Visible‐Light‐Driven Hydrogen Production and Polymerization using Triarylboron‐Functionalized Iridium(III) Complexes

The development of novel iridium(III) complexes has continued as an important area of research owing to their highly tunable photophysical properties and versatile applications. In this report, three heteroleptic dimesitylboron‐containing iridium(III) complexes, [Ir(p‐B‐ppy)₂(N^N)]⁺ {p‐B‐ppy=2‐(4‐dimesitylborylphenyl)pyridine; N^N=dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) ( 1 ), dipyrido[3,2‐d:2′,3′‐f]quinoxaline (dpq) ( 2 ), and 1,10‐phenanthroline (phen) ( 3 )}, were prepared and fully characterized electrochemically, photophysically, and computationally. Altering the conjugated length of the N^N ligands allowed us to tailor the photophysical properties of these complexes, especially their luminescence wavelength, which could be adjusted from λ=583 to 631 nm in CH₂Cl₂. All three complexes were evaluated as visible‐light‐absorbing sensitizers for the photogeneration of hydrogen from water and as photocatalysts for the photopolymerization of methyl methacrylate. The results showed that all of them were active in both photochemical reactions. High activity for the photosensitizer (over 1158 turnover numbers with 1 ) was observed, and the system generated hydrogen even after 20 h. Additionally, poly(methyl methacrylate) with a relatively narrow molecular‐weight distribution was obtained if an initiator (i.e., ethyl α‐bromophenylacetate) was used. The living character of the photoinduced polymerization was confirmed on the basis of successful chain‐extension experiments.     

An Efficient RuII–RhIII–RuII Polypyridyl Photocatalyst for Visible‐Light‐Driven Hydrogen Production in Aqueous Solution

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single‐component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen‐evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy‐conversion applications. Herein, we introduce a new ruthenium–rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H₂ production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components.     

A Covalent Organic Framework–Cadmium Sulfide Hybrid as a Prototype Photocatalyst for Visible‐Light‐Driven Hydrogen Production

CdS nanoparticles were deposited on a highly stable, two‐dimensional (2D) covalent organic framework (COF) matrix and the hybrid was tested for photocatalytic hydrogen production. The efficiency of CdS‐COF hybrid was investigated by varying the COF content. On the introduction of just 1 wt % of COF, a dramatic tenfold increase in the overall photocatalytic activity of the hybrid was observed. Among the various hybrids synthesized, that with 10 wt % COF, named CdS‐COF (90:10), was found to exhibit a steep H₂ production amounting to 3678 μmol h^?1 g^?1, which is significantly higher than that of bulk CdS particles (124 μmol h^?1 g^?1). The presence of a π‐conjugated backbone, high surface area, and occurrence of abundant 2D hetero‐interface highlight the usage of COF as an effective support for stabilizing the generated photoelectrons, thereby resulting in an efficient and high photocatalytic activity.     

[RhIII(dmbpy)2Cl2]+ as a Highly Efficient Catalyst for Visible‐Light‐Driven Hydrogen Production in Pure Water: Comparison with Other Rhodium Catalysts

We report a very efficient homogeneous system for the visible‐light‐driven hydrogen production in pure aqueous solution at room temperature. This comprises [Rh^III(dmbpy)₂Cl₂]Cl ( 1 ) as catalyst, [Ru(bpy)₃]Cl₂ ( PS1 ) as photosensitizer, and ascorbate as sacrificial electron donor. Comparative studies in aqueous solutions also performed with other known rhodium catalysts, or with an iridium photosensitizer, show that 1) the PS1 / 1 /ascorbate/ascorbic acid system is by far the most active rhodium‐based homogeneous photocatalytic system for hydrogen production in a purely aqueous medium when compared to the previously reported rhodium catalysts, Na₃[Rh^I(dpm)₃Cl] and [Rh^III(bpy)Cp*(H₂O)]SO₄ and 2) the system is less efficient when [Ir^III(ppy)₂(bpy)]Cl ( PS2 ) is used as photosensitizer. Because catalyst 1 is the most efficient rhodium‐based H₂‐evolving catalyst in water, the performance limits of this complex were further investigated by varying the PS1 / 1 ratio at pH 4.0. Under optimal conditions, the system gives up to 1010 turnovers versus the catalyst with an initial turnover frequency as high as 857 TON h^?1. Nanosecond transient absorption spectroscopy measurements show that the initial step of the photocatalytic H₂‐evolution mechanism is a reductive quenching of the PS1 excited state by ascorbate, leading to the reduced form of PS1 , which is then able to reduce [Rh^III(dmbpy)₂Cl₂]⁺ to [Rh^I(dmbpy)₂]⁺. This reduced species can react with protons to yield the hydride [Rh^III(H)(dmbpy)₂(H₂O)]²⁺, which is the key intermediate for the H₂ production.     

