Substituents lead to differences in the formation of two different butterfly-shaped Ni II2Dy III2 clusters: structures and multistep assembly mechanisms

Abstract

The most effective way to understand reaction mechanisms and kinetics is to identify the reaction intermediates and determine the possible reaction patterns. The influencing factors that must be considered in the self-assembly of clusters are the type of ligand, metal ion, coordination anion and the pH of the solution. However, changes in ligand substituents resulting in different self-assembly processes to obtain different types of structures are still very rare, especially with –H and –CH₃ substituents, which do not exert significant steric hindrance effects. In this study, planar mononuclear Ni(L¹)₂ (L¹ = 2-ethoxy-6-(iminomethyl)phenol) was dissolved in methanol and combined with Dy(NO₃)₃·6H₂O for 48 h at room temperature to obtain a butterfly-like Ni₂Dy₂ cluster ([Dy₂Ni₂(L¹)₄(CH₃O)₂(NO₃)₄], 1). The Dy(III) ions in cluster 1 are in an O₈N coordination environment, and the Ni(II) ions are in an O₅N coordination environment. High-resolution electrospray ionization mass spectrometry (HRESI-MS) was used to track species changes during the formation of cluster 1. Six key intermediate fragments were screened, and the self-assembly mechanism was proposed as Ni(L¹)₂ → HL¹ + NiL¹ → DyL¹/Ni(L¹)₂′ → DyNi(L¹)₂ → Dy₂Ni₂(L¹)₄. Through this assembly mechanism, we found that Ni(L¹)₂ was first cleaved into HL¹ + NiL¹ and then further assembled to obtain 1. Another butterfly-like tetranuclear heterometallic cluster ([Dy₂Ni₂(L²)₄(CH₃O)₂(NO₃)₄], 2) was obtained using planar mononuclear Ni(L²)₂ (L² = (E)-2-ethoxy-6-((methylimino)methyl)phenol) with –CH₃ substitution on the nitrogen atom under the same reaction conditions. The structural analysis of cluster 2 showed that the Dy(III) ions are in an O₉ coordination environment, and the Ni(II) ions are in an O₄N₂ coordination environment. HRESI-MS was used to trace species changes during the formation of 2, and the assembly mechanism was proposed as Ni(L²)₂ → DyNi(L²)₂ → Dy₂Ni(L²)₂ → Dy₂Ni₂(L²)₄. Analysis of the assembly mechanism of 2 showed that Ni(L²)₂ was twisted during the reaction, and its coordination point was exposed to capture the Dy(III) ions. Finally, Dy(NO₃)₃·6H₂O was replaced with NaN₃ to obtain a [Ni₂Na₂(L²)₄(N₃)₄] cluster (3) under the same reaction conditions and verify the above-mentioned torsion step. HRESI-MS was also used to trace the assembly process, and the assembly mechanism was proposed as Ni(L²)₂ → NiNa(L²)₂ → NiNa₂(L²)₂ → Ni₂Na₂(L²)₄. Herein, the effect of interference from substitution and the regulation self-assembly process were discovered in the formation of 3d–4f heterometallic clusters, and different types of coordination clusters were obtained.