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·NH·CO·CO·NH·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.