Capturing HBCy2: Using N,O‐Chelated Complexes of Rhodium(I) and Iridium(I) for Chemoselective Hydroboration

1,3‐N,O‐chelated complexes of Rh^I and Ir^I cooperatively and reversibly stabilized the B?H bond of HBCy₂ to afford six‐membered metallaheterocycles (M=Rh ( 7 ) or Ir ( 8 )) having a δ‐[M]???H‐B agostic interaction. Treatment of these Shimoi‐type borane adducts 7 or 8 with both an aldehyde and an alkene resulted in chemoselective aldehyde hydroboration and reformation of the 1,3‐N,O‐chelated starting material. The observed chemoselectivity is inverted from that of free HBCy₂, which is selective for alkene hydroboration.     

Iridium(III) Photocatalysis: A Visible‐Light‐Induced Dearomatizative Tandem [4+2] Cyclization to Furnish Benzindolizidines

A photocatalytic dearomatizative tandem [4+2] cyclization between N‐(2‐iodoethyl)indoles and a variety of alkenes leads to tri‐ and tetracyclic benzindolizidines with high diastereoselectivity and yield. The intermolecular annulation reaction is performed under visible‐light irradiation and employs [Ir(ppy)₃] or [Ir(dtbbpy)(ppy)₂] PF₆ as photocatalysts, in combination with tertiary amines as electron and hydrogen atom donors.     

ZnCr Layered Double Hydroxide (LDH) Nanosheets Assisted Formation of Hierarchical Flower‐Like CdZnS@LDH Microstructures with Improved Visible‐Light‐Driven H2 Production

The development of new semiconductor photocatalysts toward splitting water has supplied a promising way to obtain sustainable and clean hydrogen energy. Herein, CdZnS@layered double hydroxide (LDH) composites with a hierarchical flower‐like microstructure have been fabricated with the aid of ZnCr–LDH nanosheets as templates. XRD, SEM and HRTEM show that the ZnCr–LDH nanosheets are uniformly dispersed within the composites. The surface of the hierarchical structures is rough and composed of numerous nanocrystals of CdZnS. The HRTEM images indicate that the surface of CdZnS nanocrystals is mainly composed of the (111) plane. Moreover, the visible‐light‐driven H₂ production performance of the CdZnS in the presence and absence of ZnCr–LDH nanosheets has been measured. The results show that ZnCr–LDH nanosheets play an important role in the hierarchical morphology and photocatalytic activity of the as‐prepared samples. In the water‐splitting process, the visible‐light‐driven H₂‐production rate of hierarchical flower‐like CdZnS@LDH is 4.03 times and nearly 10 times higher than that of pristine CdZnS microsphere and pure commercial CdS, respectively. Therefore, this work not only achieves enhanced catalytic performance of the CdZnS by the introduction of ZnCr–LDH nanosheets, but also supplies an insight into the relationship between the hierarchical morphology and the semiconductor photocatalytic activity.     

Hydride‐Rhodium(III)‐N‐Heterocyclic Carbene Catalysts for Vinyl‐Selective H/D Exchange: A Structure–Activity Study

A series of neutral and cationic Rh^III‐hydride and Rh^III‐ethyl complexes bearing a NHC ligand has been synthesized and evaluated as catalyst precursors for H/D exchange of styrene using CD₃OD as a deuterium source. Various ligands have been examined in order to understand how the stereoelectronic properties can modulate the catalytic activity. Most of these complexes proved to be very active and selective in the vinylic H/D exchange, without deuteration at the aromatic positions, displaying very high selectivity toward the β‐positions. In particular, the cationic complex [RhClH(CH₃CN)₃(IPr)]CF₃SO₃ showed excellent catalytic activity, reaching the maximum attainable degree of β‐vinylic deuteration in only 20 min. By modulation of the catalyst structure, we obtained improved α/β selectivity. Thus, the catalyst [RhClH(κ²‐O,N‐C₉H₆NO)(SIPr)], bearing an 8‐quinolinolate ligand and a bulky and strongly electron‐donating SIPr as the NHC, showed total selectivity for the β‐vinylic positions. This systematic study has shown that increased electron density and steric demand at the metal center can improve both the catalytic activity and selectivity. Complexes bearing ligands with very high steric hindrance, however, proved to be inactive.     

