Copper (II) complexes of 3,5‐di‐tert‐butyl‐2‐hydroxybenzoylhydrazones of 2‐formylpyridine and 2‐acetylpyridine, with tautomeric azine‐scaffold‐based architecture: Synthesis, crystal structures, the effect of counteranions on complexation, and their anti‐microbial and anti‐tuberculosis evaluation

Abstract

A series of air‐ and moisture‐stable copper (II) complexes of 3,5‐di‐tert‐butyl‐2‐hydroxy‐N′‐(pyridin‐2‐ylmethylene)benzohydrazide (L1H) and 3,5‐di‐tert‐butyl‐2‐hydroxy‐N′‐(1‐(pyridin‐2‐yl)ethylidene)benzohydrazide (L2H) were synthesized. The two newly designed hydrazone Schiff base ligands with the tautomeric azine‐scaffold involving N, N and O ligating architecture were derived from the condensation of 3,5‐di‐tert‐butyl‐2‐hydroxybenzoylhydrazide with 2‐formylpyridine and 2‐acetylpyridine, respectively. The resultant ligands and their copper complexes were characterized using various spectro‐analytical techniques, like infrared (IR), 1H‐NMR, 13C‐NMR, electron paramagnetic resonance, electronic spectroscopy, thermogravimetric analysis and conductance measurements. The interesting inter‐convertible E/Z geometrical isomeric forms of the ligand L1H was established by NMR and IR spectra. In addition, the molecular structures of L2H, C3 and C4 were unambiguously established using the single‐crystal X‐ray diffraction technique. The dichlorido‐bridged dinuclear square pyramidal complexes, with the [Cu2(μ‐Cl)2L2] type of core units, were resulted for the complexes C1 and C3, whereas z‐in compressed mononuclear 1:2 (M:L) octahedral complex geometries resulted for C2, C4 and the rest of the complexes even when various copper (II) precursor salts with different counteranions were used. The ligands and their complexes were investigated for the anti‐microbial activities against gram‐positive and gram‐negative bacteria. The ligand L2H and its complexes C3 and C4 exhibited better anti‐tuberculosis activity than the controls.