Cooperative Sulfate Binding Drives Hierarchical Assembly of Supramolecular Trimers

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 Pd²⁺ or Pt²⁺, 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-Pd²⁺ 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.