Pathway Complexity in Nanotubular Supramolecular Polymerization: Metal-Organic Nanotubes with a Planar-Chiral Monomer.

Here, we report an anomalous pathway complexity in the supramolecular polymerization of a chiral monomer, which displays an unusual chiroptical feature that does not follow any of the known stereochemical rules such as "chiral self-sorting" and "majority rule". We newly developed a planar-chiral ferrocene-cored tetratopic pyridyl monomer FcL , which underwent AgBF ₄ -mediated supramolecular polymerization to give nanotubes FcNT s composed of metal-organic nanorings FcNR s. Although FcNR s must be homochiral because of a strong geometrical constraint, FcNR s were formed even efficiently from racemic FcL and AgBF ₄ . Detailed studies revealed the presence of two competing pathways for producing homochiral FcNR s as the constituents of FcNT s: (i) spontaneous cyclization of initially formed acyclic polymers -[ FcL -Ag ⁺ ] _n - and (ii) template ( FcNR )-assisted cyclization via a Ag ⁺ ···Ag ⁺ metallophilic interaction. The dominance of the two pathways changes depending on the % ee of chiral FcL . Namely, when the % ee of FcL is high, -[ FcL -Ag ⁺ ] _n - must contain sufficiently long homochiral sequences that can be readily cyclized into FcNR s. Meanwhile, when the % ee of FcL is low, the homochiral sequences in -[ FcL -Ag ⁺ ] _n - must be short and therefore are hardly eligible for spontaneous cyclization. Why were FcNR s formed? Even though the probability is very low, homochiral -[ FcL -Ag ⁺ ] _n - can be statistically generated and undergo spontaneous cyclization to give FcNR s minutely. We found that FcNR s can be amplified by heterochirally templating their own synthesis using metallophilic interaction. Because of this stereochemical preference, the growth of FcNR s into FcNT s via the template-assisted mechanism occurs only when both ^{( R,R )} FcL and ^{( S,S )} FcL are present in the polymerization system.