Tuning Anisotropy Axis Orientation and Dipolar Coupling in Triple-Stranded Dinuclear Dy³⁺ Helicates Via Ligand Protonation State

We report the synthesis, crystal structures, and magnetic properties of two novel triple-stranded Dy3+ helicates, [Dy2(H3TB)3]Br3 (1) and [Dy2(H2TB)2(H3TB)](NO3) (2), which share an identical framework but differ in the protonation state of the bridging ligand. These protonation-state variations induce a pronounced reorientation of each Dy3+ ion’s local anisotropy axis. While the weakly axial environment is not favorable for observing zero-field slow relaxation of the magnetization in both complexes, we demonstrate the computed dipole–dipole coupling constant switches from ferromagnetic in 1 to antiferromagnetic in 2, while the net exchange interaction remains antiferromagnetic. These results demonstrate that fine control over ligand protonation provides a powerful and molecular handle for tuning the orientation of anisotropy axes and magnetic interactions in the context of lanthanide single-molecule magnets.