Considerations on Gated CO₂ Adsorption Behavior in One-Dimensional Porous Coordination Polymers Based on...

Low-dimensional coordination polymers such as one-dimensional chains often exhibit gated guest sorption accompanying structural transition at a temperature (T_G), which is associated with an external pressure of the guest (P_G) characteristic to the material and guest used. This phenomenon can be evaluated using the Clausius-Clapeyron relationship with the equation d(ln P_G)/d(1/T_G) = ΔH_G/R, where ΔH_G and R are the transition enthalpy and gas constant, respectively. In this study, gated CO₂ adsorption behavior was investigated in a one-dimensional chain based on a benzoate-bridged paddlewheel diruthenium(II,II) complex with a phenazine (phz) linker, [Ru₂(p-MeOPhCO₂)₄(phz)] (1; p-MeOPhCO₂^- = p-anisate). Surprisingly, 1 underwent gate opening (GO)/closing (GC) at a much higher T_G, e.g., 385 K for GC, under P_CO = 100 kPa than those previously reported for such chain compounds, which usually appeared in the temperature range of 200-270 K. The transition entropy ΔS_G in each system plays a key role in shifting T_G; 1 results in a much smaller |ΔS_G| in the series. Only 1 produced a CO₂-accessible two-dimensional topological pore in its CO₂-adsorbed phase 1⊃CO, whereas the others reported previously produced one-dimensional or discrete topological pores for CO₂ accommodation, strongly reflecting the degree of freedom of CO₂ molecules in pores, which is related to ΔS_G.