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

Pyridinethiolate-Capped CdSe Quantum Dots for Red-Light-Driven H2 Production in Water

The utilization of low-energy sunlight to produce renewable fuels is a subject of great interest. Here we report the first example of metal chalcogenide quantum dots (QDs) capped with a pyridinethiolate carboxylic acid (pyS-COOH) for red-light-driven H2 production in water. The precious-metal-free system is robust over 240 h, and achieves a turnover number (TON) of 43910±305 (vs Ni) with a rate of 31570±1690 μmol g-1 h-1 for hydrogen production. In contrast to the inactive QDs capped with other thiolate ligands, the CdSe-pyS-COOH QDs give a significantly higher singlet oxygen quantum yield [ΦΔ (1O2)] in solution.

An intramolecular cobalt-peptoid complex as an efficient electrocatalyst for water oxidation at low overpotential

Water electrolysis is the simplest way to produce hydrogen, as a clean renewable fuel. However, the high overpotential and slow kinetics hamper its applicability. Designing efficient and stable electrocatalysts for water oxidation (WO), which is the first and limiting step of the water splitting process, can overcome this limitation. However, the development of such catalysts based on non-precious metal ions is still challenging. Herein we describe a bio-inspired Co(iii)-based complex i.e., a stable and efficient molecular electrocatalyst for WO, constructed from a peptidomimetic oligomer called peptoid - N-substituted glycine oligomer - bearing two binding ligands, terpyridine and bipyridine, and one ethanolic group as a proton shuttler. Upon binding of a cobalt ion, this peptoid forms an intramolecular Co(iii) complex, that acts as an efficient electrocatalyst for homogeneous WO in aqueous phosphate buffer at pH 7 with a high faradaic efficiency of up to 92% at an overpotential of about 430 mV, which is the lowest reported for Co-based homogeneous WO electrocatalysts to date. We demonstrated the high stability of the complex during electrocatalytic WO and that the ethanolic side chain plays a key role in the stability and activity of the complex and also in facilitating water binding, thus mimicking an enzymatic second coordination sphere.

Mononuclear Iron Pyridinethiolate Complex Promoted CO2 Photoreduction via Rapid Intramolecular Electron Transfer

Designing earth-abundant metal complexes as efficient molecular photocatalysts for visible light-driven CO2 reduction is a key challenge in artificial photosynthesis. Here, we demonstrated the first example of a mononuclear iron pyridine-thiolate complex that functions both as a photosensitizer and catalyst for CO2 reduction. This single-component bifunctional molecular photocatalyst efficiently reduced CO2 to formate and CO with a total turnover number (TON) of 46 and turnover frequency (TOF) of 11.5 h-1 in 4 h under visible light irradiation. Notably, the quantum yield was determined to be 8.4 % for the generation of formate and CO at 400 nm. Quenching experiments indicate that high photocatalytic activity is mainly attributed to the rapid intramolecular quenching protocol. The mechanism investigation by DFT calculation and electrochemical studies revealed that the protonation of Febpy(pyS)2 is indispensable step for photocatalytic CO2 reduction.

Emerging trends in the development and applications of triazine-based covalent organic polymers: a comprehensive review

Owing to unique structural features, triazine-based covalent organic polymers (COPs) have attracted significant attention and emerged as novel catalysts or support materials for an array of applications. Typically formed by reacting triazine-based monomers or the in situ creation of triazine rings from nitrile monomers, these COPs possess 2D/3D meso/microporous structures held together via strong covalent linkages. The quest for efficient, stable and recyclable catalytic systems globally necessitates the need for a well-structured and comprehensive review summarizing the synthetic methodologies and applications of triazine-based COPs. This review explores the various synthetic routes and applications of these COPs in photocatalysis, heterogeneous catalysis, electrocatalysis, adsorption and sensing. By exploring the latest advancements and future directions, this review offers valuable insights into the synthesis and applications of triazine-based COPs.

