Diacylhydrazone-assembled {Ln11} nanoclusters featuring a “double-boats conformation” topology: synthesis, structures and magnetism

Abstract

A family of novel Ln nanoclusters, namely, [Ln₁₁(ovpho)₄(μ-CH₃O)₂(μ-H₂O)₂(μ₃-OH)₆(CH₃OH)₄(H₂O)₂(NO₃)₈](OH)·xH₂O·yCH₃OH [Ln = Gd (1), x = 1, y = 3; Ln = Tb (2), x = 1, y = 3; Ln = Dy (3), x = 0, y = 3], was obtained via solvothermal reactions of Ln(NO₃)₃ with a diacylhydrazone ligand N,N′-bis(o-vanillidene)pyridine-2,6-dicarbohydrazide N-oxide (H₄ovpho). Their isostructural molecular structures are composed of two crystallographically symmetric {Ln₆} rings sharing a Ln³⁺ ion, and display an unprecedented “double boat conformation” topology that, to our knowledge, has not yet been reported. O_phenol, O_enol and O_oxynitride from ovpho⁴⁻ ligands, as well as O_methanol, O_water and O_hydroxyl help to bridge the Ln³⁺ ions. The structural variation between these {Ln₁₁} clusters and a previously reported {Gd₁₈} nanowheel, both of which are assembled by H₄ovpho under the same synthetic method and reaction conditions, is caused only by changing the anions of Ln salts. Magnetic investigations revealed a large magnetocaloric effect (MCE) of 1, whose maximum −ΔS_m value reaches 30.1 J kg⁻¹ K⁻¹ for ΔH = 50 kOe at 2.0 K. Additionally, it was found that 3 shows single-molecule magnets (SMMs) behavior, with an approximated energy barrier U_eff = 6.13 K and pre-exponential factor τ₀ = 1.70 × 10⁻⁶ s.