Record-Breaking Asymmetric Hysteresis Width Greater Than 100 K Spanning Room Temperature in Mn(III)-Based...

Spin crossover (SCO) materials capable of exhibiting wide thermal hysteresis loops near room temperature are highly desirable for molecular switches, memory devices, and sensors. While such behavior has been extensively studied in Fe(II), Fe(III), and Co(II) complexes, examples in Mn(III)-based systems remain limited due to the typically favored high-spin state and subtle structural changes accompanying spin-state switching. Herein, we report a Mn(III)-based spin-crossover complex [Mn(3-MeO-5-Me-sal₂-323)]·BF₄ (1), which exhibits a record-breaking asymmetric thermal hysteresis width exceeding 100 K, spanning room temperature. Detailed magnetic, calorimetric, and crystallographic studies reveal that this exceptional cooperativity is driven by an unprecedented two-dimensional (2D) hydrogen-bonding network formed between the complex cations and ordered BF₄ ^- counterions. High-resolution transmission electron microscopy (HRTEM) reveals locally ordered nanoscale domains, confirming retention of the supramolecular stacking motif at the nanoscale, consistent with the interplanar spacings obtained from single-crystal x-ray analysis. Variable-temperature Raman spectroscopy further corroborates Mn(III) spin crossover via spin-state-dependent metal-ligand vibrational signatures. Overall, the present report demonstrates the critical role of dimensionality in supramolecular hydrogen-bonding cooperativity in governing bistability in Mn(III)-SCO systems, offering a powerful design strategy for developing next-generation switchable materials.