Discovering the Balance of Steric and Electronic Factors Needed To Provide a New Structural Motif for Photocatalytic Hydrogen Production from Water

Accumulative Charge Separation in a Modular Quaterpyridine Bridging Ligand Platform and Multielectron Transfer Photocatalysis of π-Linked Dinuclear Ir(III)-Re(I) Complex for CO2 Reduction

Four sterically distorted quaterpyridyl (qpy) ligand-bridged Ir(III)-Re(I) heterometallic complexes (Ir-qpy_mm-Re, Ir-qpy_mp-Re, Ir-qpy_pm-Re, and Ir-qpy_pp-Re), in which the position of the coupling pyridine unit of the two 2,2'-bipyridine ligands was varied (meta (m)- or para (p)-position), pypy_x-py_xpy (x = m and m, qpy_mm; x = m and p, qpy_mp; x = p and m, qpy_pm; x = p and p, qpy_pp), were prepared, along with the fully π-conjugated Ir(III)-[π linker]-Re(I) complexes (π linker = 2,2'-bipyrimidine (bpm), Ir-bpm-Re; π linker = 2,5-di(pyridin-2-yl)pyrazine (dpp), Ir-dpp-Re) to elucidate the electron mediating and accumulative charge separation properties of the bridging π-linker in a bimetallic system (photosensitizer-π linker-catalytic center). From the photophysical and electrochemical studies, it was found that the quaterpyridyl (qpy) bridging ligand (BL), in which the two planar Ir/Re metalated bipyridine (bpy) ligands were connected but slightly canted relative to each other, linking the heteroleptic Ir(III) photosensitizer, [(^piqC^N)₂Ir^III(bpy)]⁺, and catalytic Re(I) complex, (bpy)Re^I(CO)₃Cl, minimized the energy lowering of the qpy BL, which hampers the forward photoinduced electron transfer (PET) process from [(^piqC^N)₂Ir^III(N^N)]⁺ to (N^N)Re^I(CO)₃Cl (E_red1 = -(0.85-0.93) V and E_red2 = -(1.15-1.30) V vs SCE). This result contrasts with the fully π-delocalized bimetallic systems (Ir-bpm-Re and Ir-dpp-Re) that show a significant energy reduction due to the considerable π-extension and deshielding effect caused by the neighboring Lewis acidic metals (Ir and Re) on the electrochemical scale (E_red1 = -0.37 V and E_red2 = -1.02 and -0.99 V vs SCE). Based on a series of anion absorption studies and spectroelectrochemical (SEC) analyses, all Ir(III)-BL-Re(I) bimetallic complexes were found to exist as dianionic form (Ir(III)-[BL]^2--Re(I)) after a fast reductive-quenching process in the presence of excess electron donor. In the photolysis experiment, the four Ir-qpy-Re complexes displayed the reasonable photochemical CO₂-to-CO conversion activities (TON of 366-588 for 19 h) owing to the moderated electronic coupling between two functional Ir(III) and Re(I) centers through the slightly distorted qpy ligand, whereas Ir-bpm-Re and Ir-dpp-Re displayed negligible performances as a result of the strong electronic coupling via π-conjugation between the two functional components resulting in the energetic constraints for PET and an unwanted side reactions competing with the forward processes. These results confirm that the qpy unit can be utilized as an efficient BL platform in π-linked bimetallic systems.

Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water

Molecular hydrogen (H₂) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H₂ production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, [Rh^III(tpy)(CH₃CN)Cl₂](CF₃SO₃) (Rh-tpy), and shown that it can act as a catalyst for the light-induced proton reduction into H₂ in water in the presence of the [Ru(bpy)₃]Cl₂ (Ru) photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 μM of Ru, the Rh-tpy catalyst produces H₂ with turnover number versus catalyst (TON_Cat*) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of Ru/Rh-tpy/HA^-/H₂A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh^III(dmbpy)₂Cl₂]⁺ (Rh2) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh^I(tpy)Cl] (Rh^I-tpy), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Ep_c(Rh^III/Rh^I) = -0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru, [Ru^II(bpy)(bpy^•-)]⁺ (Ru^-) (E_1/2(Ru^II/Ru^-) = -1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state (*Ru) by ascorbate during photocatalysis. A Stern-Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of *Ru by ascorbate. The resulting reduced Ru species (Ru^-) were then able to activate the Rh^III-tpy H₂-evolving catalyst by reduction generating Rh^I-tpy, which can react with a proton on a sub-nanosecond time scale to form a Rh^III(H)-tpy hydride, the key intermediate for H₂ evolution.

