Electronic and steric control of phase changes caused by methylamine insertion into copper paddlewheel metal-organic...

Gas adsorption through phase changes, in which guest molecules induce reversible structural transformations in crystalline porous solids, offers a powerful yet underdeveloped strategy for selective molecular capture. Metal-organic frameworks (MOFs) containing copper paddlewheel nodes are among the few systems known to undergo such transformations, but the structural factors governing guest insertion, adsorption capacity, formed phases, and reversibility remain poorly defined. Herein, we establish how linker basicity and sterics affect phase changes facilitated by methylamine insertion across three copper-carboxylate frameworks: Cu(bpdc) (bpdc^2- = biphenyl-4,4'-dicarboxylate), Cu(fumarate), and Cu(F₄bdc) (F₄bdc^2- = 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylate). Gas adsorption, spectroscopy, and diffraction analyses reveal that linker basicity is the primary descriptor dictating the extent of mono- and dimethylamine insertion, the stability of the resulting coordination solid, and the ability to regenerate the parent framework, while linker length plays a secondary role. These trends uncover an intrinsic tradeoff between adsorption capacity and cyclability that arises from the strength of the copper-carboxylate bond. Guided by these structure-reactivity relationships, we identify Cu(bpdc) as a balanced adsorbent that combines moderate basicity with a long and flexible linker, enabling a two-step temperature vacuum swing adsorptive separation of monomethylamine, dimethylamine, and trimethylamine at 25 °C in under 3 h. This work provides the first general framework for predicting and controlling phase-changing amine insertion in copper paddlewheel MOFs and establishes design principles for adsorbents that exploit coordination chemistry to achieve selective gas separations.