A Tetracoordinated Phosphasalen Nickel(III) Complex

Abstract

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...

Electronic Structures of Mono‐Oxidized Copper and Nickel Phosphasalen Complexes

“…This is why the bands have been tentatively assigned to charge‐transfer bands. This assignment is in good agreement with the literature and our previous assignments for the [Ni(Psalen t Bu )](SbF 6 ) complex . Therefore, in the case of a delocalized orbital, it is conceivable that the transition from a primarily ligand loaded orbital to a primarily empty metal orbital (LMCT) would have a similar intensity than the transition of a primarily metal‐loaded orbital to a primarily ligand empty orbital (MLCT).…”

“…This contraction is accompanied by an elongation of the P−N bonds by about 0.025 Å, translating the increased donation of the iminophosphorane moieties to the metal. Similar variations were already observed for [Ni(Psalen t Bu )](SbF 6 ) . As found in the neutral complexes, the NOON torsion angle is larger in [Ni(Psalen OMe )](SbF 6 ) (16.0 ° compared to 11.5 ° in [Ni(Psalen t Bu )](SbF 6 ) ).…”

“…The CV of [Ni(Psalen t Bu )] exhibits three oxidation waves at 0.01, 0.82 and 1.24 V . Similarly, [Ni(Psalen OMe )] exhibits three oxidation waves: a reversible one at −0.06 V, a pseudo‐reversible at 0.47 V and an irreversible one at 1.00 V (see Figures and S12).…”

“…Given our interest in electron‐rich iminophosphorane (P=N)‐based ligands as alternatives to imine ones, we began to study the phosphorous analogues of salen, that we termed phosphasalen (or Psalen) . In a previous study, we investigated the electronic structure of [Ni(Psalen t Bu )] (Figure ) and its one‐electron oxidized product, which was characterized as a rare, temperature‐persistent, high‐valent tetracoordinated Ni III complex . This was in contrast with the salen derivatives for which one‐electron oxidation led to a radical complex …”

Heteroatomic Ni, Sn Clusters-Grafted Anatase TiO2 Photocatalysts: Structure, Electron Delocalization, and Synergy for Solar Hydrogen Production

“…It clearly demonstrates that the Sn grafting induces the decrease in valence electron of Ni 2+ species, further implying the bond of Sn 4+ to Ni 2+ by an oxygen bridge in the Sn 1.12 /Ni 0.98 /TiO 2 . Under visible and solar light irradiation, the g = 2.13 signal of Sn 1.12 /Ni 0.98 /TiO 2 lacks a change in intensity (Figure A), but a new and weak ESR signal, which also belongs to Ni 3+ , , occurs at g = 2.07 under and follows the same light excitation as the Ti–O–Ni linkages in Ni 1.0 /TiO 2 . Interestingly, under solar light irradiation, the Sn 1.12 /Ni 0.98 /TiO 2 gives an ESR spectrum different completely from that of Ni 1.0 /TiO 2 .…”