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A Five-Coordinate Heme Dioxygen Adduct Isolated within a Metal–Organic Framework

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journal contribution
posted on 26.11.2014 by John S. Anderson, Audrey T. Gallagher, Jarad A. Mason, T. David Harris
The porphyrinic metal–organic framework (MOF) PCN-224 is metalated with FeII to yield a 4-coordinate ferrous heme-containing compound. The heme center binds O2 at −78 °C to give a 5-coordinate heme-O2 complex. For the first time, this elusive species is structurally characterized, revealing an FeIII center coordinated to superoxide via an end-on, η1 linkage. Mössbauer spectroscopy supports the structural observations and indicates the presence of a low-spin electronic configuration for FeIII. Finally, variable-temperature O2 adsorption data enable quantification of the Fe–O2 interaction, providing a binding enthalpy of −34(4) kJ/mol. This value is nearly half of that observed for comparable 6-coordinate, imidazole-bound heme-O2 complexes, a difference that further illustrates the importance of axial ligands in biological heme-mediated O2 transport and storage. These results demonstrate the ability of a MOF, by virtue of its rigid solid-state structure, to enable isolation and thorough characterization of a species that can only be observed transiently in molecular form.