The Middle-Earth between Homogeneous and Heterogeneous Catalysis in Water Oxidation with Iridium

Harnessing the electronic structure of active metals to lower the overpotential of the electrocatalytic oxygen evolution reaction

Despite substantial advancements in the field of the electrocatalytic oxygen evolution reaction (OER), the efficiency of earth-abundant electrocatalysts remains far from ideal. The difficulty stems from the complex nature of the catalytic system, which limits our fundamental understanding of the process and thus the possibility of a rational improvement of performance. Herein, we shed light on the role played by the tunable 3d configuration of the metal centers in determining the OER catalytic activity by combining electrochemical and spectroscopic measurements with an experimentally validated computational protocol. One-dimensional coordination polymers based on Fe, Co and Ni held together by an oxonato linker were selected as a case study because of their well-defined electronic and geometric structure in the active site, which can be straightforwardly correlated with their catalytic activity. Novel heterobimetallic coordination polymers were also considered, in order to shed light on the cooperativity effects of different metals. Our results demonstrate the fundamental importance of electronic structure effects such as metal spin and oxidation state evolutions along the reaction profile to modulate ligand binding energies and increase catalyst efficiency. We demonstrated that these effects could in principle be exploited to reduce the overpotential of the electrocatalytic OER below its theoretical limit, and we provide basic principles for the development of coordination polymers with a tailored electronic structure and activity.

Dinuclear Cobalt Complexes for Homogeneous Water Oxidation: Tuning Rate and Overpotential through the Non-Innocent Ligand

In this study, dinuclear cobalt complexes (1 and 2) featuring bis(benzimidazole)pyrazolide-type ligands (H₂ L and Me₂ L) were prepared and evaluated as molecular electrocatalysts for water oxidation. Notably, 1 bearing a non-innocent ligand (H₂ L) displayed faster catalytic turnover than 2 under alkaline conditions, and the base dependence of water oxidation and kinetic isotope effect analysis indicated that the reaction mediated by 1 proceeded by a different mechanism relative to 2. Spectroelectrochemical, cold-spray ionization mass spectrometric and computational studies found that double deprotonation of 1 under alkaline conditions cathodically shifted the catalysis-initiating potential and further altered the turnover-limiting step from nucleophilic water attack on (H₂ L)Co^III ₂ (superoxo) to deprotonation of (L)Co^III ₂ (OH)₂ . The rate-overpotential analysis and catalytic Tafel plots showed that 1 exhibited a significantly higher rate than previously reported Ru-based dinuclear electrocatalysts at similar overpotentials. These observations suggest that using non-innocent ligands is a valuable strategy for designing effective metal-based molecular water oxidation catalysts.

Single-Site Iridium Picolinamide Catalyst Immobilized onto Silica for the Hydrogenation of CO2 and the Dehydrogenation of Formic Acid

The development of an efficient heterogeneous catalyst for storing H₂ into CO₂ and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol–gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L₃-edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO₂. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

We report the synthesis of a heterogeneous immobilized catalyst (Ir_PicaSi_SiO₂) and its successful application in aqueous CO₂ hydrogenation and FA dehydrogenation. The information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and XAS indicates the presence of cationic iridium sites in Ir_PicaSi_SiO₂. The latter shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO₂. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

Beyond Water Oxidation: Hybrid, Molecular-Based Photoanodes for the Production of Value-Added Organics

The political and environmental problems related to the massive use of fossil fuels prompted researchers to develop alternative strategies to obtain green and renewable fuels such as hydrogen. The light-driven water splitting process (i.e., the photochemical decomposition of water into hydrogen and oxygen) is one of the most investigated strategies to achieve this goal. However, the water oxidation reaction still constitutes a formidable challenge because of its kinetic and thermodynamic requirements. Recent research efforts have been focused on the exploration of alternative and more favorable oxidation processes, such as the oxidation of organic substrates, to obtain value-added products in addition to solar fuels. In this mini-review, some of the most intriguing and recent results are presented. In particular, attention is directed on hybrid photoanodes comprising molecular light-absorbing moieties (sensitizers) and catalysts grafted onto either mesoporous semiconductors or conductors. Such systems have been exploited so far for the photoelectrochemical oxidation of alcohols to aldehydes in the presence of suitable co-catalysts. Challenges and future perspectives are also briefly discussed, with special focus on the application of such hybrid molecular-based systems to more challenging reactions, such as the activation of C–H bonds.

