Role of the ancillary ligand in determining the antimicrobial activity of Pd(II) complexes with N^N^N-tridentate coligand

Two [PdN₃L]PF₆ complexes (L = 2,6-bis(1-ethyl-benzimidazol-2́-yl)pyridine (L^BZ) and 4-(2-pyridyl)-2,2′:6′,2′'-terpyridine (L^PY)) underwent catalyst-free 1,3-dipolar cycloaddition coupling with phenyl isothiocyanate at the room temperature giving the corresponding tetrazole–thiolato complexes. ¹H NMR studies showed that the N1-isomer appeared first in the catalyst-free coupling, then gradually isomerized to the more stable tetrazole–thiolato isomer. The interconversion of N1-isomer into the S-isomer is accompanied by a decrease of the greater steric demand of the 5-membered ring. Tetrazolate complex of L^BZ displayed higher antifungal activity (MIC ≤ 0.25 μg/mL) than the chloride analogue (MIC = 0.5–1.0 μg/mL) and the reference drug, Fluconazole (MIC = 0.125–8.0 μg/mL) against Candida albicans and Cryptococcus neoformans. The treated non-cancerous human embryonic kidney cells (HEK293) showed no cytotoxicity and no haemolysis release at the measured concentrations of the chloride complex of L^BZ and tetrazolate complex of L^PY.