Anion-Driven Conformational Polymorphism in Homochiral Helical Coordination Polymers

Three homochiral 3D frameworks are assembled based on periodically ordered arrays of helices built from axial chiral 3,3′-bipyridine-5,5′,6,6′-tetramethyl-2,2′-dimethoxy-1,1′-biphenyl ligands and linearly coordinated Ag(I) ions. The aggregation behavior of silver salts and the ditopic ligand in solutions was investigated by a variety of techniques, including ¹H NMR, UV−vis, CD, GPC and MALDI-TOF. The cationic polymer skeleton exhibits an unprecedented conformational polymorphism in the solid-state, folding into two-, three- and four-fold helices with NO₃⁻, PF₆⁻ and ClO₄⁻ as the counteranion, respectively. The two-fold helices cross-link via argentophilic Ag−Ag interactions to form sextuple helices, which lead to a three-dimensional (3D) chiral framework. The three-fold or four-fold helices, on the other hand, self-associates in pairs to form three-dimensional tubular architectures. This anion-dependent self-assembly behavior can be rationalized by considering the sizes, geometries and binding abilities of the counteranions and subsequent chain conformation to minimize steric repulsions and maximize secondary interactions.