Near-quantitative Removal of Oxalate and Terephthalate from Water by Precipitation with a Rigid Bis-amidinium Compound

A simple, readily-prepared precipitant (1.Cl2) precipitates oxalate or terephthalate from water with very high efficacy, removing these anions at sub-millimolar concentrations using only one equivalent of precipitant. A simple aqueous base/acid cycle can be used to regenerate 1.Cl2 after use. The resulting precipitates, 1.oxalate and 1.terephthalate, are anhydrous and closely-packed, with each anion receiving eight charge-assisted hydrogen bonds from amidinium N–H donors. Precipitation of oxalate and terephthalate occurs at much lower concentrations than other dicarboxylates, and direct competition experiments with the biologically/environmentally relevant divalent anions CO32–, HPO42– and SO42– reveal very high selectivity for oxalate or terephthalate over these competitors.