A new family of tetra-anionic tetradentate amidate ligands, N1,N1'-(1,2-phenylene)bis(N2-methyloxalamide) (H4L1), and its derivatives containing electron-donating groups at the aromatic ring have been prepared and characterized, together with their corresponding anionic Cu(II) complexes, [(LY)Cu](2-). At pH 11.5, the latter undergoes a reversible metal-based III/II oxidation process at 0.56 V and a ligand-based pH-dependent electron-transfer process at 1.25 V, associated with a large electrocatalytic water oxidation wave (overpotential of 700 mV). Foot-of-the-wave analysis gives a catalytic rate constant of 3.6 s(-1) at pH 11.5 and 12 s(-1) at pH 12.5. As the electron-donating capacity at the aromatic ring increases, the overpotential is drastically reduced down to a record low of 170 mV. In addition, DFT calculations allow us to propose a complete catalytic cycle that uncovers an unprecedented pathway in which crucial O-O bond formation occurs in a two-step, one-electron process where the peroxo intermediate generated has no formal M-O bond but is strongly hydrogen bonded to the auxiliary ligand.

Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal-oxide photoanodes remains a significant experimental and theoretical challenge. Here we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly upon hematite. We observe that hematite is able to access a reaction mechanism third order in surface hole density, assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits a remarkably low activation energy (Ea ~ 60 meV). Density functional theory is employed to determine the energetics of charge accumulation and O-O bond formation on a model hematite 110 surface. The proposed mechanism shows parallels with the function of oxygen evolving complex of photosystem II, and provides new insights to the mechanism of heterogeneous water oxidation on a metal oxide surface.

There is an urgent need to transition from fossil fuels to solar fuels -not only to lower CO2 emissions that cause global warming, but also to ration fossil resources. Splitting H2O with sunlight emerges as a clean and sustainable energy conversion scheme that can afford practical technologies in the short to midterm. A crucial component in such a device is a water oxidation catalyst (WOC). These artificial catalysts have mainly been developed over the last two decades, which is in contrast to Nature's WOCs, which have featured in its photosynthetic apparatus for more than a billion years. This time period has seen the development of increasingly active molecular WOCs, the study of which affords an understanding of catalytic mechanisms and decomposition pathways. This Perspective offers a historical description of the landmark molecular WOCs, particularly ruthenium systems, that have guided research to our present degree of understanding.

Energy has been a central subject for human development from Homo erectus to date. The massive use of fossil fuels during the last 50 years has generated a large CO concentration in the atmosphere that has led to the so-called global warming. It is very urgent to come up with C-neutral energy schemes to be able to preserve Planet Earth for future generations to come and still preserve our modern societies' life style. One of the potential solutions is water splitting with sunlight (hν-WS) that is also associated with "artificial photosynthesis", since its working mode consists of light capture followed by water oxidation and proton reduction processes. The hydrogen fuel generated in this way is named as "solar fuel". For this set of reactions, the catalytic oxidation of water to dioxygen is one of the crucial processes that need to be understood and mastered in order to build up potential devices based on hν-WS. This tutorial describes the different important aspects that need to be considered to come up with efficient and oxidatively robust molecular water oxidation catalysts (Mol-WOCs). It is based on our own previous work and completed with essential contributions from other active groups in the field. We mainly aim at describing how the ligands can influence the properties of the Mol-WOCs and showing a few key examples that overall provide a complete view of today's understanding in this field.

Electrocatalytic approaches to the activation of unsaturated substrates via reductive concerted proton-electron transfer (CPET) must overcome competing, often kinetically dominant hydrogen evolution. We introduce the design of a molecular mediator for electrochemically triggered reductive CPET through the synthetic integration of a Brønsted acid and a redox mediator. Cathodic reduction at the cobaltocenium redox mediator substantially weakens the homolytic nitrogen-hydrogen bond strength of a Brønsted acidic anilinium tethered to one of the cyclopentadienyl rings. The electrochemically generated molecular mediator is demonstrated to transform a model substrate, acetophenone, to its corresponding neutral α-radical via a rate-determining CPET.

