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 <i>h</i>_ads = −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.