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.

Recent Progress in Advanced Electrocatalyst Design for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers hold great significance for renewable energy storage and conversion. The acidic oxygen evolution reaction (OER) is one of the main roadblocks that hinder the practical application of PEM water electrolyzers. Highly active, cost-effective, and durable electrocatalysts are indispensable for lowering the high kinetic barrier of OER to achieve boosted reaction kinetics. To date, a wide spectrum of advanced electrocatalysts has been designed and synthesized for enhanced acidic OER performance, though Ir and Ru based nanostructures still represent the state-of-the-art catalysts. In this Progress Report, recent research progress in advanced electrocatalysts for improved acidic OER performance is summarized. First, fundamental understanding about acidic OER including reaction mechanisms and atomic understanding to acidic OER for rational design of efficient electrocatalysts are discussed. Thereafter, an overview of the progress in the design and synthesis of advanced acidic OER electrocatalysts is provided in terms of catalyst category, i.e., metallic nanostructures (Ir and Ru based), precious metal oxides, nonprecious metal oxides, and carbon based nanomaterials. Finally, perspectives to the future development of acidic OER are provided from the aspects of reaction mechanism investigation and more efficient electrocatalyst design.

Accessing Molecular Dimeric Ir Water Oxidation Catalysts from Coordination Precursors

One ongoing challenge in the field of iridium-based water oxidation catalysts is to develop a molecular precatalyst affording well-defined homogeneous active species for catalysis. Our previous work by using organometallic precatalysts Cp*Ir(pyalk)OH and Ir(pyalk)(CO)₂ (pyalk = (2-pyridyl)-2-propanolate) suggested a μ-oxo-bridged Ir dimer as the probable resting state, although the structure of the active species remained elusive. During the activation, the ligands Cp* and CO were found to oxidatively degrade into acetic acid or other products, which coordinate to Ir centers and affect the catalytic reaction. Two related dimers bearing two pyalk ligands on each iridium were crystallized for structural analysis. However, preliminary results indicated that these crystallographically characterized dimers are not active catalysts. In this work, we accessed a mixture of dinuclear iridium species from a coordination precursor, Na[Ir(pyalk)Cl₄], and assayed their catalytic activity for oxygen evolution by using NaIO₄ as the oxidant. This catalyst showed comparable oxygen-evolution activity to the ones previously reported from organometallic precursors without demanding oxidative activation to remove sacrificial ligands. Future research along this direction is expected to provide insights and design principles toward a well-defined active species.

Photoelectrochemical Water Oxidation by Cobalt Cytochrome C Integrated-ATO Photoanode

Here, we report the immobilization of Co-protoporphyrin IX (Co-PPIX) substituted cytochrome c (Co-cyt c) on Antimony-doped Tin Oxide (ATO) as a catalyst for photoelectrochemical oxidation of water. Under visible light irradiation (λ > 450 nm), the ATO-Co-cyt c photoanode displays ~6-fold enhancement in photocurrent density relative to ATO-Co-PPIX at 0.25 V vs. RHE at pH 5.0. The light-induced water oxidation activity of the system was demonstrated by detecting evolved stoichiometric oxygen by gas chromatography, and incident photon to current efficiency was measured as 4.1% at 450 nm. The faradaic efficiency for the generated oxygen was 97%, with a 671 turnover number (TON) for oxygen. The current density had a slow decay over the course of 6 h of constant irradiation and applied potential, which exhibits the robustness of catalyst-ATO interaction.

