Six new 1,3-diorganylimidazolidin-2-ylidene (NHC) gold(I) complexes of the type [Au(NHC) 2 ] + (1-6), were synthesized by reacting [AuCl(PPh) 3 ] with 1,3-dimesitylimidazolidin-2-ylidene or bis(1,3-dialkylimidazolidin-2-ylidene). The complexes 1-6 were fully characterized by elemental analyses and spectroscopic data. The placement of mesityl or para-substituted benzyl groups on the nitrogen atoms of the ring of the complexes leads to the particularly active antibacterial agents evaluated in this work. It is worth noting that the p-methoxybenzyl derivative (2) inhibited the growth of Pseudomona aeruginosa, Staphylococcus epidermidis, Staphylococcus aureus and Enterococcus faecalis with minimum inhibitory concentration (MIC) values of 3.12 µg ml −1 , 6.25 µg ml −1 , 3.12 µg ml −1 and 3.12 µg ml −1 respectively. In contrast, the analogous p-dimethylaminobenzyl derivative (3) is effective only against Escherichia coli (MIC = 3.12 µg ml −1 ).

In vitro antibacterial and antifungal activities of a series of 25 diazolidinium salts, 1,3-diorganylimidazolidinium (1), together with 1,3-dialkylpyrimidinium (2), were evaluated against standard strains: Escherichia coli (ATCC 25922), Staphylococcus epidermidis (ATCC 12228), Staphylococcus aureus (ATCC 29213), Enterococcus faecalis (ATCC 29212), Enterobacter cloacae (ATCC 13047), Pseudomonas aeruginosa (ATCC 27853) and Candida albicans (ATCC 10239). Selective and effective antibacterial activity against one gram-negative (P. aeruginosa) and two gram-positive (E. faecalis and S. aureus) bacteria were found in salts 1a, 1i and 1j, in contrast to modest to poor activity observed in the rest of the salts. The enhanced antibacterial activity is clearly linked to the introduction of two bulky mesityl or mesitylbenzyl substituents on the nitrogen atoms of the imidazoline skeleton, while the side chain of the backbone of the molecule has no influence.

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