Tailoring Second Coordination Sphere for Tunable Solid-Liquid Interfacial Charge Transfer toward Enhanced Photoelectrochemical H2 Production

The recombination of photogenerated charge carriers severely limits the performance of photoelectrochemical (PEC) H2 production. Here, we demonstrate that this limitation can be overcome by optimizing the charge transfer dynamics at the solid-liquid interface via molecular catalyst design. Specifically, the surface of a p-Si photocathode is modulated using molecular catalysts with different metal atoms and organic ligands to improve H2 production performance. Co(pda-SO3H)2 is identified as an efficient and durable catalyst for H2 production through the rational design of metal centers and first/second coordination spheres. The modulation with Co(pda-SO3H)2, which contains an electron-withdrawing -SO3H group in the second coordination sphere, elevates the flat-band potential of the polished p-Si photocathode and nanoporous p-Si photocathode by 81 mV and 124 mV, respectively, leading to the maximized energy band bending and the minimized interfacial carrier transport resistance. Consequently, both the two photocathodes achieve the Faradaic efficiency of more than 95 % for H2 production, which is well maintained during 18 h and 21 h reaction, respectively. This work highlights that the band-edge engineering by molecular catalysts could be an important design consideration for semiconductor-catalyst hybrids toward PEC H2 production.

Modulating the Primary and Secondary Coordination Spheres of Single Ni(II) Sites in Metal-Organic Frameworks for Boosting Photocatalysis

Though the coordination environment of single metal sites has been recognized to be of great importance in promoting catalysis, the influence of simultaneous precise modulation of primary and secondary coordination spheres on catalysis remains largely unknown. Herein, a series of single Ni(II) sites with altered primary and secondary coordination spheres have been installed onto metal-organic frameworks (MOFs) with UiO-67 skeleton, affording UiO-Ni-X-Y (X = S, O; Y = H, Cl, CF3) with X and Y on the primary and secondary coordination spheres, respectively. Upon deposition with CdS nanoparticles, the resulting composites present high photocatalytic H2 production rates, in which the optimized CdS/UiO-Ni-S-CF3 exhibits an excellent activity of 13.44 mmol g-1, ∼500 folds of the pristine catalyst (29.6 μmol g-1 for CdS/UiO), in 8 h, highlighting the key role of microenvironment modulation around Ni sites. Charge kinetic analysis and theoretical calculation results demonstrate that the charge transfer dynamics and reaction energy barrier are closely correlated with their coordination spheres. This work manifests the advantages of MOFs in the fabrication of structurally precise catalysts and the elucidation of particular influences of microenvironment modulation around single metal sites on the catalytic performance.

Photocatalytic Hydrogen Production with A Molecular Cobalt Complex in Alkaline Aqueous Solutions

The thermodynamic favorability of an alkaline solution for the oxidation of water suggests the need for developing hydrogen evolution reaction (HER) catalysts that can function in basic aqueous solutions so that both of the half reactions in overall water splitting can occur in mutually compatible solutions. Although photocatalytic HERs have been reported mostly in acidic solutions and a few at basic pHs in mixed organic aqueous solutions, visible-light driven HER catalyzed by molecular metal complexes in purely alkaline aqueous solutions remains largely unexplored. Here, we report a new cobalt complex with a tetrapyridylamine ligand that catalyzes photolytic HER with turnover number up to 218 000 in purely aqueous solutions at pH 9.0. Density functional theory (DFT) calculations suggested a modified electron transfer (E)-proton transfer (C)-electron transfer (E)-proton transfer (C) (mod-ECEC) pathway for hydrogen production from the protonation of CoII-H species. The remarkable catalytic activity resulting from subtle structural changes of the ligand scaffold highlights the importance of studying structure-function relationships in molecular catalyst design. Our present work significantly advances the development of a molecular metal catalyst for visible-light driven HER in more challenging alkaline aqueous solutions that holds substantial promise in solar-driven water-splitting systems.

Synergy of redox-activity and hemilability in thioamidato cobalt(III) complexes for the chemoselective reduction of nitroarenes to anilines: catalytic and mechanistic investigation

Reduction of nitro-compounds to amines is one of the most often employed and challenging catalytic processes in the fine and bulk chemical industry. Herein, we present two series of mononuclear homoleptic and heteroleptic Co(III) complexes, i.e., [Co(LNS)3] and [Co(LNS)2L1L2]x+, respectively (x = 0 or 1, LNS = pyrimidine- or pyridine-thioamidato, L1/L2 = thioamidato, phosphine or pyridine), which successfully catalyze the transformation of nitroarenes to anilines by methylhydrazine. The catalytic reaction can be accomplished for a range of electronically and sterically diverse nitroarenes, using mild experimental conditions and low catalyst loadings, resulting in the corresponding anilines in high yields, with high chemoselectivity, and no side-products. Electronic and steric properties of the ligands play pivotal role in the catalytic efficacy of the respective complexes. In particular, complexes bearing ligands of high hemilability/lability and being capable of stabilizing lower metal oxidation-states exhibit the highest catalytic activity. Mechanistic investigations suggest the participation of the Co(III) complexes in two parallel reaction pathways: (a) coordination-induced activation of methylhydrazine and (b) reduction of nitroarenes to anilines by methylhydrazine, through the formation of Co(I) and Co-hydride intermediates.

Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons

Alkyne semihydrogenation is a broadly important transformation in chemical synthesis. Here, we introduce an electrochemical method for the selective semihydrogenation of terminal alkynes using a dihydrazonopyrrole Ni complex capable of storing an H2 equivalent (2H+ + 2e-) on the ligand backbone. This method is chemoselective for the semihydrogenation of terminal alkynes over internal alkynes or alkenes. Mechanistic studies reveal that the transformation is concerted and Z-selective. Calculations support a ligand-based hydrogen-atom transfer pathway instead of a hydride mechanism, which is commonly invoked for transition metal hydrogenation catalysts. The synthesis of the proposed intermediates demonstrates that the catalytic mechanism proceeds through a reduced formal Ni(I) species. The high yields for terminal alkene products without over-reduction or oligomerization are among the best reported for any homogeneous catalyst. Furthermore, the metal-ligand cooperative hydrogen transfer enabled with this system directs the efficient flow of H atom equivalents toward alkyne reduction rather than hydrogen evolution, providing a blueprint for applying similar strategies toward a wide range of electroreductive transformations.

Enzymatic and Bioinspired Systems for Hydrogen Production

The extraordinary potential of hydrogen as a clean and sustainable fuel has sparked the interest of the scientific community to find environmentally friendly methods for its production. Biological catalysts are the most attractive solution, as they usually operate under mild conditions and do not produce carbon-containing byproducts. Hydrogenases promote reversible proton reduction to hydrogen in a variety of anoxic bacteria and algae, displaying unparallel catalytic performances. Attempts to use these sophisticated enzymes in scalable hydrogen production have been hampered by limitations associated with their production and stability. Inspired by nature, significant efforts have been made in the development of artificial systems able to promote the hydrogen evolution reaction, via either electrochemical or light-driven catalysis. Starting from small-molecule coordination compounds, peptide- and protein-based architectures have been constructed around the catalytic center with the aim of reproducing hydrogenase function into robust, efficient, and cost-effective catalysts. In this review, we first provide an overview of the structural and functional properties of hydrogenases, along with their integration in devices for hydrogen and energy production. Then, we describe the most recent advances in the development of homogeneous hydrogen evolution catalysts envisioned to mimic hydrogenases.

Assembly of copper-clusters into a framework: enhancing the structural stability and photocatalytic HER performance

We synthesized a novel Cu₈ cluster and employed it as a building block to further obtain a copper cluster-based MOF material (CCMOF). Compared with the Cu₈ cluster precursor, the periodically constructed CCMOF exhibited better structural stability, and showed enhanced photocatalytic H₂ evolution performance due to efficient mass/charge transportation benefitting from the ordered porous framework structure. This two-step grafting construction strategy is of great significance to the exploration of copper clusters in catalytic applications.

Molecular Nickel Thiolate Complexes for Electrochemical Reduction of CO2 to C1-3 Hydrocarbons

We report the unprecedented electrocatalytic activity of a series of molecular nickel thiolate complexes (1-5) in reducing CO₂ to C_1-3 hydrocarbons on carbon paper in pH-neutral aqueous solutions. Ni(mpo)₂ (3, mpo=2-mercaptopyridyl-N-oxide), Ni(pyS)₃ ^- (4, pyS=2-mercaptopyridine), and Ni(mp)₂ ^- (5, mp=2-mercaptophenolate) were found to generate C₃ products from CO₂ for the first time in molecular complex. Compound 5 exhibits Faradaic efficiencies (FEs) of 10.6 %, 7.2 %, 8.2 % for C₁ , C₂ , C₃ hydrocarbons respectively at -1.0 V versus the reversible hydrogen electrode. Addition of CO to the system significantly promotes the FE_C1-C3 to 41.1 %, suggesting that a key Ni-CO intermediate is associated with catalysis. A variety of spectroscopies have been performed to show that the structures of nickel complexes remain intact during CO₂ reduction.

