Immobilization of an Amphiphilic Molecular Cobalt Catalyst on Carbon Black for Ligand-Assisted Water Oxidation

A bioinspired cobalt catalyst based on a tripodal imidazole/pyridine platform capable of water reduction and oxidation

The prototypical drug carrier [Co^II(L¹)Cl]PF₆ (1), where L¹ is a tripodal amine bound to pyridine and methyl-imidazoles, had its electrocatalytic water splitting activity studied under different pH conditions. This species contains a high-spin 3d⁷ Co^II metal center, and is capable of generating both H₂ from water reduction and O₂ from water oxidation_. Turnover numbers reach 390 after 3 h for water reduction. Initial water oxidation activity is molecular, with TONs of 71 at pH 7 and 103 at pH 11.5. The results reveal that species 1 can undergo several redox transformations, including reduction to the 3d⁸ Co^I species that precedes a ^LS3d⁶ hydride for water reduction, as well as nominal Co^IVO and Co^III-OOH species required for water oxidation. Post-catalytic analyses confirm the molecular nature of reduction and support initial molecular activity for oxidation.

Non-heme oxoiron complexes as active intermediates in the water oxidation process with hydrogen/oxygen atom transfer reactions

In this study, we explore the water oxidation process with the help of density functional theory. The formation of an oxygen-oxygen bond is crucial in the water oxidation process. Here, we report the formation of the oxygen-oxygen bond by the N5-coordinate oxoiron species with a higher oxidation state of Fe^IV and Fe^V. This bond formation is studied through the nucleophilic addition of water molecules and the transfer of the oxygen atom from meta-chloroperbenzoic acid (mCPBA). Our study reveals that the oxygen-oxygen bond formation by reacting mCPBA with Fe^VO requires less activation barrier (13.7 kcal mol^-1) than the other three pathways. This bond formation by the oxygen atom transfer (OAT) pathway is more favorable than that achieved by the hydrogen atom transfer (HAT) pathway. In both cases, the oxygen-oxygen bond formation occurs by interacting the σ*d_z²-2p_z molecular orbital of the iron-oxo intermediate with the 2p_x orbital of the oxygen atom. From this study, we understand that the oxygen-oxygen bond formation by Fe^IVO with the OAT process is also feasible (16 kcal mol^-1), suggesting that Fe^VO may not always be required for the water oxidation process by non-heme N5-oxoiron. After the oxygen-oxygen bond formation, the release of the dioxygen molecule occurs with the addition of the water molecule. The release of dioxygen requires a barrier of 7.0 kcal mol^-1. The oxygen-oxygen bond formation is found to be the rate-determining step.

Highly Efficient and Durable Electrocatalysis by a Molecular Catalyst with Long Alkoxyl Chains Immobilized on a Carbon Electrode for Water Oxidation

A dinuclear Ru complex, proximal,proximal-[Ru₂L(C₈Otpy)₂(OH)(OH₂)]³⁺ (C₈Otpy = 4'-octyloxy-2,2'; 6',2″-terpyridine) (1) with long alkoxyl chains, was synthesized to be immobilized on a carbon paper (CP) electrode via hydrophobic interactions between the long alkoxyl chains and the CP surface. The 1/CP electrode demonstrated efficient electrocatalytic water oxidation with a low overpotential (η_onset) of 0.26 V (based on the onset potential for water oxidation) in an aqueous medium at pH 7.0, which is compared advantageously with those of hitherto-reported molecular anodes for water oxidation. The active species of Ru^IIIRu^III(μ-OO) with a μ-OO bridge was involved in water oxidation at 0.95 V versus Ag/AgCl. As the applied potential increased to 1.40 V, water oxidation was promoted by participation of the more active species of Ru^IIIRu^IV(μ-OO), and very durable electrocatalysis was gained for more than 35 h without elution of 1 into the electrolyte solution. The introduced long alkoxyl chains act as a dual role of the linker of 1 on the CP surface and decrease the η value. Theoretical investigation provides insights into the O-O bond formation mechanism and the activity difference between Ru^IIIRu^III(μ-OO) and Ru^IIIRu^IV(μ-OO) for electrocatalytic water oxidation.

Polymer-Supported-Cobalt-Catalyzed Regioselective Cyclotrimerization of Aryl Alkynes

A convoluted poly(4-vinylpyridine) cobalt(II) (P4VP-CoCl2) system was developed as a stable and reusable heterogeneous catalyst. The local structure near the Co atom was determined on the basis of experimental data and theoretical calculations. This immobilized cobalt catalyst showed high selectivity and catalytic activity in the [2 + 2 + 2] cyclotrimerization of terminal aryl alkynes. With 0.033 mol % P4VP-CoCl2, the regioselective formation of 1,3,5-triarylbenzene was realized without 1,2,4-triarylbenzene formation. Further, a multigram-scale (11 g) reaction proceeded efficiently. In addition, the polymer-supported catalyst was successfully recovered and used three times. X-ray photoelectron spectroscopy analysis of the recovered catalyst suggested that cobalt was in the +2 oxidation state. The 1,3,5-triarylbenzene derivatives were applied to the synthesis of a molecular beam electron resist and a polycyclic aromatic hydrocarbon.

