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

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.