Further investigations into tetrahedral M4L6 cage complexes containing guest anions: new structures and NMR spectroscopic studies

A series of ligands L-Ph, L-naph and L-anth, which contain two bidentate pyrazolyl-pyridine termini separated by an aromatic (1,2-phenyl,2,3-naphthyl or 2,3-anthracenyl, respectively) spacer have been used to prepare tetrahedral cage complexes of the form [M4L6]X-n, in which a bis-bidentate bridging ligand spans each of the six edges of the M-4 tetrahedron and one anion is bound in the central cavity. Several new examples have been structurally characterised, including an example with a new ligand (L-anth), the first example with a second-row transition metal ion [Cd(II)], and the first example of a cage containing a dianionic guest (hexafluorosilicate). The series of structurally similar Co(II) complexes [Co4L6(BF4)](BF4)(7) (L = L-Ph, L-naph and L-anth) have been examined in detail by NMR spectroscopy. The H-1 NMR spectra are highly shifted between -110 and +90 ppm, but the spectra can be completely assigned by correlation of measured T-1 relaxation times with distances of the protons in the complexes from the paramagnetic Co(II) centres. H-1 DOSY measurements have been used to estimate diffusion constants which confirm the structural integrity of the cages in solution, and F-19 DOSY measurements on the anions show that (i) the trapped [BF4](-) anion diffuses at the same rate as the cage superstructure surrounding it, indicating that it is trapped inside the cage cavity; and (ii) the 'free' [BF4](-) anions have diffusion rates consistent with substantial retardation due to ion-pairing with the 7+ complex cation.