Copper Versus Thioether‐Centered Oxidation: Mechanistic Insights into the Non‐Innocent Redox Behavior of Tripodal Benzimidazolylaminothioether Ligands

Abstract

A series of Cu(+) complexes with ligands that feature varying numbers of benzimidazole/thioether donors and methylene or ethylene linkers between the central nitrogen atom and the thioether sulfur atoms have been spectroscopically and electrochemically characterized. Cyclic voltammetry measurements indicated that the highest Cu(2+)/Cu(+) redox potentials correspond to sulfur-rich coordination environments, with values decreasing as the thioether donors are replaced by nitrogen-donating benzimidazoles. Both Cu(2+) and Cu(+) complexes were studied by DFT. Their electronic properties were determined by analyzing their frontier orbitals, relative energies, and the contributions to the orbitals involved in redox processes, which revealed that the HOMOs of the more sulfur-rich copper complexes, particularly those with methylene linkers (-N-CH2-S-), show significant aromatic thioether character. Thus, the theoretically predicted initial oxidation at the sulfur atom of the methylene-bridged ligands agrees with the experimentally determined oxidation waves in the voltammograms of the NS3- and N2S2-type ligands as being ligand-based, as opposed to the copper-based processes of the ethylene-bridged Cu(+) complexes. The electrochemical and theoretical results are consistent with our previously reported mechanistic proposal for Cu(2+)-promoted oxidative C-S bond cleavage, which in this work resulted in the isolation and complete characterization (including by X-ray crystallography) of the decomposition products of two ligands employed, further supporting the novel reactivity pathway invoked. The combined results raise the possibility that the reactions of copper-thioether complexes in chemical and biochemical systems occur with redox participation of the sulfur atom.

Hydroxyl Radical Generation and DNA Nuclease Activity: A Mechanistic Study Based on a Surface‐Immobilized Copper Thioether Clip‐Phen Derivative

“…In addition, as can be ascertained from the inset in Figure , the potential shifts observed at different pH values are consistent with a two‐electron process and are tentatively assigned to the oxidation of a dimeric species of copper(I). In fact, the formation of reactive intermediates containing oxygen species as bridging ligands has been claimed by several groups, particularly those attempting to isolate compounds that mimic the active site of the copper monooxygenase enzymes to understand their Cu I /O 2 reactivity …”

“…However, on the surface and with the complex in the bulk electrolyte solution, the SPR data indicate a mass increase consistent with another molecule of the complex being attached to the previously adsorbed one through an O−O bridge. In fact, the formation of putative dimeric intermediates containing oxygen species has been claimed by a number of authors . In most of these reports, however, the detection of such intermediates is proposed based on observations at very low temperatures (e.g., T <−50 °C).…”

Non-innocent ligands in copper complexes catalyzed oxidation of thiols

“…Oxidation can be catalyzed by metal complexes not only via cyclic variation of the oxidation state of the transition metal but also via transformations of donor ligands. [20][21][22][23][24][25] In this study, we used industrially important oxidation of heavy thiols with air oxygen leading to the formation of disulfides as the model reaction:…”

Controlled Synthesis of Thiazole-Based Polymers and Block Copolymers by RAFT Polymerization of Azolyl S-Vinyl Sulfides and Metal Complexation

“…These azolyl S -vinyl sulfides have many attractive features, including an aromatic nucleus, high dipole moment, electron-accepting heterocycles, roles as ligands and coordination sites for metal species, and access to structurally diverse materials. Another attractive point is that the thioether group in the monomer unit can be readily converted to other functional groups. − In addition, the binding of thioethers to transition metals, particularly concerning the coordination chemistry of Cu­(II)-S species, has attracted significant interest because of their analogy to various class of metalloproteins, such as blue copper proteins. − …”