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

Abstract

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 Ic.

Discarding metal incorporation in pyrazole macrocycles and the role of the substrate on single-layer assemblies

“…or three-dimensional arrangements based on interactions between pyrazole derivatives through hydrogen bonds have been described. [7][8][9][10][11] Additionally, interesting properties like gel and liquid crystal behavior, luminescence or materials for energy storage have been reported within such kind of supramolecular organizations. [12][13][14] The pyrazole-based structures and geometries are manifold expanded whenever mixed with metals.…”