Bioinspired Water Preorganization in Confined Space for Efficient Water Oxidation Catalysis in Metallosupramolecular Ruthenium Architectures

ConspectusNature has established a sustainable way to maintain aerobic life on earth by inventing one of the most sophisticated biological processes, namely, natural photosynthesis, which delivers us with organic matter and molecular oxygen derived from the two abundant resources sunlight and water. The thermodynamically demanding photosynthetic water splitting is catalyzed by the oxygen-evolving complex in photosystem II (OEC-PSII), which comprises a distorted tetramanganese-calcium cluster (CaMn4O5) as catalytic core. As an ubiquitous concept for fine-tuning and regulating the reactivity of the active site of metalloenzymes, the surrounding protein domain creates a sophisticated environment that promotes substrate preorganization through secondary, noncovalent interactions such as hydrogen bonding or electrostatic interactions. Based on the high-resolution X-ray structure of PSII, several water channels were identified near the active site, which are filled with extensive hydrogen-bonding networks of preorganized water molecules, connecting the OEC with the protein surface. As an integral part of the outer coordination sphere of natural metalloenzymes, these channels control the substrate and product delivery, carefully regulate the proton flow by promoting pivotal proton-coupled electron transfer processes, and simultaneously stabilize short-lived oxidized intermediates, thus highlighting the importance of an ordered water network for the remarkable efficiency of the natural OEC.Transferring this concept from nature to the engineering of artificial metal catalysts for fuel production has fostered the fascinating field of metallosupramolecular chemistry by generating defined cavities that conceptually mimic enzymatic pockets. However, the application of supramolecular approaches to generate artificial water oxidation catalysts remained scarce prior to our initial reports, since such molecular design strategies for efficient activation of substrate water molecules in confined nanoenvironments were lacking. In this Account, we describe our research efforts on combining the state-of-the art Ru(bda) catalytic framework with structurally programmed ditopic ligands to guide the water oxidation process in defined metallosupramolecular assemblies in spatial proximity. We will elucidate the governing factors that control the quality of hydrogen-bonding water networks in multinuclear cavities of varying sizes and geometries to obtain high-performance, state-of-the-art water oxidation catalysts. Pushing the boundaries of artificial catalyst design, embedding a single catalytic Ru center into a well-defined molecular pocket enabled sophisticated water preorganization in front of the active site through an encoded basic recognition site, resulting in high catalytic rates comparable to those of the natural counterpart OEC-PSII.To fully explore their potential for solar fuel devices, the suitability of our metallosupramolecular assemblies was demonstrated under (electro)chemical and photocatalytic water oxidation conditions. In addition, testing the limits of structural diversity allowed the fabrication of self-assembled linear coordination oligomers as novel photocatalytic materials and long-range ordered covalent organic framework (COF) materials as recyclable and long-term stable solid-state materials for future applications.

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures

The conversion of light into chemical energy is the game-changer enabling technology for the energetic transition to renewable and clean solar fuels. The photochemistry of interest includes the overall reductive/oxidative splitting of water into hydrogen and oxygen and alternatives based on the reductive conversion of carbon dioxide or nitrogen, as primary sources of energy-rich products. Devices capable of performing such transformations are based on the integration of three sequential core functions: light absorption, photo-induced charge separation, and the photo-activated breaking/making of molecular bonds via specific catalytic routes. The key to success does not rely simply on the individual components' performance, but on their optimized integration in terms of type, number, geometry, spacing, and linkers dictating the photosynthetic architecture. Natural photosynthesis has evolved along this concept, by integrating each functional component in one specialized "body" (from the Greek word "soma") to enable the conversion of light quanta with high efficiency. Therefore, the natural "quantasome" represents the key paradigm to inspire man-made constructs for artificial photosynthesis. The case study presented in this perspective article deals with the design of artificial photosynthetic systems for water oxidation and oxygen production, engineered as molecular architectures then rendered on electrodic surfaces. Water oxidation to oxygen is indeed the pervasive oxidative reaction used by photosynthetic organisms, as the source of reducing equivalents (electrons and protons) to be delivered for the processing of high-energy products. Considering the vast and abundant supply of water (including seawater) as a renewable source on our planet, this is also a very appealing option for photosynthetic energy devices. We will showcase the progress in the last 15 years (2009-2023) in the strategies for integrating functional building blocks as molecular photosensitizers, multi-redox water oxidation catalysts and semiconductor materials, highlighting how additional components such as redox mediators, hydrophilic/hydrophobic pendants, and protective layers can impact on the overall photosynthetic performance. Emerging directions consider the modular tuning of the multi-component device, in order to target a diversity of photocatalytic oxidations, expanding the scope of the primary electron and proton sources while enhancing the added-value of the oxidation product beyond oxygen: the selective photooxidation of organics combines the green chemistry vision with renewable energy schemes and is expected to explode in coming years.