Tuning the Charge Transport Property and Photocatalytic Activity of Anthracene-Based 1D π-d Conjugated Coordination Polymers by Interlayer Stacking

One-dimensional (1D) π-d-conjugated coordination polymers (CCPs) with charge delocalization have attracted significant attention due to their potential application in energy conversion and storage. However, the fundamental understanding of the correlation of their structural parameters with photophysical and photocatalytic properties remains underexplored. Herein, we report three novel Cu-node anthracene-based 1D π-d CCPs with systematic variation of steric groups (Ph > Me > H) at the 9 and 10 position of anthracene (denoted as An^Ph, An^Me, and An^H), which is aimed at altering the stacking of the polymer chains and its impact on the inter-chain charge transport property. Using the combination of steady-state X-ray absorption spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and electrochemical impedance spectroscopy, we show that the linear ligands (An^Ph, An^Me, and An^H) with different degrees of steric groups (Ph > Me > H) introduced at the 9 and 10 position of anthracene can alter the stacking of the polymer chains and thus impact their crystallinity, charge separation, and charge transport property, which in turn impacts their photocatalytic performance for hydrogen evolution reaction.

Improving Photocatalytic Hydrogen Production through Incorporating Copper to Organic Photosensitizers

Organic dyes have been investigated extensively as promising photosensitizers in noble-metal-free photocatalytic systems for hydrogen production. However, other than functional group optimization, there are very few methods reported to be effective in improving their photocatalytic activity. Herein, we report the incorporation of Cu²⁺ into purpurin and gallein dyes for visible-light-driven hydrogen production. These Cu-dye chromophores significantly promote the photocatalytic activity of homogeneous systems when paired with a series of molecular Ni or Fe catalysts. Under optimal conditions, the Cu-purpurin and Cu-gallein photosensitizers exhibit more than 20-fold increases in turnover frequencies for hydrogen evolution when compared with purpurin and gallein. Catalytic systems with the Cu-purpurin chromophore show no decrease in activity over 120 h. Based on electrochemical and fluorescence quenching experiments, the enhancement of photocatalytic activity is likely due to the fact that Cu²⁺ can facilitate the transfer of electrons from the photosensitizers to the catalysts through creating highly reducing centers.

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.

Dissociation of Pyridinethiolate Ligands during Hydrogen Evolution Reactions of Ni-Based Catalysts: Evidence from X-ray Absorption Spectroscopy

The protonation of several Ni-centered pyridine-2-thiolate photocatalysts for hydrogen evolution is investigated using X-ray absorption spectroscopy (XAS). While protonation of the pyridinethiolate ligand was previously thought to result in partial dechelation from the metal at the pyridyl N site, we instead observe complete dissociation of the protonated ligand and replacement by solvent molecules. A combination of Ni K-edge and S K-edge XAS of the catalyst Ni(bpy)(pyS)₂ (bpy = 2,2′-bipyridine; pyS = pyridine-2-thiolate) identifies the structure of the fully protonated catalyst as a solvated [Ni(bpy)(DMF)₄]²⁺ (DMF = dimethylformamide) complex and the dissociated ligands as the N-protonated 2-thiopyridone (pyS-H). This surprising result is further supported by UV–vis absorption spectroscopy and DFT calculations and is demonstrated for additional catalyst structures and solvent environments using a combination of XAS and UV–vis spectroscopy. Following protonation, electrochemical measurements indicate that the solvated Ni bipyridine complex acts as the primary electron-accepting species during photocatalysis, resulting in separate protonated ligand and reduced Ni species. The role of ligand dissociation is considered in the larger context of the hydrogen evolution reaction (HER) mechanism. As neither the pyS-H ligand nor the Ni bipyridine complex acts as an efficient HER catalyst alone, the critical role of ligand coordination is highlighted. This suggests that shifting the equilibrium toward bound species by addition of excess protonated ligand (2-thiopyridone) may improve the performance of pyridinethiolate-containing catalysts.

Protonation of hydrogen-evolving Ni pyridinethiolate catalysts is investigated using X-ray absorption spectroscopy supported by UV−vis absorption spectroscopy and density functional theory. While pyridinethiolate ligand protonation was previously assumed to result in a partially coordinated species with a dissociated Ni−N bond, it is instead observed here to fully dissociate from the metal. The results are considered in the context of the electro- and photocatalytic hydrogen evolution reaction mechanisms of Ni pyridinethiolate complexes.

