Highly stable drone-shaped lanthanide clusters: structure, assembly mechanism, and crystalline–amorphous transitions

Abstract

Great progress has been made in the design and synthesis of high-nuclear lanthanide clusters with impressive structural connections, high stability, and special topologies, but still lack reliable regularity. Herein, a drone-shaped lanthanide cluster (1) was obtained by mixing stoichiometric ratios of 2-pyridinecarbohydrazide, 2,3,4-trihydroxybenzaldehyde, and a mixed-metal dysprosium salt (Dy(NO₃)₃·6H₂O : DyCl₃·6H₂O) under solvothermal conditions. Notably, a four-coordinated bridging chloride ion (μ₄-Cl⁻) was found in the structure of cluster 1. The eight ligands were distributed on the periphery of the cluster core and tightly wrapped around the cluster core. The freely rotatable pyridine ring at the end of the ligand can effectively prevent the attack of solvent molecules on the cluster core and promote the stability of cluster 1. The self-assembly process of cluster 1 was tracked by time-dependent high-resolution electrospray-ionization mass spectrometry, and the trend of various intermediate molecular ion peaks over time was identified. We speculated that the possible self-assembly mechanism of cluster 1 was L + Dy → DyL → Dy₂L₂ → Dy₃L₃ → Dy₄L₄ → 2Dy₄L₄ → Dy₈L₈ → Dy₉L₈. This study finds that the self-assembly mechanism of drone-shaped lanthanide clusters can guide the construction of high-nuclear lanthanide clusters with special shapes. Interestingly, cluster 1 can undergo obvious color transitions in air and under UV light conditions. IR and PXRD results indicated that the above color change was caused by a transition from the crystalline to amorphous state and did not trigger the change in functional groups. This work explores the crystalline-to-amorphous transition of high-nuclear lanthanide clusters. Magnetic studies revealed that cluster 1 exhibited distinct SMM behavior under zero-field conditions. We provided a vivid example of the synthesis of high-nuclear lanthanide clusters with special shapes and provided insights into building multifunctional lanthanide clusters.