Effects of metal co-ordination geometry on self-assembly: a dinuclear double helicate complex and a tetranuclear cage complex of a new bis-bidentate bridging ligand

Reaction of 3-(2-pyridyl)pyrazole with 3,3'-bis(bromomethyl)biphenyl resulted in the new ligand L-1 which contains two bidentate chelating pyrazolyl/pyridine fragments separated by a meta-biphenyl spacer; this ligand is designed to act only as a bridging ligand, as the two bidentate sites are too far apart to co-ordinate to the same metal ion. The dinuclear copper(II) complex [Cu-2(L-1)(2)(OAc)(2)][BF4](2) is a double helicate in which each copper(II) centre is in a square pyramidal co-ordination geometry, arising from two bidentate pyrazolyl/pyridine groups (one from each ligand L-1) and a monodentate acetate. The structure is stabilised by extensive inter-ligand pi-stacking interactions. The complex [Ag-2(L-1)(2)][ClO4](2) is also assumed to be a double helicate. In contrast, reaction with Co-II afforded the tetranuclear cage complex [Co-4(L-1)(6)][BF4](8), in which each bridging ligand links two metal centres by spanning one edge of the Co-4 tetrahedron. Each metal is therefore in a pseudo-octahedral tris-chelate geometry, with the three bidentate chelating arms each coming from a different ligand L-1. Again there is substantial inter-ligand stacking. Unlike other complexes with the same {M4L6} tetrahedral cage structure, the central cavity is not occupied by a counter ion, showing that although the templating effect of a counter ion can be beneficial in the assembly of such cages it is clearly not essential. H-1 NMR spectroscopy suggests that there is a mixture of species in solution arising from other metal : ligand combinations; B-11 NMR spectroscopy shows that at -40 degrees C a [BF4](-) anion can become trapped in the cavity of the cage, giving a characteristic high-field resonance in addition to that for the free [BF4](-) anions. Reaction of L-1 with Pd-II afforded a mixture of products arising from ligand decomposition, of which [Pd-2(L-1)(pypz)(2)][BF4][OH] was structurally characterised. It has a near-planar {Pd-2(mu-pypz)(2)}(2+) core [Hpypz = 3-(2-pyridyl)pyrazole, which has arisen from decomposition of L-1] with an additional bridging ligand L-1 co-ordinating in a 'basket-handle' mode, straddling the central core.