Metal-Diamidobenzoquinone Frameworks via Post-Synthetic Linker Exchange

Metal-organic frameworks with amidic linkers often exhibit exceptional physical properties, but, owing to their strong metal-nitrogen bonds, are exceedingly challenging to isolate through direct synthesis. Here, we report a route to access metal-diamidobenzoquinone frameworks from their dihydroxobenzoquinone counterparts via postsynthetic linker exchange. The parent compounds (Me₂NH₂)₂[M₂L₃] (M = Zn, Mn; H₂L = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) undergo linker exchange upon exposure to a solution of monodeprotonated 2,5-diamino-3,6-dibromo-1,4-benzoquinone or 2,5-diamino-3,6-dichloro-1,4-benzoquinone, proceeding through single-crystal-to-single-crystal reactions. The presence of both types of linker in the resulting frameworks is confirmed by a combination of NMR, Raman, and energy-dispersive X-ray (EDX) spectroscopies. Moreover, the extent of linker exchange in the Zn frameworks is quantified using ¹³C NMR spectroscopy, and spatially resolved EDX spectroscopy reveals the two types of linker to be homogeneously distributed within a crystal. Finally, we propose a tentative mechanism of linker exchange based on pK_a measurements, considerations of framework solubility, and powder X-ray diffraction analysis. This work provides the first method to exchange organic linkers with different donor atoms in metal-organic frameworks and in doing so demonstrates exchange between linkers with donor atoms differing in acidity by a remarkable 11 units of pK_a. Together, these results offer a potentially general synthetic strategy toward new materials with exotic metal-linker coordination modes.