Self-Assembly of Enantiopure Pd12 Tetrahedral Homochiral Nanocages with Tetrazole Linkers and Chiral Recognition

Abstract

Enantiopure acceptors (R,R)M and (S,S)M [where M = (N1,N1,N2,N2-tetramethylcyclohexane-1,2-diamine)Pd(NO3)2] have been used to design enantiopure Pd(II) tetrahedral cages. Self-assembly of [1,4-di(1H-tetrazol-5-yl)benzene] (H2L1) with chiral acceptors (R,R)M and (S,S)M yielded enantiopure homochiral tetrahedral cages (ΛΛΛΛ)T1 and (ΔΔΔΔ)T1, respectively. This strategy was further extended by using [2,6-di(1H-tetrazol-5-yl)naphthalene] (H2L2) with (R,R)M and (S,S)M to obtain water-soluble enantiopure tetrahedral nanocages (ΛΛΛΛ)T2 and (ΔΔΔΔ)T2, respectively. In order to obtain assembly with a larger cavity for potential use in enantioselective recognition, [4,4′-di(1H-tetrazol-5-yl)-1,1′-biphenyl] (H2L3) was used as the linker, which also resulted in the formation of water-soluble enantiopure tetrahedral cages (ΛΛΛΛ)T3 and (ΔΔΔΔ)T3 upon treatment with (R,R)M and (S,S)M, respectively. The present cages represent unusual examples of enantiopure tetrahedral cages of square-planar metal ions. Finally, T3 cages have been employed in a host–guest study as they offer the largest hydrophobic cavity. Encapsulation of chiral guest molecules such as [(R/S)-1,1′-binaphthalene]-2,2′-diol] (B) and [(R/S)-2,2′-diethoxy-1,1′-binaphthalene] (EtB) has been performed in order to successfully establish the asymmetric nature and enantiopurity of the tetrahedral cavity. The host T3 showed certain selectivity toward one enantiomer over the other. (ΛΛΛΛ)T3 preferred R-EtB over S-EtB (75:25) because of better fitting within the chiral cavity (Λ/R pair), whereas (ΔΔΔΔ)T3 favored S-EtB instead of R-EtB (Δ/S pair ratio = 73:27) with similar selectivity.