Catalytic Water Oxidation by Ruthenium Complexes Containing Negatively Charged Ligand Frameworks

Artificial photosynthesis represents an attractive way of converting solar energy into storable chemical energy. The H₂O oxidation half‐reaction, which is essential for producing the necessary reduction equivalents, is an energy‐demanding transformation associated with a high kinetic barrier. Herein we present a couple of efficient Ru‐based catalysts capable of mediating this four‐proton‐four‐electron oxidation. We have focused on the incorporation of negatively charged ligands, such as carboxylate, phenol, and imidazole, into the catalysts to decrease the redox potentials. This account describes our work in designing Ru catalysts based on this idea. The presence of the negatively charged ligands is crucial for stabilizing the metal centers, allowing for light‐driven H₂O oxidation. Mechanistic details associated with the designed catalysts are also presented.

     

Efficient Water Oxidation Catalyzed by Mononuclear Ruthenium(II) Complexes Incorporating Schiff Base Ligands

Four new charge‐neutral ruthenium(II) complexes containing dianionic Schiff base and isoquinoline or 4‐picoline ligands were synthesized and characterized by NMR and ESI‐MS spectroscopies, elemental analysis, and X‐ray diffraction. The complexes exhibited excellent chemical water oxidation activity and high stability under acidic conditions (pH 1.0) using (NH₄)₂Ce(NO₃)₆ as a sacrificial electron acceptor. The high catalytic activities of these complexes for water oxidation were sustained for more than 10 h at low concentrations. High turnover numbers of up to 3200 were achieved. A water nucleophilic attack mechanism was proposed. A Ru^V?O intermediate was detected during the catalytic cycle by high‐resolution mass spectrometry.     

Photochemical,Electrochemical, and Photoelectrochemical Water Oxidation Catalyzed by Water‐Soluble Mononuclear Ruthenium Complexes

Two mononuclear ruthenium complexes [Ru(H₂tcbp)(isoq)₂] ( 1 ) and [Ru(H₂tcbp)(pic)₂] ( 2 ) (H₄tcbp=4,4′,6,6′‐tetracarboxy‐2,2′‐bipyridine, isoq=isoquinoline, pic=4‐picoline) are synthesized and fully characterized. Two spare carboxyl groups on the 4,4′‐positions are introduced to enhance the solubility of 1 and 2 in water and to simultaneously allow them to tether to the electrode surface by an ester linkage. The photochemical, electrochemical, and photoelectrochemical water oxidation performance of 1 in neutral aqueous solution is investigated. Under electrochemical conditions, water oxidation is conducted on the deposited indium‐tin‐oxide anode, and a turnover number higher than 15,000 per water oxidation catalyst (WOC) 1 is obtained during 10 h of electrolysis under 1.42 V vs. NHE, corresponding to a turnover frequency of 0.41 s^?1. The low overpotential (0.17 V) of electrochemical water oxidation for 1 in the homogeneous solution enables water oxidation under visible light by using [Ru(bpy)₃]²⁺ ( P1 ) (bpy=2,2′‐bipyridine) or [Ru(bpy)₂(4,4′‐(COOEt)₂‐bpy)]²⁺ ( P2 ) as a photosensitizer. In a three‐component system containing 1 or 2 as a light‐driven WOC, P1 or P2 as a photosensitizer, and Na₂S₂O₈ or [CoCl(NH₃)₅]Cl₂ as a sacrificial electron acceptor, a high turnover frequency of 0.81 s^?1 and a turnover number of up to 600 for 1 under different catalytic conditions are achieved. In a photoelectrochemical system, the WOC 1 and photosensitizer are immobilized together on the photoanode. The electrons efficiently transfer from the WOC to the photogenerated oxidizing photosensitizer, and a high photocurrent density of 85 μA cm^?2 is obtained by applying 0.3 V bias vs. NHE.     

Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands

We describe herein the synthesis and characterization of ruthenium complexes with multifunctional bipyridyl diphosphonate ligands as well as initial water oxidation studies. In these complexes, the phosphonate groups provide redox‐potential leveling through charge compensation and σ donation to allow facile access to high oxidation states. These complexes display unique pH‐dependent electrochemistry associated with deprotonation of the phosphonic acid groups. The position of these groups allows them to shuttle protons in and out of the catalytic site and reduce activation barriers. A mechanism for water oxidation by these catalysts is proposed on the basis of experimental results and DFT calculations. The unprecedented attack of water at a neutral six‐coordinate [Ru^IV] center to yield an anionic seven‐coordinate [Ru^IV?OH]^? intermediate is one of the key steps of a single‐site mechanism in which all species are anionic or neutral. These complexes are among the fastest single‐site catalysts reported to date.     

Synthesis and Catalytic Water Oxidation Activities of Ruthenium Complexes Containing Neutral Ligands

Two dinuclear and one mononuclear ruthenium complexes containing neutral polypyridyl ligands have been synthesised as pre‐water oxidation catalysts and characterised by ¹H and ¹³C NMR spectroscopy and ESI‐MS. Their catalytic water oxidation properties in the presence of [Ce(NH₄)₂(NO₃)₆] (Ce^IV) as oxidant at pH 1.0 have been investigated. At low concentrations of Ce^IV (5 mM ), high turnover numbers of up to 4500 have been achieved. An ¹⁸O‐labelling experiment established that both O atoms in the evolved O₂ originate from water. Combined electrochemical study and electrospray ionisation mass spectrometric analysis suggest that ligand exchange between coordinated 4‐picoline and free water produces Ru aquo species as the real water oxidation catalysts.     

Oxygen Evolution Catalyzed by a Mononuclear Ruthenium Complex Bearing Pendant SO3− Groups

Rational molecular design of catalytic systems capable of smooth O? O bond formation is critical to the development of efficient catalysts for water oxidation. A new ruthenium complex was developed, which bears pendant SO₃^? groups in the secondary coordination sphere: [Ru(terpy)(bpyms)(OH₂)] (terpy=2,2′:6′,2′′‐terpyridine, bpyms=2,2′‐bipyridine‐5,5′‐bis(methanesulfonate)). Water oxidation driven by a Ce⁴⁺ oxidant is distinctly accelerated upon introduction of the pendant SO₃^? groups in comparisons to the parent catalyst, [Ru(terpy)(bpy)(OH₂)]²⁺ (bpy=2,2′‐bipyridine). Spectroscopic, electrochemical, and crystallographic investigations concluded that the pendant SO₃^? groups promote the formation of an O? O bond via the secondary coordination sphere on the catalyst, whereas the influence of the pendant SO₃^? groups on the electronic structure of the [Ru(terpy)(bpy)(OH₂)]²⁺ core is negligible. The results of this work indicate that modification of the secondary coordination sphere is a valuable strategy for the design of water oxidation catalysts.     

Mononuclear Water Oxidation Catalysts

Recently, several mononuclear water oxidation catalysts have been reported, a breakthrough considering the dogma that at least two metal sites were required to oxidize water efficiently. In this Review various mononuclear catalysts which have been reported in the last five years are reviewed, as well as their implementation in prototype devices that allow dioxygen formation to be coupled to dihydrogen production will be discussed.     

Oxidation of Alcohols Catalyzed by Ruthenium Complexes with Iodosylbenzene as Oxidant

Five ruthenium complexes such as Phen-Ru-Phen, Phen-Ru-Bipy, Phen-Ru-Quin, Quin-Ru-Quin and Bipy-Ru-Quin (where Phen=l, 10-phenanthroline, Quin=8-hydroxyquinoline, Bipy=2, 2‘-bipyridine) were synthesized and used as catalysts for the oxidation of benzylic and primary aliphatic alcohols with iodosylbenzene as oxidant. The oxidations were carried out at room temperature, affording the corresponding aldehydes and ketones with high selectivity.     

