Dinuclear lanthanide molecular magnetic materials bridged by tetrazine derivatives containing N6 and N8 atom sites display slow magnetic relaxation behavior†

Three dinuclear dysprosium complexes 1–3 with structural formulas [Dy₂(bptz)(tta)₆]·2CH₂Cl₂ (1), [Dy₂(H₂bmtz)(tta)₆] (2) and [Dy₂(H₂bmtz)(hfac)₆(H₂O)₂] (3) (bptz = 3,6-di(2-pyridyl)-1,2,4,5-tetrazine, H₂bmtz = 3,6-di(2-pyrimidyl)-1,4-dihydro-1,2,4,5-tetrazine, tta = 2-thenoyltrifluoroacetonate, hfac = hexafluoroacetylacetonate) were synthesized by the use of two tetrazine derivative ligands containing six and eight nitrogen atoms in combination with β-diketonate co-ligands, which are a good research platform to study the synergistic influence of the local coordination environment and magnetic interaction on magnetic relaxation. Complexes 1 and 2 possess the same peripheral co-ligands and nearly the same coordination environment (approaching an ideal triangular dodecahedron), but different multi-nitrogen bridges involving N6 (bptz) and N8 (H₂bmtz) tetrazine derivatives. The structural difference leads to distinct magnetic relaxation properties with a hysteresis temperature of 2.5 K and 5.5 K at a rate of 200 Oe s⁻¹ in 1 and 2, respectively, which is most likely due to the different magnetic coupling transmitted by H₂bmtz and bptz in the direction close to the vertical magneto axis. Combined with a suitable local coordination environment, the energy barrier of complex 2 bridged by H₂bmtz can reach three times that of complex 3. Considering the local ligand field, the important influence of this multi-nitrogen bridge transmitting magnetic coupling on the SMM properties is demonstrated with the aid of ab initio calculations. Also of note is that complexes 2 and 3 are the first reported lanthanide SMMs constructed from dihydro-tetrazine.