Resolution of chiral, tetrahedral M4L6 metal-ligand hosts

The supramolecular metal-ligand assemblies of M(4)1(6) stoichiometry are chiral (M = Gall, Al-III, In-III, Fe-III, Ti-IV, or Ge-IV, H(4)1 = N,N'-bis(2,3-dihydroxybenzoyl)-1,5-diaminonaphthalene). The resolution process of Delta Delta Delta Delta- and Delta Delta Delta Delta-[M(4)1(6)](12-) by the chiral cation S-nicotinium (S-nic(+)) is described for the Ga-III, Al-III, and Fe-III assemblies, and the resolution is shown to be proton dependent. From a methanol solution of M(acac)(3), H(4)1, S-nicl, and KOH, the Delta Delta Delta Delta-KH3(S-niC)(7)[(S-nic) subset of M(4)1(6)] complexes precipitate, and the Lambda Lambda Lambda Lambda-K-6(S-niC)5[(S-nic) subset of M(4)1(6)] complexes subsequently can be isolated from the supernatant. Ion exchange enables the isolation of the (NEt4+)(12), (NMe4+)(12), and K-12(+) salts of the resolved structures, which have been characterized by CD and NMR spectroscopies. Resolution can also be accomplished with 1 equiv of NEt4+ blocking the cavity interior, demonstrating that external binding sites are responsible for the difference in S-nic+ enantiomer interactions. Circular dichroism data demonstrate that the (NMe4+)(12) and (NEt4+)(12) salts of the resolved [Ga(4)1(6)](12-) and [Al(4)1(6)](12-) structures retain their chirality over extended periods of time (> 20 d) at room temperature; heating the (NEt4+)(12)[Ga(4)1(6)] assembly to 75 degrees C also had no effect on its CD spectrum. Finally, experiments with the resolved K-12[Ga(4)1(6)] assemblies point to the role of a guest in stabilizing the resolved framework.