Benign synthesis of quinolinecarboxamide ligands, H₂bqbenzo and H₂bqb and their Pd(II) complexes: X-ray crystal structure, electrochemical and antibacterial studies

Abstract

Two carboxamide ligands, H₂bqbenzo {3,4-bis(2-quinolinecarboxamido)benzophenone} and H₂bqb {N,N′-bis[(2-quinolinecarboxamide)-1,2-benzene]}, have been prepared using tetrabutylammonium bromide as an environmentally benign reaction medium. Two new Pd(II) complexes, [Pd^II(bqbenzo)] (1) and [Pd^II(bqb)] (2), have been synthesized, characterized, and their structures determined by single crystal X-ray diffraction. The di-anionic ligands, bqbenzo²⁻ and bqb²⁻, are coordinated via two N_amide atoms and the nitrogens of the two quinoline rings, with Pd−N_amide < Pd–N_quinoline bond lengths. The geometry around palladium(II) in both complexes is distorted square planar. The electrochemical behaviors of the ligands and their Pd(II) complexes have been investigated by cyclic voltammetry in DMF. An irreversible Pd^II/I reduction is observed at −1.06 V for 1 and at −1.177 V for 2, indicating the influence of the R substituent on the central phenyl ring of carboxamide ligands on the Pd^II/I reduction potential. The ligands and palladium complexes were also screened for in vitro antibacterial activity. The Pd(II) complexes show strong biological activity against S.typhi and E.coli as Gram −ve and B.cereus and S.aureus as Gram +ve bacteria comparable to the antibiotic penicillin. The antibacterial results also reveal that coordination of Pd(II) significantly improves the activity.

Keywords:

• Carboxamide ligands
• Pd(II) complexes
• X-ray crystal structure
• cyclic voltammetry
• antibacterial activity

Acknowledgements

Partial support of this work by the Isfahan University of Technology Research Council is gratefully acknowledged. We also thank the Swiss-Norwegian beam line at the ESRF in Grenoble, France, for beam time.