Self-assembly of Ligands Designed for the Building of a New Type of [2 × 2] Metallic Grid. Anion Encapsulation and Diffusion NMR Spectroscopy

The ligands 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm), 4,6-bis(3,5-dimethylpyrazol-1-yl)pyrimidine (bpz*pm), 4,6-bis(4-methylpyrazol-1-yl)pyrimidine (Mebpzpm), and 3,6-bis(3,5-dimethylpyrazol-1-yl)pyridazine (ppdMe) were synthesized
and were made to react with Cu(I) centers in the presence of different counteranions. Different [2 × 2] metallic
grids were obtained. With ligands bpzpm, bpz*pm, and Mebpzpm, a new type of grid was obtained where the
facing ligands were divergent and two counteranions (BF4- or PF6-) were hosted in the resulting cavities and
exhibit C−H···F and anion···π interactions in the solid state. The presence of methyl groups on the pyrazolyl rings
induced several distortions in the structure. In complexes with the ligand ppdMe, there were found two groups of
parallel ligands in the grid, and the cavities generated were smaller. The counteranions were situated outside the
grid, and the facing ligands exhibited aromatic π−π stacking interactions. Anion−π interactions involving the pyridazine
ring were found. The behavior in solution of the new derivatives with a special emphasis on the cation−anion
interactions was studied by UV−vis and NMR spectroscopy. Diffusion NMR experiments performed for some
complexes allowed us to conclude that weak cation−anion interactions exist in solution, with the counteranions
undergoing fast exchange on the diffusion time scale between the free and ion-paired states.