Folding-Induced Promotion of Proton-Coupled Electron Transfers via Proximal Base for Light-Driven Water Oxidation

Proton-coupled electron-transfer (PCET) processes play a key role in biocatalytic energy conversion and storage, for example, photosynthesis or nitrogen fixation. Here, we report a series of bipyridine-containing di- to tetranuclear Ru(bda) macrocycles 2 C-4 C (bda: 2,2'-bipyridine-6,6'-dicarboxylate) to promote O-O bond formation. In photocatalytic water oxidation under neutral conditions, all complexes 2 C-4 C prevail in a folded conformation that support the water nucleophilic attack (WNA) pathway with remarkable turnover frequencies of up to 15.5 s^-1 per Ru unit respectively. Single-crystal X-ray analysis revealed an increased tendency for intramolecular π-π stacking and preorganization of the proximal bases close to the active centers for the larger macrocycles. H/D kinetic isotope effect studies and electrochemical data demonstrate the key role of the proximal bipyridines as proton acceptors in lowering the activation barrier for the crucial nucleophilic attack of H₂ O in the WNA mechanism.

Self-assembled Ru(bda) Coordination Oligomers as Efficient Catalysts for Visible Light-Driven Water Oxidation in Pure Water

Water-soluble multinuclear complexes based on ruthenium 2,2'-bipyridine-6,6'-dicarboxylate (bda) and ditopic bipyridine linker units are investigated in three-component visible light-driven water oxidation catalysis. Systematic studies revealed a strong enhancement of the catalytic efficiency in the absence of organic co-solvents and with increasing oligomer length. In-depth kinetic and morphological investigations suggest that the enhanced performance is induced by the self-assembly of linear Ru(bda) oligomers into aggregated superstructures. The obtained turnover frequencies (up to 14.9 s^-1 ) and turnover numbers (more than 1000) per ruthenium center are the highest reported so far for Ru(bda)-based photocatalytic water oxidation systems.

Recent findings and future directions in photosynthetic hydrogen evolution using polypyridine cobalt complexes

The production of hydrogen gas using water as the molecular substrate currently represents one of the most challenging and appealing reaction schemes in the field of artificial photosynthesis (AP), i.e., the conversion of solar energy into fuels. In order to be efficient, this process requires a suitable combination of a light-harvesting sensitizer, an electron donor, and a hydrogen-evolving catalyst (HEC). In the last few years, cobalt polypyridine complexes have been discovered to be competent molecular catalysts for the hydrogen evolution reaction (HER), showing enhanced efficiency and stability with respect to previously reported molecular species. This perspective collects information about all relevant cobalt polypyridine complexes employed for the HER in aqueous solution under light-driven conditions in the presence of Ru(bpy)32+ (where bpy = 2,2'-bipyridine) as the photosensitizer and ascorbate as the electron donor, trying to highlight promising chemical motifs and aiming towards efficient catalytic activity in order to stimulate further efforts to design molecular catalysts for hydrogen generation and allow their profitable implementation in devices. As a final step, a few suggestions for the benchmarking of HECs employed under light-driven conditions are introduced.

