Electron Transfer Reactivity of Organolanthanides: Insights from Spectroscopy and Theory – ReDivaLan
Electronic transfers in organometallic complexes of lanthanides: varied geometries and original electronic structures.
Although reports of lanthanide coordination complexes are extensive and concern most of the modern coordination chemistry, materials chemistry, spectroscopy and biologically relevant chemistry; those on organometallic lanthanide complexes or organolanthanides are rarer. It is only quite recently that the amount of those molecules – in term of different elements and numbers of publications-, increased with the work of several research laboratories.
Organometallic compounds of lanthanides and reactivity of small molecules.
The project follows these objectives: i) First of all it is necessary to carry out the synthesis of so-called «classical« divalent lanthanide complexes, namely ytterbium, samarium and europium, but also those very reducing, called «non-classical«, such as thulium, and dysprosium with simple ligands borrowed from coordination chemistry and organometallic chemistry. The use of redox-active ligands is a main objective of the project. These compounds are difficult to synthesize as the redox potential of divalent lanthanides is low and because of the chemical binding model that promotes ionic interactions. Obtaining them is a first challenge whose repercussions in synthetic methodology are important. ii) Once the original objects are made, their spectroscopic studies by many tools is carried out. It should be noted that for most specific spectroscopies, the literature is almost blank, ensuring a certain impact on its work. iii) The analysis of these objects is reinforced by a theoretical study using quantum chemistry calculations at different levels with the objective of clarifying the appropriate methodology. In view of the recent literature, the electronic structures of organolanthanide compounds are not simple to explain and much more needs to be done in this area. iv) Finally, the reduction reactivity towards small molecules (O2, CO2, organic compounds) is studied for these well-characterized objects and rationalized through their electronic and geometrical nature. In the field of activation of small molecules, lanthanides allow rare transformations whose description has a good impact. Indeed, these studies relate to the transformation and recovery of greenhouse gas or industrial waste and is strongly encouraged by the societal context.
The methodology consists in synthesizing new divalent lanthanide complexes and therefore very reactive with simple ligands using synthetic methods typical of the inert atmosphere. Cyclopentadienyl ligands (Cp), cyclooctatrienyls (Cot), but also triflate ligands (OTf) or borohydride (BH4) are thus envisaged. Beyond this, the use of more original ligands, such as cyclononate tetraenyl (Cnt) must allow to achieve varied and original geometries. Routine characterization is performed using multiple spectroscopic tools such as X-ray diffraction, NMR (nuclear magnetic resonance, cyclic voltammetry, electron paramagnetic resonance), and magnetism studies. We aim to focus on the optical emission spectra of molecules, since this method allows to probe the nature of the crystal field splitting and to have precise information on the configuration of the element. Finally, the theoretical calculations are carried out at different levels of computations, DFT (density functional theory) and ab initio, with multi-reference methods (CASSCF). In fact, the multi-configurational nature of the different states can notably mask their degree of oxidation in the spectroscopic and non-formal sense. Reactivity with small molecules is followed by routine characterizations in organometallic chemistry, in particular the NMR of paramagnetic compounds and the X-ray diffraction for the characterization of the products formed. The theoretical DFT studies complete the experimental mechanistic studies to validate the mechanisms involved. These last ones make it possible to draw lessons on the new molecules to be made.
Several important results are to be taken into account for this project: i) firstly, the successful synthesis of thulium triflate, a rare, but nevertheless simple, compounds of divalent thulium. Thus, the first emission spectra for molecular thulium compounds have been realized. ii) Then a similar complex of triflate samarium demonstrated the total four-electron cleavage of molecular oxygen (O2), a first for lanthanides which are single electron reducers. iii) The methodology of this project has streamlined the design of the first divalent lanthanide complex with slow magnetic relaxation. (iv) Finally, the use of the cyclononate tetraenyl ligand (Cnt) has allowed access to rigorously linear sandwich-type complexes (metallocenes), and establishing the lanthanidocene series 50 years after uranocene, paving the way for numerous studies. on this new family. The project served as a springboard for an ERC Starting Grant for the coordinator.
The work that remains to be done concerning the synthesis of new divalent complexes will consist in synthesizing new complexes with a given symmetry and the least distorted possible to achieve a magneto-structural correlation. Further studies on the luminescence of divalent and trivalent ytterbium complexes are also underway.
Other original complexes are also being studied via theoretical studies of their electronic structure. In particular, the concept of the electronic reservoir (ie reversible electronic transfer) could be demonstrated with the Cp * 2Yb (phen) complexes with a TEMPO molecule. The work is being finalized.