Multifunctional Polyoxometalate Platforms for Supramolecular Light-Driven Hydrogen Evolution*

Multifunctional supramolecular systems are a central research topic in light-driven solar energy conversion. Here, we report a polyoxometalate (POM)-based supramolecular dyad, where two platinum-complex hydrogen evolution catalysts are covalently anchored to an Anderson polyoxomolybdate anion. Supramolecular electrostatic coupling of the system to an iridium photosensitizer enables visible light-driven hydrogen evolution. Combined theory and experiment demonstrate the multifunctionality of the POM, which acts as photosensitizer/catalyst-binding-site[1] and facilitates light-induced charge-transfer and catalytic turnover. Chemical modification of the Pt-catalyst site leads to increased hydrogen evolution reactivity. Mechanistic studies shed light on the role of the individual components and provide a molecular understanding of the interactions which govern stability and reactivity. The system could serve as a blueprint for multifunctional polyoxometalates in energy conversion and storage.

Excited state relaxation processes of H2-evolving Ru-Pd supramolecular photocatalysts containing a linear or non-linear bridge: a DFT and TDDFT study

In this study, the early-time excited state relaxation processes of bimetallic Ru-Pd supramolecular photocatalysts containing a linear 2,2':5',2''-terpyridine or a nonlinear 2,2':6',2''-terpyridine bridging ligand (BL) were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) approaches. The bridge based metal-to-ligand charge transfer triplet (³MLCT) state of the metal complex containing a linear bridging ligand was calculated to be the lowest energy triplet (T₁) state which is closely related to the photocatalytic H₂ production, while for that having a nonlinear bridging ligand, the T₁ state is a Ru metal-centered (MC) triplet (³MC_Ru) state that is short-lived and rapidly decays to the ground electronic state (S₀). Our simulation provides an alternative explanation for the smaller interligand electron transfer (ILET) rate in the Ru-Pd complex containing a linear bridge compared to the corresponding monometal Ru complex. Based on the calculation, we also suggest that the successive ³MLCT → ³MC_Ru → S₀ conversion is responsible for the inefficiency of the Ru-Pd complex containing nonlinear bridge as a photocatalyst for H₂ production. This study provides theoretical insights into the key steps of the photoinduced processes of the bimetallic H₂-evolving supramolecular photocatalyst.

A metal-organic cage incorporating multiple light harvesting and catalytic centres for photochemical hydrogen production

Photocatalytic water splitting is a natural but challenging chemical way of harnessing renewable solar power to generate clean hydrogen energy. Here we report a potential hydrogen-evolving photochemical molecular device based on a self-assembled ruthenium–palladium heterometallic coordination cage, incorporating multiple photo- and catalytic metal centres. The photophysical properties are investigated by absorption/emission spectroscopy, electrochemical measurements and preliminary DFT calculations and the stepwise electron transfer processes from ruthenium-photocentres to catalytic palladium-centres is probed by ultrafast transient absorption spectroscopy. The photocatalytic hydrogen production assessments reveal an initial reaction rate of 380 μmol h⁻¹ and a turnover number of 635 after 48 h. The efficient hydrogen production may derive from the directional electron transfers through multiple channels owing to proper organization of the photo- and catalytic multi-units within the octahedral cage, which may open a new door to design photochemical molecular devices with well-organized metallosupramolecules for homogenous photocatalytic applications.

Photocatalytic water splitting is a promising route to hydrogen generation from renewable solar power. Here, the authors report a hydrogen-evolving photochemical molecular device based on a self-assembled coordination cage, which simultaneously incorporates multiple photosensitizing and catalytic metal centres.

A new hydrophilic supramolecular photocatalyst for the production of H2 in aerobic aqueous solutions

Addition of sulfonated ligands into a Ru,Rh,Ru photocatalyst increases solubility and H₂ production in aqueous aerobic solutions.

A new Ru(II)Rh(III) bimetallic with a single Rh-Cl bond as a supramolecular photocatalyst for proton reduction

A new Ru(II)Rh(III) structural motif [(bpy)2Ru(dpp)RhCl(tpy)](4+) with one halide on the Rh(III) center demonstrates light-driven proton reduction ability, establishing that two halide ligands are not mandatory despite all prior systems containing a cis-RhCl2 catalytic site. This new design provides a novel approach to modulate Rh(III) redox behavior and catalytic activity with insight into catalytic intermediates.

