Switching from Metal‐ to Ligand‐Based Oxidation in Cobalt Complexes with Redox‐Active Bisguanidine Ligands. Issue 46 (5th July 2021)

• Record Type: Journal Article
• Authors: Lohmeyer, Lukas; Kaifer, Elisabeth; Enders, Markus; Himmel, Hans‐Jörg
• Is Part Of: Chemistry. Volume 27:Issue 46(2021)
• Page Start: 11852
• Page End: 11867
• Publication Date: 2021-07-05
• Subjects: cobalt -- guanidine -- oxidation -- radical ligands -- redox-active ligand; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202101364
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: The control of the redox reactivity, magnetic and optical properties of the different redox states of complexes with redox‐active ligands permits their rational use in catalysis and materials science. The redox‐chemistry of octahedrally coordinated high‐spin Co II complexes (three unpaired electrons) with one redox‐active bisguanidine ligand and two acetylacetonato (acac) co‐ligands is completely changed by replacing the acac by hexafluoro‐acetylacetonato (hfacac) co‐ligands. The first one‐electron oxidation is metal‐centered in the case of the complexes with acac co‐ligands, giving diamagnetic Co III complexes. By contrast, in the case of the less Lewis‐basic hfacac co‐ligands, the first one‐electron oxidation becomes ligand‐centered, leading to high‐spin Co II complexes with a radical monocationic guanidine ligand unit (four unpaired electrons). Ferromagnetic coupling between the spins on the metal and the organic radical in solution is evidenced by temperature‐dependent paramagnetic NMR studies, allowing to estimate the isotropic exchange coupling constant in solution. Second one‐electron oxidation leads to high‐spin Co II complexes with dicationic guanidine ligand units (three unpaired electrons) in the presence of hfacac co‐ligands, but to low‐spin Co III complexes with radical monocationic, peralkylated guanidine ligand (one unpaired electron) in the presence of acac co‐ligands. The analysis of the electronic structures is complemented by quantum‐chemical calculations on the spin density distributions and relative energies of the possible redox isomers. Abstract : The substitution of the methyl groups of acetylacetonato (acac) co‐ligands in a cobalt(II) complex with a redox‐active bisguanidine ligand by trifluoromethyl groups (hfacac co‐ligands) changes the redox properties. One‐electron oxidation of the complexes with acac co‐ligands is metal centered (Co(II)→Co(III)). By contrast, a radical monocationic bisguanidine ligand is formed upon one‐electron oxidation of the complexes with hfacac co‐ligands. The changes are accompanied by distinct changes in the magnetic and optical properties.