Synthesis, spectral characterization, thermal, computational and antibacterial studies of lanthanide complexes with 2-fluorobenzoic acid-(5-R-2-hydroxy-benzylidene)hydrazide {R = chloro or bromo)

Tridentate Schiff base ligands, 2-Fluorobenzoic acid-(5-chloro-2-hydroxy benzylidene)hydrazide {H2LCl} and 2-Fluorobenzoic acid-(5-bromo-2-hydroxybenzylidene)hydrazide {H2LBr} have been used to prepare a variety of lanthanide complexes [HNEt3][LnLx(NO3)2(H2O)]H2O, Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Er. The 1H and 13C NMR of lanthanum in conjunction with the infrared, elemental, thermal, molecular mass and conductance measurements enable the assignment of the formula to these complexes. The anionic unit [LnLx(NO3)2(H2O)]− contains one tridentate ONO-donor, L2−, which coordinates the metal ions via the phenolate-O, the imine-N and the deprotonated amide-O atoms in enol tautomeric form. The coordination environment around central metal ion is completed by two bidentate nitrate ligands and one coordinated water molecule to give a coordination number of eight for Ln(III). In order to get a better insight into the structural features of the complexes, their molecular geometries were fully optimized using density functional theory calculations at the M06-2X/6-31G∗ level of theory. The antibacterial activity results, on a panel of six different bacterial strains, show that the activity of the complexes is higher than that of the free ligands and in some cases higher than that of amoxicillin which is taken as standard reference drug. Compared to the free ligands, the emission spectra of the complexes exhibit a blue-shift with a clear enhancement in the emission intensity.