Pressure-Induced Spin Crossover at Room Temperature in a Nanoporous Host-Guest Framework Structure

The two-dimensional (2D) metal-organic framework compound Fe-2(azpy)(4)(NCS)(4)(azpy=trans-4,4 '-azopyridine, NCS=thiocyanate) has been pressurized in isopropanol and ethanol that act as guest molecules and pressure-transmitting media. Room temperature conversion of metal-ion sites from high-spin to low-spin states under high-pressure-driven uptake of guest molecules was investigated. Upon isopropanol guest inclusion, spin conversion of similar to 66 % low-spin abundance is attained by similar to 3 GPa. Spin-crossover progression stimulated by ethanol guest inclusion is more sluggish and similar to 56 % low-spin abundance is attained by similar to 5.5 GPa. This pressure-facilitated guest-specific triggering of spin crossover is suggestive of guest-molecule interactions with the local coordination network of the Fe(II) core, which increases the ligand field. Appreciable low-spin abundances attained at room temperature upon pressurization, are comparable to maximal similar to 50 % spin conversions only attainable below similar to 150 K at ambient pressure.

Automated summary

Pressurization of the 2D metal-organic framework compound Fe-2(azpy)(4)(NCS)(4) in isopropanol and ethanol results in metal-ion sites converting from high-spin to low-spin states under high pressure, driven by the uptake of guest molecules. The inclusion of different guest molecules leads to varying degrees of spin crossover progression, with higher pressure facilitating greater low-spin abundance at room temperature. This pressure-facilitated guest-specific triggering of spin crossover highlights interactions between guest molecules and the local coordination network of the Fe(II) core, increasing the ligand field.