Pathway control in metallosupramolecular polymerization of a monoalkynylplatinum(II) terpyridine complex through competitive complex formation

Abstract

Understanding the pathway complexity of supramolecular polymerization in biomimetic systems has been a challenging issue due to its importance in the development of rationally controlled materials and insight into self-assembly in nature. We herein report a kinetic trapping strategy as a new methodology on how to control the pathway of metallosupramolecular polymerization by employing secondary metal ions and/or ligands which form competitive complex species. For this, we proposed monoalkynylplatinum(II) metalloligand (Pt-L¹) derived from a bis(amideterpyridine) receptor with one unoccupied terpyridyl terminal as a coordination site for the secondary metal ion (Ag+ or Fe2+). The inherent pathway complexity intrinsic to the Pt-L¹-anchored supramolecular polymerization has been modulated through the incorporation of Ag⁺ or Fe²⁺. During the supramolecular polymerization of Pt-L¹ in the presence of Ag⁺ and Fe²⁺, the added secondary ligand bpy (4,4'-dimethyl-2,2'-bipyridine) or DA18C6 (1,14-diaza-18-crown-6) form complexes as kinetic species, thereby inhibiting spontaneous polymerizations. The supramolecular polymer (SP-I), with a spherical structure composed of Pt-L¹ in the absence of metal ions as a kinetic product, did not transform into the thermodynamic product, namely supramolecular polymer (SP-III) with a left-handed fiber structure, due to a high energy barrier. However, the supramolecular polymer (SP-II) with a left-handed fiber structure, which was formed by Pt-L¹ in the presence of AgNO₃, converted to SP-III upon the addition of NaCl. Additionally, SP-II transformed into supramolecular polymer (SP-IV) upon the addition of Fe(BF₄)₂, through an on-pathway process. Both the morphological and emissive characteristics of the resulting supramolecular polymers can be fine-tuned via the Pt···Pt or Ag···Ag interactions as well as through the changes of the coordination geometry depending on the existing Ag⁺ or Fe²⁺ ions. The present results have important implications in expanding the scope of pathway complexity to produce a variety of products via kinetically controlled processes involving secondary metal ions and ligands.