Dioxygen adducts of molecular metalloporphyrin compexes have garnered interest for decades, owing to their relevance as models of proteins that carry out biological processes such as oxygen transport and catalytic oxidation. Studies of molecular metalloprophyrin adducts, have been limited by the propensity of these complexes to undergo deleterious bimolecular condensation reactions to form kinetically inert oxo-bridged dinuclear species. To overcome the challenges associated with studying dioxygen adducts of molecular metalloporphyrin complexes, we have sought to stabilize these species in metal-organic frameworks (MOFs). Utilizing porphyrin-based framework, PCN-224, we have isolated a rare 5-coordinate cobalt and iron porphyrin dioxygen adduct at low temperature, both of which have previously eluded structural and spectroscopic characterization in the molecular form. In addition, we were able to ascertain key information regarding the electronic structure of these rare species and have examined the thermodynamics of O2 binding to the metal centers. From this study, we have discovered that the strength of O2 binding to four coordinate, metalloporphyrin centers is considerably lower than what has been observe for substituted proteins and model complexes that feature axial ligands, further highlighting the importance of axial ligation in biological 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 have only be observed transiently in molecular form.

Isolation of metalloporphyrin dioxygen adducts in metal-organic frameworks

253rd National Meeting (2017)

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