Electron-Transfer-Induced Structural Modification in a Thermochromic Hofmann Clathrate Derivative.

This study explores a Hofmann-type clathrate, {Fe(2,4-(OCH)-bpy)(HO)[μ-Pt(CN)][μ-Pt(CN)]·4HO}, which exhibits a thermally activated electron transfer (ET) without spin crossover. Synthesized as microcrystalline powders and single crystals, this compound reveals significant structural changes upon variation of the temperature, including alterations in the iron coordination sphere due to the large movement of the bipyridinium ligand. These changes correlate with a thermochromic hysteretic transition from yellow to orange.

Combining electron transfer, spin crossover, and redox properties in metal-organic frameworks.

Hofmann coordination polymers (CPs) that couple the well-studied spin transition of the Fe central ion with electron-responsive ligands provide an innovative strategy toward multifunctional metal-organic frameworks (MOFs). Here, we developed a 2D planar network consisting of metal-cyanide-metal sheets in an unusual coordination mode, brought about by infinitely π-stacked redox-active bipyridinium derivatives as axial ligands. The obtained family of materials show vivid thermochromism attributed to electron transfer and/or electronic spin state change processes that can occur either independently or concomitantly.

Hofmann Clathrates: A "Blue Box" Approach to Modulate Spin-Crossover Properties.

Hofmann coordination polymers (CPs) with cationic ligands provide an innovative strategy for recognizing π-electron-rich aromatic molecules - similar to the "little blue box". In this study, we demonstrate that hydroquinone molecules can be incorporated into these coordination polymers when redox-active bipyridinium derivatives are used as axial ligands. The insertion leads to a significant structural modification, resulting in a shift of the spin transition by 150 K and an approximate 23 % increase in volume, caused by the strong donor-acceptor π-π stacking interaction formed between the ligands and the guest molecule.