Solid-State Landscape of 4,4′-Azobis(3,5-dimethyl‑1H‑pyrazole) with the Isolation of Conformer-Dependent Polymorphs

The molecule 4,4′-azobis­(3,5-dimethyl-1H-pyrazole) (H₂azbpz) can exist in different planar conformers that differ in the relative orientation of the NH atoms with respect to the central azo bond and are related by azo-pedaling, rotation of the pyrazolyl group around the C4–N­(azo) bond, or N1–H to N2–H proton transfer. In the two polymorphs H₂azbpz-I and H₂azbpz-II, the two symmetrical forms 4,4′-a,a-E (I) and 4,4′-s,s-E (II) assemble in their own solid-state packing as a case of conformational polymorphism. Lattice energy and gas-phase conformer calculations point to the harder-to-obtain polymorph H₂azbpz-II, as the thermodynamic more stable form at lower temperatures but having the higher energy conformer form II. Both polymorphs H₂azbpz-I and H₂azbpz-II were reproducibly obtained by their own crystallization experiments which initially included ball milling to transform H₂azbpz-I into -II. The polymorph structures were established by single-crystal and powder X-ray diffraction in combination with Raman spectroscopy and solid-state ¹³C­{¹H} cross-polarization magic angle spinning NMR. The supramolecular assembly of H₂azbpz in its two polymorphs features supramolecular honeycomb sheets (hcb topology) in a staggered AA′-packing (in H₂azbpz-I) or in an eclipsed AB-fashion (in H₂azbpz-II). In addition, the hemihydrate H₂azbpz·0.5H₂O crystallized reproducibly from water-containing solutions, based on a tetrahedral (T_d) {H₂O­(pz)₄}-subunit with pseudo T_d-water molecules, thereby giving a network structure with dia-topology, which can be regarded as a supramolecular analogue of the cubic ice polymorph I_c.