A structurally-characterized peroxomanganese(iv) porphyrin from reversible O2 binding within a metal–organic framework

摘要

The role of peroxometal species as reactive intermediates in myriad biological processes has motivated the synthesis and study of analogous molecular model complexes. Peroxomanganese( IV ) porphyrin complexes are of particular interest, owing to their potential ability to form from reversible O _(2) binding, yet have been exceedingly difficult to isolate and characterize in molecular form. Alternatively, immobilization of metalloporphyrin sites within a metal–organic framework (MOF) can enable the study of interactions between low-coordinate metal centers and gaseous substrates, without interference from bimolecular reactions and axial ligation by solvent molecules. Here, we employ this approach to isolate the first rigorously four-coordinate manganese( II ) porphyrin complex and examine its reactivity with O _(2) using infrared spectroscopy, single-crystal X-ray diffraction, EPR spectroscopy, and O _(2) adsorption analysis. X-ray diffraction experiments reveal for the first time a peroxomanganese( IV ) porphyrin species, which exhibits a side-on, η ~(2) binding mode. Infrared and EPR spectroscopic data confirm the formulation of a peroxomanganese( IV ) electronic structure, and show that O _(2) binding is reversible at ambient temperature, in contrast to what has been observed in molecular form. Finally, O _(2) gas adsorption measurements are employed to quantify the enthalpy of O _(2) binding as h _(ads) = ?49.6(8) kJ mol ~(?1) . This enthalpy is considerably higher than in the corresponding Fe- and Co-based MOFs, and is found to increase with increasing reductive capacity of the M ~(II/III) redox couple.