Primary, secondary (1°,2°) alkylethylenediamine- and alkylpropylenediamine-appended variants of metal-organic framework are provided for CO2 capture applications. Increasing the size of the alkyl group on the secondary amine enhances the stability to diamine volatilization from the metal sites. Two-step adsorption/desorption profiles are overcome by minimzing steric interactions between adjacent ammonium carbamate chains. For instance, the isoreticularly expanded framework Mg2(dotpdc) (dotpdc4−=4,4″-dioxido-[1,1′:4′,1″-terphenyl]-3,3″-dicarboxylate), yields diamine-appended adsorbents displaying a single CO2 adsorption step. Further, use of the isomeric framework Mg-IRMOF-74-II or Mg2(pc-dobpdc) (pc-dobpdc4−=3,3-dioxidobiphenyl-4,4-dicarboxylate, pc=para-carboxylate) also leads to a single CO2 adsorption step with bulky diamines. By relieving steric interactions between adjacent ammonium carbamate chains, these frameworks enable step-shaped CO2 adsorption, decreased water co-adsorption, and increased stability to diamine loss. Variants of Mg2(dotpdc) and Mg2(pc-dobpdc) functionalized with large diamines such as N-(n-heptyl)ethylenediamine have utility as adsorbents for carbon capture applications.