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

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 (Lambda Lambda Lambda Lambda)T1 and (Delta Delta Delta Delta)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 (Lambda Lambda Lambda Lambda)T2 and (Delta Delta Delta Delta)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 (Lambda Lambda Lambda Lambda)T3 and (Delta Delta Delta Delta)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. (Lambda Lambda Lambda Lambda)T3 preferred R-EtB over S-EtB (75:25) because of better fitting within the chiral cavity (Lambda/R pair), whereas (Delta Delta Delta Delta)T3 favored S-EtB instead of R-EtB (Delta/S pair ratio = 73:27) with similar selectivity.