Crystal structures of two polymorphic forms of DOTAM‐mono‐acid dihydrate

The structural chemistry of 2‐[4,7,10‐tris(carbamoylmethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐yl]acetic acid dihydrate, C 16 H 31 N 7 O 5 ·2H 2 O, is described. The macrocyclic compound, also known by the abbreviation DOTAM‐mono‐acid, crystallized at room temperature and was isolated concomitantly as two polymorphic forms. The structures of both polymorphs were determined at 90 K. The first polymorph crystallized as a zwitterionic dihydrate [systematic name: 4,7,10‐tris(carbamoylmethyl)‐1‐(carboxylatomethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐ium dihydrate] in the space group P 2 1 / n , with Z ′ = 1. The second polymorph crystallized as a zwitterionic dihydrate in the space group P 2 1 at 90 K, with Z ′ = 2. The two independent molecules are related by a local center. In each polymorph, the zwitterion is formed between the negatively‐charged carboxylate group and the ring N atom that bears the acetate pendant arm. Extensive inter‐ and intramolecular hydrogen bonding exists in both polymorphic structures. In polymorph 1, an intermolecular hydrogen‐bonding network propagating parallel to the a direction creates an infinite chain. A second hydrogen‐bonding network is observed through a water molecule of hydration in the b direction. Polymorph 2 also has two intermolecular hydrogen‐bonding networks. One propagates parallel to the a direction, while the other propagates in the [10] direction. Increasing the temperature of polymorph 2 yields the same structure at T = 180 K, but the pseudocenter becomes exact at 299 K. The higher‐temperature structure has Z ′ = 1 in the space group P 2 1 / c .