A Nickel Phosphine Complex as a Fast and Efficient Hydrogen Production Catalyst

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.

The rigidity and chelation effect of ligands on the hydrogen evolution reaction catalyzed by Ni(II) complexes

With increasing interest in nickel-based electrocatalysts, three heteroleptic Ni(II) dithiolate complexes with the general formula [Ni(II)L(L')2] (1-3), L = 2-(methylene-1,1'-dithiolato)-5,5'-dimethylcyclohexane-1,3-dione and L' = triphenylphosphine (1), 1,1'-bis(diphenylphosphino)ferrocene (DPPF) (2), and 1,2-bis(diphenylphosphino)ethane (DPPE) (3), have been synthesized and characterized by various spectroscopic techniques (UV-vis, IR, 1H, and 31P{1H} NMR) as well as the electrochemical method. The molecular structure of complex 2 has also been determined by single-crystal X-ray crystallography. The crystal structure of complex 2 reveals a distorted square planar geometry around the nickel metal ion with a NiP2S2 core. The cyclic voltammograms reveal a small difference in the redox properties of complexes (ΔE° = 130 mV) while the difference in the catalytic half-wave potential becomes substantial (ΔEcat/2 = 670 mV) in the presence of 15 mM CF3COOH. The common S^S-dithiolate ligand provides stability, while the rigidity effect of other ligands (DPPE (3) > DPPF (2) > PPh3 (1)) regulates the formation of the transition state, resulting in the NiIII-H intermediate in the order of 1 > 2 > 3. The foot-of-the-wave analysis supports the widely accepted ECEC mechanism for Ni-based complexes with the first protonation step as a rate-determining step. The electrocatalytic proton reduction activity follows in the order of complex 1 > 2 > 3. The comparatively lower overpotential and higher turnover frequency of complex 1 are attributed to the flexibility of the PPh3 ligand, which favours the easy formation of a transition state.

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.

The hangman effect boosts hydrogen production by a manganese terpyridine complex

The manganese terpyridine complex 1 with a coordinated carboxylate in the axial position was obtained in situ. By virtue of a hangman effect, complex 1 catalyzes electrochemical hydrogen evolution from phenol in acetonitrile solution with a turnover frequency of 525 s^-1 at a low overpotential of ca. 230 mV.

Electrocatalytic H2 evolution promoted by a bioinspired (N2S2)Ni(II) complex

A bioinspired (N2S2)Ni(II) electrocatalyst is reported that produces H₂ from CF₃CO₂H with a turnover frequency (TOF) of ∼1250 s^-1 at low acid concentration (<0.043 M) in MeCN. A mechanism for the H₂ production by this electrocatalyst is proposed and its activity is benchmarked against those of other reported molecular Ni H₂ evolution electrocatalysts. The involvement of a hemilabile pyridyl group of the N2S2 ligand is proposed to mimic the role of a cysteine residue involved in the biological proton reduction performed by [NiFe] hydrogenases.

Electrocatalytic proton reduction by dinuclear cobalt complexes in a nonaqueous electrolyte

Two dinuclear CoII complexes 1 and 2 have been synthesized and characterized using various spectroscopic methods. Both the complexes were employed for H+ reduction in organic media. Faradaic efficiency of 82–90% was obtained for the H2 evolution.

Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in "PN3 P" Pincer-Supported NiII Complexes

Water is the most sustainable source for H₂ production, and the efficient electrocatalytic production of H₂ from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ³ -2,6-{Ph₂ PNR}₂ (NC₅ H₃ )Br₂ ] (R=H I, Me II) is reported. Hydrogen generation from H₂ O/CH₃ CN solutions is initiated at -2 V against Fc^+/0 , and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s^-1 . A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H₂ formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni^0I species bonded to an anionic PN³ P ligand ("L^- /Ni^0I ") and a Ni^I -hydride ("Ni-H"). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pK_a .

Effect of Counteranions in Electrocatalytic Hydrogen Generation Promoted by Bis(phosphinopyridyl) Ni(II) Complexes

We previously reported the preparation and characterization of a Ni(II) complex capable of electrocatalytic hydrogen generation. The complex [Ni(L_NH2)₂Cl]Cl (1) includes a 6-((diphenylphosphino)methyl)pyridin-2-amine ligand (L_NH2), which has an amino group as a base that acts as a proton transfer site by virtue of its location near the metal center. In order to study the effect of counteranions in hydrogen generation, two additional Ni^II(L_NH2) complexes with weakly coordinating/noncoordinating counteranions, [Ni(L_NH2)₂](OTs)₂ (OTs^- = p-toluenesulfonate) (2) and [Ni(L_NH2)₂](BF₄)₂ (3), were synthesized. Their X-ray crystal structures reveal that the Ni(II) ion is coordinated with two bidentate L_NH2 ligands in both complexes. Complex 2 contains both trans and cis isomers in the unit cell. The former is in an axially elongated square-pyramidal geometry (τ₅ = 0.17), and the latter is in a nearly square planar geometry (τ₄ = 0.11) with two weakly interacting OTs^- anions at the axial sites. Complex 3 has only the cis isomer in the solid state, which is in a nearly square planar geometry (τ₄ = 0.10). These complexes are slightly different from 1, which has a distorted-square-pyramidal geometry (τ₅ = 0.25) with a coordinated chloride anion. UV-vis spectra of 2 and 3 in MeCN show a spectral pattern characteristic of a square-planar Ni(II) complex. These spectra are slightly different from the unique spectrum of 1, which is typical of an axially coordinating Ni(II) species as a result of having a Cl^- anion at the apical position. Electrocatalytic hydrogen generation promoted by these three Ni(II) complexes (1.0 mmol) demonstrates an increase in the catalytic current induced by stepwise addition of HOAc (pK_a = 22.3 in MeCN) as a proton source. The complexes demonstrate turnover frequencies (TOF) of 3800 s^-1 for 1, 5400 s^-1 for 2, and 8800 s^-1 for 3 in MeCN (3 mL) containing 0.1 M [n-Bu₄N](ClO₄) in the presence of HOAc (145 equiv) at overpotentials of ca. 530, 490, and 430 mV, respectively.

