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.

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 to the Fe or Mn centers of isolated tetra(4-sulfonatophenyl)metalloporphyrin tetraanions. In contrast, Fe and Mn trianion homologues were found to be much less reactive-indicating an upper bound to their dioxygen binding energies of 34 kJ mol. We modeled the corresponding O adsorbates at the density functional theory and CASPT2 levels. These quantum-chemical calculations verified the stronger O binding on the Fe or Mn centers and suggested that O binds as a superoxide anion.