Homogeneous Catalysts Based on First-Row Transition-Metals for Electrochemical Water Oxidation

Strategies that enable the renewable production of storable fuels (i. e. hydrogen or hydrocarbons) through electrocatalysis continue to generate interest in the scientific community. Of central importance to this pursuit is obtaining the requisite chemical (H+ ) and electronic (e- ) inputs for fuel-forming reduction reactions, which can be met sustainably by water oxidation catalysis. Further possibility exists to couple these redox transformations to renewable energy sources (i. e. solar), thus creating a carbon neutral solution for long-term energy storage. Nature uses a Mn-Ca cluster for water oxidation catalysis via multiple proton-coupled electron-transfers (PCETs) with a photogenerated bias to perform this process with TOF 100∼300 s-1 . Synthetic molecular catalysts that efficiently perform this conversion commonly utilize rare metals (e. g., Ru, Ir), whose low abundance are associated to higher costs and scalability limitations. Inspired by nature's use of 1st row transition metal (TM) complexes for water oxidation catalysts (WOCs), attempts to use these abundant metals have been intensively explored but met with limited success. The smaller atomic size of 1st row TM ions lowers its ability to accommodate the oxidative equivalents required in the 4e- /4H+ water oxidation catalysis process, unlike noble metal catalysts that perform single-site electrocatalysis at lower overpotentials (η). Overcoming the limitations of 1st row TMs requires developing molecular catalysts that exploit biomimetic phenomena - multiple-metal redox-cooperativity, PCET and second-sphere interactions - to lower the overpotential, preorganize substrates and maintain stability. Thus, the ultimate goal of developing efficient, robust and scalable WOCs remains a challenge. This Review provides a summary of previous research works highlighting 1st row TM-based homogeneous WOCs, catalytic mechanisms, followed by strategies for catalytic activity improvements, before closing with a future outlook for this field.

Modulating the water oxidation catalytic activity of iridium complexes by functionalizing the Cp*-ancillary ligand: hints on the nature of the active species

A comparative study on the behavior of a series of iridium dimeric WOCs with modified Cp* ligands reveals the key role played by the variable substituent.

Redox-inactive metal single-site molecular complexes: a new generation of electrocatalysts for oxygen evolution?

Bypassing the metal-based oxidation in a Cu-containing water oxidation catalytic system.

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)₂(μ-κ³-O,N,O-H₄-EDTMP)] (Ir-H₄-EDTMP, H₄-EDTMP^4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)₂(μ-κ³-O,N,O-EDTA)] (Ir-EDTA, EDTA^4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO₂ nanocrystals obtaining Ir-H₄-EDTMP@TiO₂ and Ir-EDTA@TiO₂ hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO₄, providing almost quantitative yields, and TON values only limited by the amount of NaIO₄ used. As for the molecular catalysts, Ir-H₄-EDTMP (TOF up to 184 min^-1) exhibited much higher activity than Ir-EDTA (TOF up to 19 min^-1), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H₄-EDTMP@TiO₂ (up to 33 min^-1) and Ir-EDTA@TiO₂ (up to 41 min^-1) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts.

Effect of Ligand Chelation and Sacrificial Oxidant on the Integrity of Triazole-Based Carbene Iridium Water Oxidation Catalysts

We report the effect of replacing the pyridine group in the chelating trz Ir-water oxidation catalysts by a benzoxazole and a thiazole moiety. We have also evaluated if the presence of bidentate ligands is crucial for high activities and to avoid the decomposition into undesired heterogeneous layers. The catalytic performance of these benzoxazole/thiazole-triazolidene Ir-complexes in water oxidation was studied at variable pH using either CAN (pH = 1) or NaIO₄ (pH = 5.6 and 7). Electrocatalytic experiments indicated that while CAN-mediated water oxidation led to catalyst heterogeneization irrespective of the triazolylidene substituent, periodate as sacrificial oxidant preserved a homogeneously active species. Repetitive additions of sacrificial oxidant indicates higher integrity of the Ir-complex with a thiazole-substituted triazolylidene compared to ligands featuring a benzoxazole as chelating donor or no chelating group at all. Rigid chelation of the thiazole group was also established from stability measurements under highly acidic, oxidizing, and high ionic strength conditions.

