3504 H . A. Staab, F. Diederjch, C. Krieger, and D. Schweitzer Chern. Ber. 116, 3504-3512 (1983) Cycloarenes, a New Class of Aromatic Compounds, III) Molecular Structure and Spectroscopic Properties of Kekulene Heinz A. Staab·, Franrois Diederich, Claus Krieger, and Dieter Schweitzer Max-Planck-Institut fUr medizinische Forschung, Abteilung Organische Chemie, Jahnstr. 29, 0-6900 Heidelberg Received May 2, 1983 The molecular structure of kekulene (1) was determined by X-ray structure analysis. From the bond lengths of 1 a remarkable localisation of aromatic sextets and double bonds is concluded (cf. formula 1 b). - The problem of annulenoid versus benzenoid diatropicity in 1 is discussed on the basis of 1H NMR absorptions. These data, in agreement with recent theoretical calculations. support a predominant ring-current induction in the benzenoid subunits of 1 and rule out a significant contribution of annulenoid structures like 1 a. - Absorption and emission spectra of 1 are discussed as are the zero-field splitting parameters of the excited triplet state of 1. Cycloarene. eine neue Klasse aromatischer Verbindungen. III) Molekiilstruktur und spektroskopische Eigenschaften von Kekulen Oie Molekiilstruktur von Kekulen (1) wurde durch ROntgen-Strukturanalyse bestimmt. Aus den Bindungslangen von 1 wird auf eine bemerkenswerte Lokalisation von aromatischen Sextetts und Ooppelbindungen im Sinne der Formulierung 1 b geschlossen. - Oas Problem der annulenoiden oder benzoiden Oiatropie von 1 wird an Hand der lH-NMR-Spektren diskutiert. Die experimentellen Ergebnisse stiilZen in Obereinstimmung mit neueren theoretischen Berechnungen die Annahme einer vorherrschenden Ringstrom-Induktion in den benzoiden Untereinheiten von 1 und schliefien einen wesentlichen Beitrag annulenoider Strukturen wie 1a aus. - Absorptions· und Emissionsspektren von 1 sowie Nullfeld-Aufspaltungsparameter des angeregten Triplettzustands von 1 werden diskutiert. In the preceding paper 1) we described the synthesis of kekulene (1) which is the first example of "cycloarenes". This new class of aromatic compounds is defined as com· prising aromatic systems in which the circular annellation of arene units results in a I' 16 '--,..,.... 13 12 II © Verlag Chemie GmbH. 0-6940 Weinheim. 1983 0009-2940/83/ 1010-3504 $ 02.50/ 0 Cycloarenes, a New Class of Aromatic Compounds, II 3505 macrocyclic structure enclosing a cavity surrounded by carbon-hydrogen bonds. According to the nomenclature we suggested for cycloarenes I) the systematic name of 1 is "cyclo[d.e.d.e.d.e.d.e.d.e.d.eJdodecakisbenzene". In this paper we report on the molecular structure of 1 as derived from X-ray struc­ ture analysis and discuss IH NMR spectra and other spectroscopic data which are relevant to the electronic structure of 1. Molecular and Crystal Structure of 12) Crystal Preparation and X-Ray Structure Analysis: Due to the extremely low solubi­ lity of I in all solvents it was extraordinarily difficult to obtain single crystals of I suitable for an X-ray analysis. Eventually I-crystals were grown from pyrene by slow cooling from 450 to 350°C and preparing the crystals as described in detail in the Experimental Part. 1 forms monoclinic needles with the following cell parameters: a = 2795.1 (4), b = 457.9 (I), c = 2268.0 (2) pm, P = 109.64 (1); space group C2Ic; Z = 4; Dx = 1.46 gcm - 3. Intensity data were collected using graphite-monochromated Mo-Ka radiation (Enraf-Nonius CAD 4). Up to sin el A. = 6.98 nm - I 3803 symmetry­ independent reflections were measured out of which 1560 reflections with I ~ 1.920(1) were graded as observed. 2 Table 1. Atomic Coordinates and Thermal Parameters (in pm ) for Carbon Atoms and for Hydrogen Atoms of 1 (in Brackets Standard Deviations in Units of the Last Quoted Digit) • z Uoou . • z Uoou . tU) '.lrn7( 7) 1,88')7(5) 8 . 27398( 8) elB(1) C( 13) 8 . 18')88(7) -11 . 4944(5 ) 11 . 631115(8) 3&4(6) C<l ) 8.12681(7) 8 . 9974(5) 8 . 27851(8) Q Z(11 e ( 14) 8 .23438(7) -8 5928 (S) 8 .'7536(8) ....( 7) CO) 8. 11 987(6 ) ' .77.1(5 ) 8 . 314 18 (8 1 356(7) e (IS ) ' . 292Z"(6 ) - 8 .(9111'5) e . 6186UO) 357 (1) (4) ' . 17293(7) 11 . 6685(5 ) 8.312 17(8 ) 394 (7) C ( 16) 8 .21391(6 ) 1I .7~ 3 ( 5 ) II.J662 1(7) J3Zt f) ( [ 5) 8 . &6793 (&) IILCS29(S ) 8 . ] 548& (8 ) 3t](/) t(l7) 8.1641 1 (6 ) 8 6141H5> ' . )6212(7) JI'5(6' C ( 6 ) 8 . 