Photochemical Water Oxidation Using a Doubly N-Confused Hexaphyrin Dinuclear Cobalt Complex

The Catalytic Mechanism of a Highly Active Cobalt Chlorin Complex for Photocatalytic Water Oxidation

Highly active catalysts for electrocatalytic and photocatalytic water oxidation are strongly demanded to realize artificial photosynthesis. A cobalt complex with a chlorin derivative ligand (CoII(Ch)) exhibited high activity for electrocatalytic water oxidation with an overpotential of 0.45 V at pH 9.0. Spectroelectrochemistry (UV-vis) unveiled the formation of two intermediates by successive one-electron oxidations. Also, the Pourbaix diagram depicted by the pH dependence of redox potentials indicated that the water oxidation proceeded after the oxidation of both the central cobalt ion and chlorin ligand with proton-coupled electron transfer (PCET). Then, the photocatalytic activity of CoII(Ch) was examined for water oxidation using [RuII(bpy)3]2+ (bpy: 2,2'-bipyridine) and S2O82- as a photosensitizer and a sacrificial electron acceptor, respectively. The turnover number, turnover frequency, and oxygen yield reached as high as 980, 5.2 s-1, and 98%, respectively, under optimized conditions. The O2-evolution rates increased in proportion to the square of the catalyst concentration in the reaction solution, suggesting that the formation of the O-O bond regarded as the rate-determining step of water oxidation proceeded by the interaction of two metal centers (I2M) mechanism in which two molecules of high-valent metal oxo or oxyl radical species react with each other.

Enhanced catalytic activity of solubilised species obtained by counter-cation exchange of K{CoII 1.5[FeII(CN)6]} for water oxidation

A cyano-bridged coordination polymer, K{CoII 1.5[FeII(CN)6]} {(K)Co-Fe}, reported as a highly active heterogeneous catalyst for water oxidation was solubilised by a conventional counter-cation exchange of K+ with Me4N+ ions to provide the homogeneous catalyst of (Me4N){CoII 1.5[FeII(CN)6]} {(Me4N)Co-Fe}. (Me4N)Co-Fe exhibited enhanced catalytic activity for photocatalytic water oxidation using [Ru(2,2'-bipyridine)3]2+ and S2O8 2- as a photosensitiser and a sacrificial electron acceptor, respectively, in terms of the initial reaction rate (1.26 μmol min-1), which is about twice that of (K)Co-Fe (0.61 μmol min-1). Powder X-ray diffraction, pair distribution function and electrospray ionization mass spectrometry measurements of (Me4N)Co-Fe manifested that small heptanuclear clusters of {Co4[Fe(CN)6]3}4- formed by depolymerisation are catalytically active species in solution.

An intramolecular cobalt-peptoid complex as an efficient electrocatalyst for water oxidation at low overpotential

Water electrolysis is the simplest way to produce hydrogen, as a clean renewable fuel. However, the high overpotential and slow kinetics hamper its applicability. Designing efficient and stable electrocatalysts for water oxidation (WO), which is the first and limiting step of the water splitting process, can overcome this limitation. However, the development of such catalysts based on non-precious metal ions is still challenging. Herein we describe a bio-inspired Co(iii)-based complex i.e., a stable and efficient molecular electrocatalyst for WO, constructed from a peptidomimetic oligomer called peptoid - N-substituted glycine oligomer - bearing two binding ligands, terpyridine and bipyridine, and one ethanolic group as a proton shuttler. Upon binding of a cobalt ion, this peptoid forms an intramolecular Co(iii) complex, that acts as an efficient electrocatalyst for homogeneous WO in aqueous phosphate buffer at pH 7 with a high faradaic efficiency of up to 92% at an overpotential of about 430 mV, which is the lowest reported for Co-based homogeneous WO electrocatalysts to date. We demonstrated the high stability of the complex during electrocatalytic WO and that the ethanolic side chain plays a key role in the stability and activity of the complex and also in facilitating water binding, thus mimicking an enzymatic second coordination sphere.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Energy Landscape for the Electrocatalytic Oxidation of Water by a Single-Site Oxomanganese(V) Porphyrin

A cationic manganese porphyrin, Mn^III-TDMImP, is an efficient, homogeneous, single-site water oxidation electrocatalyst at neutral pH. The measured turnover frequency for oxygen production is 32 s^-1. Mechanistic analyses indicate that Mn^V(O)(OH₂), the protonated form of the corresponding trans-Mn^V(O)₂ species, is generated from the Mn^III(OH₂)₂ precursor in a 2-e^- two-proton process and is responsible for O-O bond formation with a H₂O molecule. Chloride ion is a competitive substrate with H₂O for the Mn^V(O)(OH₂) oxidant, forming hypochlorous acid with a rate constant that is 3 orders of magnitude larger than that of water oxidation. The data allow the construction of an experimental energy landscape for this water oxidation catalysis process.

High quantum yield photochemical water oxidation using a water-soluble cobalt phthalocyanine as a homogenous catalyst

We demonstrated high catalytic activity (TON = 670, TOF_max = 2.7 s^-1) of a water-soluble cobalt phthalocyanine complex (CoPcTS, PcTS = phthalocyaninetetrasulfonate) for visible light-driven photochemical water oxidation and investigated its reaction mechanism by electrochemical and spectroscopic measurements.

Homogeneous Water Oxidation Catalyzed by First-Row Transition Metal Complexes: Unveiling the Relationship between Turnover Frequency and Reaction Overpotential

The utilization of earth-abundant low-toxicity metal ions in the construction of highly active and efficient molecular catalysts promoting the water oxidation reaction is important for developing a sustainable artificial energy cycle. However, the kinetic and thermodynamic properties of the currently available molecular water oxidation catalysts (MWOCs) have not been comprehensively investigated. This Review summarizes the current status of MWOCs based on first-row transition metals in terms of their turnover frequency (TOF, a kinetic property) and overpotential (η, a thermodynamic property) and uses the relationship between log(TOF) and η to assess catalytic performance. Furthermore, the effects of the same ligand classes on these MWOCs are discussed in terms of TOF and η, and vice versa. The collective analysis of these relationships provides a metric for the direct comparison of catalyst systems and identifying factors crucial for catalyst design.

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.