Low Heat of Adsorption of Ethylene Achieved by Major Solid-State Structural Rearrangement of a Discrete Copper(I) Complex

The trinuclear copper(I) pyrazolate complex [Cu₃] rearranges to the dinuclear analogue [Cu₂ (C₂H₄)₂] when exposed to ethylene gas. Remarkably, the [Cu₃]↔[Cu₂ (C₂H₄)₂ ] rearrangement occurs reversibly in the solid state. Furthermore, this transformation emulates solution chemistry. The bond-making and breaking processes associated with the rearrangement in the solid-state result in an observed heat of adsorption (-13±1 kJ mol^-1 per Cu-C₂H₄ interaction) significantly lower than other Cu-C₂H₄ interactions (≥-24 kJ mol^-1). The low overall heat of adsorption, "step" isotherms, high ethylene capacity (2.76 mmol g^-1 ; 7.6 wt % at 293 K), and high ethylene/ethane selectivity (136:1 at 293 K) make [Cu₃] an interesting basis for the rational design of materials for low-energy ethylene/ethane separations.