Selective CO2 Reduction to Ethylene Using Imidazolium-Functionalized Copper

Electrochemical CO₂ reduction is a promising approach to obtain sustainable chemicals in energy conversion. Improving the selectivity of CO₂ reduction toward a particular C₂ product such as ethylene remains a significant challenge. Herein, we report a series of imidazolium hexafluorophosphate compounds as surface modifiers for planar Cu foils to boost the Faradaic efficiency (FE) of ethylene from 5 to 73%, which is among the highest reported using polycrystalline Cu. The modified electrodes are convenient to prepare. The structure-function study demonstrates that varying the alkyl or aromatic substituents on the imidazolium nitrogen atoms has significant effects on the morphology of the deposited films and the product selectivity of CO₂ reduction. Experimental FE_C≥2, FE_C2H4, ln(FE_C≥2/FE_CH4), and ln(FE_C2H4/FE_C2H5OH) values show generally linear relationships with FE_H2 while using different imidazolium modifiers, suggesting that factors governing proton reduction may also be directly related to both overall C_≥2 generation and ethylene selectivity. This work presents an effective and practical way in tailoring the active sites of metallic surface for selective CO₂ reduction.

Accelerating Nickel-Based Molecular Construction via DFT Guidance for Advanced Photocatalytic Hydrogen Production

Understanding the nickel-based molecular catalyst structure and functional relationship is crucial for catalytic hydrogen production in aqueous solutions. Density functional theory (DFT) provides mature theoretical knowledge for efficient catalyst design, significantly reducing catalyst synthesis time and energy consumption. In the present work, three molecular catalysts, Ni(qbz)(pys)₂ (qbz = 2-quinoline benzimidazole) (NQP 1), Ni(qbo)(pys)₂ (qbo = 2-quinoline benzothiazole) (NQP 2), and Ni(pbz)(pys)₂ (pbz = 4-chloro-2,2-pyridylbenzimidazole) (NQP 3) (pys = 2-mercaptopyridine), were designed and synthesized and exhibit a high performance for H₂ generation in aqueous solution with a lamp (λ ≥ 400 nm) under visible light irradiation. Under the optimal conditions, a H₂ evolution rate as high as 1190 μmol h^-1 can be obtained over 25 mg of NQP 1 with the best catalytic performance. DFT has been adopted in this study to unveil the relationship between the ligand qbz and catalyst NQP 1─an efficient step in the design of catalysts with an excellent catalytic performance. We show that, in addition to the presence of the triphenyl ring increasing the overall electron density, rapid electron transfer (ET) from excited fluorescein (Fl) to NQP 1 significantly improves the chance of photogenerated electrons transferring to the active site, ultimately increasing the catalytic activity for H₂ production. This work on understanding the correlation between structures and properties of complexes provides a new idea for manufacturing high-performance photocatalysts.

Computational investigation into intramolecular hydrogen bonding controlling the isomer formation and pKa of octahedral nickel (II) proton reduction catalysts

This work demonstrates the impact of intramolecular hydrogen bonding (H-bonding) on the calculated pKa of octahedral tris-(pyridinethiolato)nickel (II), [Ni(PyS)3]-, proton reduction catalysts. Density Functional Theory (DFT) calculations on a [Ni(PyS)3]-...

Excited-state structural dynamics of nickel complexes probed by optical and X-ray transient absorption spectroscopies: insights and implications

Excited states of nickel complexes undergo a variety of photochemical processes, such as charge transfer, ligation/deligation, and redox reactions, relevant to solar energy conversion and photocatalysis. The efficiencies of the aforementioned processes are closely coupled to the molecular structures in the ground and excited states. The conventional optical transient absorption spectroscopy has revealed important excited-state pathways and kinetics, but information regarding the metal center, in particular transient structural and electronic properties, remains limited. These deficiencies are addressed by X-ray transient absorption (XTA) spectroscopy, a detailed probe of 3d orbital occupancy, oxidation state and coordination geometry. The examples of excited-state structural dynamics of nickel porphyrin and nickel phthalocyanine have been described from our previous studies with highlights on the unique structural information obtained by XTA spectroscopy. We close by surveying prospective applications of XTA spectroscopy to active areas of Ni-based photocatalysis based on the knowledge gained from our previous studies.