Ruthenium-Benzothiadiazole Building Block Derived Dynamic Heterometallic Ru-Ag Coordination Polymer and Its Enhanced Water-Splitting Feature

This article deals with the development of the unprecedented redox-mediated heterometallic coordination polymer {[Ru^III(acac)₂(μ-bis-η¹-N,η¹-N-BTD)₂Ag^I(ClO₄)]ClO₄}_n (3) via the oxidation of the monomeric building block cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) by AgClO₄ (BTD = exodentate 2,1,3-benzothiadiazole, acac = acetylacetonate). Monomeric cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) and [Ru^II(acac)₂(η¹-N-BTD)(CH₃CN)] (2) were simultaneously obtained from the electron-deficient BTD heterocycle and the electron-rich metal precursor Ru^II(acac)₂(CH₃CN)₂ in refluxing CH₃CN. Molecular identities of 1-3 were authenticated by their single-crystal X-ray structures as well as by solution spectral features. These results also reflected the elusive trigonal-planar geometry of the Ag ion in Ru-Ag-derived polymeric 3. Ru(III) (S = 1/2)-derived 3 displayed metal-based anisotropic EPR with ⟨g⟩/Δg = 2.12/0.56 and paramagnetically shifted ¹H NMR. Spectroelectrochemistry in combination with DFT/TD-DFT calculations of 1ⁿ and 2ⁿ (n = 1+, 0, 1-) determined a metal-based (Ru^II/Ru^III) oxidation and BTD-based reduction (BTD/BTD^•-). The drastic decrease in the emission intensity and quantum yield but insignificant change in the lifetime of 3 with respect to 1 could be addressed in terms of static quenching and/or a paramagnetism-induced phenomenon. A homogeneously dispersed dumbbell-shaped morphology and the particle diameter of 3 were established by microscopic (TEM-EDX/SEM) and DLS analysis, respectively. Moreover, the dynamic nature of polymeric 3 was highlighted by its degradation to the η¹-N-BTD coordinated monomeric fragment 1, which could also be followed spectrophotometrically in polar protic EtOH. Interestingly, both monomeric 1 and polymeric 3 exhibited efficient electrocatalytic activity toward water oxidation processes (OER, HER) on immobilization on an FTO support, which also divulged the better intrinsic water oxidation activity of 3 in comparison to 1.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Catalysis of a Single Transition Metal Site for Water Oxidation: From Mononuclear Molecules to Single Atoms

Low-cost, nonprecious transition metal (TM) catalysts toward efficient water oxidation are of critical importance to future sustainable energy technologies. The advances in structure engineering of water oxidation catalysts (WOCs) with single TM centers as active sites, for example, single metallic molecular complexes (SMMCs), supported SMMCs, and single-atom catalysts (SACs) in recent reports are examined. The efforts made on these configurations in terms of design principle, advanced characterization, performances and theoretical studies, are critically reviewed. A clear roadmap with the correlations between the single-TM-site-based structures (coordination and geometric structure, TM species, support), and the catalytic performances in water oxidation is provided. The insights bridging SMMCs with SACs are also given. Finally, the challenges and opportunities in the single-TM-site catalysis are proposed.

A recyclable heterogeneous nanocatalyst of copper-grafted natural asphalt sulfonate (NAS@Cu): characterization, synthesis and application in the Suzuki–Miyaura coupling reaction

NAS@Cu has synthesis simplicity given the availability of natural materials and has advantages such as being eco-friendly, high reactivity and recyclability.

Recent Advances in the Development of Molecular Catalyst-Based Anodes for Water Oxidation toward Artificial Photosynthesis

Catalytic water oxidation represents a bottleneck for developing artificial photosynthetic systems that store solar energy as renewable fuels. A variety of molecular water oxidation catalysts (WOCs) have been reported over the last two decades. In view of their applications in artificial photosynthesis devices, it is essential to immobilize molecular catalysts onto the surfaces of conducting/semiconducting supports for fabricating efficient and stable water oxidation anodes/photoanodes. Molecular WOC-based anodes are essential for developing photovoltaic artificial photosynthesis devices and, moreover, the performance of molecular WOC on anodes will provide important insight into designing extended molecular WOC-based photoanodes for photoelectrochemical (PEC) water oxidation. This Review concerns recent progress in the development of molecular WOC-based anodes over the last two decades and looks at the prospects for using such anodes in artificial photosynthesis.