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

Abstract

Reaction of 3-(2-pyridyl)pyrazole with 3,3′-bis(bromomethyl)biphenyl resulted in the new ligand L1 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 [Cu2(L1)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 L1) and a monodentate acetate. The structure is stabilised by extensive inter-ligand π-stacking interactions. The complex [Ag2(L1)2][ClO4]2 is also assumed to be a double helicate. In contrast, reaction with CoII afforded the tetranuclear cage complex [Co4(L1)6][BF4]8, in which each bridging ligand links two metal centres by spanning one edge of the Co4 tetrahedron. Each metal is therefore in a pseudo-octahedral tris-chelate geometry, with the three bidentate chelating arms each coming from a different ligand L1. Again there is substantial inter-ligand π 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. 1H NMR spectroscopy suggests that there is a mixture of species in solution arising from other metal∶ligand combinations; 11B NMR spectroscopy shows that at −40 °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 L1 with PdII afforded a mixture of products arising from ligand decomposition, of which [Pd2(L1)(pypz)2][BF4][OH] was structurally characterised. It has a near-planar {Pd2(μ-pypz)2}2+ core [Hpypz = 3-(2-pyridyl)pyrazole, which has arisen from decomposition of L1] with an additional bridging ligand L1 co-ordinating in a ‘basket-handle’ mode, straddling the central core.