CO 2 ‐Induced Spin‐State Switching at Room Temperature in a Monomeric Cobalt(II) Complex with the Porous Nature

CO 2 ‐responsive spin‐state conversion between high‐spin (HS) and low‐spin (LS) states at room temperature was achieved in a monomeric cobalt(II) complex. A neutral cobalt(II) complex, [Co II (COO‐terpy) 2 ]⋅4 H 2 O ( 1⋅4 H 2 O ), stably formed cavities generated via π–π stacking motifs and hydrogen bond networks, resulting in the accommodation of four water molecules. Crystalline 1⋅4 H 2 O transformed to solvent‐free 1 without loss of porosity by heating to 420 K. Compound 1 exhibited a selective CO 2 adsorption via a gate‐open type of the structural modification. Furthermore, the HS/LS transition temperature ( T 1/2 ) was able to be tuned by the CO 2 pressure over a wide temperature range. Unlike 1 exhibits the HS state at 290 K, the CO 2 ‐accomodated form 1⊃CO 2 ( PCO2=110 kPa) was stabilized in the LS state at 290 K, probably caused by a chemical pressure effect by CO 2 accommodation, which provides reversible spin‐state conversion by introducing/evacuating CO 2 gas into/from 1 .