Design of a Family of Ln₃ Triangles with the HAT Ligand (1,4,5,8,9,12-Hexaazatriphenylene): Single-Molecule Magnetism

A series of trinuclear Ln₃ complexes (Ln^III = Yb (1), Er (2), Dy (3) and Gd (4)) were prepared from the tris-chelate bidentate ligand 1,4,5,8,9,12-hexaazatriphenylene (HAT). 1 and 2 exhibited field-induced single-molecule-magnet (SMM) behavior with estimated U_eff values of 21.30 and 13.86 K, respectively. Complex 3 behaved as a SMM even at zero field, and two different thermally assisted relaxation processes were detected with U_eff values of 29.6 K (fast relaxation process, FR) and 69 K (slow relaxation process, SR) due to the existence of two magnetically different Dy^III centers in the molecule. Ab initio studies reveal that all the Dy³⁺ centers have almost an Ising ground state. The local anisotropy axes are not coplanar but form angles with the Dy3 plane in the range 58-78°. The magnetic interaction between the anisotropic Dy³⁺ ions is antiferromagnetic in nature and very weak in magnitude. However, due to the extreme feebleness of the magnetic interaction with regard to the local excitation energies, the magnetization blockade is most probably of single-ion origin. Calculations support the existence of two relaxation processes, which take place through the first excited state following an Orbach/Raman mechanism. Finally, for complex 4, the magnetocaloric effect was simulated using the magnetic parameters extracted from the fit of the magnetization and susceptibility data and demonstrated that the simulated -ΔS_m values were almost coincident with those extracted from the integration of the field dependence of the magnetization. The simulated MCE value at 2 K and 5 T (20.46 J kg^-1 K^-1) makes complex 4 an attractive candidate for cryogenic magnetization.