Structural characterization, theoretical, and antibacterial activity study of halogen-η3-allylpalladium(II) complexes incorporating 2-, 3- and 4-pyridyl-methylen-4-methylumbelliferone esters

Abstract

A series of 2-, 3- and 4-pyridyl-methylen-4-methylumbelliferone esters ligands (1 – 3) and their chloro- and bromo-η³-allylpalladium(II) complexes (Pd1 – Pd6) were designed, synthetized, and characterized. Solution-phase studies by ¹H and ¹³C NMR spectroscopy of Pd1 – Pd6 revealed the presence of the allyl fragment which suggested the coordination of the ligands (1 – 3) towards Pd(II). GIAO/DFT studies were performed to predict the molecular structures of Pd1 – Pd6 by comparing the experimental and theoretical ¹H and ¹³C NMR chemical shifts. The molecular structure of 1, 1a, 2, 3, Pd1 and Pd4 was determined by X-ray crystallographic analysis. The molecular structure of Pd1 and Pd4 reveals that 2-pyridyl-methylen-4-methylumbelliferone ester ligand (1) is coordinated to the palladium (II) center via a monodentate fashion through the nitrogen atom of the pyridinyl fragment and allyl group is binding via a η³ fashion in an overall five-fold coordination geometry completed with a halogen atom (chloro in Pd1 and bromo in Pd4, respectively). The crystal packing is stabilized by a variety of weak intermolecular conventional and non-conventional interactions involving CH•••O/N/Hal hydrogen bonds, π•••π, lone pair•••π and CH•••π interactions, which have been analyzed by Hirshfeld surface analysis. All halogen-η³-allylpalladium(II) complexes displayed potential activities against both Gram-positive (Listeria monocytogenes and Staphylococcus aureus) and Gram-negative (Escherichia coli and Salmonella spp.)