Finally, in order to evaluate the observations made in the first preliminary results of this project, we conducted reactions with samarium diiodide in thf and benzophenone, a reaction known since the 1980s, but by isolating the various intermediates. Our studies showed that the CC bond formed to obtain the pinacolate form was reversible simply by changing the solvent and that the addition of phenanthroline or bipyridine allowed the direct coupling in the a-position of the nitrogen of the ketone on the N-heteroaromatic ring, this to form a very interesting new molecule. The prospects for this study are numerous in organic chemistry for the development of direct blends on N-hetero nitrogen cycles.
10. M. Xémard, S. Zimmer, M. Cordier, V. Goudy, L. Ricard, C. Clavaguéra, G. Nocton, J. Am. Chem. Soc. 2018, sous presse.
9. M. Xémard, M. Cordier, E. Louyriac, L. Maron, C. Clavaguéra, G. Nocton, Dalton Trans. 2018, 47, 9226-9230.
8. V. Goudy, M. Xémard, S. Karleskind, M. Cordier, C. Alvarez Lamsfus, L. Maron, G. Nocton, Inorganics, 2018, 6, 82. Open access.
7. B. Casanovas, S. Speed, O. Maury, M. S. El Fallah, M. Font-Bardía, R. Vicente Eur. J. Inorg. Chem. 2018, 3859–3867. DOI: 10.1002/ejic.201800624
6. F. Guégan, F. Riobé, O. Maury, J. Jung, B. Le Guennic, C. Morell, D. Luneau Inorg. Chem. front. 2018, 5, 1346-1353. DOI: 10.1039/C8QI00174J
5. M. Xémard, V. Goudy, A. Braun, M. Tricoire, M. Cordier, L. Ricard, L. Castro, E. Louyriac, C. Kefalidis, C. Clavaguéra, L. Maron, G. Nocton, Organometallics 2017, 36, 4660-4668.
4. V. Goudy, A. Jaoul, M. Cordier, C. Clavaguéra, G. Nocton, J. Am. Chem. Soc. 2017, 139, 10633-10636. Open access.
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2. M. Xémard, A. Jaoul, M. Cordier, F. Molton, O. Cador, B. Le Guennic, C. Duboc, O. Maury, C. Clavaguéra, G. Nocton, Angew. Chem. Int. Ed. 2017, 56, 4266-4271. Back Cover, highlighted dans CNRS News.
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In this proposal, we are discussing the opportunity of studying the electron transfer reactivity of divalent organolanthanides complexes. This type of complexes has known a growing attention in the last few years and few groups around the world contributed to the description of intriguing reactivity as well as singularities in electronic structures. The latter have been challenged by spectroscopic observations that are not in agreement with the traditional believe of pure electrostatic bonds but are also far from a classical covalent model. A better understanding of the overall bonding picture may be overcome by making new divalent lanthanides with simple ligands, typical of organometallic chemistry, such as the indenyl ligand or amides based ligands, and divalent lanthanides precursors. We will be engaged in making new divalent molecules with the classical Sm, Eu and Yb lanthanides but also the more challenging Tm, Dy and Nd. The divalent lanthanides complexes are extremely reactive toward – even weak, oxidants. We will take advantage of having stable divalent complexes to study the single electron transfer from the reactive divalent metal to redox non-innocent ligands such as N-aromatic heterocycles and P-aromatic heterocycles. Characterization of these complexes will be performed with routine experimental techniques of synthetic organometallic chemists (such as X-ray diffraction, multi- atomic NMR, absorption visible spectra, IR, electrochemistry and magnetic studies) but we are also interested in this project to have a deeper look at their electronic structure using optical spectroscopy measurments, almost an empty field of research for organolanthanides. As to reinforce the expected experimental observations, a strong feedback from theoretical chemistry is proposed in the enclosed document in order to gather as much details we may and lean toward a decent physical model. If some of the complexes considered in this proposal are expected to be stable to allow their isolation; further reactivity is also expected and examples of C-H activation and of reversible C-C coupling have been observed in our preliminary work. We would like to take advantage of such reactivity to react these molecules with small molecules such as N2, CO2 or N2O. Assisted with theoretical calculations of energy profile and by experimental kinetics, their reactivity will be carefully studied and aim at relating the main outcomes to the electronic structure of the parent molecules.
Project coordination
Grégory Nocton (Laboratoire de Chimie Moléculaire)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partner
LC ENS Laboratoire de Chimie de l'ENS de Lyon
LPCNO Laboratoire de Physique et Chimie des Nano-objets
CNRS DR ILE DE FRANCE SUD
CNRS DR ILE DE FRANCE SUD
CNRS DR ILE DE FRANCE SUD
LCM Laboratoire de Chimie Moléculaire
Help of the ANR 392,080 euros
Beginning and duration of the scientific project:
September 2015
- 36 Months
