Multifunctional Triple-Decker Inverse 12-Metallacrown-4 Sandwiching Halides

A family of six triple-decker complexes, {(MX)₂[(pz)₄]₃} (Hpz = 4-nitropyrazole, MX = NaCl, 1; NaBr, 2; NaI, 3; KCl, 4; KBr, 5, and KI, 6), exhibiting inclusion of halides into inverse 12-metallacrown-4 [inv-(12-MC_Cu(I),pz-4)] array has been realized. Single-crystal X-ray crystallography of each compound reveals a common structural feature consisting of four Cu^I ions bonded by four pz to form a square metallomacrocycle comprising four metal centers and eight N atoms, thus giving an inv-[12-MC_Cu(I),pz-4] motif. Two halides are sandwiched by three inv-[12-MC_Cu(I),pz-4] to form triple-deckers that are further extended in an offset stacking mode by ligand-unsupported cuprophilicity interactions to form a one-dimensional chain structure. Halides are attached to six Cu^I centers with weak Cu^I···halogen interaction, resembling anion templates. High-resolution electrospray ionization mass spectrometry reveals that the predominant fragments corresponding to a half of the triple-decker structures of 1-3 exist in solution. Compounds 4, 5, and 6 showed excellent electrocatalytic activities toward the reduction of nitrite and can also be used as selective "turn-off" sensors for Ag(I) in water. The present results will be helpful for the future design and synthesis of functional inverse metallacrowns and their multiple-decker complexes.