Reversible chemisorption of sulfur dioxide in a spin crossover porous coordination polymer.

The chemisorption of sulfur dioxide (SO2) on the Hofmann-like spin crossover porous coordination polymer (SCO-PCP) {Fe(pz)[Pt(CN)4]} has been investigated at room temperature. Thermal analysis and adsorption-desorption isotherms showed that ca. 1 mol of SO2 per mol of {Fe(pz)[Pt(CN)4]} was retained in the pores. Nevertheless, the SO2 was loosely attached to the walls of the host network and completely released in 24 h at 298 K. Single crystals of {Fe(pz)[Pt(CN)4]}·nSO2 (n ≈ 0.25) were grown in water solutions saturated with SO2, and its crystal structure was analyzed at 120 K. The SO2 molecule is coordinated to the Pt(II) ion through the sulfur atom ion, Pt-S = 2.585(4) Å. This coordination slightly stabilizes the low-spin state of the Fe(II) ions shifting the critical temperatures of the spin transition by 8-12 K. DFT calculations have been performed to rationalize these observations.