Phosphasalen oxidized complexes – PsalenOx

Oxidation of phosphasalen complexes is easier than for their salen congeners, as demonstrated by cyclic voltammetry. It is conducted chemically generally by adding silver salts The oxidized complexes can be studied by NMR spectroscopy even in the case of paramagnetic compounds. Indeed in some case (Co, Fe, some Mn) those complexes have a decent spectrum at room temperature and some important piece of information is hidden in NMR isotropic chemical shifts, like spin density, symmetry, conformations, distances and angles X-ray diffraction analysis is essential to determine the geometry of the complexes in the solid state and evidence how bonds and angles are modified by the removal of one electron. It also allows to «calibrate« DFT studies for which it is highly important to be sure that the chosen method (functional and basis) reproduce accurately the experimental structure before determining the molecular orbitals, spin density. UV spectroscopy may be informative as well especially when combined with calculations in order to assign the observed transitions. Magnetic measurements can be obtained from NMR by the Evans method. The main drawback of this method is the incertitude on the values that are quite high and mainly that temperature may not be lower than 180 K. This is why solid state measurement with a squid will be conducted in order to get all the temperature dependent information needed on the spin state of the molecule. Nevertheless, EPR is the most informative spectroscopy in the questioning about the location of the electrons. X-band solid state and solution (frozen or not) EPR can be conducted and if the spectrum is narrow enough, the hyperfine and superhyperfine coupling constants may be obtained. If so, they may be of high interest for the description of the metal-ligand bonding.

After a first study having established that a phosphasalen ligand can stabilize a tetracoordinated high valent nickel(III) complex, we investigated the influence of structural modifications in the ligand backbone. Therefore, new ligands were synthesized differing by the nature of the substituents on phenoxides, on the phosphorus atoms (phenyl vs alkyl), and the linker between the two nitrogen atoms (cf illustration). The study concerning the one electron oxidation of nickel(II) and copper(II) is almost finished. Results confirm that the nature of the oxidized complexes highly differ from those obtained with salens. Phosphasalens, thanks to their donor ability are more prone to stabilize high valent metals. Nevertheless, their donation can be easily tuned, in particular by introducing an aryl linker between the two nitrogen atoms. It does not only modify the electron density on the nitrogen (lower basicity) but also rigidifies the structure because of the presence of this planar aromatic group. In that case, the oxidized complexes markedly differ from those obtained with other phosphasalens and may in some cases evolve in an original manner. The study now focus on the rationalization of the experimental observations by combining experimental study and theoretical chemistry.

The structure of the oxidized complexes obtained with a Psalophen ligand which features a phenyl as a linker between the nitrogen atoms can in some cases be highly original. Therefore complementary investigations are required to understand how and why these species form. A good rationalization of the formation of these adducts is for us a requirement for publication.
Synthesis of oxo complexes has started with chromium which should give the more stable complexes taking into account that chromium oxo complexes are known. Next the synthesis and characterization of manganese and iron oxo complexes will be attempted, using for that the most donating phosphasalen ligands; those incorporating electron-donating substituents on the phenoxides, alkyl groups on the phosphorus atom, and an alkyl linker between the nitrogen atoms. If stabilization of oxo complex is easy with such a coordination sphere, the electron donating ability of the ligand would be progressively reduced until a possible synthetic limit. In parallel, the reactivity of those complexes towards olefin will be investigated.
Later the mono-oxidation of cobalt and iron complexes will be studied.

Communications
Invited conference to ICPC (Internation Conference on Phosphorus Chemistry), Dublin, 28 juin-2 juillet 2014
«Phosphasalen Nickel complexes, what phosphorus changes«
A. Auffrant
Oral communication at EUCOMC (European Conference on Organometallic Chemistry), Bratislava, 5-9 juillet 2015
«Oxidation of Phosphasalen Nickel complexes,
Irene Mustieles-Marin, Thi-Phuong-Anh Cao, Grégory Nocton,
Louis Ricard, Audrey AUFFRANT

One-electron oxidation of Copper phosphasalen complexes
Irene Mustieles-Marin, Thibault Cheisson, Grégory Nocton, Carine Clavaguéra, Audrey AUFFRANT, Gecom Concoord 2015, Lyon, 26-29 mai 2015
Poster communication awarded

In this project we will use an original ligand combining iminophosphoranes and phenolates to stabilize a variety of oxidized metal complexes. This ligand was named phosphasalen since it can be viewed as the phosphorus analogue of the well-known salen where iminophosphorane (P=N) replace imines. We had already shown that this ligand is more electron donating than its imine counterpart. Moreover, in a preliminary study, we demonstrated that the phosphasalen ligand is able to stabilize a tetracoordinated high valent Nickel(III) metal centre whereas salen oxidized nickel complexes mainly exist as highly delocalized ligand radical (Ni(II)L•+). In this proposal we aim to study the stoichiometric oxidation of a variety of phosphasalen complexes (Cu, Ni, Co, Fe, Mn) and to describe the electronic structure of the obtained complexes. The locus of the oxidation (metal or ligand centred) will be studied as well as if the outcome of the oxidation can be tuned with modifications of the ligand structure or the reaction conditions. In particular we will compare these results with the plethoric literature on salen one-electron oxidized complexes, in order to show what changes are induced by the presence of an iminophosphorane instead of an imine. Furthermore the electron donating ability of the phosphasalen ligand should allow them to stabilize complexes that are elusive in the salen chemistry and we will target the preparation and characterization of oxo complexes. In order to make conclusive statements on the chemical structure of all these complexes we will associate various characterization techniques (X-ray crystallography, NMR, EPR, and infrared spectroscopy…) as well as high-level theoretical calculations. The oxidized complexes in hand, we will evaluate their reactivity and oxidations of various substrates (phenols, alcohols, thioethers for example) will be attempted. Their reactivity for epoxidation will be also investigated. At this stage, calculations will also be used to rationalize the observed reactivity, and to compare it in silico with that of the corresponding salen complex. Finally, this may lead to second generation of ligands with improved reactivity.