Functionalization of luminescent lanthanide-gallium metallacrowns using copper-catalyzed alkyne-azide cycloaddition and thiol-maleimide Michael addition.

The synthesis and characterization of {Ln[12-MCGaIIIN(eshi)-4]}2(iph)4 and {Ln[12-MCGaIIIN(shi)-4]}2(miph)4 metallacrowns (MCs), where shi3- is salicylhydroximate, eshi3- is 4-ethynylsalicylhydroximate, iph2- is isopthalate, and miph2- is 5-maleimidoisophthalate, is reported. The ethynyl functionality allows for coupling of MCs to azides using copper(I) catalyzed alkyne-azide cycloaddition (CuAAC), while the maleimido functionality allows for coupling of the MCs to thiol-bearing compounds. We demonstrate these coupling reactions using benzyl azide for the former and cysteamine for the latter, with complete conversion shown by ESI-MS. With the Sm analogues, the MCs exhibit characteristic luminescent emission of Sm(III), which is preserved after introducing the ethynyl and maleimido groups onto the MC scaffold. Furthermore, the high stability of these compounds in solution illustrates that once functionalized, the MCs are promising for fluorescent imaging applications.