Tetrazine-Radical-Bridged Lanthanide Complexes: From Di- to Trinuclear Single-Molecule Magnets

The intrinsic shielding of lanthanide 4f orbitals leads to weak magnetic exchange, a fundamental limitation that can be addressed by using radical-bridged ligands. In this work, we employed the electron-deficient ligand 3,6-bis(2,2'-bipyridyl)-1,2,4,5-tetrazine (bbpytz) and prepared a series of di- and trinuclear lanthanide complexes by intentionally controlling the stoichiometry of bbpytz and Ln(acac)₃. This approach yielded the dinuclear complexes [Ln^III₂(bbpytz^•-)(μ₂-OH)(acac)₄] (Ln = Dy, 1; Tb, 2) and trinuclear complexes [Ln^III₃(bbpytz^•-)(μ₂-OH)(acac)₇]·2MeCN (Ln = Dy, 3; Tb, 4; Gd, 5; Y, 6), in which the reduced tetrazine ligand (bbpytz^•-) serves as both a bridging and capping radical ligand. Magnetic studies reveal antiferromagnetic coupling between the radical and Ln^III centers, with coupling constant (-2J formalism) of -3.3 (1), -6.0 (3), and -3.8 cm^-1 (5). Ab initio calculations support strong axial anisotropy for the Dy^III ions and demonstrate a near-parallel alignment of magnetic easy axes. As a result, dysprosium complexes 1 and 3 exhibit slow magnetic relaxation and function as zero-field single-molecule magnets (SMMs), with effective energy barriers of 18.8 and 19.8 K, respectively. This study underscores the dual role of tetrazine radicals in enabling both strong magnetic exchange and control over magnetic anisotropy for the design of high-performance SMMs.