Electrochemical Proton Reduction Research Articles

Hydrogen Evolving Mono‐Nuclear Manganese Complexes with Carefully Positioned Pyridine Base Ligands

AbstractEnormous efforts have been made by the scientific community in the development of bio‐mimetic/bio‐inspired hydrogen‐evolving complexes as catalysts for an alternative hydrogen energy carrier and a renewable energy sources. In this regard, new bio‐inspired mono‐nuclear Mn(I) carbonyl model complexes fac‐[(Mn(CO)3(κ2‐SN2C7H5)(κ1‐PPh2Py))] 1 and fac‐[(Mn(CO)3(κ2‐S2NC7H4)(κ1‐PPh2Py))] 2 with 2‐mercaptobenzimidazole (N,N) and 2‐mercaptobenzothiazole (S,N) ligands and a N‐based phosphine ligand (PPh2Py) have been synthesized, spectroscopically characterized and evaluated as electro‐catalysts for hydrogen generation. In contrast to related mono‐nuclear (N,N/S,N)−Mn−PPh3 complexes,, the newly introduced pyridine nitrogen atom of the PPh2Py ligand placed in an axial position in the second coordination sphere of the Mn(I) complexes, provides a first site for protonation and enables a fast intra‐molecular proton transfer to the central metal atom with a weaker acid. Theoretical DFT calculations enable a detailed picture of catalytic pathways, assign redox transitions and structural changes. Both the molecular complexes were tested as electro‐catalysts for hydrogen generation in CH3CN. They effectively catalyzed the electrochemical proton reduction with the weaker acetic acid as the proton source and displayed a turnover frequency (TOF/s−1; O.P., η/V) of 610; 1.02 and 615; 1.12, respectively. The computer‐aided design of dedicated proton transfer pathways from second coordination sphere ligands is thus able to allow a hydrogen evolution reactivity with a weaker acid (acetic acid).

A Bifunctional Novel Bipyridine Bridged Dicobaloxime Complex for Electrochemical Proton Reduction and Toxic Metanil Yellow Dye Detection

AbstractA new bipyridine bridged dicobaloxime complex BrCo(dpgh)2(4,4’‐bipyridine)Co(dpgH)2Br was synthesized and characterized by standard spectroscopic techniques. The synthesized complex was employed as an electrocatalyst for the proton reduction in the presence of acetic acid as an acid source. The catalytic efficiency was calculated to be 0.379. Subsequently the nanocomposite of the complex was prepared with g‐C3N4 nanosheet by hydrothermal method and the composite material was characterized by standard physical characterization techniques. When the nanocomposite was employed as a catalyst for electrochemical sensing of toxic food color Metanil yellow dye using gold electrode, the LOD value was found to be 0.09 nM. These studies show that the dicobaloxime acts as a bifunctional catalyst towards electrochemical proton reduction and for the detection of Metanil yellow dye.

Design and synthesis of heteroleptic Ni(II) dipyrrin complexes for electrochemical proton reduction reactions: Cyclic voltammetric and theoretical studies

The effect of nuclearity on electrochemical hydrogen generation using new heteroleptic Ni(II) complexes containing redox-active dipyrrin and dithiocarbamate ligands has been described. Complexes 1–2 have been meticulously characterized by spectroscopic studies (ESI-MS, IR, 1H, 13C NMR, UV–vis) and their structures unambiguously confirmed by X-ray single crystal analyses. Electrocatalytic properties of the complexes toward hydrogen evolution reaction have been investigated by cyclic voltammetric studies in an organic medium in the presence of acetic acid as a weak proton source. Notably, complexes 1 and 2 produce H2 via doubly reduced Ni(II) species i.e. Ni(0) in the presence of acetic acid. Further, these complexes exhibited significant electrocatalytic activity (TOF: 264 (1) and 650 s−1 (2). Controlled potential electrolysis established a minimum Faradaic efficiency of 92 (1) and 96 % (2). Complex 2 exhibited higher turnover frequency relative to 1, while 1 showed lower overpotential (0.35 V) in comparison to 2 (0.45 V). The stability of the complexes and the amount of produced H2 has been investigated by bulk electrolysis study. A tentative mechanism (ECEC; E, electrons and C, chemical steps) and involved intermediate species for the proton reduction reaction for 1 has been established by theoretical studies.

