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.