Oxidatively locked [Co2L3]6+ cylinders derived from bis(bidentate) 2-pyridyl-1,2,3-triazole "click" ligands: Synthesis, stability, and antimicrobial studies

Abstract

A small family of [Co2(Lpytrz)3]6+ cylinders was synthesised from bis(bidentate) 2-pyridyl-1,2,3-triazole "click" ligands (Lpytrz) through an "assembly-followed-by-oxidation" method. The cylinders were characterised using 1H, 13C, and DOSY NMR, IR, and UV-Vis spectroscopies, along with electrospray ionisation mass spectrometry (ESMS). Stability studies were conducted in dimethyl sulfoxide (DMSO) and D2O. In contrast to similar, previously studied, [Fe2(Lpytrz)3]4+ helicates the more kinetically inert [Co2(Lpytrz)3]6+ systems proved stable (over a period of days) when exposed to DMSO and were even more stable in D2O. The triply stranded [Co2(Lpytrz)3]6+ systems and the corresponding "free" ligands were tested for antimicrobial activity in vitro against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) microorganisms. Agar-based disk diffusion and Mueller-Hinton broth micro-dilution assays showed that the [Co2(Lpytrz)3]6+ cylinders were not active against either strain of bacteria. It is presumed that a high charge of the [Co2(Lpytrz)3]6+ cylinders is preventing them from crossing the bacterial cell membranes, rendering the compounds biologically inactive.