Identification, Characterization, and Electronic Structures of Interconvertible Cobalt-Oxygen TAML Intermediates.

Area of Science:

• Coordination Chemistry
• Bioinorganic Chemistry
• Spectroscopy and Theoretical Chemistry

Background:

• Tetraamido macrocyclic ligands (TAML) are crucial in biomimetic oxidation catalysis.
• Understanding the electronic structure of cobalt-TAML complexes is key to elucidating their catalytic mechanisms.
• Temperature-dependent transformations in coordination complexes can reveal dynamic electronic changes.

Purpose of the Study:

• To synthesize and characterize novel blue and green cobalt-TAML species (1 and 2).
• To elucidate the electronic structures of species 1 and 2 using advanced spectroscopic and theoretical methods.
• To investigate the temperature-dependent interconversion between species 1 and 2 and its implications for reactivity.

Main Methods:

• Synthesis of cobalt-TAML complexes in the presence of iodosylbenzene and Lewis/Brønsted acids.
• Characterization using X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS), and magnetic circular dichroism (MCD).
• Computational analysis using ab initio theoretical studies and reactivity studies in oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions.

Main Results:

• A blue species (1) was identified as an S=1/2 [(Sol)(TAML•+)CoIII---OH(LA)]− complex with antiferromagnetic coupling.
• A green species (2), formed reversibly upon cooling, exhibits a multiconfigurational electronic structure with contributions from Co(IV) and protonated TAML ligands.
• The interconversion between 1 and 2 involves a Lewis/Brønsted acid-associated tautomerization, altering the cobalt-oxo species character.

Conclusions:

• The study reveals a novel temperature-dependent tautomerization in cobalt-TAML complexes, impacting their electronic configurations.
• The distinct electronic structures of species 1 and 2 lead to different reactivities in OAT and HAT reactions.
• This work provides fundamental insights into the structure-reactivity relationships of cobalt-based oxidation catalysts.