Complementary hydrogen bonding in a new tridentate Schiff baseligand: X-ray, DFT and solution NMR studies

Abstract

The X-ray crystal structure of a novel mono(pyrrole)-containing tridentate Schiff base ligand, N-[(1E)-1H-pyrrol-2-ylmethylene]benzene-1,2-diamine (3), shows unexpected hydrogen bond complementarity between neighbouring pairs of molecules in the solid state (monoclinic space group C2). The supramolecular structure is best described as a hinged dimer in which the dihedral angle between the two ligand planes measures 65(1)° and is stabilized by a pair of hydrogen bonds involving the pyrrole N–H proton on one molecule and the N atom of the aryl amino group on the neighbouring molecule. The experimental H_(pyrrole)⋯N_(amine) H-bond distances are equivalent at 2.40(2) Å. The ¹H NMR spectrum of 3 in CDCl₃ shows concentration-dependent features that are consistent with endergonic dimerization by intermolecular hydrogen bonding at higher concentrations [K_D = 0.89(16) M⁻¹ at 25 °C]. DFT calculations at the B3LYP/6-31G** level of theory correctly predict the main geometrical features of the dimer, which has slightly inequivalent H_(pyrrole)⋯N_(amine) distances of 2.31 and 2.33 Å in the calculated structure. The DFT-calculated dimer shows some conformational differences from the X-ray structure that primarily reflect the fact that the individual molecules of 3 in the in vacuo model are twisted about the CN–CC torsion angle involving the imine group and adjacent phenyl ring (C5–N2–C6–C7) by 4–8° more than is the case for the experimental structure. These conformational differences reflect the role that crystal packing plays in fine-tuning the supramolecular structure of 3.