Kinetically controlled Ag+-coordinated chiral supramolecular polymerization accompanying a helical inversion

Abstract

We report kinetically controlled chiral supramolecular polymerization based on ligand-metal complex with a 3 : 2 (L : Ag+) stoichiometry accompanying a helical inversion in water. A new family of bipyridine-based ligands (d-L-1, l-L-1, d-L-2, and d-L-3) possessing hydrazine and d- or l-alanine moieties at the alkyl chain groups has been designed and synthesized. Interestingly, upon addition of AgNO3 (0.5-1.3 equiv.) to the d-L-1 solution, it generated the aggregate I composed of the d-(LAgNO3)-Ag-1 complex (d-L-1 : Ag+ = 1 : 1) as the kinetic product with a spherical structure. Then, aggregate I (nanoparticle) was transformed into the aggregate II (supramolecular polymer) based on the (d-L-1)(3)Ag-2(NO3)(2) complex as the thermodynamic product with a fiber structure, which led to the helical inversion from the left-handed (M-type) to the right-handed (P-type) helicity accompanying CD amplification. In contrast, the spherical aggregate I (nanoparticle) composed of the d-(LAgNO3)-Ag-1 complex with the left-handed (M-type) helicity formed in the presence of 2.0 equiv. of AgNO3 and was not additionally changed, which indicated that it was the thermodynamic product. The chiral supramolecular polymer based on (d-L-1)(3)Ag-2(NO3)(2) was produced via a nucleation-elongation mechanism with a cooperative pathway. In thermodynamic study, the standard Delta G degrees and Delta H-e values for the aggregates I and II were calculated using the van't Hoff plot. The enhanced Delta G degrees value of the aggregate II compared to that of the formation of aggregate I confirms that aggregate II was thermodynamically more stable. In the kinetic study, the influence of concentration of AgNO3 confirmed the initial formation of the aggregate I (nanoparticle), which then evolved to the aggregate II (supramolecular polymer). Thus, the concentration of the (d-L-1)(3)Ag-2(NO3)(2) complex in the initial state plays a critical role in generating aggregate II (supramolecular polymer). In particular, NO3- acts as a critical linker and accelerator in the transformation from the aggregate I to the aggregate II. This is the first example of a system for a kinetically controlled chiral supramolecular polymer that is formed via multiple steps with coordination structural change.