Octahedrally coordinated cobalt(II) complexes with ar edox-active bisguanidine ligand and acac co-ligands were synthesized and their redoxc hemistry analysed in detail. The N À Hfunctions in abisguanidine ligand with partially alkylated guanidino groups form N À H•••O hydrogen bonds with the acac co-ligands,t hereby massively influencing the redox chemistry. Fora ll complexes,t he first one-electron oxidation is metalcentred, leading to Co III complexes with neutral bisguanidine ligand units.Further one-electron oxidation is ligand-centred in the case of Co-bisguanidine complexes with fully alkylated guanidino groups,g iving Co III complexes with radical monocationic bisguanidine ligands.O nt he other hand, the hydrogen-bond strengthening upon oxidation of the Co-bisguanidine complex with partially alkylated guanidino groups initiates metal reduction (Co III !Co II )and two-electron oxidation of the guanidine ligand, providing the first example for the stimulation of redox-induced electron transfer by interligand hydrogen bonding.

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.

Redox-isomeric coordination compounds, in which the magnetic and optical properties could be varied by a stimulated intramolecular electron transfer between the metal and a redoxactive ligand, are of interest for several applications in catalysis and materials science. In this work, the redox chemistry of cobalt complexes with redox-active bisguanidine ligands is studied; systematic modifications at the redox-active bisguanidine and the co-ligand units allow for fine-tuning of the electronic structure, which eventually leads to the first observation of redox isomerism for cobalt complexes with redox-active guanidine ligands. Redox isomerism is triggered by a change in the solvent properties.

Newr edox-active1 ,2,5,6-tetrakis(guanidino)-naphthalenec ompounds, isolable and storable in the neutral and deep-green dicationic redox states and oxidisable furtheri n two one-electron steps to the tetracations, are reported. Protonations witches on blue fluorescence, with the fluorescence intensity( quantum yield) increasing with the degree of protonation.R eactions with N-halogenosuccinimideso rNhalogenophthalimides led to as eries of new redox-active halogeno-and succinimido-/phthalimido-substituted deriva-tives. These highly selectiver eactionsa re proposed to proceed via the tri-or tetracationic state as the intermediate. The derivatives are oxidised reversibly at slightly higher potentials than that of the unsubstituted compounds to dications andf urthert ot ri-and tetracations. The integrationo f redox-active ligands in the transition-metal complexes shifts the redox potentials to higher values anda lso allowsr eversible oxidationi nt wo potentially separated one-electron steps. Details of the quantum chemical calculationsDFT calculations were carried out with the TURBOMOLE program package. [36] The B3LYP functional [37] was used in combination with the def2-TZVP basis set. [38] All structures are stationary points on the energy potential surface as confirmed by frequency computations. [39] Details of the structural characterisationsSuitable crystals for single-crystal structure determination were taken directly from the mother liquor,i mmersed in perfluorinated polyether oil and fixed on ac ryo loop. For compound 4,af ull shell of intensity data was collected at low temperature with an Agilent

Die ersten oktaedrischkoordinierten Co II -Komplexe mit redoxaktivem Bisguanidin-Liganden und acac-Coliganden wurden synthetisiert und hinsichtlichi hrer Redoxchemie analysiert. Die N-H-Funktionen in einem Bisguanidin-Liganden mit partiell alkylierten Guanidino-Gruppen bilden N-H•••O-Brücken zu den acac-Coliganden und beeinflussen dadurch stark die Redoxchemie.Bei allen Komplexen ist die erste 1e À -Oxidation metallzentriert und führt zu Co III -Komplexen mit neutralen Bisguanidin-Ligandeneinheiten. Die zweite 1e À -Oxidation ist im Falle von Cobalt-Bisguanidin-Komplexen mit vollständig alkylierten Guanidino-Gruppen ligandenzentriert; es resultieren Co III -Komplexem it radikal-monokationischen Bisguanidin-Liganden. Bei der Oxidation eines Cobalt-Bisguanidin-Komplexes mit teilalkylierten Guanidino-Gruppen kommt es dagegen durch die Verstärkung der Wasserstoffbrücken zu einer Metallreduktion (Co III !Co II )u nd zu einer 2e À -Oxidation des Guanidin-Liganden, dem ersten Beispiel fürd as Auslçsen eines redoxinduzierten Elektronentransfers durch Interligand-Wasserstoffbrücken.