Metal Supramolecular Frameworks with Silver and Ditopic Bis(pyrazolyl)methane Ligands: Effect of the Anions and Ligand Substitution

A study of the self-assembly of different silver salts and the potentially ditopic ligands bis(pyrazol-1-yl)(pyridine-4-yl)methane (L-1) and bis(3,5-dimethylpyrazol-1-yl)(pyridine-4-yl)methane (L-2) was undertaken and species' with discrete or polymeric structures were obtained. Reactions with L-1 always yielded derivatives that had a box-like cyclic dimer structure with two metal centers and two ligands. However, on using ligand L-2, the resulting structure strongly depended on the coordination ability of the anion. On using anions that have coordinating ability (X = NO3-, CF3SO3-, PF2O2-), a similar box-like structure was formed. However, the use of anions with lower coordinating ability (X = PF6-, ClO4-) led to coordination polymers with a helical disposition. Spontaneous resolution was observed in the two compounds, and only one enantiomer was present in the crystals studied. Solid-state circular dichroism spectroscopy was applied to bunches of crystals, and the results indicate that growth of single colonies of homochiral crystals starting from single nucleation points may take place. When the helical complexes were recrystallized, box-like cyclic dimers were obtained. The presence or absence of methyl groups on the pyrazolyl rings had a strong influence on the supramolecular structure. In solution, the box-like dimers containing L-1 were more stable than those of L-2.