Effect of Spin‐Crossover‐Induced Pore Contraction on CO₂–Host Interactions in the Porous Coordination Polymers [Fe(pyrazine)M(CN)₄] (M = Ni, Pt)

Abstract

Abstract Variable‐temperature in situ ATR‐FTIR spectra are presented for the porous spin‐crossover compounds [Fe(pyrazine)Ni(CN) 4 ] and [Fe(pyrazine)Pt(CN) 4 ] under CO 2 pressures of up to 8 bar. Significant shifts in the ν 3 and ν 2 IR absorption bands of adsorbed CO 2 are observed as the host materials undergo transition between low‐ and high‐spin states. Computational models used to determine the packing arrangement of CO 2 within the pore structures show a preferred orientation of one of the adsorbed CO 2 molecules with close O=C=O ··· H contacts with the pyrazine pillar ligands. The interaction is a consequence of the commensurate distance of the inter‐pyrazine separations and the length of the CO 2 molecule, which allows the adsorbed CO 2 to effectively bridge the pyrazine pillars in the structure. The models were used to assign the distinct shifts in the IR absorption bands of the adsorbed CO 2 that arise from changes in the O=C=O ··· H contacts that strengthen and weaken in correlation with changes in the Fe–N bond lengths as the spin state of Fe changes. The results indicate that spin‐crossover compounds can function as a unique type of flexible sorbent in which the pore contractions associated with spin transition can affect the strength of CO 2 –host interactions.