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 hexadentate bridging ligands, with the central pyridyl N atom of L^Py not involved in coordination. The following complexes were prepared and structurally characterised: [M₂(L ^Ph)₂][ClO₄]₄ (M = Ni, Cu), which are dinuclear double helicates; [Ag₂-(L^Ph)(MeCN) ₂][BF₄]₂, a dinuclear complex with an Ag⋯Ag bond in which the ligand adopts a helical twist around the pair of metal ions; [Ni₂(L^Py)₂][BF₄] ₄, an achiral "mesocate" with a box-like structure and a face-to-face arrangement of ligands; [Ag₃(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 [Cu₂(L^Ph)₂][ClO₄]₄ and [Cd₆(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.