A molecular water oxidation catalyst based on the copper complex of general formula [(Lpy)Cu II ] 2-, 2 2-, (Lpy is 4-pyrenyl-1,2-phenylenebis(oxamidate) ligand) has been rationally designed and prepared to support a more extended π-conjugation through its structure in contrast with its homologue, the [(L)Cu II ] 2-water oxidation catalyst, 1 2-(L is ophenylenebis(oxamidate)). The catalytic performance of both catalysts has been comparatively studied in homogeneous phase and in heterogeneous phase by π-stacking anchorage to graphene-based electrodes. In the homogeneous system, the electronic perturbation provided by the pyrene functionality translates into a 150 mV lower overpotential for 2 2-respect to 1 2-and an impressive increase in the kcat from 6 s -1 to 128 s -1 . Upon anchorage, π-stacking interactions with the graphene sheets provide further π-delocalization that improves the catalytic performance of both catalysts. In this sense, 2 2-turned out to be the most active catalyst due to the double influence of both the pyrene and the graphene, displaying an overpotential of 538 mV, a kcat of 540 s -1 and producing more than 5300 TONs.Heterogenized water-oxidation catalysis based on earth abundant transition metals, such as Mn, Fe, Co, Ni and Cu, are highly desired for sustainable energy technologies that exploit direct solar water-splitting. 1 An advantage of heterogenized homogeneous catalysts, when compared to heterogeneous catalysts, 2 is that they can be improved by ligand design. Yet first-row transition metal complexes pose several challenges. They usually get deactivated when immobilized on electrode surfaces and they suffer from instability due to hydrolytic behavior and decomposition into metal-oxides upon oxidation of the organic ligands. 3 However, from an engineering perspective, solid-state electroanodes are desired due to the simplicity of assembly for potential devices. Therefore, it is imperative to understand the influence of the anchoring functionality on the performance of the immobilized catalysts to learn how to anchor and stabilize functional molecular catalysts on electrode surfaces. 4,5 Here, we focus on water oxidation by Cu(II) molecular catalysts heterogenized on graphene surfaces.A family of copper complexes based on tetraamide ligands, such as [(L)Cu II ] 2-, 1 2-, (L = o-phenylenebis(oxamidate)) shown in Figure 1, have been recently reported to be effective at catalyzing oxygen evolution by water oxidation at basic pH. 6 Remarkably, the rate determining step (rds) was found to involve reversible oxidation of the phenyl ring. Here, we explore whether the catalytic properties of these complexes can be manipulated by electronic perturbation of the tetraamide π-system, either by modification of the ligand or by π-stacking to graphitic electrode surfaces.

S1. General ProceduresGeneral Considerations: All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 or Ar atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with N2 gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. For electrochemical measurements under an Ar atmosphere, solvents were further degassed and then left under Ar. All solvents were stored over activated 4 Å molecular sieves prior to use. Anhydrous ammonia gas was dried by passage through a calcium oxide drying tube. All reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Tris(2pyridylmethyl)amine (TPA) 1 and tris(2-pyridylmethylamine) iron(II) triflate bis acetonitrile 2 were synthesized according to literature procedures. 15 NH4OTf was prepared from 15 NH4Cl (Cambridge Isotope Laboratories) by anion exchange with silver triflate followed by repeated recrystallization from acetonitrile. 1 H NMR chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards.Electrochemistry: Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV), Differential Pulse Voltammetry (DPV) and Controlled Potential Coulometry (CPC) experiments were carried out with a Biologic VSP-300 potentiostat using a one-compartment three-electrode cell. For CV, LSV and DPV, a Boron Doped Diamond (BDD) disk electrode (3 mm diameter) was used as the working electrode, Pt wire as the counter electrode, and a Ag/AgOTf reference electrode was employed using an acetonitrile solution containing 5 mM AgOTf and 0.1 M TBAPF6. For CPE, the same reference electrode was used, but a BDD plate (geometric area: 1 cm 2 ) and a Pt mesh were used respectively as working and counter electrode. All redox potentials in the present work are reported versus the Fc/Fc + couple, measured before each experiment to be +0.115 V versus our Ag/AgOTf reference electrode.CVs and LSVs were collected at 100 mV•s -1 unless specified otherwise. DPVs were obtained with the following parameters: amplitude = 50 mV, step height = 4 mV, pulse width = 0.05 s, pulse period = 0.5 s and sampling width = 0.0167 s. E1/2 values for the reversible waves were obtained from the half potential between the oxidative and reductive peaks. All measurements were performed applying IR compensation, compensating 85% of the resistance measured at one high frequency value (100 kHz).Gas Chromatography: Gas chromatography was performed in the Environmental Analysis Center using HP 5890 Series II instruments. Gas quantification was performed using a molecular sieve column attached to a thermal conductivity detector. Argon was the carrier gas. Standard curves were generated by direct injection of hydrogen or nitrogen gas. Quantification of background nitrogen was determined using the background oxygen signal. Isotopic measurements were performed with a separate HP 5890 Series II equipped ...

The systematic computational study of the mechanism for water oxidation in four different complexes confirms the existence of an alternative mechanism to those previously