The problem of the existence of C(Ar)–H ⋯ N intramolecular hydrogen bonds in a family of 9-azaphenyl-9H-carbazoles

Abstract

In order to demonstrate the possibility of intramolecular hydrogen bonds (IMHB) between an aromatic C–H and a pyridine nitrogen atom, the family of 9-arylcarbazoles [Ar = phenyl (1), 2-pyridyl (2) or 2-pyrimidinyl (3)], their 9-aryl-1-azacarbazole analogues [Ar = phenyl (4). 2-pyridyl (5) or 2-pyrimidinyl (6)] and 9-phenyl-1,8-diazacarbazole (7), have been synthesized and the X-ray structure of all of them, except 4, has been determined. The unit cell parameters are as follows. For 1: a= 14.526(5)Å, b= 10.985(1)Å, c= 18.094(2)Å, β= 113.45(2)°, V= 2649(1)ų, Z= 8, P2¹/n. For 2: a= 14.625(4)Å, b= 11.685(6)Å, c= 16.636(1)Å, β= 114.07(4)°, V= 2596(3)ų, Z= 8, P2¹/n. For 3: a= 25.76(2)Å, b= 5.200(6)Å, c= 19.47(1)Å, β= 112.07(5)°, V= 2417(4)ų, Z= 8, C2/c. For 5: a= 12.825(2)Å, b= 12.336(4)Å, c= 16.468(6)Å, β= 111.41(3)°, V= 2426(2)ų, Z= 8, P2₁/n. For 6: a= 10.682(3)Å, b= 29.033(9)Å, c= 7.524(3)Å, β= 90.46(3)°, V= 2333(1)ų, Z= 8, P2₁/c. For 7: a= 10.895(2)Å, b= 13.407(4)Å, c= 17.552(6)Å, β= 107.81(2)°, V= 2441(2)ų, Z= 8, P2₁/c. The data were obtained at 293 K except for compound 7 for which the experimental temperature was 200 K. All compounds except 3 exist in the crystal in two different conformations. The potential curves corresponding to the rotation about the N(9)–C(aryl) bond of nine compounds (1–7 plus the non-synthesized 2-pyridyl- and 2-pyrimidinyl-1,8-diaza analogues) have been calculated. The crystallographic torsion angles are near the zone of minimum energies according to AM1 calculations, save in the case of compound 1(9-phenylcarbazole) where one of the independent molecules has a torsion angle of 78.4°, much larger than calculated. The calculated barrier through the planar state, ΔE°, contains much information and appears to be related to the ¹H and ¹³C chemical shifts of some selected molecules. Compound 3[9-(2-pyrimidinyl)-9H-carbazole], with a torsion angle of 7.4(4)°, is the clearest representative of the existence of C–H ⋯ N IMHBs.