Metal-templated synthesis of intertwined, functionalized strands as precursors to molecularly woven materials

Abstract

Herein we propose a novel approach toward yet to be realized molecularly woven materials (MWMs), based on metal-templated precursors containing intertwined strands with functional ends. Two different potential precursors, based on terminal alkene-functionalized bis-Schiff base ligands (with either 1,2-diaminophenylene or 1,2-diaminoethylene cores) coordinated around a Cu(I) ion, have been tested. During this work, four novel organic ligands were prepared, along with the Cu(I) or Ag(I) complexes of three of them, and were characterized by X-ray diffraction and/or NMR spectroscopy. Chemical reactivity and structural studies (by single-crystal X-ray crystallography) of these novel compounds led to the assessment of their viability as precursors for MWMs. The essential requirement that terminal functionalities on the two intertwined ligand strands of the precursor must be far enough from each other so that only inter- and no intra-molecular reactivity is possible, is only met by the precursor with a 1,2-diaminophenylene core. This precursor, however, is less stable than the analogous one with a 1,2-diaminoethylene core, as it easily undergoes intramolecular cyclization/aromatization to yield a stable benzimidazole moiety, resulting in breakdown of the strands. The benzimidazole-containing compound offers an interesting example of chiral crystallization (helical arrangement about a four-fold axis) induced by hydrogen bonding of an otherwise achiral molecule. The results of this study outline the challenges involved in the preparation of a MWM using our approach, and will aid in identifying more robust ligand systems that meet the requirements of a MWM precursor set forth here.