Activating the Fe(I) State of Iron Porphyrinoid with Second-Sphere Proton Transfer Residues for Selective Reduction of CO2 to HCOOH via Fe(III/II)-COOH Intermediate(s)

The ability to tune the selectivity of CO₂ reduction by first-row transition metal-based complexes via the inclusion of second-sphere effects heralds exciting and sought-after possibilities. On the basis of the mechanistic understanding of CO₂ reduction by iron porphyrins developed by trapping and characterizing the intermediates involved ( J. Am. Chem. Soc. 2015, 137, 11214), a porphyrinoid ligand is envisaged to switch the selectivity of the iron porphyrins by reducing CO₂ from CO to HCOOH as well as lower the overpotential to the process. The results show that the iron porphyrinoid designed can catalyze the reduction of CO₂ to HCOOH using water as the proton source with 97% yield with no detectable H₂ or CO. The iron porphyrinoid can activate CO₂ in its Fe(I) state resulting in very low overpotential for CO₂ reduction in contrast to all reported iron porphyrins, which can reduce CO₂ in their Fe(0) state. Intermediates involved in CO₂ reduction, Fe(III)-COOH and a Fe(II)-COOH, are identified with in situ FTIR-SEC and subsequently chemically generated and characterized using FTIR, resonance Raman, and Mössbauer spectroscopy. The mechanism of the reaction helps elucidate a key role played by a closely placed proton transfer residue in aiding CO₂ binding to Fe(I), stabilizing the intermediates, and determining the fate of a rate-determining Fe(II)-COOH intermediate.

Self-Assembled Nickel-4 Supramolecular Squares and Assays for HER Electrocatalysts Derived Therefrom

Solid-state structures find a self-assembled tetrameric nickel cage with carboxylate linkages, [Ni(N₂S'O)I(CH₃CN)]₄ ([Ni-I]₄⁰), resulting from sulfur acetylation by sodium iodoacetate of an [NiN₂S]₂²⁺ dimer in acetonitrile. Various synthetic routes to the tetramer, best described from XRD as a molecular square, were discovered to generate the hexacoordinate nickel units ligated by N₂S_thioether, iodide, and two carboxylate oxygens, one of which is the bridge from the adjacent nickel unit in [Ni-I]₄⁰. Removal of the four iodides by silver ion precipitation yields an analogous species but with an additional vacant coordination site, [Ni-Solv]⁺, a cation but with coordinated solvent molecules. This also recrystallizes as the tetramer [Ni-Solv]₄⁴⁺. In solution, dissociation into the (presumed) monomer occurs, with coordinating solvents occupying the vacant site [Ni(N₂S'O)I(solv)]⁰, ([Ni-I]⁰). Hydrodynamic radii determined from ¹H DOSY NMR data suggest that monomeric units are present as well in CD₂Cl₂. Evans method magnetism values are consistent with triplet spin states in polar solvents; however, in CD₂Cl₂ solutions no paramagnetism is evident. The abilities of [Ni-I]₄⁰ and [Ni-Solv]₄⁴⁺ to serve as sources of electrocatalysts, or precatalysts, for the hydrogen evolution reaction (HER) were explored. Cyclic voltammetry responses and bulk coulometry with gas chromatographic analysis demonstrated that a stronger acid, trifluoroacetic acid, as a proton source resulted in H₂ production from both electroprecatalysts; however, electrocatalysis developed primarily from uncharacterized deposits on the electrode. With acetic acid as a proton source, the major contribution to the HER is from homogeneous electrocatalysis. Overpotentials of 490 mV were obtained for both the solution-phase [Ni-I]⁰ and [Ni-Solv]⁺. While the electrocatalyst derived from [Ni-Solv]⁺ has a substantially higher TOF (102 s^-1) than [Ni-I]⁰ (19 s^-1), it has a shorter catalytically active lifespan (4 h) in comparison to [Ni-I]⁰ (>18 h).

Exploring Ligand-Centered Hydride and H-Atom Transfer

The nickel(II) complex [ON(H)O]Ni(PPh₃) ([ON(H)O]^2- = bis(3,5-di-tert-butyl-2-phenoxy)amine), bearing a protonated redox-active ligand, was examined for its ability to serve as a hydrogen atom (H^•) and hydride (H^-) donor. Deprotonation of [ON(H)O]Ni(PPh₃) afforded the square-planar anion {[ONO^cat]Ni(PPh₃)}^1-, whereas hydrogen atom transfer from [ON(H)O]Ni(PPh₃) to TEMPO^• in the presence of added PPh₃ afforded five-coordinate [ONO]Ni(PPh₃)₂ that has been structurally characterized. In solution, this five-coordinate complex exists in equilibrium with four-coordinate [ONO]Ni(PPh₃), and this ligand exchange equilibrium correlates with a valence tautomerization between the redox-active ligand and the nickel center. Abstraction of a hydride from [ON(H)O]Ni(PPh₃) in the presence of PPh₃ afforded the octahedral complex, [ONO^q]Ni(OTf)(PPh₃)₂, which was characterized as an S = 1, nickel(II) complex. Bond dissociation free energy (BDFE) and hydricity (ΔG°_H-) measurements benchmark the thermodynamic propensity of this complex to participate in ligand-centered H^• and H^- transfer reactions.