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles

The luminophore Ru(bpy)2 (dcbpy)2+ (bpy=2,2'-bipyridine; dcbpy=4,4'-dicarboxy-2,2'-bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru-decorated chitosan fibers (NS-RuCh), with a 20 : 1 ratio between chitosan repeating units and RuII chromophores. The properties of the RuII compound are unperturbed by the chitosan structure, with NS-RuCh exhibiting the typical metal-to-ligand charge-transfer (MLCT) absorption and emission bands of RuII complexes. When crosslinks are made in the presence of IrO2 nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO2 ⊂NS-RuCh system, in which both RuII photosensitizers and IrO2 water oxidation catalysts are within the nanofiber structures. NS-RuCh and IrO2 ⊂NS-RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy-dispersive X-ray analysis, which indicated a 2 : 1 ratio between RuII chromophores and IrO2 species. Photochemical water oxidation has been investigated by using IrO2 ⊂NS-RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O2 with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)3 2+ and IrO2 nanoparticles (0.05) or with respect to that obtained when using NS-RuCh and "free" IrO2 nanoparticles (0.10). A fast hole-scavenging process (rate constant, 7×104  s-1 ) involving the oxidized photosensitizer and the IrO2 catalyst within the IrO2 ⊂NS-RuCh system is behind the improved photochemical quantum yield of IrO2 ⊂NS-RuCh.

Water-Assisted Concerted Proton-Electron Transfer at Co(II)-Aquo Sites in Polyoxotungstates With Photogenerated RuIII (bpy)33+ Oxidant

The cobalt substituted polyoxotungstate [Co6 (H2 O)2 (α-B-PW9 O34 )2 (PW6 O26 )]17- (Co6) displays fast electron transfer (ET) kinetics to photogenerated RuIII (bpy)33+ , 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one-electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II)-OH2 and Co6(II)-OH, respectively, whose speciation in aqueous solution is associated to a pKa of 7.6), and generates a Co(III)-OH moiety (Co6(III)-OH), as proven by transient absorption spectroscopy; (ii) at pH>pKa , the Co6(II)-OH→RuIII (bpy)33+ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH Collapse

Key Words

• Co-aquo moiety
• cobalt polyoxometalate
• flash photolysis
• hydrogen bonding
• proton coupled electron transfer

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MESH Headings

• Cobalt/chemistry
• Coordination Complexes/chemical synthesis
• Coordination Complexes/chemistry
• Electrons
• Kinetics
• Light
• Organometallic Compounds/chemistry
• Organometallic Compounds/radiation effects
• Oxidants/chemistry
• Oxidants/radiation effects
• Oxidation-Reduction
• Polymers/chemical synthesis
• Polymers/chemistry
• Protons
• Ruthenium/chemistry
• Ruthenium/radiation effects
• Tungsten Compounds/chemical synthesis
• Tungsten Compounds/chemistry
• Water/chemistry

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Grants

• Fondazione Cariparo
• 10BIRD2018-UNIPD Department of Chemical Sciences at University of Padova
• 2017PBXPN4 Department of Chemical Sciences at University of Padova
• FAR 2020 University of Ferrara
• Fondazione Cariparo
• University of Ferrara

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Affiliation(s)

Francesco Rigodanza Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly Consiglio Nazionale delle Ricerche (C.N.R.)Institute on Membrane Technology section of Padovavia Marzolo 135131PadovaItaly
Nadia Marino Department of Chemistry and Chemical TechnologiesUniversity of Calabria87036Arcavacata di Rende (CS)Italy
Alessandro Bonetto Dept. Environmental Sciences, Informatics and StatisticsUniversity Ca' Foscari Venice VegaparkVia delle Industrie 21/830175Marghera, VeniceItaly
Antonio Marcomini Dept. Environmental Sciences, Informatics and StatisticsUniversity Ca' Foscari Venice VegaparkVia delle Industrie 21/830175Marghera, VeniceItaly
Marcella Bonchio Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly Consiglio Nazionale delle Ricerche (C.N.R.)Institute on Membrane Technology section of Padovavia Marzolo 135131PadovaItaly
Mirco Natali Department of Chemical, Pharmaceutical and Agricultural Sciences (DOCPAS)University of Ferrara, and Centro Interuniversitario per la Conversione Chimica dell'Energia Solare (SOLARCHEM) sez. di Ferraravia L. Borsari 4644121FerraraItaly
Andrea Sartorel Department of Chemical SciencesUniversity of Padovavia Marzolo 135131PadovaItaly

