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Fe(III) Protoporphyrin IX Encapsulated in a Zinc Metal–Organic Framework Shows Dramatically Enhanced Peroxidatic Activity

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journal contribution
posted on 08.01.2018 by Nicola A. Dare, Lee Brammer, Susan A. Bourne, Timothy J. Egan
Two MOFs, [H2N­(CH3)2]­[Zn3(TATB)2­(HCOO)]·HN­(CH3)2·DMF·6H2O (1) and Zn-HKUST-1 (2), were investigated as potential hosts to encapsulate Fe­(III) heme (Fe­(III) protoporphyrin IX = Fe­(III)­PPIX). Methyl orange (MO) adsorption was used as an initial model for substrate uptake. MOF 1 showed good adsorption of MO (10.3 ± 0.8 mg g–1) which could undergo in situ protonation upon exposure to aqueous HCl vapor. By contrast, MO uptake by 2 was much lower (2 ± 1 mg g–1), and PXRD indicated that structural instability on exposure to water was the likely cause. Two methods for Fe­(III)­PPIX-1 preparation were investigated: soaking and encapsulation. Encapsulation was verified by SEM-EDS and showed comparable concentrations of Fe­(III)­PPIX on exposed interior surfaces and on the original surface of fractured crystals. SEM-EDS results were consistent with ICP-OES data on bulk material (1.2 ± 0.1 mass % Fe). PXRD data showed that the framework in 1 was unchanged after encapsulation of Fe­(III)­PPIX. MO adsorption (5.8 ± 1.2 mg g–1) by Fe­(III)­PPIX-1 confirmed there is space for substrate diffusion into the framework, while the UV–vis spectrum of solubilized crystals confirmed that Fe­(III)­PPIX retained its integrity. A solid-state UV–vis spectrum of Fe­(III)­PPIX-1 indicated that Fe­(III)­PPIX was not in a μ-oxo dimeric form. Although single-crystal XRD data did not allow for full refinement of the encapsulated Fe­(III)­PPIX molecule owing to disorder of the metalloporphyrin, the Fe atom and pyrrole N atoms were located, enabling rigid-body modeling of the porphine core. Reaction of 2,2′-azino-bis­(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) with H2O2, catalyzed by Fe­(III)­PPIX-1 and -2, showed that Fe­(III)­PPIX-1 is significantly more efficient than Fe­(III)­PPIX-2 and is superior to solid Fe­(III)­PPIX-Cl. Fe­(III)­PPIX-1 was used to catalyze the oxidation of hydroquinone, thymol, benzyl alcohol, and phenyl ethanol by tert-butyl-hydroperoxide with t1/2 values that increase with increasing substrate molecular volume.