valence tautomerism of 1,6-methano[ 101-annulene (1aIIIa)l and its derivatives with two bridge substituents, in particular 11,l l-dimethyl-1,6-methano[lO]annulene (DIM, Ib/IIb)? has been of considerable interest for a long time. As shown in Figure 1 these molecules either exist as an interconverting mixture of an aromatic form (I) and a bisnorcaradiene form (11) separated by an energy barrier (Figure ldf) or adopt an intermediate nonclassical structure between the extremes 1 and 11 (Figure la-c). Solution 13C NMR studies of a variety of 11,ll-disubstituted 1,6-methano[lO]annulenes exhibited a surprisingly large range for the chemical shift of the central carbon atoms 1 and 6, 124 ppm =. d1,6 > 40 ppm,where derivatives with a shift at the low-field value were assigned to form I.las2v3 Furthermore, for some of these molecules, a temperature dependence of d1,6 was found and interpreted in terms of a fast tautomerism between the two forms2 Similar conclusions were reached in a recent variabletemperature 13C CPMAS NMR study of DIM carried out at temperatures down to 140 K.3 Theoretical studies also favor interconversion over a barriere4 In contrast, room-temperature crystal structures of several 1,6-methano[ 101annulenes showed C1C6 distances varying between 1.54 and 2.27 A, indicating nonclassical structures between the two extremes I and IL5 In this we report the results of 13C CPMAS NMR experiments on DIM at very low temperatures (100-10 K) which support a nonclassical structure.A selection of the spectra, obtained at 1.4 T using an apparatus described previously,6 are shown in Figure 2. The two lines marked A and B at 80 and 74 ppm, respectively, have been assigned3 to the equivalent carbon atoms C1 and c 6 of molecules in two nonequivalent sites, A and B, in the asymmetric unit, exhibiting slightly different Clc6 distances of 1.836 and 1.780 A.7 The position of line A remains remarkably constant over the entire temperature range (10 K to ambient), a phenomenon which at first sight is consistent3 with a degenerate rearrangement between forms I and I1 characterized by an equilibrium constant K -1. On the other hand, between room temperature and 140 K, the position of line B shifts slightly to higher fieldas t Permanent address:

New Interpretation of the Valence Tautomerism of 1,6-Methano[10]annulenes and Its Application to Fullerene Derivatives

“…However, according to this interpretation these cases do not correspond to a static C-C bond length of about 1.8 Å but rather to an average resulting from a large amplitude mode along d 1,6 . This conclusion is in qualitative agreement with the recent CPMAS NMR study 13 on 1c/2c and also with Kaupp and Boy's comment. 10 The temperature dependence of the d 1,6 bond for the dimethyl derivative results, therefore, from the flat and highly anharmonic nature of the potential, as suggested by alternative 1 in the paper by Dorn et al 13 mentioned in the Introduction.…”

“…The observed range of 13 C NMR chemical shifts of atoms 1 and 6 of various 11,11-disubstituted 1,6-methano [10]annulenes also exhibits a substituent effect at C 11 . 1c,11,12 In some of these molecules, the temperature dependency of the chemical shift of atoms 1 and 6 has been observed and interpreted as a fast tautomerism 12 between the two forms, contrasting the results of X-ray experiments.…”

“…Early potential surface calculations along the d 1,6 based on Hartree-Fock theory (HF/3-21G) 2e also supported this picture. However, recently, Dorn et al 13 have performed 13 C CPMAS NMR experiments on the dimethyl derivative at low temperatures (10-100 K) and interpreted the results as a nonclassical structure having an unusual bond length, 1.8 Å corresponding to a potential energy surface such as 3e, not as a fast tautomerization between two structures as would be the case for 3a. On the basis of their NMR analysis and the earlier X-ray evidence, Dorn et al also noted that this structure exhibits a strong temperature dependence.…”

11,11-Dimethyl-1,6-methano[10]annulene—An Annulene with an Ultralong CC Bond or a Fluxional Molecule?

“…47,48 These studies were latter confirmed by Frydman and co-workers 49 as well as Dorn and co-workers, 50 who all documented a strong temperature dependence of the 13 C NMR signals of 1. Hence, the basic question was whether bridged annulenes could be considered as fluxional systems which changed their bonding structure according to the valence tautomeric rearrangement indicated in Figure 1 or whether they possessed long CC bonds.…”

“…At the DFT level, solvation effects were tested with the PCM (polarizable continuum model) of Tomasi and coworkers 65 where the dielectric constant ϵ was increased from 2 to the value of methanol (ϵ = 32.7 66 ), which was used as a solvent for some of the NMR investigations of 1. 49,50 The influence of the methyl groups on the stability of the annulene was determined by utilizing isodesmic reaction energies. For reasons of comparison, cyclopropane (4), 1,1-dicyanocyclopropane (5), and 1,1-dimethylcyclopropane (6) were investigated.…”