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) (H(2)azbpz) 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 azopedaling, rotation of the pyrazolyl group around the C4-N(azo) bond, or N1-H to N2-H proton transfer. In the two polymorphs H(2)azbpz-I and H(2)azbpz-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 H(2)azbpz-II, as the thermodynamic more stable form at lower temperatures but having the higher energy conformer form II. Both polymorphs H(2)azbpz- I and H(2)azbpz-II were reproducibly obtained by their own crystallization experiments which initially included ball milling to transform H(2)azbpz-I into -II. The polymorph structures were established by single-crystal and powder X-ray diffraction in combination with Raman spectroscopy and solid-state C-13{H-1} cross-polarization magic angle spinning NMR. The supramolecular assembly of H(2)azbpz in its two polymorphs features supramolecular honeycomb sheets (hcb topology) in a staggered AA'-packing (in H(2)azbpz-I) or in an eclipsed AB-fashion (in H(2)azbpz-II). In addition, the hemihydrate H(2)azbpz center dot 0.5H(2)O 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.