Metal-Organic Nanotube with Helical and Propeller-Chiral Motifs Composed of a C10-Symmetric Double-Decker Nanoring.

Abstract

Coassembly of an achiral ferrocene-cored tetratopic pyridyl ligand (FcL) with AgBF4 in CH2Cl2/MeCN (7:3 v/v) containing chiral Bu4N(+) (+)- or (-)-menthylsulfate (MS*(-)) results in the formation of an "optically active" metal-organic nanotube (FcNT) composed of a C10-symmetric double-decker nanoring featuring 10 FcL units and 20 Ag(+) ions. The circular dichroism spectrum of FcNT along with its 2D X-ray diffraction (2D XRD) pattern indicates that the constituent metal-organic nanorings in FcNT stack one-handed helically on top of each other. A crystal structure of the dimeric double-decker model complex (Ag2(FcL')2) from a ditopic ferrocene ligand (FcL') and AgBF4 allowed for confirming the binding of MS*(-) onto the Ag(+) center of the complex. The results of detailed spectroscopic studies indicate that in its double-decker aromatic arrays, FcNT possibly possesses propeller-chiral twists in addition to the helically chiral structure, where the former is considerably more dynamic than the latter. Notably, both chiral structural motifs responded nonlinearly to an enantiomeric excess of MS*(-) (majority rule) though with no stereochemical influence on one another.