Stepwise Fragmentation of Asymmetric Triangular Shape Ln3 Cluster and Magnetocaloric Effect of Gd3 Analogue

N-Methylbenzimidazole-2-methanol (L) and Ln(NO3)3·6H2O are dissolved in a mixed solvent of methanol and acetonitrile and then reacted at 80 °C to obtain [Ln3(L)5(NO3)4] (Ln3, Ln = Gd, Tb). In Ln3, three L− are located in the same plane as the three Ln3+ ions, and the other two L− are located above and below the plane, respectively. Two of the four NO3− ions are respectively coordinated with two Ln3+, and the other two are coordinated with one Ln3+ ion, which results in the asymmetry of the above-described triangular shape Ln3. When the Ln3 single crystal was tested by Electrospray Ionization Mass Spectrometry, the main frame [Ln3L5(NO3)3]+ was found to be stable, only one NO3−ion was lost. As the energy of the ion source increases, we observe the gradual fragmentation of the triangular shape Ln3, and propose that the fragmentation mechanism is: Ln3L5 → Ln3L4 → Ln2L4 → Ln2L3 → Ln2L2 → LnL. This provides the basis for the assembly of triangular shaped Ln3 clusters. For the first time, the magnetocaloric effect (MCE) of triangular Gd3 clusters was studied and it exhibited a large MCE of 10.65 J kg−1 K−1 at 7 K for ΔH = 5 T.