Coordination complexes of methimazole with copper: Controlling redox reactions and sulfur extrusion

Abstract

Sulfur-containing imidazole thiones are of interest for their redox chemistry and tendency to form disulfide bonds reminiscent of biologically active disulfides. Treating copper(II) with redox-active methimazole in the presence of oxygen results in concomitant copper reduction and formation of methimazole disulfide, and ultimately, extrusion of one of the sulfur atoms. Reaction conditions, including stoichiometry, presence of oxygen, and redox state of the metal ion, were varied to examine the mechanistic details that lead to the sulfur extrusion. Six unique products are reported: [Cu(κN,N-MMIMS)2(H2O)2](HSO4)2, [Cu(κN,N-MMIMS)2(H2O)][CH3SO4][HSO4][H2O], [Cu(κN,N-MMIMS)2(H2O)](CH3SO4)2, [Cu(κN,N-MMIMS)(κO,O-SO4)(CH3OH)], [Cu(κN,N-MMIMS)(κO,O-SO4)(DMSO)]∙0.5 DMSO, and [Cu(μ,κO,O,O-SO4)(κN,N-MMIMS)]n[CH3CN] (MMIMS = bis(1-methylimidazol-2-yl)sulfide). In all structures, the HSO4- and CH3SO4- counterions are generated in situ. EPR analysis of the reaction as it proceeds indicates initial formation of a four-coordinate Cu2+-S complex that shifts to square planar or octahedral Cu2+-N coordination over the course of the reaction. Formation of the thiyl radical is also observed by EPR analysis using DMPO as a spin trap. Together, these results establish a complete mechanism for sulfur extrusion that proceeds through copper-methimazole binding and redox, superoxide formation, and nucleophilic attack of water or methanol on sulfur, and subsequent nucleophilic aromatic substitution to yield the sulfur-extruded product. Understanding this mechanism lays the foundation for catalyst development for desulfurization and sulfur-containing polymerization reactions.