Synthesis, Antibacterial Studies and Crystal Structures of Tridentate Schiff Base Ligand and It’s Cobalt(III) Complex

The crystal structures of 4-[(2-hydroxyphenyl)imino]-2-pentanone (H2hpac, 1) and its cobalt(III) complex [CoIII(hpac)py3]+·PF6 − (2) have been determined by X-ray diffraction. The ligand 1 crystallizes in orthorhombic chiral P212121 space group, with a = 8.8405(4) Å, b = 10.5349(8) Å, c = 11.2292)7( Å), and the complex 2—in the centrosymmetric monoclinic P2/n space group, with a = 16.496(5) Å, b = 10.171(2) Å, c = 16.646(5) Å, and β = 95.53(3)°. In the ligand molecule quite strong intramolecular hydrogen bond closes six-membered ring. The bond length pattern within this ring suggests the significant conjugation and the structure might be therefore regarded as the intermediate between keto-enamine and zwitterionic forms, and the intramolecular hydrogen bond falls into category of resonance-assisted hydrogen bonds. In turn, intermolecular O–H···O hydrogen bonds connect the molecules of the ligand into infinite chains along [100] direction. In the complex, the Co(III) ion is hexa-coordinated, by two oxygen and one nitrogen atoms of the doubly-deprotonated ligand 1 and by three nitrogen atoms from three pyridine ligands. The coordination polyhedron is close to a slightly distorted octahedron. The in vitro antimicrobial activity of the Schiff base ligand and its corresponding complex have been tested against human pathogenic bacterias such as Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa and Escherichia coli. Resonance-assisted intramolecular hydrogen bond increases the degree of delocalization within the NCCCO fragment of the Schiff base.