Efficient Visible‐Light‐Driven Z‐Scheme Overall Water Splitting Using a MgTa2O6−xNy /TaON Heterostructure Photocatalyst for H2 Evolution

An (oxy)nitride‐based heterostructure for powdered Z‐scheme overall water splitting is presented. Compared with the single MgTa₂O_6?xN_y or TaON photocatalyst, a MgTa₂O_6?xN_y /TaON heterostructure fabricated by a simple one‐pot nitridation route was demonstrated to effectively suppress the recombination of carriers by efficient spatial charge separation and decreased defect density. By employing Pt‐loaded MgTa₂O_6?xN_y /TaON as a H₂‐evolving photocatalyst, a Z‐scheme overall water splitting system with an apparent quantum efficiency (AQE) of 6.8 % at 420 nm was constructed (PtO_x‐WO₃ and IO₃^?/I^? pairs were used as an O₂‐evolving photocatalyst and a redox mediator, respectively), the activity of which is circa 7 or 360 times of that using Pt‐TaON or Pt‐MgTa₂O_6?xN_y as a H₂‐evolving photocatalyst, respectively. To the best of our knowledge, this is the highest AQE among the powdered Z‐scheme overall water splitting systems ever reported.     

The Uranyl Cation as a Visible‐Light Photocatalyst for C(sp3)−H Fluorination

The fluorination of unactivated C(sp³)?H bonds remains a desirable and challenging transformation for pharmaceutical, agricultural, and materials scientists. Previous methods for this transformation have used bench‐stable fluorine atom sources; however, many still rely on the use of UV‐active photocatalysts for the requisite high‐energy hydrogen atom abstraction event. Uranyl nitrate hexahydrate is described as a convenient, hydrogen atom abstraction catalyst that can mediate fluorinations of certain alkanes upon activation with visible light.     

A Methylthio‐Functionalized‐MOF Photocatalyst with High Performance for Visible‐Light‐Driven H2 Evolution

Recently, the emergence of photoactive metal–organic frameworks (MOFs) has given great prospects for their applications as photocatalytic materials in visible‐light‐driven hydrogen evolution. Herein, a highly photoactive visible‐light‐driven material for H₂ evolution was prepared by introducing methylthio terephthalate into a MOF lattice via solvent‐assisted ligand‐exchange method. Accordingly, a first methylthio‐functionalized porous MOF decorated with Pt co‐catalyst for efficient photocatalytic H₂ evolution was achieved, which exhibited a high quantum yield (8.90 %) at 420 nm by use sacrificial triethanolamine. This hybrid material exhibited perfect H₂ production rate as high as 3814.0 μmol g^?1 h^?1, which even is one order of magnitude higher than that of the state‐of‐the‐art Pt/MOF photocatalyst derived from aminoterephthalate.     

TiO2 Nanotubes: Nitrogen‐Ion Implantation at Low Dose Provides Noble‐Metal‐Free Photocatalytic H2‐Evolution Activity

Low‐dose nitrogen implantation induces an ion and damage profile in TiO₂ nanotubes that leads to “co‐catalytic” activity for photocatalytic H₂‐evolution (without the use of any noble metal). Ion implantation with adequate parameters creates this active zone limited to the top part of the tubes. The coupling of this top layer and the underlying non‐implanted part of the nanotubes additionally contributes to an efficient carrier separation and thus to a significantly enhanced H₂ generation.     

Heteroatom Dopants Promote Two‐Electron O2 Reduction for Photocatalytic Production of H2O2 on Polymeric Carbon Nitride

Polymeric carbon nitride modified with selected heteroatom dopants was prepared and used as a model photocatalyst to identify and understand the key mechanisms required for efficient photoproduction of H₂O₂ via selective oxygen reduction reaction (ORR). The photochemical production of H₂O₂ was achieved at a millimolar level per hour under visible‐light irradiation along with 100 % apparent quantum yield (in 360–450 nm region) and 96 % selectivity in an electrochemical system (0.1 V vs. RHE). Spectroscopic analysis in spatiotemporal resolution and theoretical calculations revealed that the synergistic association of alkali and sulfur dopants in the polymeric matrix promoted the interlayer charge separation and polarization of trapped electrons for preferable oxygen capture and reduction in ORR kinetics. This work highlights the key features that are responsible for controlling the photocatalytic activity and selectivity toward the two‐electron ORR, which should be the basis of further development of solar H₂O₂ production.     

