Electronic Structures of Mono‐Oxidized Copper and Nickel Phosphasalen Complexes

Abstract

Non-innocent ligands render the determination of the electronic structure in metal complexes difficult. As such, a combination of experimental techniques and quantum chemistry are required to correctly elucidate them. This paper deals with the one-electron oxidation of copper(II) and nickel(II) complexes featuring a phosphasalen ligand (Psalen), which differs from salen analogues by the presence of iminophosphorane groups (P=N) instead of imines. Various experimental techniques (X-ray diffraction, cyclic voltammetry, NMR, EPR, and UV/Vis spectroscopies, and magnetic measurements) as well as quantum chemical calculations were used to define the electronic structure of the oxidized complexes. These can be modified by a small change in the ligand structure, that is, the replacement of a tert-butyl group by a methoxy on the phenoxide ring. The different techniques have allowed quantifying the amount of spin density located on the metal center and on the Psalen ligands. All complexes were found to possess a multi-configurational ground state, in which the ratio of the +II versus +III oxidation state of the metal center, and therefore the phenolate versus phenoxyl radical ligand character, varies upon the substituents. The tert-butyl group favors a strong localization on the metal center whereas with the methoxy group the metallic configurations decrease and the ligand configurations increase. The importance of the geometrical considerations compared with the electronic substituent effect is highlighted by the differences observed between the solid-state (EPR, magnetic measurements) and solution characterizations (EPR and NMR data).

Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes

“…Apart from these catalytic applications and medicinal purposes, salen complexes of the first row metal ions are also excellent candidates for electrochemical immunosensing, photoelectrochemical energy conversion, , as well as molecular magnetism. , Bulky tert -butyl (or any other electron-rich) groups are often introduced in ortho and para positions of the salicylidene moieties to improve the enantioselectivity during turnover. − ,− In some instances these groups can alternatively confer a redox noninnocence to the salen ligand − and stable radicals can be isolated and characterized. In these latter cases the site of oxidation and subsequently the properties of the salen complex are governed by several interdependent factors such as the electronics of the ligand, − the nature of the metal ion, − the solvent, ,− ,− the temperature, ,,, and others. − Among the pro-phenoxyl (sterically hindered) salen complexes, the copper derivatives ,,− have attracted particular interest due to their ability to mimic the active site of the radical enzyme galactose oxidase. − The one-electron oxidized copper complexes of the N,N’ -bis­(3- tert -butyl, 5-R-salicylidene)-1,2-cyclohexanediamine ([Cu­(Sal R )] + , Scheme ) are both models of galactose oxidase and prototypical examples of the subtle balance between the possible valence tautomers: For [Cu­(Sal tBu )] + the Cu­(II)-radical valence tautomer is favored at high temperatures, whereas the high-valent Cu­(III) complex predominates at low temperature …”

Synthesis and Reactivity of Low-Valent f-Element Iodide Complexes with Neutral Iminophosphorane Ligands

“…Finally, redox-active PDI ligands (Chart ) have been shown by the Bart group to achieve a rich and unprecedented chemistry with f elements . Because of the structural resemblance but electronic divergence of L Ph and L Cy with PDI ligands and given our activity in probing the electronic structure of iminophosphorane ligands, we were interested in studying their coordination behavior and reactivity in a highly reducing environment.…”

Redox Chemistry of Pt(II) Complex with Non-Innocent NHC Bis(Phenolate) Pincer Ligand: Electrochemical, Spectroscopic, and Computational Aspects

“…Indeed, electron-rich sterically hindered phenolates are among the most common noninnocent ligands that can assist in catalytic transformations by storing (in form of phenoxyl) 2 of 15 and delivering charge during catalytic transformations [22][23][24][25][26][27]. However, bidentate ligand scaffolds utilized in many catalytic reactions involving such phenolate ligands (or their aminophenol derivatives) often lead to unfavorable isomerization processes of the ligand field, precluding further transformations of a substrate [28,29].…”

“…However, the precise determination of the metal oxidation state in transition-metal complexes is not always possible. In reality, metal ions and ligands are both affected by the oxidation and the resulting spin density is shared over both, allowing a multi-configurational state [9,25]. Therefore, a careful examination of metal complexes via various spectroscopic methods combined with computational studies is required for a better description of their electronic structure, which is essential for the design of catalytic cycles with participation of redox non-innocent ligands [39,40].…”