A malonic acid-containing macrocycle is observed to spontaneously undergo decarboxylation in water at mildly to strongly acidic pH and at room temperature. This phenomenon has significant implications for its use in aqueous solution within various areas of chemistry, like complexation of metals.

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.