Synthesis, structures, and investigation of noncovalent interactions of 1,3-dimethyl-5-(4ʹ/3ʹ-pyridylazo)-6-aminouracil and their Ni(II) complexes

In this research paper we report synthesis of two planar ligands, namely 1,3-dimethyl-5-(4ʹ/3ʹ-pyridylazo)-6-aminouracil: HL1 (1) for 4ʹ-pyridyazo- and HL2 (2) for 3ʹ-pyridyazo-group and, moreover, their nickel (II) square planar complexes ([NiII(L1)2] (3) and ([NiII(L2)2] (4)). The report is also included solid and solution state structure of the compounds and study of noncovalent stacking interactions. Crystal structures of the ligands, 1 and 2 are monoclinic with alike space group P21/c and the complex 3 is also monoclinic with space group C2/c. The molecular electrostatic potential (MEP) analysis shows that the ligands possess both π-basic and π-acid character in the same framework over the center of the pyridine and the pyrimidine rings, respectively, favoring electrostatically enhanced π-π stacking interaction among the molecular units. Upon complexation, π-acidity of the pyrimidine ring increases, and the complex 3 exhibits lone pair (lp)-π stacking interaction involving lone pair of the pyridine N-atom and π-acid pyrimidine plane. The study evidences that the ligands exist in azo-enamine-keto tautomer in solid and hydrazone-imine-keto in solution state and get coordinated with nickel (II) via azo-N and uracil-6-NH– to form a distorted square planar complex. Electronic absorption bands/energies obtained from experimental and theoretical (TDDFT) study are indeed alike, giving reliability to the level of theory and the absorption band assignments. Moreover, the synthesized compounds containing uracil and pyridine fragments exhibit π-π/lp-π interactions with considerable energy for stabilization of their assemblies in the solid state