(E)-3,3′,4,4′,7,7′,8,8′-Octamethyl-2H,2′H-1,1′-bi(cyclopenta[fg]acenaphthylenylidene)-2,2′,5,5′,6,6′-hexaone dichloromethane monosolvate

The title compound, C36H24O6·CH2Cl2, is a dimer of two essentially planar (r.m.s., deviations of fitted plane of 14 pyracene C atoms = 0.0539 and 0.0543 Å) tetracyclic pyracene frameworks (each with four methyl groups and three carbonyl groups on the peripheral carbon atoms) twisted along a central C=C bond with an angle of 50.78 (3)° at 90 K. There are notably long Csp 2—Csp 2 bonds associated with the carbonyl groups, the longest being 1.601 (3) Å between two carbonyl C atoms. There are also intermolecular carbonyl⋯carbonyl interactions of both parallel and antiparallel types, with C⋯O distances in the range 3.041 (3) to 3.431 (2) Å. This compound is of interest with respect to the synthesis of fullerene fragments, such as corannulene and semibuckminsterfullerene derivatives (or ‘buckybowls’), and is a side product of the previously reported oxidation reaction. Structural details, such as planarity analysis of fused rings, out-of-plane deviation of substituents, intermolecular interactions, and longer than typical bond lengths, will be discussed as well as comparisons to structurally related compounds.

The title compound, C 36 H 24 O 6 ÁCH 2 Cl 2 , is a dimer of two essentially planar (r.m.s., deviations of fitted plane of 14 pyracene C atoms = 0.0539 and 0.0543 Å ) tetracyclic pyracene frameworks (each with four methyl groups and three carbonyl groups on the peripheral carbon atoms) twisted along a central C C bond with an angle of 50.78 (3) at 90 K. There are notably long Csp 2 -Csp 2 bonds associated with the carbonyl groups, the longest being 1.601 (3) Å between two carbonyl C atoms. There are also intermolecular carbo-nylÁ Á Ácarbonyl interactions of both parallel and antiparallel types, with CÁ Á ÁO distances in the range 3.041 (3) to 3.431 (2) Å . This compound is of interest with respect to the synthesis of fullerene fragments, such as corannulene and semibuckminsterfullerene derivatives (or 'buckybowls'), and is a side product of the previously reported oxidation reaction. Structural details, such as planarity analysis of fused rings, outof-plane deviation of substituents, intermolecular interactions, and longer than typical bond lengths, will be discussed as well as comparisons to structurally related compounds.

Fronczek Comment
The structure of title compound ( Figure 1 and minor product in Figure 2) can be described as a dimer of two pyracene frameworks joined together with a C═C bond. Both individual tetracyclic subunits are nearly planar (root mean square, or r.m.s., deviation of each fitted plane of 14 pyracene carbon atoms is 0.0539 Å and 0.0543 Å, respectively) and have four methyl groups and three carbonyl groups on the peripheral carbons. The least-squares planes for the two halves of this highly conjugated molecule are twisted along the central C═C bond with an angle of 50.78 (3)° at 90 K. A visual "side-view" ORTEP representation of this twist is shown in Figure 3.
The out-of-plane linear deviation of the methyl substituents from a least squares plane of the pyracene carbon atoms ranges from 0.029 (3) Å to 0.365 (3) Å. In comparison to another compound in the Cambridge Structural Database or CSD (Allen, 2002), this range is below the maximum substituent deviation reported for 1,4,5,6,7,10,11,12-octamethylindeno[1,2,3-cd]fluoranthene (CSD Refcode NOTVAT) which is a fullerene fragment with 4 methyl groups on the peripheral naphthalene carbons and 4 methyl groups on the peripheral phenyl carbons (Sygula et al., 1997). These methyl carbons were reported to deviate up to 0.4 Å with respect to a fitted least squares plane.
There are long C(sp 2 )-C(sp 2 ) bond lengths between the carbonyl carbons with a C5-C6 bond length of 1.601 (3) Å and a C5A-C6A bond length of 1.590 (3) Å. These long bond lengths involve 5-membered carbon rings that are fused to a naphthalene framework. The angle between the 3 carbon atoms shared by the 5-membered carbon rings and the naphthalene rings form angles (119.07 (16)° and 119.23 (17)° for C14-C12-C13 and C14A-C12A-C13A, respectively) that are closer to the angles observed in hexagons, 120°, instead of pentagons, 108°. The combination of a long bond length and the deviation in bond angles up to ~119° results in a significantly distorted, yet planar, ring.
An example of this type of ring distortion is found in the structure of 1,2,5,6 tetraketopyracene (Abdourazak et al., 1994), CSD Refcode YEHHAU, which contains two C(sp 2 )-C(sp 2 ) bonds between carbonyl carbons that are both separated by 1.579 (9) Å based on diffraction data collected at T = 115 K. This example also has two enlarged bond angles; both measuring 119.2 (3)° between the fused carbons connecting the 5-membered carbon rings to the naphthalene framework. In the publication containing the 1,2,5,6 tetraketopyracene crystal structure, calculation results were also published and are in good agreement with this structural elongation (using either the PM3 or ab initio method).
Also, there are long C(sp 2 )-C(sp 2 ) bond lengths of 1.545 (3) Å for the C1-C2 bond and 1.553 (3) Å for the C1A-C2A bond. These bond lengths are shorter than the bonds discussed in the previous paragraphs. This observation coincides with less bond angle distortion for the carbons in the 5-membered rings that are fused with the naphthalene subunit. The bond angles between fused bonds are 116.98 (16)° and 116.85 (16)° for C9-C10-C11 and C9A-C10A-C11A, respectively, and near the average of 120° (hexagon) and 108° (pentagon).

Refinement
All non-hydrogen atoms were identified and subsequently refined anisotropically. With the remaining unaccounted electron densities visible in SXGRAPH (Farrugia, 1999) difference Fourier map, hydrogen atomic sites were generated using HFIX commands and refined as idealized "riding" positions. The extinction parameter had a refined value of zero and, therefore, was omitted from the model. Final refinement cycles included the SHELXL97 (Sheldrick, 2008) recommended weighting scheme. Missing symmetry was checked using ADDSYM feature in PLATON (Spek, 2003).
The highest remaining undetermined electron density above 0.45 e Å -3 , identified as "Q1", at the conclusion of the refinement in the difference Fourier map is 0.80 e Å -3 . This electron density is located ~1.2 Å from O2A and "Q1"-O2A -C5A forms an angle of ~100°. "Q1" is also ~1.9 Å from C5A and the angles for "Q1"-C5A-C13A and "Q1"-C5A -C6A are ~91° and ~163°, respectively. No chemically reasonable solution was identified for "Q1".      Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.