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. The N-H functions in a bisguanidine 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-centred, leading to CoIII 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, giving CoIII 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 (CoIII →CoII ) and two-electron oxidation of the guanidine ligand, providing the first example for the stimulation of redox-induced electron transfer by interligand hydrogen bonding.