Abiotic Metallofoldamers as Electrochemically Responsive Molecules

Described are the design, synthesis, and study of nonbiological molecules based on salophen
and salen ligands that fold into single-stranded helices in the presence of either Ni(II) or Cu(II). X-ray
diffraction studies show that the materials fold into helical structures in the solid state, and a series of NMR
studies provide strong evidence that the folded structures are conserved in solution. Metal coordination is
required for folding, as NMR and X-ray show that the free ligands do not adopt helical structures. Two of
the racemic metallofoldamers spontaneously resolve during crystallization from CHCl3/acetonitrile, and CD
spectroscopy and optical rotation show that the resolved, crystalline materials racemize quickly when
dissolved at 5 °C. This shows that the secondary structures can reorganize easily and can, therefore,
provide the basis for responsive materials. By comparison, an analogue from enantiomerically pure (R,R)-(−)-trans-cyclohexanediamine showed a strong CD signal and a large specific rotation. Electrochemical
experiments show that a structural reorganization occurs upon metal-centered reduction of a Cu(II)-containing
foldamer. When the reduction is carried out in the presence of coordinating ligands, it is proposed that
apical binding of those ligands gives square pyramidal complexes. Semiempirical (AM1) calculations support
that the helical structure would be disrupted by the reduction to Cu(I) with concomitant reorganization to a
square pyramidal complex.