Immobilization of Ir(OH)3 Nanoparticles in Mesospaces of Al-SiO2 Nanoparticles Assembly to Enhance Stability for Photocatalytic Water Oxidation

Iridium hydroxide (Ir(OH)3) nanoparticles exhibiting high catalytic activity for water oxidation were immobilized inside mesospaces of a silica-nanoparticles assembly (SiO2NPA) to suppress catalytic deactivation due to agglomeration. The Ir(OH)3 nanoparticles immobilized in SiO2NPA (Ir(OH)3/SiO2NPA) catalyzed water oxidation by visible light irradiation of a solution containing persulfate ion (S2O82−) and tris(2,2′-bipyridine)ruthenium(II) ion ([RuII(bpy)3]2+) as a sacrificial electron acceptor and a photosensitizer, respectively. The yield of oxygen (O2) based on the used amount of S2O82− was maintained over 80% for four repetitive runs using Ir(OH)3/SiO2NPA prepared by the co-accumulation method, although the yield decreased for the reaction system using Ir(OH)3/SiO2NPA prepared by the equilibrium adsorption method or Ir(OH)3 nanoparticles without SiO2NPA support under the same reaction conditions. Immobilization of Ir(OH)3 nanoparticles in Al3+-doped SiO2NPA (Al-SiO2NPA) results in further enhancement of the catalytic stability with the yield of more than 95% at the fourth run of the repetitive experiments.

Iridium-Doped Nanosized Zn-Al Layered Double Hydroxides as Efficient Water Oxidation Catalysts

Layered double hydroxides (LDHs) are an ideal platform to host catalytic metal centers for water oxidation (WO) owing to the high accessibility of water to the interlayer region, which makes all centers potentially reachable and activated. Herein, we report the syntheses of three iridium-doped zinc-aluminum LDHs (Ir-LDHs) nanomaterials (1-3, with about 80 nm of planar size and a thickness of 8 nm as derived by field emission scanning electron microscopy and powder X-ray diffraction studies, respectively), carried out in the confined aqueous environment of reverse micelles, through a very simple and versatile procedure. These materials exhibit excellent catalytic performances in WO driven by NaIO4 at neutral pH and 25 °C, with an iridium content as low as 0.5 mol % (∼0.8 wt %), leading to quantitative oxygen yields (based on utilized NaIO4, turnover number up to ∼10,000). Nanomaterials 1-3 display the highest ever reported turnover frequency values (up to 402 min-1) for any heterogeneous and heterogenized catalyst, comparable only to those of the most efficient molecular iridium catalysts, tested under similar reaction conditions. The boost in activity can be traced to the increased surface area and pore volume (>5 times and 1 order of magnitude, respectively, higher than those of micrometric materials of size 0.3-1 μm) estimated for the nanosized particles, which guarantee higher noble metal accessibility. X-ray absorption spectroscopy (XAS) studies suggest that 1-3 nanomaterials, as-prepared and after catalysis, contain a mixture of isolated, single octahedral Ir(III) sites, with no evidence of Ir-Ir scattering from second-nearest neighbors, excluding the presence of IrO2 nanoparticles. The combination of the results obtained from XAS, elemental analysis, and ionic chromatography strongly suggests that iridium is embedded in the brucite-like structure of LDHs, having four hydroxyls and two chlorides as first neighbors. These results demonstrate that nanometric LDHs can be successfully exploited to engineer efficient WOCs, minimizing the amount of iridium used, consistent with the principle of the noble-metal atom economy.

Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble Ni^II complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm^-2 for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiO_x . Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO₄ ^2- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [Ni^III (TAML^2- )-O^. ] is formed via several proton-coupled electron transfer processes and reacts with H₂ O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.

