Structures, host–guest chemistry and mechanism of stepwise self-assembly of M4L6 tetrahedral cage complexes

The ligand Lbip, containing two bidentate pyrazolyl–pyridine termini separated by a 3,3′-biphenyl spacer, has been used to prepare tetrahedral cage complexes of the form [M4(Lbip)6]X8, in which a bridging ligand spans each of the six edges of the M4 tetrahedron. Several new examples have been structurally characterized with a variety of metal cation and different anions in order to examine interactions between the cationic cage and various anions. Small anions such as BF4− and NO3− can occupy the central cavity where they are anchored by an array of CH⋯F or CH⋯O hydrogen-bonding interactions with the interior surface of the cage, but larger anions such as naphthyl-1-sulfonate or tetraphenylborate lie outside the cavity and interact with the external surface of the cage via CH⋯π interactions or CH⋯O hydrogen bonds. The cages with M = Co and M = Cd have been examined in detail by NMR spectroscopy. For [Co4(Lbip)6](BF4)8 the 1H NMR spectrum is paramagnetically shifted over the range −85 to +110 ppm, but the spectrum has been completely assigned by correlation of measured T1 relaxation times of each peak with Co⋯H distances. 19F DOSY measurements on the anions show that at low temperature a [BF4] − anion diffuses at a similar rate to the cage superstructure surrounding it, indicating that it is trapped inside the central cage cavity. Furthermore, the equilibrium step-by-step self-assembly of the cage superstructure has been elucidated by detailed modeling of spectroscopic titrations at multiple temperatures of an acetonitrile solution of Lbip into an acetonitrile solution of Co(BF4)2. Six species have been identified: [Co2Lbip]4+, [Co2(Lbip)2]4+, [Co4(Lbip)6]8+, [Co4(Lbip)8]8+, [Co2(Lbip)5]4+, and [Co(Lbip)3]2+. Overall the assembly of the cage is entropy, and not enthalpy, driven. Once assembled, the cages show remarkable kinetic inertness due to their mechanically entangled nature: scrambling of metal cations between the sites of pure Co4 and Cd4 cages to give a statistical mixture of Co4, Co3Cd, Co2Cd2, CoCd3 and Cd4 cages takes months in solution at room temperature.