Metal-free electrocatalytic upcycling of polyethylene terephthalate plastic to C2 products.

Polyethylene terephthalate (PET) is one of the most used polymers, but the non-degradable and persistent nature of PET waste in the environment is a global menace. Hence upcycling PET waste becomes indispensable. Herein, we introduce the first metal-free electrochemical-upcycling of PET into value-added chemicals and H2 fuel using an organo-electrocatalyst (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO). Electrolysis at pH 10 produces glycolate and oxalate exclusively while at pH 14, over-oxidation and subsequent C-C bond cleavage produce formate and carbonate as well. Tuning the rate and product selectivity via pH regulation with mechanistic insight displays a sustainable route to implement waste PET recycling.

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective.

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Invited for this month's cover are the groups of Menny Shalom at the Ben-Gurion University of the Negev, Israel and Dr. Biswajit Mondal at Indian Institute of Technology Gandhinagar, India. The image shows the connection between two half-cells: an electron transfer-mediated [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl] (TEMPO)-catalyzed benzylamine oxidation at the anode and a proton-coupled electron transfer at the cathode for hydrogen generation. The difference in pH dependence of the anodic and cathodic processes enables hybrid water electrolysis at low cell potential (∼1.0 V) by adjusting only the pH value of the electrolytic medium. The Research Article itself is available at 10.1002/cssc.202202271.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Electrolysis of water is a sustainable route to produce clean hydrogen. Full water-splitting requires a high applied potential, in part because of the pH-dependency of the H2 and O2 evolution reactions (HER and OER), which are proton-coupled electron transfer (PCET) reactions. Therefore, the minimum required potential will not change at different pHs. TEMPO [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl], a stable free-radical that undergoes fast electro-oxidation by a single-electron transfer (ET) process, is pH-independent. Here, we show that the combination of PCET and ET processes enables hydrogen production from water at low cell potentials below the theoretical value for full water-splitting by simple pH adjustment. As a case study, we combined the HER with the oxidation of benzylamine by anodically oxidized TEMPO. The pH-independent electrocatalytic oxidation of TEMPO permits the operation of a hybrid water-splitting cell that shows promise to perform at a low cell potential (≈1 V) and neutral pH conditions.

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes.

The redox non-innocent behavior of the diaryl-azo-oxime ligand LNOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(LNO-)Cl(CO)(PPh3)2]+2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(LNO-)Cl(CO)(PPh3)2]+3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(LNO-)Cl2(PPh3)2] 4 and [Ir(LNO-)2Cl(PPh3)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(LNO-)Cl(CO2H)(PPh3)2] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO2H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 is primarily azo-oxime centric with very low contribution from the iridium center.