Combining electron transfer, spin crossover, and redox properties in metal-organic frameworks
Livia Getzner, Damian Paliwoda, Laure Vendier, Latévi Max Lawson Daku, Aurelian Rotaru, Gábor Molnár, Saioa Cobo, Azzedine Bousseksou
Abstract
Hofmann coordination polymers (CPs) that couple the well-studied spin transition of the FeII 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. Importantly, the redox activity of the ligands within the structure leads to the quasi-reversible electrochemical reduction reaction on a spin-crossover complex at solid state. These observations have been confirmed via temperature-dependent single-crystal X-ray diffraction, magnetic measurements, Mössbauer, EPR, optical and vibrational spectroscopies as well as quantum chemical calculations. Hofmann-type MOFs with an uncommon structural arrangement show thermochromism based on spin crossover behaviour and/or an intramolecular electron transfer between non-bridged CN units and the employed redox-active cationic bipyridinium ligand.