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A Calix[4]arene-Based Cyclic Dinuclear Ruthenium Complex for Light-Driven Catalytic Water Oxidation

A cyclic dinuclear ruthenium(bda) (bda: 2,2'-bipyridine-6,6'-dicarboxylate) complex equipped with oligo(ethylene glycol)-functionalized axial calix[4]arene ligands has been synthesized for homogenous catalytic water oxidation. This novel Ru(bda) macrocycle showed significantly increased catalytic activity in chemical and photocatalytic water oxidation compared to the archetype mononuclear reference [Ru(bda)(pic)2 ]. Kinetic investigations, including kinetic isotope effect studies, disclosed a unimolecular water nucleophilic attack mechanism of this novel dinuclear water oxidation catalyst (WOC) under the involvement of the second coordination sphere. Photocatalytic water oxidation with this cyclic dinuclear Ru complex using [Ru(bpy)3 ]Cl2 as a standard photosensitizer revealed a turnover frequency of 15.5 s-1 and a turnover number of 460. This so far highest photocatalytic performance reported for a Ru(bda) complex underlines the potential of this water-soluble WOC for artificial photosynthesis.

The role of the capping agent and nanocrystal size in photoinduced hydrogen evolution using CdTe/CdS quantum dot sensitizers

Hydrogen production via light-driven water splitting is a key process in the context of solar energy conversion. In this respect, the choice of suitable light-harvesting units appears as a major challenge, particularly as far as stability issues are concerned. In this work, we report on the use of CdTe/CdS QDs as photosensitizers for light-assisted hydrogen evolution in combination with a nickel bis(diphosphine) catalyst (1) and ascorbate as the sacrificial electron donor. QDs of different sizes (1.7-3.4 nm) and with different capping agents (MPA, MAA, and MSA) have been prepared and their performance assessed in the above-mentioned photocatalytic reaction. Detailed photophysical studies have been also accomplished to highlight the charge transfer processes relevant to the photocatalytic reaction. Hydrogen evolution is observed with remarkable efficiencies when compared to common coordination compounds like Ru(bpy)₃²⁺ (where bpy = 2,2'-bipyridine) as light-harvesting units. Furthermore, the hydrogen evolution performance under irradiation is strongly determined by the nature of the capping agent and the QD size and can be related to the concentration of the surface defects within the semiconducting nanocrystal. Overall, the present results outline how QDs featuring large quantum yields and long lifetimes are desirable to achieve sustained hydrogen evolution upon irradiation and that a precise control of the structural and photophysical properties thus appears as a major requirement towards profitable photocatalytic applications.

Impact of substituents on molecular properties and catalytic activities of trinuclear Ru macrocycles in water oxidation

Herein we report a broad series of new trinuclear supramolecular Ru(bda) macrocycles bearing different substituents at the axial or equatorial ligands which enabled investigation of substituent effects on the catalytic activities in chemical and photocatalytic water oxidation. Our detailed investigations revealed that the activities of these functionalized macrocycles in water oxidation are significantly affected by the position at which the substituents were introduced. Interestingly, this effect could not be explained based on the redox properties of the catalysts since these are not markedly influenced by the functionalization of the ligands. Instead, detailed investigations by X-ray crystal structure analysis and theoretical simulations showed that conformational changes imparted by the substituents are responsible for the variation of catalytic activities of the Ru macrocycles. For the first time, macrocyclic structure of this class of water oxidation catalysts is unequivocally confirmed and experimental indication for a hydrogen-bonded water network present in the cavity of the macrocycles is provided by crystal structure analysis. We ascribe the high catalytic efficiency of our Ru(bda) macrocycles to cooperative proton abstractions facilitated by such a network of preorganized water molecules in their cavity, which is reminiscent of catalytic activities of enzymes at active sites.

