Double butterfly-shaped octanuclear dysprosium clusters: structure, magnetism and assembly mechanism

Abstract

Lanthanide clusters with pleasing structures and excellent molecular magnetic properties have attracted extensive attention, but there are still very few examples to explore and dissect their self-assembly process, which limits the precise synthesis of lanthanide clusters with specific structures and functions. Herein, two examples of Dy₈ clusters (1 and 2) formed by the superposition of butterfly-shaped Dy₄ bilayers with opposite directions were obtained using salicylic acylhydrazone ligands (2-hydroxybenzoic hydrazide and 4-dimethyl/diethylaminosalicylaldehyde) and Dy(NO₃)₃·6H₂O in the presence of CH₃OH : CH₃CN as a mixed solvent. Structural analysis shows that the butterfly shaped Dy₄L₂ is an independent unit of Dy₈, which is composed of two ligands located in the butterfly wings and the cluster nucleus Dy₄O₆. Magnetic studies show that clusters 1 and 2 exhibit single-molecule magnet (SMM) behavior under zero-field conditions. The effective energy barriers (U_eff) and relaxation times (τ) obtained by fitting clusters 1 and 2 by the Arrhenius formula are U_eff = 1.91 (0.11) and 5.8 (1.04) K, and τ = 1.77 × 10⁻² and 1.88 × 10⁻⁵ s, respectively. Notably, the octanuclear dysprosium clusters can exist stably under the conditions of high-resolution electrospray ionization mass spectrometry (HRESI-MS), providing an opportunity to track their self-assembly process. The formation of the above mentioned Dy₈ clusters was further followed using time-dependent HRESI-MS, and fragments of six reaction intermediates were identified. Combined with the change trend of the reaction intermediate fragments, we speculate that the possible self-assembly mechanism is as follows: L² + Dy → Dy(L²) → Dy₂(L²) → Dy₃(L²) → Dy₃(L²)₂ → 2Dy₄(L²)₂ → Dy₈(L²)₄ or L² + Dy → Dy(L²) → Dy₂(L²) → Dy₃(L²) → 2Dy₄(L²)₂ → Dy₈(L²)₄ or L² + Dy → Dy(L²) → Dy₂(L²) → Dy₂(L²)₂ → Dy₃(L²)₂ → 2Dy₄(L²)₂ → Dy₈(L²)₄.