Front Cover: Use of Crown Ether Functions as Secondary Coordination Spheres for the Manipulation of Ligand–Metal Intramolecular Electron Transfer in Copper–Guanidine Complexes (Chem. Eur. J. 3/2021)

Haaf, Sebastian; Kaifer, Elisabeth; Wadepohl, Hubert; Himmel, Hans‐Jörg

doi:10.1002/chem.202004294

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“…Redox‐active guanidines, comprising guanidino‐functionalized aromatics (GFAs), were established by our group as a new class of versatile redox‐active ligands [31–33] . IET in mono‐ or dinuclear copper complexes with a GFA ligand could be triggered thermally (VT), [34–37] by redox reactions (RIET), [36] by co‐ligand addition [38,39] or substitution, [40] and by metal coordination to a secondary coordination sphere [41] . Also, first applications were disclosed, for example catalytic aerobic phenol homo‐ and cross‐coupling reactions [42] …”

Section: Introductionmentioning

confidence: 99%

“…[ 31 , 32 , 33 ] IET in mono‐ or dinuclear copper complexes with a GFA ligand could be triggered thermally (VT),[ 34 , 35 , 36 , 37 ] by redox reactions (RIET), [36] by co‐ligand addition[ 38 , 39 ] or substitution, [40] and by metal coordination to a secondary coordination sphere. [41] Also, first applications were disclosed, for example catalytic aerobic phenol homo‐ and cross‐coupling reactions. [42] …”

Section: Introductionmentioning

confidence: 99%

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Switching from Metal‐ to Ligand‐Based Oxidation in Cobalt Complexes with Redox‐Active Bisguanidine Ligands

Lohmeyer

Kaifer

Enders

et al. 2021

Chemistry A European J

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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 CoII 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 CoIII 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 CoII 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 CoII complexes with dicationic guanidine ligand units (three unpaired electrons) in the presence of hfacac co‐ligands, but to low‐spin CoIII 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.

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