A copper complex, [Cu(dpaq)](ClO 4 ) (1), of a monoanionic pentadentate amidate ligand (dpaq) has been isolated and characterized to study its efficacy toward electrocatalytic reduction of oxygen in neutral aqueous medium. The Cu(II) mononuclear complex, poised in a distorted trigonal bipyramidal structure, reduces oxygen at an onset potential of 0.50 V vs RHE. Kinetics study by hydrodynamic voltammetry and chronoamperometry suggests a stepwise mechanism for sequential reduction of O 2 to H 2 O 2 to H 2 O at a single-site Cu-catalyst. The foot-of-the-wave analysis records a turnover frequency of 5.65 × 10 2 s −1 . At pH 7.0, complex 1 undergoes a quasi-reversible mixed metal−ligand-based reduction and triggers the reduction of dioxygen to water. Electrochemical studies in tandem with quantum chemical investigation, conducted at different redox states, portray the active participation of ligand in completing the process of proton-coupled electron transfer internally. The protonated carboxamido moiety acts as a proton relay, while the quinoline-based orbital supplies the necessary redox equivalent for the conversion of complex 1 to Cu(II)-hydroperoxo species. Thus, a suitable combination of redox non-innocence and proton shuttling functionality in the ligand makes it an effective electron−proton-transfer mediator and subsequently assists the process of oxygen reduction.

A Co(III)−hydroxo complex, [CoIII(dpaq)OH]ClO4 (1‐OH) bearing a pentadentate ligand, H‐dpaq, (H‐dpaq=(2‐[bis(pyridin‐2‐ylmethyl)]amino‐N‐quinolin‐8‐yl‐acetamidate]) catalyses water oxidation in mildly alkaline medium (pH 8.0) at a potential of 1.4 VNHE with an average Turn‐Over‐Frequency (TOFmax) of 2.8×104 s−1 and faradaic efficiency of 88 %. Post‐electrolysis characterization of the electrode rules out the formation of any heterogeneous electroactive species. Electrochemical results and theoretical calculations confirm the occurrence of both metal and ligand centered PCET processes during anodic scanning. The resulting formally Co(V)−oxo/oxyl intermediate undergoes water nucleophilic attack to install the O−O bond. The role of axial ligand in water oxidation by Co(III)−dpaq system has been examined by comparing the reactivity of the Co‐hydroxide complex (1‐OH) with that of its chloride‐ligated counterpart, [CoIII(dpaq)Cl]Cl (1‐Cl). The results confirm the ability of the Co‐dpaq complexes to bind water/or water derived ligands over chloride or non‐aqueous solvents. The interplay of ligand redox non‐innocence and σ‐donating ability of the N5‐carboxamido ligand helps to store oxidizing equivalents and triggers O−O bond formation.

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Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help to understand the mechanism of O―O bond formation presumably occurring at the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscores the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species, [(dpaq)MnIV(O)]+ (2), through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability and could be reverted to the starting complex (1) by switching the potential to 0.25 V vs. NHE. The formation of 2 via PCET oxidation of 1 demonstrates an alternate pathway for the generation of the oxomanganese(IV) transient (2) without the requirement of redox-inactive metal ions or acid additives as proposed earlier. Theoretical studies predict that one-electron oxidation of [(dpaq)MnIV(O)]+ (2) forms a manganese(V)-oxo (3) species, which can be oxidized further by one-electron to a formally manganese(VI)-oxo transient (4). Theoretical analyses suggest that the first oxidation event (2 to 3) takes place at the metal-based d-orbital whereas, in the second oxidation process (3 to 4), the electron eliminates from an orbital composed of equitable contribution from metal and ligand, leaving a single electron in the quinoline-dominated orbital in the doublet ground spin state of the manganese(VI)-oxo species (4). This mixed metal- ligand (quinoline)-based oxidation is proposed to generate a formally Mn(VI) species (4), a non-heme analogue of the species ‘compound I’, formed in the catalytic cycle of cytochrome P-450. We propose that the highly electrophilic species 4 catches water during cyclic voltammetry experiments and results in O―O bond formation leading to electrocatalytic oxidation of water to hydrogen peroxide.

A pentadentate Cu(II) complex, [CuII(dpaq)](ClO4) (1), featuring a redox active ligand, H-dpaq (H-dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), catalyses four-electron reduction of dioxygen by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CF3COOH)...