Mechanism-Guided Kinetic Analysis of Electrocatalytic Proton Reduction Mediated by a Cobalt Catalyst Bearing a Pendant Basic Site.

Cobalt polypyridyl complexes stand out as efficient catalysts for electrochemical proton reduction, but investigations into their operating mechanisms, with broad-reaching implications in catalyst design, have been limited. Herein, we investigate the catalytic activity of a cobalt(II) polypyridyl complex bearing a pendant pyridyl base with a series of organic acids spanning 20 pKa units in acetonitrile. Structural analysis, as well as electrochemical studies, reveals that the Co(III) hydride intermediate is formed through reduction of the Co(II) catalyst followed by direct metal protonation in the initial EC step despite the presence of the pendant base, which is commonly thought of as a more kinetically accessible protonation site. Protonation of the pendant base occurs after the Co(III) hydride intermediate is further reduced in the overall ECEC pathway. Additionally, when the acid used is sufficiently strong, the Co(II) catalyst can be protonated, and the Co(III) hydride can react directly with acid to release H2. With thorough mechanistic understanding, the appropriate electroanalytical methods were identified to extract rate constants for the elementary steps over a range of conditions. Thermodynamic square schemes relating catalytic intermediates proposed in the three electrocatalytic HER mechanisms were constructed. These findings reveal a full description of the HER electrocatalysis mediated by this molecular system and provide insights into strategies to improve synthetic fuel-forming catalysts operative through metal hydride intermediates.

Synthesis of phosphine derivatives of [Fe2(CO)6(μ-sdt)] (sdt = SCH2SCH2S) and investigation of their proton reduction capabilities

The reactions of [Fe2(CO)6(μ-sdt)] (1) (sdt = SCH2SCH2S) with phosphine ligands have been investigated. Treatment of 1 with dppm (bis(diphenylphosphino)methane) or dcpm (bis(dicyclohexylphosphino)methane) affords the diphosphine-bridged products [Fe2(CO)4(μ-sdt)(μ-dppm)] (2) and [Fe2(CO)4(μ-sdt)(μ-dcpm)] (3), respectively. The complex [Fe2(CO)4(μ-sdt)(κ2-dppv)] (4) with a chelating diphosphine was obtained by reacting 1 with dppv (cis-1,2-bis(diphenylphosphino)ethene). Reaction of 1 with dppe (1,2-bis(diphenylphosphino)ethane) produces [{Fe2(CO)4(μ-sdt)}2(μ-κ1-dppe)] (5) in which the diphosphine forms an intermolecular bridge between two diiron cluster fragments. Three products were obtained when dppf (1,1′-bis(diphenylphosphino)ferrocene) was introduced to complex 1; they were [Fe2(CO)5(μ-sdt)(κ1-dppfO)] (6), the previously known [{Fe2(CO)5(μ-sdt)}2(μ-κ1-κ1-dppf)] (7), and [Fe2(CO)4(μ-sdt)(μ-dppf)] (8), with complex 8 being produced in highest yield. Single crystal X-ray diffraction analysis was performed on compounds 2, 3 and 8. All structures reveal the adoption of an anti-arrangement of the dithiolate bridges, while the diphosphines occupy dibasal positions. Infra-red spectroscopy indicates that the mono-substituted complexes 5, 6, and 7 are inert to protonation by HBF4.Et2O, but complexes 2, 3, 4 and [Fe2(CO)5(μ-sdt)(κ1-PPh3)] (9) show shifts of their ν(C-O) resonances that indicate that protons bind to the metal cores of the clusters. Addition of the one-electron oxidant [Cp2Fe]PF6 does not lead to any discernable shift in the IR resonances. The redox chemistry of the complexes was investigated by cyclic voltammetry, and the abilities of complexes to catalyze electrochemical proton reduction were examined.

Recent Application of Nanomaterials to Overcome Technological Challenges of Microbial Electrolysis Cells.