The Influence of the Ligand in the Iridium Mediated Electrocatalyic Water Oxidation

Electrochemical water oxidation is the bottleneck of electrolyzers as even the best catalysts, iridium and ruthenium oxides, have to operate at significant overpotentials. Previously, the position of a hydroxyl on a series of hydroxylpicolinate ligands was found to significantly influence the activity of molecular iridium catalysts in sacrificial oxidant driven water oxidation. In this study, these catalysts were tested under electrochemical conditions and benchmarked to several other known molecular iridium catalysts under the exact same conditions. This allowed us to compare these catalysts directly and observe whether structure–activity relationships would prevail under electrochemical conditions. Using both electrochemical quartz crystal microbalance experiments and X-ray photoelectron spectroscopy, we found that all studied iridium complexes form an iridium deposit on the electrode with binding energies ranging from 62.4 to 62.7 eV for the major Ir 4f_7/2 species. These do not match the binding energies found for the parent complexes, which have a broader binding energy range from 61.7 to 62.7 eV and show a clear relationship to the electronegativity induced by the ligands. Moreover, all catalysts performed the electrochemical water oxidation in the same order of magnitude as the maximum currents ranged from 0.2 to 0.6 mA cm^–2 once more without clear structure–activity relationships. In addition, by employing ¹H NMR spectroscopy we found evidence for Cp* breakdown products such as acetate. Electrodeposited iridium oxide from ligand free [Ir(OH)₆]^2– or a colloidal iridium oxide nanoparticles solution produces currents almost 2 orders of magnitude higher with a maximum current of 11 mA cm^–2. Also, this deposited material contains, apart from an Ir 4f_7/2 species at 62.4 eV, an Ir species at 63.6 eV, which is not observed for any deposit formed by the molecular complexes. Thus, the electrodeposited material of the complexes cannot be directly linked to bulk iridium oxide. Small IrO_x clusters containing few Ir atoms with partially incorporated ligand residues are the most likely option for the catalytically active electrodeposit. Our results emphasize that structure–activity relationships obtained with sacrificial oxidants do not necessarily translate to electrochemical conditions. Furthermore, other factors, such as electrodeposition and catalyst degradation, play a major role in the electrochemically driven water oxidation and should thus be considered when optimizing molecular catalysts.

A Remote 'Imidazole'-Based Ruthenium(II) Para-Cymene Pre-catalyst for the Selective Oxidation Reaction of Alkyl Arenes and Alcohols

Herein we disclosed the use of a remote 'imidazole'-based precatalyst [(para-cymene)Ru^II (L)Cl]⁺ , C-1 where L=2-(4-substituted-phenyl)-1H-imidazo[4,5-f]^[1,10] phenanthroline) for the selective oxidation of a variety of alkyl arenes/heteroarenes and alcohols to their corresponding aldehydes or ketones in presence of tert-butyl hydroperoxide (TBHP). The remote 'imidazole' moiety present in the complex facilitates the activation of oxidant and subsequent generation of active species via the release of para-cymene from C-1, which in-turn was less effective without the 'imidazole' moiety. The mechanistic features of C-1 promoted oxidation of alkyl arenes were also assessed from spectroscopic, kinetic, and few control experiments. The substrate scope for C-1 promoted oxidation reaction was assessed based on the selective oxidation of 27-different alkyl arenes/heteroarenes and 25 different alcohols to their corresponding aldehydes/ketones in moderate to good yields.