81938(7) 8 . 338S(5) 8 , ] 5 ]8.(8) 045 ] (7) CCl 8) B.1'58'5'5 (6 ) 8 .408e ( '5 J 1I . 40-'lK' (B) ]52( 1) t ( 7) B. etSle(6 ) 8. 1487(5 ) e .3955 21~) 449(7) t U9' e .II232(6) 8 .3"523(5) 8 . .a220 <81 326(6) t(8) e 8S953«(;) 8 . B314(SJ 8.44~ B I ( 8 ) 36'5(7) ( liB 8 t8188(6) 8 . 1367(5 ) .....716 (8) 328(6) C(' , 8 . 85637(7) -". 1126(5) 8.40'°0 ( 0 ) " 11 (1 ) 8 . 15144(1; ) 8 . 8287 (5) 8."92911(8) 355(7) Cctl) B. e:r.J79 (6' -8. 27GS (5) 1I .53532(C) 36S(6) 8 . 14836(6) -a. 1763{S) 8 .53728 (8 ) 31Be6' t n!) 8 . 8973!H71 -e. 485IHS) 8 .'58167(&) 433 (7) C(2:J ) 8 . 19369 (6 ) - 8 2899( '5 ) 1I .58!1 l g(8 ) 332(6 ) C( 12) 8. 139]3(7) -8.587<:. (5 ) IL (;26a910 ) <4 1 (7) t ( 24) " 2 .. t61C6J -e. 193''(5) 8 !lB82 HB l 37&(71 • UI ~ CI. ..... UI,o . lI . lr.51(5 ) 1.248C. ) 8.2429 (6 ) 60'.H 78) H U t) a .Klees) -11 . 552(4) ' . ~Z(6 ) ""(68) 8 8949 (5 ) I .~". ' 8. 2] 70 (6 ' 438 ( 51) IUIZ) 8 . 1)83(5) -8 n 6 14 , 8 659Z'" 6" B(68' 8 . 8422 (. ) ' . 738 ( 3) 8 .288] (6 ) 322(58) He ! .. ) 8 .2318 (5) -8.738(4 ) '. 7M 1(6' 43(61) -8. 8 123 (5 ) e ... IIC.) 8 . 328] <5 ' 526(68 ) HI lS} 8 . 18a9(4 ) 8 . ]91 (3) 8437 1(5) 214(41) - 8 . 817'(5 ) 11 . 878 (. > e. 3'''] (6) 523 (68 ) H(ZI ) 8. 1852 (5 ) 8 . e941] ) 8 ...g]9 (5) ]89(5el H (g) 8 BZIS<Sl -8 249( 4 ) e 4875(6 ) 618<68 ' M(24 ) ., 24.l9(5 ) - 8 . 851"4) 8 .5'565 (6) ]]9(511 ) A solution of the structure was not possible by using direct methods. From a Patterson synthesis, however, the position of the molecular planes in the elementary cell and an interplanar distance of the I-molecules of approximately 340 pm were derived. The model based on the molecular C, symmetry of I then was improved by slightly turning the rigid molecule around the central symmetry axis (normal to the molecular plane). Full-matrix least-squares refinement of the atomic coordinates using Chern. Ber. J/6 (1983) 3506 H. A. Staab. F. Diederich. C. Krieger. andD. Schweitzer anisotropic thennal parameters for the carbon atoms and isotropic thennal parameters for the hydrogen atoms led to a convergence at R = 0.052. Atomic coordinates and thennal parameters for the carbon and hydrogen atoms of I are listed in Table 1 (for the numbering of atoms see Figure 3)3). Crystal Lattice oj 1: In the I-crystals the molecules are stacked along the b-axis with the stacking axis forming an angle of 42.9 0 with the molecular planes. Within such a stack of equidistant molecules the interplanar distance is 335 pm; neighbouring molecules are parallel-shifted by 312 pm resulting in an overlap as shown in Figure 1. 0 The molecular planes in adjacent stacks are inclined to each other by 86 • Thus. a typical "herringbone pattern" results which is shown in Figure 2 in a view along the a-axis. Figure 1. Overlap Diagram of Two Neighbouring I-Molecules Figure 2. Crystal Packing of I as Shown in the Projection along a (for the Sake of Clearness Only Stacks Centered at 1/ 4 a are Drawn; Corresponding Stacks at 3/4 a which Result from C-Centra­ tion are Omitted) Chern. Ber. /16 (\983) Cyc1oarenes. a New Class of Aromatic Compounds. II 3507 Molecular Structure 0/1: The kekulene molecule has an almost perfectly planar structure. The mean deviation of the carbon atoms from the least-squares plane through all 48 carbon atoms amounts to only 3 pm; the maximum deviation from this plane is 7 pm. The close approach to planarity includes even the six internal hydrogens Figure 3. ORTEP Plot of 1 Figure 4. Bond Lengths (in pm) and Bond Angles (in 0) of 1 Chem. Ber. 116 (1983) 3508 H. A . Staab. F. Diederich. C. Krieger. and D. Schweitzer (maximum deviation 9 pm) although the non-bonding distances between adjacent hydrogens in the central cavity of 1 with 192 (2) pm are unusually small. With regard to the electronic structure of 1 the carbon-carbon bond lengths in 1 were of special interest. From the data given in Figure 4 it is seen that there is a remarkable difference in bond lengths between the two groups of benzene units A,C.E,G,I.K and B,D,F,H,J,L, respectively. Only for the last mentioned group "normal" arene bond lengths are oberved; for these rings the mean value for carbon-carbon bonds in the outer perimeter amounts to 139.5 (2) pm, that of the bonds in the inner perimeter to 138.6 (3) pm whereas the radial bonds are slightly stretched to a mean value of 141.8 (2) pm.