Unveiling the Decarboxylation of a Malonic Acid-Containing Macrocycle at Acidic pH: Implications and Applications of an Overlooked Phenomenon

1,10-Diaza-18-crown-6-1,10-bis(malonic acid) (oddm or macromal) is a promising chelator for large hard metal cations due to its unique structure, which combines the complexing properties of aza-crown Kryptofix 22 with those of malonic acid. This study uncovers the previously overlooked decarboxylation of macromal under mildly to strongly acidic pH conditions at room temperature. As a result, the acidity constants of macromal are carefully re-evaluated, providing precise data for its accurate and informed handling in aqueous media. The implications of these findings for metal complexation and new possible applications of macromal are also presented.