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Gas-Phase Ion Chemistry of Metalloporphyrin Anions with Molecular Oxygen: Probing the Influence of the Oxidation and Spin State of the Central Transition Metal by Experiment and Theory

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journal contribution
posted on 09.04.2018, 00:00 by Tatjana Archipov, Justin K. Kirkland, Konstantinos D. Vogiatzis, Annika Steiner, Gereon Niedner-Schatteburg, Patrick Weis, Karin Fink, Oliver Hampe, Manfred M. Kappes
We performed a comprehensive gas-phase experimental and quantum-chemical study of the binding properties of molecular oxygen to iron and manganese porphyrin anions. Temperature-dependent ion–molecule reaction kinetics as probed in a Fourier-transform ion-cyclotron resonance mass spectrometer reveal that molecular oxygen is bound by, respectively, 40.8 ± 1.4 and 67.4 ± 2.2 kJ mol–1 to the FeII or MnII centers of isolated tetra­(4-sulfonatophenyl)­metalloporphyrin tetraanions. In contrast, FeIII and MnIII trianion homologues were found to be much less reactiveindicating an upper bound to their dioxygen binding energies of 34 kJ mol–1. We modeled the corresponding O2 adsorbates at the density functional theory and CASPT2 levels. These quantum-chemical calculations verified the stronger O2 binding on the FeII or MnII centers and suggested that O2 binds as a superoxide anion.