Novel iridium complexes with N-heterocyclic dicarbene ligands in light-driven water oxidation catalysis: photon management, ligand effect and catalyst evolution

Iridium complexes [IrClCp*diNHC]PF6, with N-heterocyclic dicarbene (diNHC) and pentamethylcyclopentadienyl (Cp*) ligands, have been investigated in light driven water oxidation catalysis within the Ru(bpy)32+/S2O82- cycle (bpy = 2,2'-bipyridine). In particular, the effect of different diNHC ligands was evaluated by employing the complex 1a (diNHC = 1,1'-dimethyl-3,3'-ethylenediimidazol-2,2'-diylidene) and the novel and structurally characterised 2 (diNHC = 1,1'-dimethyl-3,3'-ethylene-5,5'-dibromodiimidazol-2,2'-diylidene) and 3 (diNHC = 1,1'-dimethyl-3,3'-ethylene-dibenzimidazol-2,2'-diylidene). The presented results include: (i) a photon management analysis of the 1a/Ru(bpy)32+/S2O82- system, revealing two regimes of O2 evolution rate, being dependent on the light intensity at low photon flux, where the system reaches an overall quantum yield up to 0.17 ± 0.01 (quantum efficiency 34 ± 2%), while being independent of light intensity at high photon flux thus indicating a change of limiting step; (ii) a trend of O2 evolution activity that follows the order 1a > 2 > 3 both under low and high photon flux conditions, with the reactivity that is favoured by the electron donating nature of the diNHC ligand, quantified on the basis of the carbene carbon chemical shift; (iii) an analogous trend also in the bimolecular rate constants of electron transfer kET from the iridium species to photogenerated Ru(bpy)33+, with kET values in the range 4.2-6.1 × 104 M-1 s-1, thus implying a significant reorganisation energy to the iridium sphere; (iv) the evolution of 1a, as the most active Ir species in the series, to mononuclear iridium species with lower molecular weight and originating from oxidative transformation of the organic ligand scaffold, as proven by converging UV-Vis, MALDI-MS and 1H-NMR evidences. These results can be used for the further design and engineering of novel catalysts.

An iridium-based nanocomposite prepared from an iridium complex with a hydrocarbon-based ligand

For the first time, a chlorobis(cyclooctene)iridium(i) dimer with only a simple hydrocarbon-based ligand is investigated as a heterogeneous catalyst for the oxygen-evolution reaction in the presence of cerium(iv) ammonium nitrate.

Ir- and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation

Doping low-cost LDHs with noble metal atoms represents a promising approach to develop effective heterogeneous Water Oxidation Catalysts.

Light-Driven Water Oxidation with the Catalyst and the Ru(bpy)32+/S2O82- Cycle: Photogeneration of Active Dimers, Electron-Transfer Kinetics, and Light Synchronization for Oxygen Evolution with High Quantum Efficiency

Light-driven water oxidation is achieved with the Ru(bpy)₃²⁺/S₂O₈^2- cycle employing the highly active Ir-blue water oxidation catalyst, namely, an Ir^IV,IV₂(pyalc)₂ μ-oxo-dimer [pyalc = 2-(2'-pyridyl)-2-propanoate]. Ir-blue is readily formed by stepwise oxidation of the monomeric Ir(III) precursor 1 by the photogenerated Ru(bpy)₃³⁺, with a quantum yield ϕ of up to 0.10. Transient absorption spectroscopy and kinetic evidence point to a stepwise mechanism, where the primary event occurs via a fast photoinduced electron transfer from 1 to Ru(bpy)₃³⁺, leading to the Ir(IV) monomer I₁ (k₁ ∼ 10⁸ M^-1 s^-1). The competent Ir-blue catalyst is then obtained from I₁ upon photooxidative loss of the Cp* ligand and dimerization. The Ir-blue catalyst is active in the Ru(bpy)₃²⁺/S₂O₈^2- light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)₃³⁺ [with k_Ir-blue = (3.00 ± 0.02) × 10⁸ M^-1 s^-1 for the primary event, outperforming iridium oxide nanoparticles by ca. 2 orders of magnitude], leading to a Ir^V,V₂ steady-state intermediate involved in O-O bond formation. The quantum yield for oxygen evolution depends on the photon flux, showing a saturation regime and reaching an impressive value of ϕ(O₂) = 0.32 ± 0.01 (corresponding to a quantum efficiency of 64 ± 2%) at low irradiation intensity. This result highlights the key requirement of orchestrating the rate of the photochemical events with dark catalytic turnover.

