Group 11 Metal Compounds with Tripodal Bis(imidazole) Thioether Ligands. Applications as Catalysts in the Oxidation of Alkenes and as Antimicrobial Agents

Abstract: New group 11 metal complexes have been prepared using the previously described tripodal bis(imidazole) thioether ligand (N-methyl-4,5-diphenyl-2-imidazolyl)₂C(OMe)C(CH₃)₂S(tert-Bu) ({BIT^OMe,StBu}, 2). The pincer ligand offers a N₂S donor atom set that can be used to coordinate the group 11 metals in different oxidation states [Au^I, Au^III, Ag^I, Cu^I and Cu^II]. Thus the new compounds [Au{BIT^OMe,StBu}Cl][AuCl₄]₂ (3), [Au{BIT^OMe,StBu}Cl] (4), [Ag{BIT^OMe,StBu}X] (X = OSO₂CF₃^- 5, PF₆^- 6) and [Cu{BIT^OMe,StBu}Cl₂] (7) have been synthesized from reaction of 2 with the appropriate metal precursors, and characterized in solution. While attempting characterization in the solid state of 3, single crystals of the neutral dinuclear mixed Au^III-Au^I species [Au₂{BIT^OMe,S}Cl₃] (8) were obtained and its crystal structure was determined by X-ray diffraction studies. The structure shows a Au^III center coordinated to the pincer ligand through one N and the S atom. The soft Au^I center coordinates to the ligand through the same S atom that has lost the tert-butyl group, thus becoming a thiolate ligand. The short distance between the Au^I-Au^III atoms (3.383 Å) may indicate a weak metal-metal interaction. Complexes 2-7 and the previously described Cu^I compound [Cu{BIT^OMe,StBu}]PF₆ (9) have been evaluated in the oxidation of biphenyl ethylene with tert-butyl hydrogen peroxide (TBHP) as the oxidant. Results have shown that the Au^I and Ag^I complexes 4 and 6 (at 10 mol % loading) are the more active catalysts in this oxidative cleavage. The antimicrobial activity of compounds 2-5, 7 and 9 against Gram-positive and Gram-negative bacteria and yeast has also been evaluated. The new gold and silver compounds display moderate to high antibacterial activity, while the copper derivatives are mostly inactive. The gold and silver complexes were also potent against fungi. Their cytotoxic properties have been analyzed in vitro utilizing HeLa human cervical carcinoma cells. The compounds displayed a very low cytotoxicity on this cell line (5 to 10 times lower than cisplatin) and on normal primary cells derived from C57B6 mouse muscle explants, which may make them promising candidates as potential antimicrobial agents and safer catalysts due to low toxicity in human and other mammalian tissues.