Enhanced Hydrogen Evolution in Neutral Water Catalyzed by a Cobalt Complex with a Softer Polypyridyl Ligand

To explore the structure-function relationships of cobalt complexes in the catalytic hydrogen evolution reaction (HER), we studied the substitution of a tertiary amine with a softer pyridine group and the inclusion of a conjugated bpy unit in a Co complex with a new pentadentate ligand, 6-[6-(1,1-di-pyridin-2-yl-ethyl)-pyridin-2-ylmethyl]-[2,2']bipyridinyl (Py3Me-Bpy). These modifications resulted in significantly improved stability and activity in both electro- and photocatalytic HER in neutral water. [Co(Py3Me-Bpy)(OH₂ )](PF₆ )₂ catalyzes the electrolytic HER at -1.3 V (vs. SHE) for 20 hours with a turnover number (TON) of 266 300, and photolytic HER for two days with a TON of 15 000 in pH 7 aqueous solutions. The softer ligand scaffold possibly provides increased stability towards the intermediate Co^I species. DFT calculations demonstrate that HER occurs through a general electron transfer/proton transfer/electron transfer/proton transfer pathway, with H₂ released from the protonation of Co^II -H species.

Efficient Hydrogen Generation from the NaBH4 Hydrolysis by Cobalt-Based Catalysts: Positive Roles of Sulfur-Containing Salts

Development of a simple and efficient strategy for improving the catalytic activity of cobalt-based catalysts toward hydrogen evolution from sodium borohydride (NaBH₄) is paramount but remains challenging. Here, we reported a facile and efficient approach to tune the catalytic performance for NaBH₄ hydrolysis with Co-based catalysts prepared by using cobalt sulfate as a precursor or a mixture of sulfur-containing sodium salts/cobalt salts as a raw material. With the use of cobalt sulfate as the precursor, the CoSO₄-doped Co₃O₄ sample was formed and it exhibited excellent activity with the generation of ∼500 mL of hydrogen gas from NaBH₄ hydrolysis under mild conditions. In terms of sulfur-free cobalt salts (e.g., cobalt chloride, cobalt nitrate, and cobalt acetate) as precursors, the obtained Co-based samples were found to be entirely ineffective for hydrogen production. Interestingly, during the cobalt-based catalyst preparation, the introduction of sodium sulfate or sodium sulfide can considerably accelerate hydrogen production. On the contrary, adding sulfur-bearing salts did not inspire any activity improvement only during the hydrogen generation reaction. Control experiments indicate that during catalyst preparation, the presence of Na₂SO₄ and Na₂S is beneficial for the in situ transformation of Co₃O₄ into catalytically active Co-B alloys, accompanying a positive change in surface morphology during the NaBH₄ hydrolysis, thereby inducing an excellent hydrogen generation rate of up to 4425 mL·min^-1·g_cat^-1.

A bioinspired thiolate-bridged dinickel complex with a pendant amine: synthesis, structure and electrocatalytic properties

By employing X(CH₂CH₂S^-)₂ (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel complexes supported by phosphine ligands, [(dppe)Ni(μ-1SSS':2SS-tpdt)Ni(dppe)][PF₆]₂ (1[PF₆]₂, dppe = Ph₂P(CH₂)₂PPh₂), [(dppe)Ni(μ-1SSN:2SS-npdt)Ni(dppe)][PF₆]₂ (2[PF₆]₂) and [(dppe)Ni(t-Cl)(μ-1SSX:2SS-C₄H₈S₂X)Ni(dppe)][PF₆] (3[PF₆], X = S; 4[PF₆], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF₆]₂, elongation of the Ni-N bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two nickel centers in solution, which is evidenced by ³¹P{¹H} NMR spectroscopic results. Furthermore, these thiolate-bridged dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF₆]₂ featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at a relatively low overpotential.

Recent advances in the mechanisms of the hydrogen evolution reaction by non-innocent sulfur-coordinating metal complexes

This review comments on the homogeneous HER mechanisms for catalysts carrying S-non-innocent ligands in the light of experimental and computational data.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Ligand dechelation effect on a [Co(tpy)2]2+ scaffold towards electro-catalytic proton and water reduction

This study presents the synthesis of the 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)quinoline (4Ql-tpy) ligand and H₂ evolution by corresponding cobalt complex, i.e. [Co(4Ql-tpy)₂]Cl₂.

Effect of ligand substituents on nickel and copper [N4] complexes: electronic and redox behavior, and reactivity towards protons

The ligand substituents have a major effect on the redox potentials, catalytic efficiency and robustness of the complexes in HER.