The effect of molecular planarity and resonant effects on supramolecular structures of N-(5-pyrazolyl)imines by X-ray crystallographic analysis

Abstract

A series of N-(5-pyrazolyl)imines 3 were synthesized in high yields (81–90%) by a solvent-free condensation reaction of 5-aminopyrazole derivatives 1 with arylaldehydes 2 under microwave irradiation. These compounds were recrystallized from methanol. X-ray diffraction analyses of N-(5-pyrazolyl)imines 3 show that substituents can affect the degree of electronic delocalization between phenyl and pyrazole rings through the bridging imine moiety. The resonance effect changes the dihedral angles of these planar rings generating conformations close or far from planarity. This effect influences the supramolecular assembly allowing or not short intermolecular hydrogen bonds involving the methoxy group in compounds 3a-c. Additionally, the degree of electronic delocalization is related to the acidic character of the hydrogen atoms in the methyl group bonded to the pyrazole ring facilitating or not the formation of short hydrogen bonds in 3a-c. In the case of 3d, the resonance effect of chlorine substituents allows the hydrogen atom in meta position to perform short intermolecular bonds. CE-B3LYP interaction energies show that, in the more planar molecules, dispersion forces act in higher proportion to build the crystals.