Cooperative Sulfate Binding Drives Hierarchical Assembly of Supramolecular Trimers

Abstract

ABSTRACT Metallosupramolecular assembly traditionally employs rigid ligands to ensure structural predictability, yet flexible ligands‐based assembly remains a formidable challenge due to their conformational indeterminacy. In this study, we report a unique supramolecular trimer assembled from a flexible tripodal ligand, where each arm features a bis(urea) anion‐binding site and a terminal pyridine metal‐binding site. Upon coordination with sulfate and Pd 2+ or Pt 2+ , sulfate–bis(urea) hydrogen‐bonding directs the spatial arrangement of pyridine units. Single‐crystal x‐ray diffraction reveals racemic mixtures of supramolecular trimers with clockwise ( M ) or counterclockwise ( P ) helicity, composed of three stacked figure‐eight subunits and resembling the morphology of a Calochortus venustus tulip. Such assembly is co‐stabilized by metal–pyridine coordination (first‐sphere coordination), N–H···O/C–H···O H‐bonding with sulfates (second‐sphere coordination), and sulfate–Pd 2+ electrostatic attraction. The trimer remains stable in solution, as confirmed by NMR, DOSY, and ESI‐MS experiments. Concentration‐dependent NMR studies elucidate a multistep assembly pathway from a 1:1 sulfate complex, through a figure‐eight subunit, to the final architecture. Moreover, replacing the non‐coordinating pyridine unit with a chiral group successfully induces enantiopure trimers. This work establishes a paradigm of anion‐dictated assembly through cooperative first‐ and second‐sphere coordination, offering a new avenue for supramolecular assemblies.