Stable Cu⁺, Ag⁺ complexes of aza-bridged macrocyclic molecules: Structure and chemical properties

The cage type compounds 1 and 2 form stable Cu⁺ or Ag⁺ complexes, which have been employed for the preparation of cation-free host molecules. A reaction between the potassium complex K⁺⊂1 and a Cu^II salt generates a Cu^I complex. The Cu^II/Cu^I redox potential is observed at +0.43 V (vs. SCE) in the cyclic voltammetry, which shows that the Cu⁺ state is stabilized by its rigid molecular skeleton and spatially fixed coordination sites. A reaction between Ag⁺ and K⁺⊂2 yields the dinuclear complex 2Ag⁺⊂2, which has a short Ag⁺ ⋯ Ag⁺ distance (2.78 Å). The halide anions (Cl^-, Br^-, I^-) remove one Ag⁺ from 2Ag⁺⊂2 to give Ag⁺⊂2, but further demetallation does not occur. CV measurements show that these silver complexes are electrochemically stable. Both silver complexes are stable to sunlight. The first preparations of guest-free hosts have been achieved by treating Cu⁺⊂1 or 2Ag⁺⊂2 with CN^-. Inclusions of neutral guests (NH₃, BH₃) have been attempted using these guest-free hosts.