Redox-Active Ligand Platforms for Spin-Crossover Materials: Influence of Halogen Substitution in Pt(II) and Ni(II) Hofmann-Type Frameworks

We report the synthesis and structural characterization of a series of halogen-based Hofmann clathrates having two different general formulas {Fe(<b>X</b>_<b>a</b><b>-pbpy</b>^<b>+</b>)₂[μ₂-M_a(CN)₄]₂·2H₂O} (where X_a = Cl, Br, or I with M_a = Ni; X_a = Br or I with M_a = Pt) and {Fe(<b>X</b>_<b>b</b><b>-pbpy</b>^<b>+</b>)₂(H₂O)₂[μ₂-M_b(CN)₄]·[M_b(CN)₄]} (where X_b = F with M_b = Ni; X_b = Cl or F with M_b = Pt), by incorporating halogen-substituted bipyridinium derivatives as ligands. We demonstrate that halogen substitution and the choice of the tetracyanometalate bridge synergistically modulate the structural motifs and therefore magnetic properties. In addition, we show that the previously established Marcus theory-based model, describing the correlation between the ligand redox potential (influenced by the nature of the halogen) and the resulting crystal space group as well as spin crossover (SCO) temperature (<i>T</i>_1/2) for a related series of Pt-based compounds, also extends to the Ni-based analogues. This confirms the general validity of the Marcus-type redox-SCO correlation across this family of Hofmann-type clathrates. Single-crystal X-ray diffraction, cyclic voltammetry, magnetic susceptibility, and Mössbauer spectroscopy confirm the structural and electronic divergence between staggered and layered architectures and establish a unified redox-structure-SCO relationship. These findings offer valuable insight into the rational design of multifunctional molecular materials.