1,5-Bis(2,5-dimethyl-1H-pyrrol-1-yl)naphthalene

In the title compound, C22H22N2, the asymmetric unit contains one half-molecule. A crystallographic inversion centre is located at the mid-point of the bond common to both rings, in the central naphthalene unit. Quantum-mechanical ab initio calculations on the isolated molecule showed that the minimum energy configuration occurs when the naphthalene ring system and the pyrrolyl groups deviate only slightly from perpendicularity. In the crystal, due to the effects of crystal packing, the molecule deviates by approximately 4° from the a priori expected ideal value of 90° [C—C—N—C torsion angle = 86.11 (15)°].

In the title compound, C 22 H 22 N 2 , the asymmetric unit contains one half-molecule. A crystallographic inversion centre is located at the mid-point of the bond common to both rings, in the central naphthalene unit. Quantum-mechanical ab initio calculations on the isolated molecule showed that the minimum energy configuration occurs when the naphthalene ring system and the pyrrolyl groups deviate only slightly from perpendicularity. In the crystal, due to the effects of crystal packing, the molecule deviates by approximately 4 from the a priori expected ideal value of 90 [C-C-N-C torsion angle = 86.11 (15) ].

Comment
Complex pyrroles are important synthons in macromolecular chemistry, environmental chemistry, medical chemistry and nano-technologies based on polymeric organic materials. Following our endeavor in synthesizing new pyrrolic compounds for material chemistry (Andrade et al., 2008;Ramos Silva et al., 2002;Sobral & Rocha Gonsalves 2001a, 2001bSobral, 2006), we prepared the title compound, by the Paal-Knorr methodology, using iodine as catalyst. Each molecule contains a crystallographic inversion centre at the mid-point of the bond common to both rings of the naphthalene moiety. All bond lengths and valency angles of the molecule lie within the expected range of values for naphtalene derivatives.
Approximate free rotation of the pyrrolyl group around the formal σ C-N bond is expected. Thus, the conformation observed for such groups in the solid state should be determined by steric rather than electronic effects. We observe in this structure a value of 86.11 (15)° for the C9-C8-N1-C1 dihedral angle, which is close to the a priori expected ideal value of 90° where the steric effects should be at a minimum.
In order to gain some insight into how the crystal packing might affect the molecular geometry we have performed a quantum chemical calculation on the equilibrium geometry of the free molecule. These calculations were performed with the computer program GAMESS (Schmidt et al., 1993). A molecular orbital Roothan Hartree-Fock method was used with an extended 6-31 G(d,p) basis set. Tight conditions for convergence of both the self-consistent field cycles and maximum density and energy gradient variations were imposed (10 -6 atomic units). The program was run on the Milipeia cluster of UC-LCA (using 16 Opteron cores, 2.2 GHz runing Linux).
The ab-initio calculations reproduce well the observed experimental bond length and angles of the molecule. All angles match the experimental values within 1°. Calculated and experimental bond distances agree within 0.023 Å. The calculated C9-C8-N1-C1 dihedral angle is 91.82°, a value closer to the ideal value of 90° than the experimental value in the solid state.
A check for weak intermolecular interactions in the crystal on the basis of short contacts revealed that a possible C-H···π interaction may exist between atoms C2 and the pyrrole ring [C2-H2···C g : 3.7791 (16) Å, 159°] Experimental 0.680 g (4.3 mmol) of 1,4-phenylenedimethanamine and 1 ml (8.5 mmol) of hexane-2,5-dione were dissolved in 20 ml of tetrahydrofuran, under nitrogen atmosphere. 0.172 g (0.678 mmol) of iodine was added to the stirred solution at 40°C.
The procedure was monitored by TLC. After completion of the reaction (6 h), 20 ml of dichlorometane were added to the mixture. The resulting mixture was washed successively with 5% Na 2 S 2 O 3 solution (2 ml), NaHCO 3 solution (2 ml) and brine (2 ml). The organic layer was then dried with anhydrous sodium sulfate and concentrated.

Refinement
The methyl H atoms were constrained to an ideal geometry (C-H = 0.96 Å) with U iso (H)= 1.5U eq (C), but were allowed to rotate freely about the C-C bonds. All remaining H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (parent atom). Fig. 1. Ellipsoid plot of the title compound. Displacement ellipsoids are drawn at the 50% level. Unlabelled atoms are generated by inversion through the origin.