Stimulation of Redox‐Induced Electron Transfer by Interligand Hydrogen Bonding in a Cobalt Complex with Redox‐Active Guanidine Ligand

Octahedrally-coordinated cobalt(II) complexes with a redox-active bisguanidine ligand and acac co-ligands were synthesized and their redox chemistry analysed in detail. Three different bisguanidines were used, exhibiting either fully or partially alkylated guanidino groups. The N-H functions in the ligand with partially alkylated guanidino groups form N-H···O hydrogen bonds with the acac co-ligands, thereby massively influencing the redox chemistry. For all complexes, the first one-electron oxidation is metal-centered, leading to Co III complexes with neutral bisguanidine ligand units. Further one-electron oxidation is ligand-centered in the case of Co-bisguanidine complexes with fully-alkylated guanidino groups, giving Co III complexes with radical monocationic bisguanidine ligands. On the 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 (RIET) by interligand hydrogen bonding. The results of this study demonstrate the huge impact of interligand hydrogen bonding on the outcome of redox reactions, providing the key for understanding the role of weak interactions in biological (enzymatic) redox processes and for a sophisticated control of redox-activity.