The crystal structures of 4-benzyl-1H-pyrazole (C10H10N2, 1) and 3,5-di-amino-4-benzyl-1H-pyrazole (C10H12N4, 2) were measured at 150 K. Although its different conformers and atropenanti-omers easily inter-convert in solution by annular tautomerism and/or rotation of the benzyl substituent around the C(pyrazole)-C(CH2) single bond (as revealed by 1H NMR spectroscopy), 1 crystallizes in the non-centrosymmetric space group P21. Within its crystal structure, the pyrazole and phenyl aromatic moieties are organized into alternating bilayers. Both pyrazole and phenyl layers consist of aromatic rings stacked into columns in two orthogonal directions. Within the pyrazole layer, the pyrazole rings form parallel catemers by N-H⋯N hydrogen bonding. Compound 2 adopts a similar bilayer structure, albeit in the centrosymmetric space group P21/c, with pyrazole N-H protons as donors in N-H⋯π hydrogen bonds with neighboring pyrazole rings, and NH2 protons as donors in N-H⋯N hydrogen bonds with adjacent pyrazoles and other NH2 moieties. The crystal structures and supra-molecular features of 1 and 2 are contrasted with the two known structures of their analogs, 3,5-dimethyl-4-benzyl-1H-pyrazole and 3,5-diphenyl-4-benzyl-1H-pyrazole.

Supramolecular polymers offer tremendous potential to produce new \"smart\" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.

Only two 4-halo-1H-pyrazole crystal structures are known to date (chloro and bromo, the structure of 4-iodo-1H-pyrazole has not been reported yet). The triclinic structure of 4-fluoro-1H-pyrazole, C3H3FN2 (P ), reported here is not isomorphous with those of the chloro and bromo analogues (which are isomorphous, ortho-rhom-bic Pnma). To avoid sublimation during the measurement, diffraction data were collected at 150 K. Two crystallographically unique 4-fluoro-1H-pyrazole moieties linked by an N-H⋯N hydrogen bond are found in the asymmetric unit. Unlike the trimeric supra-molecular motifs found in the structures of the chloro and bromo analogues, 4-fluoro-1H-pyrazole forms one-dimensional chains by inter-molecular hydrogen bonding in the crystal.

The polymeric title complex, poly[hexa-μ-aqua-diaquatetra-μ-cyanurato-tetralithium] [Li4(C3H2N3O3)4(H2O)7] n , synthesized at room temperature from an aqueous solution of lithium hydroxide and cyanuric chloride, crystallizes in the triclinic space group P . There are two distinct Li+ cations in the asymmetric unit, one of which, Li1, has distorted trigonal-bipyramidal geometry and is coordinated via oxygen to two cyanurate anions occupying equatorial positions, and three water mol-ecules, two in the axial positions and the third in an equatorial position. One of the axial water ligands and the equatorial water ligand are involved in bridging to a crystallographically equivalent Li1 cation. A centre of inversion lies between the two Li1 cations and the Li1⋯Li1 distance is 3.037 (5) Å. The remaining axial water ligand bridges to the second Li cation, Li2, which is disordered over two crystallographic sites with approximately equal occupancy, and has an Li1⋯Li2 distance of 3.438 (7) Å. The terminal Li2 cation is coordinated to three water mol-ecules and an oxygen atom from a cyanuric anion and has a distorted tetra-hedral geometry. A three-dimensional network of inter-molecular hydrogen bonds involving N-H⋯O, O-H⋯O and O-H⋯N inter-actions serves to hold the structure together. The title compound was further characterized using IR and UV-vis spectroscopy and TG-DTA analysis.

Hydrogen bonding between urea functionalities is a common structural motif employed in crystal-engineering studies. Crystallization of 1,3-bis(3-fluorophenyl)urea, C13H10F2N2O, from many solvents yielded concomitant mixtures of at least two polymorphs. In the monoclinic form, one-dimensional chains of hydrogen-bonded urea molecules align in an antiparallel orientation, as is typical of many diphenylureas. In the orthorhombic form, one-dimensional chains of hydrogen-bonded urea molecules have a parallel orientation rarely observed in symmetrically substituted diphenylureas.

Derivatives of 4-hydroxypyrimidine are an important class of biomolecules. These compounds can undergo keto-enol tautomerization in solution, though a search of the Cambridge Structural Database shows a strong bias toward the 3H-keto tautomer in the solid state. Recrystallization of 2-amino-5,6-dimethyl-4-hydroxypyrimidine, C6H9N3O, from aqueous solution yielded triclinic crystals of the 1H-keto tautomer, denoted form (I). Though not apparent in the X-ray data, the IR spectrum suggests that small amounts of the 4-hydroxy tautomer are also present in the crystal. Monoclinic crystals of form (II), comprised of a 1:1 ratio of both the 1H-keto and the 3H-keto tautomers, were obtained from aqueous solutions containing uric acid. Forms (I) and (II) exhibit one-dimensional and three-dimensional hydrogen-bonding motifs, respectively.