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

Abstract

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.

The dioxygen adducts of iron and manganese porphyrins: electronic structure and binding energy

“…This extensive list is expected to contain all quartet and sextet states previously investigated. [22][23][24][25][26][27][28]30 The electronic configurations of all states considered can be found in Figures S1-S5.…”

“…However, the results of previous theoretical studies quite often conflicted with experimental observations. 17,19 To the best of our knowledge, theoretical studies on MnP−O 2 have only been performed with DFT and single-reference wave function methods, except a very recent work of Kappes et al 22 The earliest work was done by Dedieu and Rohmer, 23 in which the authors employed Hartree-Fock to study 14 probable electronic configurations of both side-on (Griffiths model) and end-on (Pauling model) geometries ( Figure 1).…”

“…28 In some of these works, 28,30 the binding energies of O 2 to manganese porphyrins were also estimated, however, the calculated binding energies were usually too low by ∼4-6 kcal mol −1 as compared to the gas phase data. 21 During preparation of this manuscript a report by Kappes et al 22 was published, studying the electronic structures and binding energies of MP−O 2 (M = Fe, Mn) using CASPT2. Employing very limited and unbalanced active spaces, as well as small basis sets, the authors found a quartet GS for MnP and a quartet end-on GS for MnP−O 2 .…”

Heme ligation in the gas phase

“…The reaction rate of [PL] + formation can vary between values given by Langevin cross sections [20] or 1 -2 orders of magnitude less [21]; however a thermodynamic equilibrium between [P + ]+[L] and [PL] + can be established, depending on the forward association frequency k[L] (k is a gas kinetic constant) and the dissociation rate kdiss. Note that with kdiss described by RRKM (Rice-Ramsperger-Kassel-Marcus), the back reaction will be insomuch faster that the binding energy of [PL] + is small.…”

The dramatic effect of N-methylimidazole on trans axial ligand binding to ferric heme: experiment and theory

“…While solution measurements hardly access the observation of 5C hemes because of the blocking ligation to H2O (or the solvent), only gas phase measurements [7][8][9][10][11][12] render the sole or sequential addition of ligands in axial position to heme rather simple, by physically separating the ligation events and avoiding the spurious ligation by H2O (or the solvent). Following our recent studies on the binding of small ligands to ferric heme 11 and protonated heme 12 in a cold, temperature controlled, ion trap, we herein use the same approach with the presence of N- Indeed, the Fe III atom in the high spin state S=5/2 inflates due to electron repulsion and pops out of the porphyrin cage.…”