Double oxidation localizes spin in a Ni bis-phenoxyl radical complex

Area of Science:

• Inorganic Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Nickel salen complexes are important in catalysis and materials science.
• Understanding the electronic structure of oxidized metal complexes is crucial for predicting reactivity.
• Previous studies on mono-oxidized NiSal(tBu) showed delocalized radical character.

Purpose of the Study:

• To investigate the electronic structure of the doubly oxidized NiSal(tBu) complex.
• To determine the locus of oxidation and the spectroscopic signature of the doubly oxidized species.
• To compare the electronic structure of the doubly oxidized complex with its mono-oxidized analogue.

Main Methods:

• Experimental methods: Resonance Raman spectroscopy, UV-vis-NIR spectroscopy, and Electron Paramagnetic Resonance (EPR) spectroscopy.
• Theoretical methods: Density Functional Theory (DFT) calculations.
• Analysis of spectroscopic data to identify radical character and spin localization.

Main Results:

• Double oxidation of NiSal(tBu) yields a bis-ligand radical species in solution.
• Phenoxyl radical bands at 1504 cm⁻¹ (ν7a) and 1579 cm⁻¹ (ν8a) in the Raman spectrum confirm the radical nature.
• The intense NIR transition observed for the mono-radical complex is absent in the doubly oxidized species.
• Spectroscopic and DFT results indicate that the two radical spins are primarily localized on the phenolate groups.

Conclusions:

• The doubly oxidized NiSal(tBu) complex exists as a bis-ligand radical with localized spin density on the phenolate moieties.
• This localization contrasts with the extensive delocalization observed in the mono-radical analogue.
• The findings provide insights into the electronic structure and oxidation behavior of Ni salen complexes.