Large dielectric modulation triggered by spin crossover in polar Hofmann-type polymers

The integration of switchable magnetic and electric properties in a single-phase material remains a key goal in magnetoelectric material design. Herein, we report two Fe(II)-based Hofmann-type coordination polymers exhibiting ligand-directed spin crossover (SCO) coupled with tunable dielectric responses. By systematically varying the geometry and electronic nature of the bridging ligands, we achieve distinct symmetry arrangements and bistabilities. A bent Bib ligand yields a centrosymmetric phase (C2/c, {Fe^II(Bib)₂[Ag^I(CN)₂]}·ClO₄·2DMF (1), Bib = 1,3-bis(1H-imidazol-1-yl)benzene) with moderate dielectric contrast (Δε' ≈ 0.7) and thermal hysteresis (∼15 K), while a linear Bpt ligand induces a polar chiral phase (P6₅, {Fe^II₃(Bpt)₃[Ag^I(CN)₂]₆}₂ (2), Bpt = 2,5-bis(4-pyridyl)thiophene) with a gradual SCO spanning 75-218 K and a large dielectric change (Δε' ≈ 5). This study demonstrates that ligand-enabled symmetry control is an effective approach to realizing magnetoelectric bistability, offering promising implications for multifunctional material design.