The crystal and molecular structure of 2,7-di-tert-butyl-4,5,9,10-tetraphenylbenzo[1,2,:4,5]dicyclobutadiene: an exceptionally long C–C aromatic bond

Abstract

The X-ray determined structure of the title compound is reported; it was found that the annelated bonds are the longest observed in a benzene derivative [1.540(5) Å]; ab initio calculations (at the B3LYP/6-31G* and MP2/6-31G* levels of theory) were used in order to understand the electronic and structural properties of the compound.

References

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7. Crystal data for 2 C₄₂H₃₈, crystal dimensions 0.13 × 0.11 × 0.07 mm³, green, mounted with polyfluorinated oil and measured on a Siemens SMART-CCD diffractometer (three-axis platform) with Mo-Kα-radiation at 130 K; a= 21.3735(5), b= 6.11890(10), c= 23.4144(6)Å, β= 99.8480(10)○, V= 3017.07(7)ų; monoclinic crystal system, Z= 4, D_c= 1.195 g cm^–3, µ= 0.067 mm^–1, space group C2/c, data collection of 8535 intensities (2Θ_max= 45°, one run 0.3°ω-scans, 120 frames at ϕ= 0°, four runs 0.3°ϕ-scans with 600 frames at angles 135, 143, 156 and 169° in ω, more than 97% of the data covered), absorption correction with Siemens SADABS (R_merg before/after = 0.115/0.063, max/min equivalent transmission = 1.00/0.60), 1910 independent intensities (R_merg= 0.103), 1479 ‘observed’ data [F_°≥ 4σ(F)], structure solution with direct methods (Siemens SHELXS) and refined on F²(Siemens SHELXTL-Plus, ver. 5.01)(190 parameters), the hydrogen atom positions were calculated and refined as riding groups with 1.2-fold (1.5 for methyl groups) isotropic U values. R1 = 0.0827, wR2 = 0.2251, w^–1=σ2(F_o²)+(0.103P)²+ 6.50P, where P=[(maxF_o²)+(2F_c²)]/3, max/min residual electron density = 0.264/–0.246 eÅ^–3. CCDC 182/1129.
8. According to Schulman and Disch (ref. 4): (a) the experimentally obtained structure is a transition state between the two Kekulé isomers 2b and 2c, and (b)2a was predicted to be more stable than 2b by 17.9–20 kcal mol^–1.
9. GAUSSIAN 94, Revision E.1, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1995.
10. N. L. Frank, K. K. Baldridge and J. A. Siegel, J. Am. Chem. Soc., 1995, 117, 2102.
11. The discussion presented here for 4 regards the tetraethenyl derivative of isomer 2d. However, 2a also has a stable tetravinyl derivative. The two bond-shift isomers (i.e.2a and 2d) also have other stable derivatives that show this isomerism, such as the tetrahydroxy and tetracyano derivatives (A. Stanger, unpublished results). These are, however, beyond the scope of this paper.
12. The agreement between the experimental and calculated structures is even better if the experimental σ value (0.005 Å for all the discussed bonds) is considered.