Considerations on Gated CO 2 Adsorption Behavior in One-Dimensional Porous Coordination Polymers Based on Paddlewheel-Type Dimetal Complexes: What Determines Gate-Opening Temperatures?

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 Δ H _G and R are the transition enthalpy and gas constant, respectively. In this study, gated CO ₂ adsorption behavior was investigated in a one-dimensional chain based on a benzoate-bridged paddlewheel diruthenium(II,II) complex with a phenazine (phz) linker, [Ru ₂ ( p -MeOPhCO ₂ ) ₄ (phz)] ( 1 ; p -MeOPhCO ₂ ^- = 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 2} = 100 kPa than those previously reported for such chain compounds, which usually appeared in the temperature range of 200-270 K. The transition entropy Δ S _G in each system plays a key role in shifting T _G ; 1 results in a much smaller |Δ S _G | in the series. Only 1 produced a CO ₂ -accessible two-dimensional topological pore in its CO ₂ -adsorbed phase 1⊃CO ₂ , whereas the others reported previously produced one-dimensional or discrete topological pores for CO ₂ accommodation, strongly reflecting the degree of freedom of CO ₂ molecules in pores, which is related to Δ S _G .