Structural aspects of intermolecular interactions in the solid state of 1,4-dibenzylpiperazines bearing nitrile or amidine groups

Abstract

X-ray diffraction analyses for new pentamidine analogs are presented: 1,4-bis(4-cyanobenzyl)piperazine (1) crystallizes in the triclinic space group ( ) and 1,4-bis(4-amidinobenzyl)piperazine tetrahydrochloride tetrahydrate (2) in the monoclinic space group ( P2 ₁/ n) revealing a complex system of hydrogen bonds for (2).

Abstract

The crystal structures of the title 1,4-bis(4-cyanobenzyl)piperazine (1) and 1,4-bis(4-amidinobenzyl)piperazine tetrahydrochloride tetrahydrate (2) are reported. Compound (1) crystallizes in the triclinic space group and compound (2) in the monoclinic space group P2 ₁ /n. In both (1) and (2) the asymmetric unit contains one half of the molecule because the central piperazine rings were located across a symmetry center. The packing of both molecules was dominated by hydrogen bonds. The crystal lattice of (1) was formed by weak C—H⋯N and C—H⋯π interactions. The crystal structure of (2) was completely different, with cations as well as chloride anions and water molecules taking part in intermolecular interactions. Single-crystal X-ray diffraction studies combined with density functional theory (DFT) calculations allowed the characterization of the intermolecular interactions in those two systems having different types of very strong electrophilic groups: non-ionic nitrile and ionic amidine. Chemical shift data from ¹³C CP/MAS (Cross Polarization Magic Angle Spinning) NMR spectra were analyzed using the different procedures for the theoretical computation of shielding constants.