Pd(ii) and Ni(ii) complexes featuring a “phosphasalen” ligand: synthesis and DFT study

Cao, Thi-Phuong-Anh; Labouille, Stéphanie; Auffrant, Audrey; Jean, Yves; Goff, Xavier F. Le; Floch, Pascal Le

doi:10.1039/c1dt10906e

Cited by 26 publications

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References 72 publications

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“…It can be rationalized by the better electron-donating ability of iminophosphoranes compared to imines, and by the decreased electron density at the phenol rings which are substituted by an electron-demanding phosphorus atom. [12] Preliminary investigations concerning the reactivity of 2 + SbF 6 À showed it to have a high but difficult to control oxidation capacity. Our current efforts are devoted to this, considering a fine tuning of the ligand structure.…”

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“…We have recently demonstrated that these ligands are more flexible and electron-donating than their carbon analogues [12] and that they provide efficient and selective initiators for rac-lactide polymerization. [13] In the present work, we wish to report the synthesis and characterization of a Ni II phosphasalen complex and its one-electron oxidation which leads to a temperaturepersistent high-valent tetracoordinated Ni III complex.…”

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A Tetracoordinated Phosphasalen Nickel(III) Complex

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The oxidation of a Ni II complex bearing a tetradentate phosphasalen ligand, which differs from salen by the presence of an iminophosphorane (P = N) in place of an imine unit, was easily achieved by addition of a silver salt. The site of this oxidation was investigated with a combination of techniques (NMR, EPR, UV/Vis spectroscopy, X-ray diffraction, magnetic measurements) as well as DFT calculations. All data are in agreement with a high-valent Ni III center concentrating the spin density. This markedly differs from precedents in the salen series for which oxidation on the metal was only observed at low temperature or in the presence of additional ligands or anions. Therefore, thanks to the good electron-donating properties of the phosphasalen ligand, [Ni(Psalen)]+ represents a rare example of a tetracoordinated high-valent nickel complex in presence of a phenoxide ligand.N,N' -ethylenebis(salicylimine) (salen) derivatives constitute a very successful class of ligands that have found numerous applications in inorganic chemistry and catalysis. [1] Their extraordinary popularity comes from their easy synthesis, allowing a variety of skeleton variations, and their ability to coordinate to a great number of different metal ions. Moreover the presence of two phenoxide and imine functions is reminiscent of the coordination environment generated by two histidine and two tyrosine residues, encountered in several enzymes. Thus, in 1998, the one-electron oxidation of a (salen)copper(II) complex was described, and the resulting persistent phenoxy radical was established as a biomimetic functional model of galactose oxidase.[2] Since then, one-electron-oxidized metal salen complexes of firstrow transition metals have received considerable attention. [3] Depending on the nature of the redox-active orbitals, oxidation of salen complexes leads either to high-valent metal centers (metal-centered oxidation) or to species where the ligand is oxidized to a radical (ligand-centered oxidation). Therefore, various factors may affect the oxidation site, as it was nicely illustrated in the case of nickel salen complexes. In noncoordinating solvents, one-electron oxidation occurs on the ligand, and the unpaired electron is highly delocalized over the salen moiety with a small contribution of the nickel orbitals.[3b, 4] However, in coordinating solvents, or in the presence of exogenous ligands such as pyridine or coordinating anions, the geometry of the complex is modified and so is the site of the oxidation, allowing the formation of Ni III complexes.[4a,b, 5] Temperature-dependent behavior, that is, valence tautomerism, was also described in a few examples.[5b, 6] Moreover, the overall electronic distribution may be finely tuned by the substituents of the phenoxide rings. They influence not only the site of the oxidation (which takes place at the most electron-rich phenoxide ring) as evidenced with complexes featuring unsymmetrical salen, [3c, 7] but also the extent of radical delocalization. [3b, 8] The hybridization o...

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A Tetracoordinated Phosphasalen Nickel(III) Complex

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“…In the cerium complex, the NCeOC O torsion angle (see Figure for labeling) was measured at about 45° in the Psalfen complex vs. 4.5° in the salfen analogue. A larger distortion was also observed in [Pd(Psalen H )] and [Ni(Psalen H )] compared to their salen counterpart: the NMOC O torsion angle was measured at 25.64 and 22.57° in [Pd(Psalen H )] and [Ni(Psalen H )] respectively, whereas angles of 3.02 and 3.30° are featured in the corresponding salen derivatives. This evidences the bending of the phenoxide rings relative to the main coordination plane.…”

Section: Synthesis Structure and Electronic Properties Of Phosphamentioning

confidence: 93%

“…We preferred to rely on a modified Kirsanov reaction allowing the use of diamines, thus avoiding the employ of potentially explosive azido compounds (Scheme , bottom). So, we isolated the corresponding bis(aminophosphonium) salts, which are rather stable and can be stored almost indefinitely on the bench without any specific precautions.…”

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Phosphasalen vs. Salen Ligands: What Does the Phosphorus Change?

Marín

Auffrant

2018

Eur J Inorg Chem

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Phosphasalen ligands can be viewed as the phosphorus analogues of salen, in which the imine functions were replaced by iminophosphoranes (P=N). Their divergent stereoelectronic properties compared to salen, namely larger electron‐donation, higher flexibility, and larger steric hindrance due to the presence of two substituents on the tetrahedral phosphorus atoms, were exploited in coordination chemistry and catalysis. They were proven to be able to stabilize metals in unusual oxidation states; square planar copper(III) and nickel(III) complexes were isolated. Because of the redox non‐innocent character of these ligands, the electronic structure of the one‐electron oxidized complexes was studied in depth to determine the metallic vs. ligand contribution in the overall delocalization. In addition, the catalytic activity of different phosphasalen complexes was proven for the ring opening polymerization of lactide, and rare‐earth phosphasalen complexes were shown to be powerful, rapid, controlled, and stereoselective initiators. Importantly, the stereochemical outcome of the reaction can be tuned by changing the ligand structure or the metal. In particular, efficient iso‐selective initiators were developed.

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A Tetracoordinated Phosphasalen Nickel(III) Complex

Cao¹,

Nocton²,

Ricard³

et al. 2013

Angew Chem Int Ed

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The oxidation of a Ni(II) complex bearing a tetradentate phosphasalen ligand, which differs from salen by the presence of an iminophosphorane (PN) in place of an imine unit, was easily achieved by addition of a silver salt. The site of this oxidation was investigated with a combination of techniques (NMR, EPR, UV/Vis spectroscopy, X-ray diffraction, magnetic measurements) as well as DFT calculations. All data are in agreement with a high-valent Ni(III) center concentrating the spin density. This markedly differs from precedents in the salen series for which oxidation on the metal was only observed at low temperature or in the presence of additional ligands or anions. Therefore, thanks to the good electron-donating properties of the phosphasalen ligand, [Ni(Psalen)](+) represents a rare example of a tetracoordinated high-valent nickel complex in presence of a phenoxide ligand.

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Pd(ii) and Ni(ii) complexes featuring a “phosphasalen” ligand: synthesis and DFT study

Cited by 26 publications

References 72 publications

A Tetracoordinated Phosphasalen Nickel(III) Complex

A Tetracoordinated Phosphasalen Nickel(III) Complex

Phosphasalen vs. Salen Ligands: What Does the Phosphorus Change?

A Tetracoordinated Phosphasalen Nickel(III) Complex

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