Selective Reduction of Nitroarenes to Arylamines by the Cooperative Action of Methylhydrazine and a Tris(N-heterocyclic thioamidate) Cobalt(III) Complex

We report an efficient catalytic protocol that chemoselectively reduces nitroarenes to arylamines, by using methylhydrazine as a reducing agent in combination with the easily synthesized and robust catalyst tris(N-heterocyclic thioamidate) Co(III) complex [Co(κS,N-tfmp2S)₃], tfmp2S = 4-(trifluoromethyl)-pyrimidine-2-thiolate. A series of arylamines and heterocyclic amines were formed in excellent yields and chemoselectivity. High conversion yields of nitroarenes into the corresponding amines were observed by using polar protic solvents, such as MeOH and ⁱPrOH. Among several hydrogen donors that were examined, methylhydrazine demonstrated the best performance. Preliminary mechanistic investigations, supported by UV-vis and NMR spectroscopy, cyclic voltammetry, and high-resolution mass spectrometry, suggest a cooperative action of methylhydrazine and [Co(κS,N-tfmp2S)₃] via a coordination activation pathway that leads to the formation of a reduced cobalt species, responsible for the catalytic transformation. In general, the corresponding N-arylhydroxylamines were identified as the sole intermediates. Nevertheless, the corresponding nitrosoarenes can also be formed as intermediates, which, however, are rapidly transformed into the desired arylamines in the presence of methylhydrazine through a noncatalytic path. On the basis of the observed high chemoselectivity and yields, and the fast and clean reaction processes, the present catalytic system [Co(κS,N-tfmp2S)₃]/MeNHNH₂ shows promise for the efficient synthesis of aromatic amines that could find various industrial applications.

Reactivity and Mechanisms of Photoactivated Heterometallic [RuII NiII ] and [RuII NiII RuII ] Catalysts for Dihydrogen Generation from Water

Two heterometallic photocatalysts were designed and probed for water reduction. Both [(bpy)₂ Ru^II Ni^II (L¹ )](ClO₄ )₂ (1) and [(bpy)₂ Ru^II Ni^II (L² )₂ Ru^II (bpy)₂ ](ClO₄ )₂ (2) can generate the low-valent precursor involved in hydride formation prior to dihydrogen generation. However, while the bimetallic [Ru^II Ni^II ] (1) requires the presence of an external photosensitizer to trigger catalytic activity, the trimetallic [Ru^II Ni^II Ru^II ] (2) displays significant coupling between the catalytic and light-harvesting units to promote intramolecular multielectron transfer and perform photocatalysis at the Ni center. A concerted experimental and theoretical effort proposes mechanisms to explain why 1 is unable to achieve self-supported catalysis, while 2 is fully photocatalytic.

Thiolate Coordination vs C-S Bond Cleavage of Thiolates in Dinickel(II) Complexes

