Magneto-structural and theoretical insights into Ni ₂ Dy ₂ butterfly single-molecule magnets with diverse anionic co-ligands

Five defective dicubane Ni₂Dy₂ complexes with the general formula [Ni₂Dy₂(L)₄X₂(solvent)_<i>n</i>], where (X = NO₃^- (1), acetylacetonate (2), NCS^- (3), OAc^- (4), and pivalate (5)), were synthesized and structurally characterized to investigate how co-ligand variation influences magnetic exchange interactions, anisotropy, and relaxation dynamics. Single-crystal X-ray diffraction reveals that the Dy^III ions adopt highly axial coordination environments ranging from distorted square-antiprismatic and triangular dodecahedral geometries in complexes 1-4 to a near pentagonal-bipyramidal geometry in 5. All complexes exhibit zero-field single-molecule magnet behavior, consistent with strong axial ligand fields. CASSCF calculations confirm that the shortest Dy-O(phenoxide) bonds govern the orientation of the magnetic easy axes, aligning toward terminal phenoxide donors. Broken-symmetry DFT calculations indicate uniformly positive Dy-Ni coupling constants, in line with typical ferromagnetic 3d-4f interactions, while the Ni-Ni coupling strength and sign are dictated by the Ni-O-Ni bridging angle, with a crossover between ferro- and antiferromagnetic regimes near 99°. This structural sensitivity rationalizes the comparatively weaker relaxation dynamics observed in complex 3, which features a larger Ni-O-Ni bridging angle accompanied by antiferromagnetic Ni-Ni interactions. These combined experimental and theoretical results establish robust structure-property correlations that provide a rational strategy for tuning anisotropy and exchange topology in Ni-Ln butterfly clusters to advance 3d-4f single-molecule magnet design.