Self-Assembly of Enantiopure Pd12 Tetrahedral Homochiral Nano-Cages 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 (LambdaLambdaLambdaLambda)T1 and (DeltaDeltaDeltaDelta)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 nano-cages (LambdaLambdaLambdaLambda)T2 and (DeltaDeltaDeltaDelta)T2, respectively. In order to obtain assembly with 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 (LambdaLambdaLambdaLambda)T3 and (DeltaDeltaDeltaDelta)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 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 success-fully establish the asymmetric nature and enantiopurity of the tetrahedral cavity. The host T3 showed certain selectivity towards one enantiomer over the other. (LambdaLambdaLambdaLambda)T3 preferred R-EtB over S-EtB (75:25) because of better fitting within the chiral cavity (Lambda/R pair), whereas (DeltaDeltaDeltaDelta)T3 favoured S-EtB instead of R-EtB (Delta/S pair ratio = 73:27) with almost similar selectivity.