Structure, assembly mechanism and magnetic properties of heterometallic dodecanuclear nanoclusters Dy III4M II8 (M = Ni, Co)

Abstract

The formation of heterometallic clusters usually involves a much more complex self-assembly process than that of homometallic clusters. Thus the tracking of their assembly mechanism is challenging. To achieve this task, we designed one OH-containing Schiff base ligand of 1-[[(2-hydroxyethyl)imino]methyl]-2-naphthalenol (H₂L) with coordination pockets facile for selective coordination with Co^II/Ni^II and 4f metal ions. Its reactions with the corresponding metal ions gave two new isostructural dodecanuclear complexes [Dy₄Co₈(μ₃-OH)₈(L)₈(OAc)₄(H₂O)₄]·3EtOH·3CH₃CN·H₂O (1) and [Dy₄Ni₈(μ₃-OH)₈(L)₈(OAc)₄(H₂O)₄]·3.5EtOH·0.5CH₃CN·5H₂O (2), which feature a vertex-sharing tetracubane cyclic skeleton with the four Dy^III ions wrapped by the eight 3d metal ions. High-resolution electrospray mass spectrometry (HRESI-MS) tests showed that the skeletons of clusters 1 and 2 have high stability even under gradually increasing energy of the ion source. Most notably, the intermediates formed in the reaction courses for clusters 1 and 2 were tracked using time-dependent HRESI-MS, which gave different proposed assembly mechanisms for 1 (H₂L → DyL → DyCoL → Dy₂CoL → Dy₃CoL → Dy₄CoL₂ → Dy₄Co₂L₃ → Dy₄Co₄L₄ → Dy₄Co₈L₈) and 2 (H₂L → NiL → Dy₂NiL₂ → Dy₃NiL₂ → Dy₄NiL₂ → Dy₄Ni₂L₃ → Dy₄Ni₄L₃ → Dy₄Ni₆L₄ → Dy₄Ni₈L₈), respectively. This is the first time that different assembly mechanisms for isostructural heterometallic complexes are shown. Detailed magnetic studies revealed the absence of slow magnetic relaxation for 1 and the presence of slow magnetic relaxation for 2 with an energy barrier of 7.66 K and a pre-exponential factor of 1.45 × 10⁻⁶ s. The different magnetic performances of the two title complexes might be caused by the different metal ions of Co(II) and Ni(II).