Programming local confinements in crystalline frameworks through reticular chemistry

Controlling local chemical environments within porous crystalline materials is essential for selective adsorption and catalysis, yet remains difficult in stable frameworks with precisely oriented functional sites. Here we use reticular chemistry to programme tunable confinement in triazolate metal-organic frameworks constructed from Kuratowski-type Zn₅Cl₄ nodes. Linker geometry directs the formation of the ith-d topology, in which terminal Zn-bound groups point inwards to generate confined and chemically addressable pores. This strategy yields two isoreticular frameworks, NU-6000 and NU-6001, with distinct cage dimensions and apertures, while preserving the same topology. Post-synthetic chloride-to-hydroxide exchange installs dense arrays of inward-facing Zn-OH groups without loss of crystallinity, enabling reversible CO₂ chemisorption through bicarbonate formation. Single-crystal analysis of a CO₂ adduct reveals that confinement imposes a geometric accessibility limit on reactive hydroxyl sites within the smallest cage of NU-6000. Under this confinement regime, NU-6000 exhibits strong low-pressure CO₂ capture, including at 30 ppm, and achieves 61.4% site utilization at 420 ppm, among the highest reported for metal-organic frameworks under comparable conditions.