Conformational changes induced by ligand substituents in macrocyclic Ru complexes strongly affect their chemical and photocatalytic efficiencies in water oxidation.

Ru(ii) water oxidation catalysts with 2,3-bis(2-pyridyl)pyrazine and tris(pyrazolyl)methane ligands: assembly of photo-active and catalytically active subunits in a dinuclear structure

Two mononuclear Ru(ii) complexes, i.e. [RuCl(κ³N-terpy)(κ²N-dpp)]PF₆ ([1]PF₆; terpy = 2,2':6',2''-terpyridine; dpp = 2,3-bis(2'-pyridyl-pyrazine) and [RuCl(κ³N-tpm)(κ²N-dpp)]Cl ([2]Cl; tpm = tris(1-pyrazolyl)methane), and one dinuclear complex, i.e. [Ru₂Cl(κ³N-tpm)(μ-κ²N:κ²N-dpp)Ru(κ²N-bpy)₂][PF₆]₃ ([3][PF₆]₃; bpy = 2,2'-bipyridine), have been synthesized and their water oxidation catalytic properties have been investigated. A combined DFT and experimental (³⁵Cl NMR and conductivity measurements) study aimed to elucidate the nature of [1]⁺ and [2]⁺ in aqueous solution has also been performed, indicating that one water molecule is allowed to enter the first coordination sphere of [2]⁺ in the ground state, replacing one tpm nitrogen. Conversely, in the case of [1]⁺, water coordination, assumed to be needed for the water oxidation process, presumably occurs following the oxidation of the metal. For all complexes, a catalytic wave has been detected in acetonitrile/water 1 : 1 (v/v) solution in the range 1.4-1.7 V vs. SCE. In all cases, water oxidation (investigated at pH < 8) takes place initially via a proton-coupled two-electron, two-proton process with the formation of an Ru(iv)[double bond, length as m-dash]O moiety, followed by one electron oxidation and water nucleophilic attack. The TON and TOF values (within the range of 16-33 and 1.3-2.2 h^-1, respectively) of the complexes are higher than those of the benchmark [Ru(LLL)(LL)(OH₂)]²⁺-type species (LLL and LL are tridentate and bidentate polypyridine ligands, respectively), which is [Ru(terpy)(bpm)(OH₂)]²⁺.

Polyoxometalate-based composite materials in electrochemistry: state-of-the-art progress and future outlook

Polyoxometalates (POMs) have been developed as a class of promising smart material candidates not only due to their multitudinous architectures but also their good redox activities and outstanding electron and proton transport capacities. Recently, abundant studies on POMs composited with metal nanoparticles (NPs), carbon materials (e.g., carbon nanotubes (CNTs), carbon quantum dots (CQDs), graphene), and conducting polymers or highly-porous framework materials (e.g., MOFs, ZIFs) have been performed and POM-based composite materials (PCMs) undoubtedly show enhanced stability and improved electrochemical performances. Therefore, POMs and PCMs are of increasing interest in electrocatalysis, electrochemical detection and energy-related fields (such as fuel cells, redox flow batteries and so on), thus, developing novel PCMs has long been the key research topic in POM chemistry. This review mainly summarizes some representative advances in PCMs with electrochemical applications in the past ten years, expecting to provide some useful guidance for future research.

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.

Photoanodes for water oxidation with visible light based on a pentacyclic quinoid organic dye enabling proton-coupled electron transfer

A novel pentacyclic quinoid photosensitizer with extended absorption in the visible region and enabling proton-coupled electron transfer is employed in photoelectrodes for water oxidation in combination with a ruthenium polyoxometalate catalyst.

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.