Structure and crystal packing of 4-aminobenzonitriles and 4-amino-3,5-dimethylbenzonitriles at various temperatures

The amino N atom in 4-aminobenzonitrile (ABN) and 4-(dimethylamino)benzonitrile (DMABN) has a pyramidal character, with values of 34(3) and 11.9 (3) c~, respectively, for the angle between the planes of the amino group and the phenyl ring. In 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD) at 173 K, the dimethylamino group is twisted over an angle of 59.3 (2) "~ with respect to the phenyl plane. In addition, the amino N(1) atom is not located in the plane of the phenyl ring, with an out-of-plane displacement of 0.117 (5)/~. The N-phenyl bond length in MMD of 1.414 (3)/~ is larger that that in the four other aminobenzonitriles, for which an average bond length of 1.367,~ is found. With DMABN, the crystal structure consists of stacks with an alternating orientation of the molecules. In the crystals of ABN, 3,5-dimethyl-4-(methylamino)benzonitrile (MHD) and 4-amino-3,5-dimethylbenzonitrile (HHD), hydrogen bonding is an important structural element. In ABN and MHD, hydrogen bonds are formed between an amino H atom and the cyano N atom of adjacent molecules. In HHD, each H atom is linked to a cyano N atom of two different neighbouring molecules, in a layered structure consisting of squares and octagons. Temperature-dependent solid-solid phase transitions are observed with all five aminobenzonitriles studied here. Crystals of the low-temperature phase could only be obtained from ABN, with lattice constants which are considerably different from those observed at room temperature.