Carbene based photochemical molecular assemblies for solar driven hydrogen generation

Novel photocatalysts based on ruthenium complexes with NHC (N-heterocyclic carbene)-type bridging ligands have been prepared and structurally and photophysically characterised. The identity of the NHC-unit of the bridging ligand was established unambiguously by means of X-ray structural analysis of a heterodinuclear ruthenium-silver complex. The photophysical data indicate ultrafast intersystem crossing into an emissive and a non-emissive triplet excited state after excitation of the ruthenium centre. Exceptionally high luminescence quantum yields of up to 39% and long lifetimes of up to 2 μs are some of the triplet excited state characteristics. Preliminary studies into the visible light driven photocatalytic hydrogen formation show no induction phase and constant turnover frequencies that are independent on the concentration of the photocatalyst. In conclusion this supports the notion of a stable assembly under photocatalytic conditions.

Parallelization of photocatalytic gas-producing reactions

High-throughput screening has been widely utilized in the pharmaceutical and manufacturing industry targeting the development of new molecules and materials for numerous applications. To enable more rapid progress in photocatalytic water-splitting reactions, the construction of high-throughput combinatorial photoreactors enabling the parallel optimization of relevant compositions under varieties of experimental conditions seems appropriate. This contribution describes a 16-photoreactor apparatus permitting the kinetic evaluation of photocatalytic gas-producing reactions using head-space pressure, gas chromatography, and mass spectrometry operating in parallel, illustrated with molecular-based homogeneous photocatalytic H2-generating compositions.

A new Ru,Ru,Pt supramolecular architecture for photocatalytic H2 production

New RuRuPt complexes photocatalytically reduce H₂O to H₂ with enhanced efficiency and stability compared to Ru₂RuPt analogues.

Nickel pyridinethiolate complexes as catalysts for the light-driven production of hydrogen from aqueous solutions in noble-metal-free systems

A series of mononuclear nickel(II) thiolate complexes (Et4N)Ni(X-pyS)3 (Et4N = tetraethylammonium; X = 5-H (1a), 5-Cl (1b), 5-CF3 (1c), 6-CH3 (1d); pyS = pyridine-2-thiolate), Ni(pySH)4(NO3)2 (2), (Et4N)Ni(4,6-Y2-pymS)3 (Y = H (3a), CH3 (3b); pymS = pyrimidine-2-thiolate), and Ni(4,4'-Z-2,2'-bpy)(pyS)2 (Z = H (4a), CH3 (4b), OCH3 (4c); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that 2 is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H2O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex 4c achieves over 7300 turnovers of H2 in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter's greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni-H(-)/N-H(+) intermediate that forms the H-H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H2.

Solar fuels generation and molecular systems: is it homogeneous or heterogeneous catalysis?

Catalysis is a key enabling technology for solar fuel generation. A number of catalytic systems, either molecular/homogeneous or solid/heterogeneous, have been developed during the last few decades for both the reductive and oxidative multi-electron reactions required for fuel production from water or CO(2) as renewable raw materials. While allowing for a fine tuning of the catalytic properties through ligand design, molecular approaches are frequently criticized because of the inherent fragility of the resulting catalysts, when exposed to extreme redox potentials. In a number of cases, it has been clearly established that the true catalytic species is heterogeneous in nature, arising from the transformation of the initial molecular species, which should rather be considered as a pre-catalyst. Whether such a situation is general or not is a matter of debate in the community. In this review, covering water oxidation and reduction catalysts, involving noble and non-noble metal ions, we limit our discussion to the cases in which this issue has been directly and properly addressed as well as those requiring more confirmation. The methodologies proposed for discriminating homogeneous and heterogeneous catalysis are inspired in part by those previously discussed by Finke in the case of homogeneous hydrogenation reaction in organometallic chemistry [J. A. Widegren and R. G. Finke, J. Mol. Catal. A, 2003, 198, 317-341].

Powering the future of molecular artificial photosynthesis with light-harvesting metallosupramolecular dye assemblies

Chemical ingenuity will play a significant role in solving the greatest challenge currently facing society: providing clean and carbon neutral energy for all of humanity. Molecular artificial photosynthesis is an emerging technology based on principles learned from Nature where individual components perform the essential light-harvesting, charge-separation, and water splitting functions to store solar energy in the form of chemical bonds. This tutorial review focuses specifically on the application of metallosupramolecular self-assembly strategies to interface solar fuel catalysts with photosensitizers and construct light-harvesting antennae capable of achieving panchromatic absorption and directional energy concentration.