A detailed study for the synthesis of dinickel(II)-thiolate and dinickel(II)-hydrosulfide complexes and the complete characterization of the relevant intermediates involved in the C-S bond cleavage of thiolates are presented. Hydrated Ni(II) salts mediate the hydrolytic C-S bond cleavage of thiolates (NaSR/RSH; R = Me, Et, ⁿBu, ^tBu), albeit inefficiently, to yield a mixture of a dinickel(II)-hydrosulfide complex, [Ni₂(BPMP)(μ-SH)(DMF)₂]²⁺ (1), and the corresponding dinickel(II)-thiolate complexes, such as [Ni₂(BPMP)(μ-SEt)(ClO₄)]¹⁺ (2) (HBPMP is 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol). A systematic study for the reactivity of thiolates with Ni(II) was therefore pursued which finally yielded 1 as a pure product which has been characterized in comparison with the dinickel(II)-dichloride complex, [Ni₂(BPMP)(Cl)₂(MeOH)₂]¹⁺ (3). While the reaction of thiolates with anhydrous Ni(OTf)₂ in dry conditions could only yield [Ni₂(BPMP)(OTf)₂]¹⁺ (5) instead of the expected dinickel(II)-thiolate compound, the C-S bond cleavage could be suppressed by the use of a chelating thiol, such as PhCOSH, to yield [Ni₂(BPMP)(SCOPh)₂]¹⁺ (6). Finally, with the suitable choice of a monodentate thiol, a dinickel(II)-monothiolate complex, [Ni₂(BPMP)(SPh)(DMF)(MeOH)(H₂O)]²⁺ (7), was isolated as a pure product within 1 h of reaction, which after a longer time of reaction yielded 1 and PhOH. Complex 7 may thus be regarded as the intermediate that precedes the C-S bond cleavage and is generated by the reaction of a thiolate with an initially formed dinickel(II)-solvento complex, [Ni₂(BPMP)(MeOH)₂(H₂O)₂]³⁺(4). Selected dinickel(II) complexes were explored further for the scope of substitution reactions, and the results include the isolation of a dinickel(II)-bis(thiolate) complex, [Ni₂(BPMP)(μ-SPh)₂]¹⁺ (8).

Copper(i) pyrimidine-2-thiolate cluster-based polymers as bifunctional visible-light-photocatalysts for chemoselective transfer hydrogenation of α,β-unsaturated carbonyls

The photoinduced chemoselective transfer hydrogenation of unsaturated carbonyls to allylic alcohols has been accomplished using cluster-based MOFs as bifunctional visible photocatalysts.

Mononuclear Mn complexes featuring N,S-/N,N-donor and 1,3,5-triaza-7-phosphaadamantane ligands: synthesis and electrocatalytic properties

Mononuclear Mn(i) carbonyl complexes incorporating 2-mercaptobenzothiazole or 2-mercaptobenzimidazole and phosphaadamantane ligands were evaluated as electrocatalysts for the HER both in acetonitrile and acetonitrile/water.

Nickel(II) complexes based on dithiolate-polyamine binary ligand systems: crystal structures, hirshfeld surface analysis, theoretical study, and catalytic activity study on photocatalytic hydrogen generation

To ascertain the influence of binary ligand systems [1,1-dicyanoethylene-2,2-dithiolate (i-mnt-2) and polyamine {tetraen = tris(2-aminoethyl)amine, tren = diethylene triamine and opda = o-phenylenediamine}] on the coordination modes of the Ni(ii) metal center and resulting supramolecular architectures, a series of nickel(ii) thiolate complexes [Ni(tetraen)(i-mnt)](DMSO) (1), [Ni2(tren)2(i-mnt)2] (2), and [Ni2(i-mnt)2(opda)2]n (3) have been synthesized in high yield in one step in water and structurally characterized by single crystal X-ray crystallography and spectroscopic techniques. X-ray diffraction studies disclose the diverse i-mnt-2 coordination to the Ni+2 center in the presence of active polyamine ligands, forming a slightly distorted octahedral geometry (NiN4S2) in 1, square planar (NiS4) and distorted octahedral geometries (NiN6) in the bimetallic co-crystallized aggregate of cationic [Ni(tren)2]+2 and anionic [Ni(i-mnt)2]-2 in 2, and a one dimensional (1D) polymeric chain along the [100] axis in 3, having consecutive square planar (NiS4) and octahedral (NiN6) coordination kernels. The N-HO, N-HS, N-HN, N-HS, N-HN, and N-HO type hydrogen bonds stabilize the supramolecular assemblies in 1, 2, and 3 respectively imparting interesting graph-set-motifs. The molecular Hirshfeld surface analyses (HS) and 2D fingerprint plots were utilized for decoding all types of non-covalent contacts in the crystal networks. Atomic HS analysis of the Ni+2 centers reveals significant Ni-N metal-ligand interactions compared to Ni-S interactions. We have also studied the unorthodox interactions observed in the solid state structures of 1-3 by QTAIM and NBO analyses. Moreover, all the complexes proved to be highly active water reduction co-catalysts (WRC) in a photo-catalytic hydrogen evolution process involving iridium photosensitizers, wherein 2 and 3 having a square planar arrangement around the nickel center(s) - were found to be the most active ones, achieving 1000 and 1119 turnover numbers (TON), respectively.