Programming Local Confinements in Crystalline Frameworks through Reticular Chemistry

Controlling chemical microenvironments within porous crystalline materials is central to advancing selective adsorption, separation, and catalytic processes, yet remains difficult to achieve in stable frameworks with precisely oriented functional sites. Here, we leverage reticular chemistry to program tunable confinement in triazolate metal–organic frameworks (MOFs) built from Kuratowski-type Zn5Cl4 nodes. Rational modulation of linker geometry targets the ith-d topology in which terminal Zn-bound groups point inward to define confined, chemically addressable pores. This design yields two isoreticular frameworks, NU-6000 and NU-6001, that preserve the overall topology while reconfiguring cage dimensions, apertures, and pore functionality through linker inversion. The frameworks are structurally well-defined and thermally stable beyond 525 °C, and post-synthetic chloride-to-hydroxide exchange installs dense, oriented Zn-OH arrays without loss of crystallinity, enabling strong yet reversible CO2 binding through bicarbonate formation. Single-crystal analysis of a CO2 adduct reveals a geometric accessibility rule in the smallest cage of NU-6000, where only a subset of inward-facing hydroxyls can bind to form bicarbonate, thereby setting an intrinsic upper bound to uptake that is dictated by cage architecture rather than linker count. Under this tunable local-confinement regime in NU-6000, the framework achieves high CO2 uptake across a broad low-pressure range, including at concentrations as low as 30 ppm, and attains a site utilization of 61.4 % at 420 ppm, representing the highest efficiency reported for MOFs under comparable conditions. This work establishes a generalizable approach for encoding functional confinement into robust crystalline frameworks, bridging molecular design with solid-state functionality for selective gas capture and other confinement-driven applications.