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Lanthanides as electron Dimmer switch in organometallic catalysis

Periodic Reporting for period 3 - LanAsCat (Lanthanides as electron Dimmer switch in organometallic catalysis)

Reporting period: 2020-04-01 to 2021-09-30

Complexes containing redox non-innocent ligands have been well developed in the last decade with transition metal ions and have led to very important chemical transformations at lowest environmental and economic costs. Nonetheless examples with f-element are very rare and the field is almost empty with lanthanides. This is unfortunate since divalent lanthanides are excellent sources of single electron and would provide a good control of the ligand reduction because of strong electron correlation in these systems. Thus, this proposal aims at developing this field with organolanthanides. The synthesis of original complexes containing lanthanides, redox non-innocent ligands and transitions metals is herein proposed: the first providing reversible electron(s) source(s) (remote control), the second acting as electron(s) reservoir and controlling the electron correlation strength (“dimmer switch”), and the last being the site of the selective catalytic reaction. Because of this regulated electron transfer, the oxidation state of the transition metal will be modified only at specific steps, allowing the establishment of a new paradigm is organometallic catalysis with group 9 and 10 transition metals. These original complexes will be synthetized and deeply characterized by specific spectroscopic and theoretical analyses. The principal goal is to synthetize active catalysts toward C-H bonds activation, and methane activation is regarded as an ultimate achievement as is hydroalkylation of olefins. To increase the chances of success, the proposal is based on preliminary results obtained recently in the group. Several examples of original heterobimetallic and heterotrimetallic complexes containing lanthanides and transition metals of group 10 will be discussed as a good starting point for the feasibility of this challenging project that aims at answering a large societal concern: the reduction of atmospheric pollutants, such as methane, and transformation in valuable products.
The objective of this project is to synthesize heterometallic complexes combining an organolanthanide fragment, a redox non-innocent ligand and a reactive transition metal fragment. The final goal is C-H bonds activation and olefins transformation. Since the beginning of the project, we have been able to synthesize such challenging heterometallic complexes with divalent ytterbium, samarium and thulium fragments, six different redox non-innocent ligands and group 10 transition metals (Ni, Pd, Pt). All synthetic details have been investigated and the adapted conditions for their synthesis have been well worked out. All the complexes have been characterized with routine characterization means, such as 1H NMR of paramagnetic compounds, UV-Visible absorption, X-ray diffraction studies as well as specific experimental characterization such as magnetism and Electron Paramagnetic Resonance (EPR) spectroscopy. Several complexes were also studied by theoretical computations at different levels. Methods for electronic density analysis have been used. All these experimental and theoretical methods have allowed specifying the electronic structure of these complexes and showed that the divalent lanthanide fragments, the redox non-innocent ligands as well as the transition metal fragments are all responsible for modification the nature of the ground and first excited states, so that it is possible to tune them at will, a rare opportunity in organometallic chemistry. Stoichiometric reactions have been performed on these heterometallic complexes with oxidants and small pollutant molecules (CO). Important results have been obtained such as the isolation of high-valent palladium and platinum species, and insertion of carbon monoxide in Ni-C bonds and alkene isomerization with simple nickel complexes while the Pd(IV) chemistry has expended. Additionally, very interesting magnetic properties have been observed for low-valent complexes of thulium and outstanding reactivity of similar low-valent thulium complexes have been discovered with CO. 8 articles including one in the Journal of American Chemical Society and one in Angewandte Chemie have been published so far and several other are submitted or in preparation. Over the period, the team has been involved in 40 disseminations in many different countries, for the scientific community (participation to conferences, invitation in workshop or in seminars) but also the general public (article, video, general public conference).
The heterometallic compounds proposed within this project are original in the sense that they combine very different metal types bridged by a redox non-innocent ligand. The electron transfer that occurs leads to an unusual situation where a substantial amount of spin density is delocalized in a large extent over the molecule. It has led to the easier oxidative addition of Pd(II) to Pd(IV). In particular, where a typical Pd(IV) species starts to eliminate before the end of its formation, we have been able to make its formation fast enough so that the synthesis of Pd(IV)-trisalkyl species is possible. We continued in this direction with Pd and Pt species and have worked on the Ni species as well with the development of reactivity with CO and alkenes. Particularly, we have been able to push further the Pd(IV) chemistry and developed a new area of stoichiometric and possibly catalytic reactions from Pd(II)-Pd(IV) redox system. Additionally, we have developed new lanthanide fragments with very different redox potentials and geometry, allowing considering very different situations for the spin density localization and subsequent unusual reactivity. These new complexes have opened an interesting scope for the study of their magnetism. Small pollutant molecules such as CO have been used and transformed. Work in direction of alkenes substitution and C-H activation of arenes and alcanes, such as methane, are in still progress with catalytic developments. Concerning the group 9, preliminary results are already promising for Ir chemistry, notably for the formation of unusual oxidation states and concerning the new original development with the photo-activation of the complexes. Unusual reactivity with alkanes and olefins are envisaged. Additionally, heterometallic complexes with Co and bipyrimidine have been considered and are under study with the objective of transformation of pollutants, such as methane, CO2 and CO.
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