Hydroxamate Linkage Transformation in Porphyrin Functionalized Metal–Organic Frameworks

This study reports the discovery of five porphyrin-based metal-organic frameworks (MOFs) constructed from carboxylate, amide, and hydroxamate linkages. The MOFs, named the MOF-610 series (MOF-610-Co, MOF-610-Fe-1, and MOF-610-Fe-2), MOF-612, and MOF-614, were synthesized by implementing a high-throughput screening approach applying the porphyrin-based hydroxamic acid linker, 4,4',4″,4‴-(porphine-5,10,15,20-tetrayl)tetrakis(benzhydroxamic acid) (H₆PTBHA), in combination with an array of metal ions. After their synthesis, both the MOF-610 series and MOF-612 revealed in situ transformation of the hydroxamic acid linker, resulting in carboxylate or amide coordination species, while MOF-614 was able to retain the hydroxamate moiety. The chemical composition and structural configuration of the MOFs were characterized using a combination of techniques, including elemental analysis, high-resolution mass spectrometry, solid-state nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, single crystal X-ray diffraction, and microcrystal electron diffraction. Adsorption properties were evaluated through nitrogen, water, carbon dioxide, and hydrogen isotherms, providing a correlation between the MOF structures and their applications. MOF-614 demonstrated a BET surface area of 701 m² g^-1 and notable sorption capacities: water uptake initiating at 10% relative humidity and reaching 17.9 mmol g^-1 (0.32 g g^-1) at saturation, CO₂ adsorption of 2.41 mmol g^-1 at 1 bar, and hydrogen uptake of 6.16 mmol g^-1 (1.23 wt %) at 1 bar.