A Cationic Metal–Organic Solid Solution Based on Co(II) and Zn(II) for Chromate Trapping

We report the synthesis and characterization of a solid solution series of cationic metal–organic materials with full compositional range from pure Co­(II) to Zn­(II) end-members. The materials consist of [Zn_x­Co_1–x­(H₂O)₄­(4,4′-bipy)₂]²⁺ metal–organic clusters that π–π stack into 2-D positively charged layers, with the metal ratio tunable by molar ratio under hydrothermal conditions. The interlamellar α,ω-alkane­disulfonate serves as an anionic template and noncovalently interacts with the cationic layers. The weak interaction allows anion exchange for toxic oxometal anions, such as chromate, CrO₄^2–. The highest chromate adsorption capacity was 68.5 mg/g (0.43 mol/mol) for the as-synthesized 50 mol % Co­(II)-incorporated material. Our cationic material can also selectively trap these toxic oxo-anions when nontoxic anions (e.g., nitrate, sulfate) were present in an over 50-fold excess concentration.