Pentanuclear and Heptanuclear Helicates By Self-Assembly of d¹⁰ Metal Ions and a Rigid Aromatic Bis-Bidentate Chelator

The self-assembly of Zn(II) and Cd(II) ions with a bis-bidentate ligand 3,5-bis(benzimidazol-2-yl)pyrazole (H3L) was studied by Electrospray ionization mass spectrometry, 1H NMR measurements, and single-crystal X-ray diffraction analyses. Reaction of Zn(ClO4)2·6H2O and Cd(ClO4)2·6H2O with H3L in DMF gave two pentanuclear complexes [(Zn5(μ3-O)(H2L)6)(ClO4)2·DMF·9.5H2O (1) and [Cd5(μ3-O)(H2L)6](ClO4)(OH)·4.75DMF·0.25EtOH·10.5H2O (2), in which the trigonal-bipyramidal core structures are bridged by μ3-oxo and pyrazolate rings of the monodeprotonated H2L. When Na3PO4·12H2O was used in the reaction system of CdBr2·4H2O and H3L, [Cd5(μ3-O)(H2L)6]Br2·4.5DMF·6.5H2O (3) and [Cd7(μ6-PO4)(μ-Br)3(H2L)6](HPO4)·DMF·10H2O (4) were isolated. 3 displays the same core structure as that of 2, whereas 4 exhibits a turbinate, heptanuclear core which is bridged by a μ6-PO4, three μ-Br, and three pyrazolate rings. All of the pentanuclear and heptanuclear cores are surrounded by three pairs of bis-bidentate H2L− ligands with offset π−π stacking, showing propeller-like molecular structures and triple-stand helicates. Electrospray ionization mass spectrometry studies and 1H NMR measurements demonstrate that the pentanuclear complexes have different stability in the solution, depending on the metal ions and the counteranions. Furthermore, both 1 and 2 emit blue fluorescence with nanosecond luminescent lifetimes in DMF at room temperature.