Structure-Activity Relationship for Di- up to Tetranuclear Macrocyclic Ruthenium Catalysts in Homogeneous Water Oxidation

Two di- and tetranuclear Ru(bda) (bda: 2,2′-bipyridine-6,6′-dicarboxylate) macrocyclic complexes were synthesized and their catalytic activities in chemical and photochemical water oxidation investigated in a comparative manner to our previously reported trinuclear congener. Our studies have shown that the catalytic activities of this homologous series of multinuclear Ru(bda) macrocycles in homogeneous water oxidation are dependent on their size, exhibiting highest efficiencies for the largest tetranuclear catalyst. The turnover frequencies (TOFs) have increased from di- to tetranuclear macrocycles not only per catalyst molecule but more importantly also per Ru unit with TOF of 6 s⁻¹ to 8.7 s⁻¹ and 10.5 s⁻¹ in chemical and 0.6 s⁻¹ to 3.3 s⁻¹ and 5.8 s⁻¹ in photochemical water oxidation per Ru unit, respectively. Thus, for the first time, a clear structure–activity relationship could be established for this novel class of macrocyclic water oxidation catalysts.     

Ligand Modification Transforms a Catalase Mimic into a Water Oxidation Catalyst

The catalytic reactivity of the high‐spin Mn^II pyridinophane complexes [(Py₂NR₂)Mn(H₂O)₂]²⁺ (R=H, Me, tBu) toward O₂ formation is reported. With small macrocycle N‐substituents (R=H, Me), the complexes catalytically disproportionate H₂O₂ in aqueous solution; with a bulky substituent (R=tBu), this catalytic reaction is shut down, but the complex becomes active for aqueous electrocatalytic H₂O oxidation. Control experiments are in support of a homogeneous molecular catalyst and preliminary mechanistic studies suggest that the catalyst is mononuclear. This ligand‐controlled switch in catalytic reactivity has implications for the design of new manganese‐based water oxidation catalysts.     

Computational Mechanistic Insights on Homogeneous Water Oxidation Versus Catalyst Deactivation: A Case Study with Mononuclear Nickel and Copper Complexes

Water splitting is a potential pathway for hydrogen gas evolution and thereby realization of a carbon-neutral sustainable energy scheme. However, oxidation of water to dioxygen is the major impediment in conversion of solar energy to fuel. Herein, density functional studies are conducted to explore the reactivity conduits of two molecular electro-catalysts consisting of nickel and copper tetra-anionic tetradentate amide ligand complexes of the type [(L1)M^II]²⁻, where L1=o-phenylenebis(oxamidate), and their substitutionally modified analogues. While nickel complexes demonstrate complex borderline chemistry between homogeneous and heterogeneous pathways, showing competition between water oxidation and molecular species degradation, copper complexes display robust and efficient molecular water oxidation behavior. Our analysis predict that this disparity is primarily due to the reversible O−O bond formation in nickel complexes, which provide the platform necessary for a direct attack of OH⁻/H⁺ on the metal and terminally accessible amidate groups of the 2e⁻ oxidized anionic intermediate, [(L1⋅)Ni^III(OH)]¹⁻, respectively. This intermediate streamline ligand deactivation with a comparatively higher driving force for nickel complexes in acidic medium. Contrarily, the copper complexes display radical character on the hydroxyl ligand in the corresponding intermediate, [(L1⋅)Cu^II(OH⋅)]¹⁻, that expedite O−O interaction, leading to predominant homogeneous water oxidation under all conditions.     

