Metallacarborane Assemblies as Effective Antimicrobial Agents, Including a Highly Potent Anti-MRSA Agent

The salts Na­[ROC­(O)­Ph], Na­[1-ROC­(O)-1,12-C₂B₁₀H₁₁], K₂[1,4-(ROC­(O))₂-C₆H₄], and K₂[1,12-(ROC­(O))₂-1,12-C₂B₁₀H₁₀], where R is the cobaltabis­(dicarbollide)-diethylene glycol group [3,3′-Co­(8-(OCH₂CH₂OCH₂CH₂)-1,2-C₂B₉H₁₀)­(1′,2′-C₂B₉H₁₁)]⁻, were synthesized from the corresponding carboxylate salt and the zwitterion [3,3′-Co­(8-(CH₂CH₂O)₂-1,2-C₂B₉H₁₀)­(1′,2′-C₂B₉H₁₁)]. The dianion in K₂[1,12-(ROC­(O))₂-1,12-C₂B₁₀H₁₀] showed at least one K⁺ cation to be tightly bound to the dianion via K···O and K···H–B interactions with both p-carborane and cobaltabis­(dicarbollide) clusters on the basis of NMR and MS data and support of hybrid-DFT computations. An evaluation of the antimicrobial properties of these compounds revealed all of the salts to be highly effective antibacterial agents for four Gram-positive bacteria strains (standard minimum inhibitory concentration, MIC, of 1 mg/L for Na­[ROC­(O)­Ph]; Na­[3]) and antifungal agents for three Candida albicans strains (MIC 4 mg/L for the salts K₂[4] and K₂[6]). One of the four Gram-positive bacteria strains tested was a life-threatening superbug methicillin-resistant Staphylococcus aureus (MRSA) isolate, which is resistant to many commercial antimicrobial drugs. The cobaltabis­(dicarbollide) derivative Na­[3] has a remarkable inhibitory effect on the MRSA strain with an MIC of only 1 mg/L and a minimum bactericidal concentration (MBC) of 2 mg/L, thus suggesting its potential as an antibacterial agent against MRSA.