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.

Electronic Structures of Mono‐Oxidized Copper and Nickel Phosphasalen Complexes

“…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).…”

“…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 ) ).…”

“…All complexes, except [Ni(Psalen OMe )] , were investigated over the 5–300 K temperature range. The [Ni(Psalen t Bu )](SbF 6 ) complex was already measured in a previous study, and showed the typical Curie behavior of a d 7 square‐planar complex ( C =0.46 cm 3 K mol −1 ) with a measured g iso value of 2.22, in good agreement with typical unquenched orbital moment of a first row late transition metal. Accordingly, if [Ni(Psalen OMe )] is diamagnetic in agreement with d 8 square‐planar complexes, [Ni(Psalen OMe )](SbF 6 ) shows a behavior very similar to that of [Ni(Psalen t Bu )](SbF 6 ) with a χT value of 0.427 cm 3 K mol −1 at room temperature (see Figure S8).…”

“…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 …”

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

“…For this study, a ligand featuring ortho and para ‐substituted phenoxide rings was employed, tert ‐butyl substituents were chosen for solubility reason (Psalen t Bu ) . In order to determine the influence of the phenoxide substituents, the Psalen OMe ligand, for which the para position on each phenoxide ring is occupied by a methoxy group, was also employed…”