Coordination behaviour of bis-terdentate N-donor ligands: Double- and single-stranded helicates, mesocates, and cyclic oligomers

Three new ligands have been prepared in which two terdentate chelating pyrazolyl-bipyridine units are connected by a central aromatic spacer via methylene "hinges": the spacers are o-phenylene (L-Ph), 2,6-pyridine-diyl (L-Py) and 2,3-naphthalenediyl (L-naph). The ligands act as potentially hexadenlate bridging ligands, with the central pyridyl N atom of L-Py not involved in coordination. The following complexes were prepared and structurally characterised: [M-2(L-Ph)(2)][ClO4](4) (M = Ni, Cu), which are dinuclear double helicates; [Ag-2(L-Ph)(MeCN)(2)][BF4](2), a dinuclear complex with an Ag center dot center dot center dot Ag bond in which the ligand adopts a helical twist around the pair of metal ions; [Ni-2(L-Py)(2)][BF4](4), an achiral "mesocate" with a box-like structure and a face-to-face arrangement of ligands; [Ag-3(L-naph)(2)](BF4)(3), which contains a linear trinuclear array of Ag-I ions with the two ligands arranged in a shallow helical twist, each ligand spanning one terminal and the central metal ion; and [Cd-6(L-naph)(6)](ClO4)(12), a cyclic hexanuclear helicate with a perchlorate anion in the central cavity. Both [Cu-2(L-Ph)2][ClO4](4) and [Cd-6(L-naph)(6)](ClO4)(12), which have architecturally similar bridging ligands, show evidence by electrospray mass spectrometry for formation of a range of cyclic oligomers in solution up to 11-mers for the Cd-II/L-naph system. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)