Sandwich-type (η6-p-cymene)Ru(η5-thiophene) complexes: synthesis, spectroscopic and electrochemical characterization, and antimicrobial activity

Abstract

Ruthenium-arene derivatives have displayed, in recent years, interesting in vitro and in vivo biological activity. The features of sulphur-containing molecules as active components of drugs are known as well. Therefore, combining a Ru(arene)-core with an η-coordinated thiophene ligand can allow tuning of the structure and the chemical and biological behavior of the resulting species. In this context, the synthesis of two hetero-sandwich ruthenium(II) complexes with an [(η⁶-arene)Ru(η⁵-R-thiophene)]²⁺ general formula has been performed, with the aim of investigating the effect of an alkyl substituent of the heterocycle ring on the spectroscopic and redox behavior. p-Cymene has been selected as an arene ligand, whereas 3-methylthiophene and 3-octylthiophene acted as η⁵-ligands to complete the sandwich structure. The proposed structure was confirmed by full ¹H and ¹³C NMR characterization. UV-vis spectra and voltammetric responses show that the length of the alkyl chain does not affect significantly the spectroscopic and redox behavior of these ruthenium complexes. On the other side, their activity towards Gram-positive and Gram-negative bacteria seems to depend on steric effects ascribed to the alkyl chain. The evaluated compounds are promising as bases for developing new antimicrobials, for example, to treat skin or oral infections.