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

CO -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 (COO-terpy) ]⋅4 H O (1⋅4 H 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 O transformed to solvent-free 1 without loss of porosity by heating to 420 K. Compound 1 exhibited a selective CO adsorption via a gate-open type of the structural modification. Furthermore, the HS/LS transition temperature (T ) was able to be tuned by the CO pressure over a wide temperature range. Unlike 1 exhibits the HS state at 290 K, the CO -accomodated form 1⊃CO (P =110 kPa) was stabilized in the LS state at 290 K, probably caused by a chemical pressure effect by CO accommodation, which provides reversible spin-state conversion by introducing/evacuating CO gas into/from 1.