Engineering Water Adsorbing Fumarate‐Based Mixed‐Metal Trimeric Oxo‐Cluster MOFs for Slow Release of Micronutrients

Although iron‐based metal–organic frameworks (MOFs) have been investigated as controlled‐release fertilizers, mixed‐metal (Fe‐M) MOFs remain largely unexplored as platforms for the controlled, simultaneous delivery of multiple micronutrients. This study presents the rational design of biosourced mixed‐metal cluster‐based MOFs, Fe 2 M‐fumarate (M = Cu, Zn, Mn), and isostructural with MIL‐88A (Fe). The frameworks serve a dual purpose: as degradable micronutrient fertilizers that enable controlled metal release through the slow breakdown of secondary building units and as reservoirs for atmospheric moisture due to their water‐adsorption characteristics. Physicochemical characterization confirmed the successful formation of homogeneous mixed‐metal clusters, yielding spindle‐shaped crystallites with distinct porosity profiles. Notably, the Fe 2 Cu–MIL‐88A analogue exhibited a unique “breathing” behavior, achieving a remarkable water vapor uptake of ~630 mg g −1 , significantly outperforming the Zn and Mn analogues. In contrast to the rapid dissolution characteristic of conventional salts, in vitro release assays revealed a biphasic profile, with a gradual initial release of Cu, Zn, or Mn, followed by a more sustained release of Fe, driven by hydrolytic matrix degradation. Kinetic modeling indicated that the Weibull model ( R 2 ≤ 0.98) best describes this complex mechanism, which combines erosion and diffusion. The release profiles can be tuned by varying the secondary metal within the trimeric cluster, illustrating the versatility of this approach. These findings position Fe 2 M–fumarate nanorods as a promising platform for moisture‐responsive, sustained‐release micronutrient delivery in agricultural environments.