New [2 x 2] Copper(I) Grids as Anion Receptors. Effect of Ligand Functionalization on the Ability to Host Counteranions by Hydrogen Bonds

Several complexes with a [2 x 2] grid structure have been obtained by the self-assembly of different copper(I) salts and ligands of the type 4,6-bis(prazol-1-yl)pyrimidine containing different substituents on the heterocycles. The main goal has been to evaluate the influence over the solid state and solution behavior of the functionalization of the pyrimidine ring with a primary amino substituent. The molecular and crystalline structures of some derivatives have been determined by X-ray diffraction. The grids contain two open voids formed by pairs of ligands facing one another on opposite sides of the grid in a somewhat divergent manner. One counteranion is hosted in each void interacting through hydrogen bonds and anion-pi interactions. The presence of the amino group that points toward the inside of the cavity dominates the interact:ons in the void and apparently determines the orientation of the hosted counteranion and that of the ligands. With the exception of the derivative with chloride as the anion, the grid structure is preserved in solution (NMR and UV-vis) and some cation-anion interaction, increased by the presence of the amino group, exists also in solution (DOSY experiments). The experiments of anion interchange performed in solution indicate that a higher stability is found for the host-guest aggregates with OTs- (p-Me-C6H4SO3) and NO3-. While for these anions a 1:2 stoichiometry is reached, for the rest of the anions tested (ReO4, OTf-, and PF6-), only weaker 1:1 complexes are formed. Computational studies support the presence of anion-pi interactions.