How Ligand Geometry Affects the Reactivity of Co(II) Cyclam Complexes

• Published in: European journal of inorganic chemistry 2023-10, Vol.26 (30)
• Main Authors: Iffland‐Mühlhaus, Linda, Battistella, Beatrice, Siegmund, Daniel, Ray, Kallol, Apfel, Ulf‐Peter
• Format: Article
• Language: English
• Subjects: Biomimetics; Cobalt compounds; Hydrogen bonds; Inorganic chemistry; Ligands; Oxidation; Oxygen; Phosphine oxide
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.202300418

Cobalt complexes are extensively studied as bioinspired models for non‐heme oxygenases as they facilitate both the stabilization and characterization of metal‐oxygen intermediates. As an analog to the well‐known Co(cyclam) complex Co{N 4 } (cyclam=1,4,8,11‐tetraazacyclotetradecane), the Co II complex Co{i‐N 4 } with the isomeric isocyclam ligand (isocyclam=1,4,7,11‐tetraazacyclotetradecane) was synthesized and characterized. Despite the identical N 4 donor set of both complexes, Co{i‐N 4 } enables the 2e − /2H + reduction of O 2 with a lower overpotential ( η eff of 385 mV vs. 540 mV for Co{N 4 } ), albeit with a diminished turnover frequency. Characterization of the intermediates formed upon O 2 activation of Co{i‐N 4 } reveals a structurally identified stable μ ‐peroxo Co III dimer as the main product. A superoxo Co III species is also formed as a minor product, as indicated by EPR spectroscopy. In further reactivity studies, the electrophilicity of these in situ generated Co−O 2 species was demonstrated by the oxidation of the O−H bond of TEMPO−H (2,2,6,6‐tetramethylpiperidin‐1‐ol) via a H atom abstraction process. Unlike the known Co(cyclam), Co{i‐N 4 } can be employed in oxygen atom transfer reactions oxidizing triphenylphosphine to the corresponding phosphine oxide highlighting the impact of geometrical modifications of the ligand while preserving the ring size and donor atom set on the reactivity of biomimetic oxygen activating complexes.