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 CO₃ ^2-, HPO₄ ^2- and SO₄ ^2- reveal very high selectivity for oxalate or terephthalate over these competitors.