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, H-1 NMR measurements, and single-crystal X-ray diffraction analyses. Reaction of Zn(ClO4)(2)center dot 6H(2)O and Cd(ClO4)(2)center dot 6H(2)O with H3L in DMF gave two pentanuclear complexes [(Zn-5(mu(3)-O)(H2L)(6))(ClO4)(2) center dot DMF center dot 9.5H(2)O (1) and [Cd-5(mu(3)-O)(H2L)(6)](ClO4)(OH) center dot 4.75DMF center dot 0.25EtOH center dot 10.5H(2)O (2), in which the trigonal-bipyramidal core structures are bridged by mu(3)-oxo and pyrazolate rings of the monodeprotonated H2L. When Na3PO4 center dot 12H(2)O was used in the reaction system of CdBr2 center dot 4H(2)O and H3L, [Cd-5(mu(3)-O)(H2L)(6)]-Br-2 center dot 4.5DMF center dot 6.5H(2)O (3) and (Cd-7(mu(6)-PO4)(mu-Br)(3)(H2L)(6)](HPO4) center dot DMF center dot 10H(2)O (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 mu 6-PO4, three mu Br, and three pyrazolate rings. All of the pentanuclear and heptanuclear cores are surrounded by three pairs of bis-bidentate H2L- ligands with offset pi-pi stacking, showing propeller-like molecular structures and triple-stand helicates. Electrospray ionization mass spectrometry studies and H-1 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.