Abundant polymorphism in a system with multiple hydrogen-bonding opportunities: Oxalyl dihydrazide

To date, only one crystal structure has been reported in the literature for oxalyl dihydrazide [ H2N center dot NH center dot CO center dot CO center dot NH center dot NH2]. In the present paper, we report the discovery of four new polymorphs of oxalyl dihydrazide, obtained by crystallization from solution under different conditions, including the use of different crystallization solvents. All polymorphs have the trans-trans-trans conformation of the N-N-C-C-N-N backbone, but the positions of the hydrogen atoms of the NH2 groups relative to this backbone differ between the different polymorphs through variation of the torsion angle around each NH-NH2 bond. The different polymorphs display a range of different hydrogen-bonding arrangements, constructed from different types of hydrogen-bonded array. The existence of several different potential hydrogen-bond donor and hydrogen-bond acceptor groups in the oxalyl dihydrazide molecule, together with the fact that the N- H bonds of the NH2 groups adopt different orientations with respect to the molecular plane, leads to several possible geometric permutations for hydrogen-bonding arrangements in the solid state. It would not be surprising if even more polymorphs of oxalyl dihydrazide are discovered in the future.