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 Co 2+ 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 Co 2+ centers at different temperatures. Because of the Jahn-Teller effect of the Co 2+ centers, the C 2 -Co-N 4 and C-Co-N 4 combinations undergo different tetragonal elongation distortions along the C 4 axis, characterized by the relative elongation ÁZ and displacement ÁZ 0 of Co 2+ 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.