Understanding dioxygen activation by manganese complexes using mass spectrometry – ManOx

The characterization of Mn-O2 adducts and the determination of their kinetic and thermodynamic parameters will be carried out by ion-molecule reaction or (photo)dissociation in the gas phase using tandem mass spectrometry techniques. Advanced calculations will be carried out in parallel of this project, from the study of the main manganese intermediates to the study of the different mechanistic pathways of dioxygen activation. Using such approach, combining and comparing information obtained from experimental gas-phase studies and theoretical calculations, we will gain a good understanding of the key factors involved in the activation of O2 by manganese complexes used in solution.

By combining synthetic experiments with stoichiometric/substoichiometric amounts of endoperoxides as 18O2 precursors, mass spectrometry and DFT theoretical studies, we were able to demonstrate in this work that the mechanism underlying the oxidative coupling reaction of Ar-Li derivatives involving manganese in the presence of air is considerably more complex than previously assumed, and that more than one molecule of O2 per manganese is required to complete the reaction.

Negative-mode mass spectrometry studies enabled us to produce and identify an ion of interest, [MnPh3]-, and to demonstrate its possible involvement in the mechanism. Using a tandem MS/MS setup, an ion-molecule reaction with O2 yielded the expected biaryl product via a coupling reaction. To our knowledge, this is the first time such a transformation has been carried out in the gas phase, and further work along these lines is in progress.

Upon exposure to O2, a novel iron complex [FeIIL]3(BF4)3 possessing a bis(2-pyridylmethyl)amine/thiolate ligand enabled the generation of a μ-oxo di-iron(III) complex, which subsequently evolves into an adduct featuring an original {Fe4OF5}3+ core. In addition, the use of mass spectrometry enabled the identification of a highly unstable μ-oxo, μ-peroxo diiron(III) species, relevant in the oxygen activation step.

However, the generation of this intermediate species directly in the gas phase using ion-molecule reactions, as well as its isolation in solution or solid form, were unsuccessful. This iron complex also showed selective catalytic activity for the generation of water in the oxygen reduction reaction. Future modifications of the ligand are envisaged to improve its stability.

The creation of a strong network of collaborators in inorganic chemistry, homogeneous catalysis, and theory enables us to continue our research into the activation of oxygen by synthetic complexes of the first row of transition metals (Mn, Fe, Co). It would be interesting to modify the reactivity of the metal by changing the ligand in order to improve the stability of the complex. This would make it easier to characterize the complex and assess its reactivity. It would also be interesting to study complexes that have shown good catalytic results in oxygen activation. Indeed, efficient catalysts are difficult to characterize due to rapid reaction kinetics, but are particularly well suited to ion-molecule reactions.

The results obtained in two years are extremely promising, and the study of reaction mechanisms will not stop at oxygen activation. The mass spectrometric study of the activation of small molecules such as H2 for energy production, or CO2 for the environment, will also be studied in the near future, in order to respond to society's challenges.

Although many biological processes involve manganese to selectively activate dioxygen, the complexity of this process makes the development of effective synthetic catalysts extremely challenging. It is therefore crucial to understand in detail the role of the metal ion and of the mechanism at a molecular level, particularly through the characterization of intermediate species.
The challenging objective of this proposal is the generation and identification by mass spectrometry of manganese-oxygen species, generated by reacting Mn(II) precursors with O2, in order to understand the mechanistic detail of how O2 activation by manganese occurs and to investigate the connections between different “Mn-O2” intermediates. The characterization of Mn-O2 adducts and the determination of their kinetic and thermodynamic parameters will be carried out by ion-molecule reactions or (photo)dissociation in the gas phase using tandem mass spectrometry techniques.
The first part of the project will focus on the homo- and heterocoupling reaction of aryl lithium or aryl Grignard reagents catalyzed by manganese salts in the presence of dioxygen. The objective is to understand the involvement of O2 in the catalytic activity of manganese complexes by characterizing the reaction intermediates in the gas phase using ion-molecule reaction and obtaining accurate kinetic data. In a second phase, our efforts will aim to elucidate the mechanistic detail of oxygen activation by manganese biomimetic complexes. The last part of the project will concern the determination of the thermochemical properties of O-O or Mn-O bonds by measuring the bond dissociation energy (BDE) and the analysis of the “Mn-O2” compounds by infrared spectroscopy integrated with mass spectrometry (IRMPD). The main intermediates of manganese, the study of the different mechanistic pathways of oxygen activation, will be studied in parallel by theoretical calculations.
Using such complementary approach, combining and comparing information obtained from experimental gas-phase studies and theoretical calculations, we will gain a good understanding of the key factors involved in the activation of O2 by manganese complexes used in solution, with the ultimate goal of proposing new complexes for the development of optimal catalytic systems.