CO Binding at a Four-Coordinate Cobaltous Porphyrin Site in a Metal–Organic Framework: Structural, EPR, and Gas Adsorption Analysis

The variable-temperature
CO binding at a four-coordinate cobaltous porphyrin complex within
a metal–organic framework is investigated using a combined
array of infrared, X-ray diffraction, EPR, and CO adsorption analysis.
Single-crystal X-ray diffraction experiments provide the first crystallographically
characterized example of a noniron first-row transition metal porphyrin
carbonyl adduct. Variable-temperature electron paramagnetic resonance
spectroscopy supports the structural observation and reveals conversion
of the dicarbonyl complex to a monocarbonyl species as temperature
is increased. Finally, CO adsorption analysis data enable quantification
of the Co–CO interaction, providing a binding enthalpy of hads = −29(2) kJ/mol. This value is nearly
twice that observed for O2 binding in the same compound
and is attributed to a stronger σ interaction for Co and CO
vs O2. These results demonstrate the ability of MOFs to
enable a thorough investigation of CO binding in metalloporphyrins.