Issue 6, 2000

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.

Supplementary files

Article information

Article type
Paper
Submitted
09 Dec 1999
Accepted
03 Feb 2000
First published
28 Feb 2000

J. Chem. Soc., Dalton Trans., 2000, 845-851

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

R. L. Paul, S. M. Couchman, J. C. Jeffery, J. A. McCleverty, Z. R. Reeves and M. D. Ward, J. Chem. Soc., Dalton Trans., 2000, 845 DOI: 10.1039/A909702C

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