Considerations on Gated CO2 Adsorption Behavior in One-Dimensional Porous Coordination Polymers Based on Paddlewheel-Type Dimetal Complexes: What Determines Gate-Opening Temperatures?

Abstract

Low-dimensional coordination polymers such as one-dimensional chains often exhibit gated guest sorption accompanying structural transition at a temperature (TG), which is associated with an external pressure of the guest (PG) characteristic to the material and guest used. This phenomenon can be evaluated using the Clausius-Clapeyron relationship with the equation d(ln PG)/d(1/TG) = ΔHG/R, where ΔHG 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 TG, e.g., 385 K for GC, under PCO2 = 100 kPa than those previously reported for such chain compounds, which usually appeared in the temperature range of 200-270 K. The transition entropy ΔSG in each system plays a key role in shifting TG; 1 results in a much smaller |ΔSG| in the series. Only 1 produced a CO2-accessible two-dimensional topological pore in its CO2-adsorbed phase 1⊃CO2, 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 ΔSG.