Structures and anion-binding properties of M4L6 tetrahedral cage complexes with large central cavities

Abstract

Reaction of the bis-bidentate bridging ligand L³, in which two bidentate chelating 3(2-pyridyl)pyrazole units are separated by a 3,3′-biphenyl spacer, with Co(II) salts affords tetranuclear cage complexes of composition [Co₄(L³)₆]X₈ (X = [BF₄]⁻, [ClO₄]⁻, [PF₆]⁻ or I⁻) in which four 6-coordinate Co(II) ions in an approximately tetrahedral array are connected by six bis-bidentate bridging ligands, one spanning each of the six edges of the Co₄ tetrahedron. In every case, X-ray crystallography reveals that the ‘apical’ Co(II) ion has a fac tris-chelate geometry, whereas the other three Co(II) ions have mer tris-chelate geometries, resulting in (non-crystallographic) C₃ symmetry for the cages; that this structure is retained in solution is confirmed by ¹H NMR spectroscopy of the paramagnetic cages. In every case one of the anions is located inside the central cavity of the cage, with the remaining seven outside. We found no clear evidence for an anion-based templating effect. The cage superstructure is sufficiently large to leave gaps in the centres of the faces through which the internal and external anions can exchange. Variable-temperature ¹⁹F NMR spectroscopy was used to investigate the dynamic behaviour of the cages with X = [BF₄]⁻ and [PF₆]⁻ in MeCN solution: in both cases two separate signals, corresponding to external and internal anions, are clear at 233 K which have coalesced to a single signal at room temperature. Analysis of the linewidth of the minor signal (for the internal anion) at various temperatures below coalescence gave an activation energy for anion exchange of ca. 50 kJ mol⁻¹ in each case, a figure which suggests that anion exchange can occur via a conformational rearrangement of the cage superstructure in solution rather than opening of the cavity by cleavage of metal–ligand bonds.