Eosin Y‐Functionalized Conjugated Organic Polymers for Visible‐Light‐Driven CO2 Reduction with H2O to CO with High Efficiency

Visible‐light‐driven photoreduction of CO₂ to energy‐rich chemicals in the presence of H₂O without any sacrifice reagent is of significance, but challenging. Herein, Eosin Y‐functionalized porous polymers (PEosinY‐N, N=1–3), with high surface areas up to 610 m² g^?1, are reported. They exhibit high activity for the photocatalytic reduction of CO₂ to CO in the presence of gaseous H₂O, without any photosensitizer or sacrifice reagent, and under visible‐light irradiation. Especially, PEosinY‐1 derived from coupling of Eosin Y with 1,4‐diethynylbenzene shows the best performance for the CO₂ photoreduction, affording CO as the sole carbonaceous product with a production rate of 33 μmol g^?1 h^?1 and a selectivity of 92 %. This work provides new insight for designing and fabricating photocatalytically active polymers with high efficiency for solar‐energy conversion.     

A Dewetted‐Dealloyed Nanoporous Pt Co‐Catalyst Formed on TiO2 Nanotube Arrays Leads to Strongly Enhanced Photocatalytic H2 Production

Pt nanoparticles are typically decorated as co‐catalyst on semiconductors to enhance the photocatalytic performance. Due to the low abundance and high cost of Pt, reaching a high activity with minimized co‐catalyst loadings is a key challenge in the field. We explore a dewetting‐dealloying strategy to fabricate on TiO₂ nanotubes nanoporous Pt nanoparticles, aiming at improving the co‐catalyst mass activity for H₂ generation. For this, we sputter first Pt‐Ni bi‐layers of controllable thickness (nm range) on highly ordered TiO₂ nanotube arrays, and then induce dewetting‐alloying of the Pt‐Ni bi‐layers by a suitable annealing step in a reducing atmosphere: the thermal treatment causes the Pt and Ni films to agglomerate and at the same time mix with each other, forming on the TiO₂ nanotube surface metal islands of a mixed PtNi composition. In a subsequent step we perform chemical dealloying of Ni that is selectively etched out from the bimetallic dewetted islands, leaving behind nanoporous Pt decorations. Under optimized conditions, the nanoporous Pt‐decorated TiO₂ structures show a>6 times higher photocatalytic H₂ generation activity compared to structures modified with a comparable loading of dewetted, non‐porous Pt. We ascribe this beneficial effect to the nanoporous nature of the dealloyed Pt co‐catalyst, which provides an increased surface‐to‐volume ratio and thus a more efficient electron transfer and a higher density of active sites at the co‐catalyst surface for H₂ evolution.     

A Dinuclear Cobalt Cryptate as a Homogeneous Photocatalyst for Highly Selective and Efficient Visible‐Light Driven CO2 Reduction to CO in CH3CN/H2O Solution

A dinuclear cobalt complex [Co₂(OH)L¹](ClO₄)₃ ( 1 , L¹=N[(CH₂)₂NHCH₂(m ‐C₆H₄)CH₂NH(CH₂)₂]₃N) displays high selectivity and efficiency for the photocatalytic reduction of CO₂ to CO in CH₃CN/H₂O (v/v=4:1) under a 450 nm LED light irradiation, with a light intensity of 100 mW cm⁻². The selectivity reaches as high as 98 %, and the turnover numbers (TON) and turnover frequencies (TOF) reach as high as 16896 and 0.47 s⁻¹, respectively, with the calculated quantum yield of 0.04 %. Such high activity can be attributed to the synergistic catalysis effect between two Co^II ions within 1 , which is strongly supported by the results of control experiments and DFT calculations.     

The Simplest Amino‐borane H2B=NH2 Trapped on a Rhodium Dimer: Pre‐Catalysts for Amine–Borane Dehydropolymerization

The μ‐amino–borane complexes [Rh₂(L^R)₂(μ‐H)(μ‐H₂B=NHR′)][BAr^F₄] (L^R=R₂P(CH₂)₃PR₂; R=Ph, ⁱPr; R′=H, Me) form by addition of H₃B?NMeR′H₂ to [Rh(L^R)(η⁶‐C₆H₅F)][BAr^F₄]. DFT calculations demonstrate that the amino–borane interacts with the Rh centers through strong Rh‐H and Rh‐B interactions. Mechanistic investigations show that these dimers can form by a boronium‐mediated route, and are pre‐catalysts for amine‐borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis.     

Functionalization of Boron Dipyrrin (BODIPY) Dyes through Iridium and Rhodium Catalysis: A Complementary Approach to α‐ and β‐Substituted BODIPYs

Two series of BODIPY dyes with substituents either in the α positions or in the β positions and different conjugation lengths were synthesized by means of iridium‐catalyzed borylation and rhodium‐catalyzed Heck‐type addition (see scheme). The α‐ and β‐substituted series show completely different photophysical properties. BODIPY=boron dipyrrin.