Post Synthetic Defect Engineering of UiO-66 Metal–Organic Framework with An Iridium(III)-HEDTA Complex and Application in Water Oxidation Catalysis

Clean production of renewable fuels is a great challenge of our scientific community. Iridium complexes have demonstrated a superior catalytic activity in the water oxidation (WO) reaction, which is a crucial step in water splitting process. Herein, we have used a defective zirconium metal–organic framework (MOF) with UiO-66 structure as support of a highly active Ir complex based on EDTA with the formula [Ir(HEDTA)Cl]Na. The defects are induced by the partial substitution of terephthalic acid with smaller formate groups. Anchoring of the complex occurs through a post-synthetic exchange of formate anions, coordinated at the zirconium clusters of the MOF, with the free carboxylate group of the [Ir(HEDTA)Cl]− complex. The modified material was tested as a heterogeneous catalyst for the WO reaction by using cerium ammonium nitrate (CAN) as the sacrificial agent. Although turnover frequency (TOF) and turnover number (TON) values are comparable to those of other iridium heterogenized catalysts, the MOF exhibits iridium leaching not limited at the first catalytic run, as usually observed, suggesting a lack of stability of the hybrid system under strong oxidative conditions.

Catalytic Oxidation of Water to Dioxygen by Mononuclear Ru Complexes Bearing a 2,6-Pyridinedicarboxylato Ligand

The synthesis, purification, and isolation of mononuclear Ru complexes containing the tridentate dianionic meridional ligand pyridyl-2,6-dicarboxylato (pdc^2- ) of general formula [Ru^III (pdc-κ³ -N¹ O² )(bpy)Cl] (1^III ) and [Ru^II (pdc-κ² -N¹ O¹ )(bpy)₂ ] (2^II ) (bpy is 2,2'-bipyridine) is reported. These two complexes and their derivatives were thoroughly characterized through spectroscopic (UV/Vis, NMR) and electrochemical (cyclic voltammetry, differential pulse voltammetry, and coulometry) analyses, and three of the complexes were analyzed by single-crystal X-ray diffraction techniques. Under a high anodic applied potential, both complexes evolve towards the formation of Ru-aquo/oxo derivative species, namely, [Ru^III (pdc-κ³ -N¹ O² )(bpy)(OH₂ )]⁺ (1-O) and [Ru^IV (O)(pdc-κ² -N¹ O¹ )(bpy)₂ ] (2-O). These two complexes are active catalysts for the oxidation of water to dioxygen and their catalytic activity was analyzed through electrochemical techniques. A maximum turnover frequency (TOF_max )=2.4-3.4×10³  s^-1 was calculated for 2-O.

High-performance water oxidation catalysts based on the spontaneous deposition of ruthenium on electrochemically exfoliated graphene oxide

Sustainable highly active water oxidation reaction in acid over ruthenium loaded electrochemically exfoliated graphene oxide.

Molecular water oxidation catalysis by zwitterionic carboxylate bridge-functionalized bis-NHC iridium complexes

Carboxylate functionalized bis-NHC ligands allow for the stabilization of high-valent iridium intermediate species involved in homogeneous water oxidation catalysis.

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)₂(mcp)] (1α; mcp = N, N'-dimethyl- N, N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion) and β-[Fe(OTf)₂(mcp)] (1β) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1β generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1β with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation are shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation-sensitive sites. This information has served in creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. The deuterated analogues D₄-α-[Fe(OTf)₂(mcp)] (D₄-1α), D₄-β-[Fe(OTf)₂(mcp)] (D₄-1β), and D₆-β-[Fe(OTf)₂(mcp)] (D₆-1β) were prepared, and their catalytic activity has been studied. D₄-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O₂ yield); it exhibits initial reaction rates identical with those of its protio analogue, but it is substantially more robust toward oxidative degradation and yields more than 3400 TON ( n(O₂)/ n(Fe)). Altogether this evidences that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.