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

Abstract

Enantiopure acceptors (<i>R</i>,<i>R</i>)<b>M</b> and (<i>S,S</i>)<b>M</b> [where <b>M</b> = (<i>N</i>1,<i>N</i>1,<i>N</i>2,<i>N</i>2-tetramethylcyclohexane-1,2-diamine)Pd(NO₃)₂] have been used to design enantiopure Pd(II) tetrahedral cages. Self-assembly of [1,4-di(1<i>H</i>-tetrazol-5-yl)benzene] (H₂<b>L</b>^<b>1</b>) with chiral acceptors (<i>R</i>,<i>R</i>)<b>M</b> and (<i>S</i>,<i>S</i>)<b>M</b> yielded enantiopure homochiral tetrahedral cages (ΛΛΛΛ)<b>T1</b> and (ΔΔΔΔ)<b>T1</b>, respectively. This strategy was further extended by using [2,6-di(1<i>H</i>-tetrazol-5-yl)naphthalene] (H₂<b>L</b>^<b>2</b>) with (<i>R</i>,<i>R</i>)<b>M</b> and (<i>S</i>,<i>S</i>)<b>M</b> to obtain water-soluble enantiopure tetrahedral nanocages (ΛΛΛΛ)<b>T2</b> and (ΔΔΔΔ)<b>T2</b>, respectively. In order to obtain assembly with a larger cavity for potential use in enantioselective recognition, [4,4'-di(1<i>H</i>-tetrazol-5-yl)-1,1'-biphenyl] (H₂<b>L</b>^<b>3</b>) was used as the linker, which also resulted in the formation of water-soluble enantiopure tetrahedral cages (ΛΛΛΛ)<b>T3</b> and (ΔΔΔΔ)<b>T3</b> upon treatment with (<i>R</i>,<i>R</i>)<b>M</b> and (<i>S</i>,<i>S</i>)<b>M</b>, respectively. The present cages represent unusual examples of enantiopure tetrahedral cages of square-planar metal ions. Finally, <b>T3</b> cages have been employed in a host-guest study as they offer the largest hydrophobic cavity. Encapsulation of chiral guest molecules such as [(<i>R</i>/<i>S</i>)-1,1'-binaphthalene]-2,2'-diol] (<b>B</b>) and [(<i>R</i>/<i>S</i>)-2,2'-diethoxy-1,1'-binaphthalene] (Et<b>B</b>) has been performed in order to successfully establish the asymmetric nature and enantiopurity of the tetrahedral cavity. The host <b>T3</b> showed certain selectivity toward one enantiomer over the other. (ΛΛΛΛ)<b>T3</b> preferred <i>R</i>-Et<b>B</b> over <i>S</i>-Et<b>B</b> (75:25) because of better fitting within the chiral cavity (Λ/<i>R</i> pair), whereas (ΔΔΔΔ)<b>T3</b> favored <i>S</i>-Et<b>B</b> instead of <i>R</i>-Et<b>B</b> (Δ/<i>S</i> pair ratio = 73:27) with similar selectivity.