2,2'-[4,10-Bis(carb-oxy-meth-yl)-4,10-diaza-1,7-diazo-niacyclo-dodecane-1,7-di-yl]diacetate dihydrate.

In the title compound, C(16)H(28)N(4)O(8)·2H(2)O, the 12-membered macrocycle has twofold crystallographic symmetry and the asymmetric unit comprises one half-mol-ecule. The four carbox-yl/carboxyl-ate groups reside on the same side of the macrocycle. The mol-ecule is a double zwitterion with two of the carb-oxy-lic acid H atoms transferred to the two N atoms on the opposite sides of the macrocycle, resulting in both N atoms having positive charges and leaving the two resulting carboxyl-ate groups with negative charges. The two remaining carb-oxy-lic acid groups and the carboxyl-ate groups form O-H⋯O hydrogen bonds with the crystal water mol-ecules. The H atoms bound to the N atoms within the macrocyle are engaged in two equivalent hydrogen bonds with the adjacent N atoms.