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In the transition to a clean-energy future, CO₂ separations will play a critical role in mitigating current greenhouse gas emissions and facilitating conversion to cleaner-burning and renewable fuels. New materials with high selectivities for CO₂ adsorption, large CO₂ removal capacities, and low regeneration energies are needed to achieve these separations efficiently at scale. Here, we present a detailed investigation of nine diamine-appended variants of the metal–organic framework Mg₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) that feature step-shaped CO₂ adsorption isotherms resulting from cooperative and reversible insertion of CO₂ into metal–amine bonds to form ammonium carbamate chains. Small modifications to the diamine structure are found to shift the threshold pressure for cooperative CO₂ adsorption by over 4 orders of magnitude at a given temperature, and the observed …