Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-Coupled Electron Transfer with Redox-Active Guanidines in C-H Activation Processes

Aqueous Electrochemical and pH Studies of Redox-Active Guanidino Functionalized Aromatics for CO2 Capture

Escalating levels of carbon dioxide (CO2) in the atmosphere have motivated interest in CO2 capture and concentration from dilute streams. A guanidino-functionalized aromatic 1,4-bis(tetramethylguanidino)benzene (1,4-btmgb) was evaluated both as a redox-active sorbent and as a pH swing mediator for electrochemical CO2 capture and concentration. Spectroscopic and crystallographic studies demonstrate that 1,4-btmgb reacts with CO2 in water to form 1,4-btmgbH2(HCO3 -)2. The product suggests that 1,4-btmgb could be used in an aqueous redox pH swing cycle for the capture and concentration of CO2. The synthesis and characterization of the mono- and diprotonated forms (1,4-btmgbH+ and 1,4-btmgbH2 2+) and their pK a values were measured to be 13.5 and 11.0 in water, respectively. Electrochemical pH swing experiments indicate the formation of an intermediate radical species and other degradation pathways, which ultimately inhibited fully reversible redox-induced pH cycling.

Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Solvent-Induced Redox Isomerism of Cobalt Complexes with Redox-Active Bisguanidine Ligands

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 redox-active 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.

Proton-Coupled Electron Transfer (PCET) with 1,4-Bisguanidino-Benzene Derivatives: Comparative Study and Use in Acid-Initiated C-H Activation

Proton‐coupled electron transfer (PCET) is of key importance in modern synthetic chemistry. Redox‐active guanidines were established by our group as valuable alternatives to toxic high‐potential benzoquinones in a variety of different PCET reactions. In this work, the PCET reactivity of a series of 1,4‐bisguanidino‐benzenes varying in their redox potentials and proton affinities is evaluated. The relevant redox and protonation states are fully characterized, and the compounds sorted with respect to their PCET reactivity by comparative PCET experiments supplemented by quantum‐chemical calculations. Depending on the studied reactions, the driving force is either electron transfer or proton transfer; thereby the influence of both processes on the overall reactivity could be assessed. Then, two of the PCET reagents are applied in representative oxidative aryl‐aryl coupling reactions, namely the intramolecular coupling of 3,3’’‐4,4’’‐tetramethoxy‐o‐terphenyl to give the corresponding triphenylene, the intermolecular coupling of N‐ethylcarbazole to give N,N’‐diethyl‐3,3’‐bicarbazole, and in the oxidative lactonization of 2‐[(4‐methoxyphenyl)methyl]‐benzoic acid. Under mild conditions, the reactions proceed fast and efficient. Only small amounts of acid are needed, in clear contrast to the corresponding coupling reactions with traditional high‐potential benzoquinones such as DDQ or chloranil requiring a large excess of a strong acid.

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.

Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-Coupled Electron Transfer with Redox-Active Guanidines in C-H Activation Processes

Proton-coupled electron transfer (PCET) is currently intensively studied because of its importance in synthetic chemistry and biology. In recent years it was shown that redox-active guanidines are capable PCET reagents for the selective oxidation of organic molecules. In this work, the scope of their PCET reactivity regarding reactions that involve C-H activation is explored and kinetic studies carried out to disclose the reaction mechanisms. Organic molecules with potential up to 1.2 V vs. ferrocenium/ferrocene are efficiently oxidized. Reactions are initiated by electron transfer, followed by slow proton transfer from an electron-transfer equilibrium.