Combined Oxidation, Deprotonation, and Metal Coordination of a Redox-Active Guanidine Ligand

On the metal-ligand bonding in dinuclear complexes with redox-active guanidine ligands

Coordination compounds with redox-active ligands are currently intensively studied. Within this research theme, redox-active guanidines have been established as a new, eminent class of redox-active ligands. In this work the variation of metal-guanidine bonding in dinuclear transition metal complexes with bridging redox-active tetrakisguanidine ligands is analysed. A series of dinuclear complexes with different metals (Mn, Fe, Co, Ni, Cu and Zn) is synthesized, using either newly prepared redox-active tetrakisguanidino-dioxine or previously reported tetrakisguanidino-benzene ligands. The discussion of the bond properties in this work is predominantly based on the trends of structural parameters, derived from determination of single-crystal structures by X-ray diffraction and quantum chemical calculations. In addition, the trends in the redox potentials and magnetometric (SQUID) measurements on some of the complexes are included. Due to their combined σ- and π-electron donor capability, redox-active guanidine ligands are weak-field ligands; the σ- and π-bonding contributions vary with the metal. The results highlight the peculiarity of copper-guanidine bonding with a high π-bond contribution to metal-guanidine bonding, enabled by structural distortion of the coordination mode from tetrahedral in the direction of square-planar, short copper-guanidine bonds and minor displacement of the copper atoms from the ligand aromatic plane.

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

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.

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 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, 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 by interligand hydrogen bonding.

1,2,4,5‐Tetrakis(tetramethylguanidino)‐3,6‐diethynyl‐benzenes: Fluorescent Probes, Redox‐Active Ligands and Strong Organic Electron Donors

In this work, the change of reactivity induced by the introduction of two para‐ethynyl substituents (CCSi(iPr)₃ or CCH) to the organic electron‐donor 1,2,4,5‐tetrakis(tetramethylguanidino)‐benzene is evaluated. The redox‐properties and redox‐state dependent fluorescence are evaluated, and dinuclear Cu^I and Cu^II complexes synthesized. The Lewis‐acidic B(C₆F₅)₃ substitutes the proton of the ethynyl −CCH groups to give new anionic −CCB(C₆F₅)₃⁻ substituents, leading eventually to a novel dianionic strong electron donor in its diprotonated form. Its two‐electron oxidation with dioxygen in the presence of a copper catalyst yields the first redox‐active guanidine that is neutral (instead of cationic) in its oxidized form.