Diversified lanthanide-directed self-assembly using a tritopic tetradentate acylhydrazone ligand

Abstract

Lanthanide–organic assemblies exhibit considerable promise for optical, electronic, magnetic, and catalytic applications, yet their preparation remains challenging. Herein, we present a comprehensive study of lanthanide-directed coordination self-assembly using a tritopic tetradentate acylhydrazone ligand H₆L³. Assembly of H₆L³ with a broad range of lanthanide ions under varying conditions produced diverse outcomes, characterized by NMR spectroscopy, ESI-TOF-MS, and single-crystal X-ray diffraction. Under neutral conditions, early lanthanide ions (Ln: La^III, Ce^III) rarely adopted orthogonal coordination, yielding tetranuclear Ln₄(H₃L³)₂ and trinuclear Ln₃(H₃L³)₂ species. For the middle lanthanide ion (Eu^III), the Eu₆(H₃L³)₄ complex converts into anti-triangular prismatic Eu₆(H₃L^3′)₆ during crystallization or a mixture of Eu₃(H₃L³)₂ and Eu₄(H₃L³)₂ when the metal-to-ligand ratio is increased to 3 : 2. In the presence of base, the orthogonal coordination is observed to minimize steric hindrance and octahedral Ln₆L³₄ was accessed across the entire lanthanide series. Interestingly, the homochiral Δ₆/Λ₆-Ce₆L³₄ and Δ₆/Λ₆-Lu₆L³₄ display different binding sequences toward the alkylammonium cations. The rich structural diversity and size selectivity toward guest molecules were attributed to differences in the radii of lanthanide ions and coordination strength. These findings furnish general design principles for coordination-driven self-assembly of multinuclear lanthanide architectures.