Resolution of Chiral, Tetrahedral M4L6 Metal−Ligand Hosts1

The supramolecular metal−ligand assemblies of M4 1 6 stoichiometry are chiral (M = GaIII, AlIII, InIII, FeIII, TiIV, or GeIV, H4 1 = N,N‘-bis(2,3-dihydroxybenzoyl)-1,5-diaminonaphthalene). The resolution process of ΔΔΔΔ- and ΛΛΛΛ-[M4 1 6]12- by the chiral cation S-nicotinium (S-nic+) is described for the GaIII, AlIII, and FeIII assemblies, and the resolution is shown to be proton dependent. From a methanol solution of M(acac)3, H4 1, S-nicI, and KOH, the ΔΔΔΔ-KH3(S-nic)7[(S-nic) ⊂ M4 1 6] complexes precipitate, and the ΛΛΛΛ-K6(S-nic)5[(S-nic) ⊂ M4 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 [Ga4 1 6]12- and [Al4 1 6]12- structures retain their chirality over extended periods of time (>20 d) at room temperature; heating the (NEt4 +)12[Ga4 1 6] assembly to 75 °C also had no effect on its CD spectrum. Finally, experiments with the resolved K12[Ga4 1 6] assemblies point to the role of a guest in stabilizing the resolved framework.