Water Oxidation Catalyzed by a Dinuclear Cobalt–Polypyridine Complex

The dinuclear Co complex [(TPA)Co(μ‐OH)(μ‐O₂)Co(TPA)](ClO₄)₃ ( 1 , TPA=tris(2‐pyridylmethyl)amine) catalyzes the oxidation of water. In the presence of [Ru(bpy)₃]²⁺ and S₂O₈^2?, photoinduced oxygen evolution can be observed with a turnover frequency (TOF) of 1.4±0.1 mol(O₂) mol( 1 )^?1 s^?1 and a maximal turnover number (TON) of 58±5 mol(O₂) mol( 1 )^?1. The complex is shown to act as a molecular and homogeneous catalyst and a mechanism is proposed based on the combination of EPR data and light‐driven O₂ evolution kinetics.     

Homogeneous Oxidation of Water by Iron Complexes with Macrocyclic Ligands

The activity of eleven separated iron complexes and nine in situ‐generated iron complexes towards catalytic water oxidation have been examined in aqueous solutions with Ce(NH₄)₂(NO₃)₆ as the oxidant. Two iron complexes bearing tridentate and tetradentate macrocyclic ligands were found to be novel water oxidation catalysts. The one with tetradentate ligand exhibited a promising activity with a turnover number of 65 for oxygen evolution.     

Design of Mononuclear Ruthenium Catalysts for Low‐Overpotential Water Oxidation

Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Inspired by energy challenges and the emerging understanding of photosystem II, the development of artificial molecular catalysts for water oxidation has become a highly active area of research in recent years. In this Focus Review, we describe recent achievements in the development of single‐site ruthenium catalysts for water oxidation with a particular focus on the overpotential of water oxidation. First, we introduce the general scheme to access the high‐valent ruthenium–oxo species, the key species of the water‐oxidation reaction. Next, the mechanisms of the O? O bond formation from the active ruthenium–oxo species are described. We then discuss strategies to decrease the onset potentials of the water‐oxidation reaction. We hope this Focus Review will contribute to the further development of efficient catalysts toward sustainable energy‐conversion systems.     

Stepwise Metal–Ligand Cooperation by a Reversible Aromatization/Deconjugation Sequence in Ruthenium Complexes with a Tetradentate Phenanthroline‐Based Ligand

The synthesis and reactivity of ruthenium complexes containing the tetradentate phenanthroline‐based phosphine ligand 2,9‐bis((di‐tert‐butylphosphino)methyl)‐1,10‐phenanthroline (PPhenP) is described. The hydrido chloro complex [RuHCl(PPhenP)] ( 2 ) undergoes facile dearomatization upon deprotonation of the benzylic position, to give [RuH(PPhenP‐H)] ( 4 ). Addition of dihydrogen to 4 causes rearomatization of the phenanthroline moiety to trans‐[Ru(H)₂(PPhenP)] ( 5 ), followed by hydrogenation of an aromatic heterocycle in the ligand backbone, to give a new dearomatized and deconjugated complex [RuH(PPhenP*‐H)] ( 6 ). These aromatization/deconjugation steps of the coordinated ligand were demonstrated to be reversible and operative in the dehydrogenation of primary alcohols without the need for a hydrogen acceptor. This aromatization/deconjugation sequence constitutes an unprecedented mode of a stepwise cooperation between the metal center and the coordinated ligand.     

Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (qpy) Complexes: Evidence for Ruthenium(III) qpy‐N,N′′′‐dioxide as the Real Catalysts

Polypyridyl and related ligands have been widely used for the development of water oxidation catalysts. Supposedly these ligands are oxidation‐resistant and can stabilize high‐oxidation‐state intermediates. In this work a series of ruthenium(II) complexes [Ru(qpy)(L)₂]²⁺ (qpy=2,2′:6′,2′′:6′′,2′′′‐quaterpyridine; L=substituted pyridine) have been synthesized and found to catalyze Ce^IV‐driven water oxidation, with turnover numbers of up to 2100. However, these ruthenium complexes are found to function only as precatalysts; first, they have to be oxidized to the qpy‐N,N′′′‐dioxide (ONNO) complexes [Ru(ONNO)(L)₂]³⁺ which are the real catalysts for water oxidation.