Investigations of the temperature-dependent electron paramagnetic resonance spectra and local structures for a cobalt(II) porphyrin complex within a metal–organic framework

Abstract

The interaction between an adsorbed CO molecule and the unsaturated coordinated Co2+ center in the metal–organic framework (MOF) PCN-224 is investigated by analyzing the electron paramagnetic resonance (EPR) parameters (g factors and hyperfine structure constants) and the adsorption energies at various temperatures. Six- and five-coordinated octahedral models (four planar N with two and one axial CO molecules, respectively) are constructed to simulate the local structures of the Co2+ centers at different temperatures. Because of the Jahn–Teller effect of the Co2+ centers, the C2–Co–N4 and C–Co–N4 combinations undergo different tetragonal elongation distortions along the C 4 axis, characterized by the relative elongation ΔZ and displacement ΔZ′ of Co2+ at different temperatures. Given the agreement between the calculated and experimental EPR parameters, as well as the adsorption properties, the six- and five-coordinated models are regarded as suitable for low- and high-temperature systems, respectively. These studies may be helpful to understand the properties of similar MOFs with adsorbed molecules under the effect of ambient temperature.