Microbial electrolysis cells (MECs) have attracted significant interest as sustainable green hydrogen production devices because they utilize the environmentally friendly biocatalytic oxidation of organic wastes and electrochemical proton reduction with the support of relatively lower external power compared to that used by water electrolysis. However, the commercialization of MEC technology has stagnated owing to several critical technological challenges. Recently, many attempts have been made to utilize nanomaterials in MECs owing to the unique physicochemical properties of nanomaterials originating from their extremely small size (at least <100 nm in one dimension). The extraordinary properties of nanomaterials have provided great clues to overcome the technological hurdles in MECs. Nanomaterials are believed to play a crucial role in the commercialization of MECs. Thus, understanding the technological challenges of MECs, the characteristics of nanomaterials, and the employment of nanomaterials in MECs could be helpful in realizing commercial MEC technologies. Herein, the critical challenges that need to be addressed for MECs are highlighted, and then previous studies that used nanomaterials to overcome the technological difficulties of MECs are reviewed.

Low Voltage Voltammetry Probes Proton Dissociation Equilibria of Amino Acids and Peptides

Platinum-catalyzed electrochemical reduction of dissociable protons at low potentials was used to investigate proton dissociation equilibria of freely diffusing and peptide-incorporated charged amino acids. We first demonstrate with five charged essential amino acids and their analogs that the electrochemically induced deprotonation of each amino acid occurs at distinct formal reduction potential. Moreover, the observed direct reduction for all the charged species, excluding arginine, occurs at low potentials suitable for investigation under aqueous conditions (-0.4 to -0.9 V vs Ag/AgCl). The direct proton reduction was resolved via deconvolution of the observed differential pulse voltammogram (DPV) from background hydronium reduction and water electrolysis. A linear correlation was found between the formal reduction potentials and the pKa values of the dissociable protons hosted by various molecular moieties in the amino acids and their analogs and further verified with tripeptides. DPV of poly(l-lysine) decamer (Lys10) distinctively resolved the pKa values of the amino groups in the side chains and N-terminus, at a resolution not possible by conventional acid-base titration. This work demonstrates selective electrochemical titration of dissociable protons in charged amino acids in the free state and as residues in biomolecules, as well as the utility of DPV to indirectly interrogate local electrostatic environments that are essential to the stability and function of biomolecules.

Mechanism of Diiron Hydrogenase Complexes Controlled by Nature of Bridging Dithiolate Ligand.

Bio‐inorganic complexes inspired by hydrogenase enzymes are designed to catalyze the hydrogen evolution reaction (HER). A series of new diiron hydrogenase mimic complexes with one or two terminal tris(4‐methoxyphenyl)phosphine and different μ‐bridging dithiolate ligands and show catalytic activity towards electrochemical proton reduction in the presence of weak and strong acids. A series of propane‐ and benzene‐dithiolato‐bridged complexes was synthesized, crystallized, and characterized by various spectroscopic techniques and quantum chemical calculations. Their electrochemical properties as well as the detailed reaction mechanisms of the HER are elucidated by density functional theory (DFT) methods. The nature of the μ‐bridging dithiolate is critically controlling the reaction and performance of the HER of the complexes. In contrast, terminal phosphine ligands have no significant effects on redox activities and mechanism. Mono‐ or di‐substituted propane‐dithiolate complexes afford a sequential reduction (electrochemical; E) and protonation (chemical; C) mechanism (ECEC), while the μ‐benzene dithiolate complexes follow a different reaction mechanism and are more efficient HER catalysts.

Heterometallic cobalt(ii) calix[6 and 8]arenes: synthesis, structure and electrochemical activity.

Heterometallic cobalt p-tert-butylcalix[6 and 8]arenes have been generated from the in situ reaction of lithium reagents (n-BuLi or t-BuOLi) or NaH with the parent calix[n]arene and subsequent reaction with CoBr2. The reverse route, involving the addition of in situ generated Li[Co(Ot-Bu)3] to p-tert-butylcalix[6 and 8]arene, has also been investigated. X-ray crystallography reveals the formation of complicated products incorporating differing numbers of cobalt and lithium or sodium centers, often with positional disorder, as well as, in some cases, the retention of halide. The electrochemical analysis revealed several oxidation events related to the subsequent oxidation of Co(ii) centers and the reduction of the metal cation at negative potentials. Moreover, the electrochemical activity of the phenol moieties of the parent calix[n]arenes resulted in dimerized products or quinone derivatives, leading to insoluble oligomeric products that deposit and passivate the electrode. Preliminary screening for electrochemical proton reduction revealed good activity for a number of these systems. Results suggest that [Co6Na(NCMe)6(μ-O)(p-tert-butylcalix[6]areneH)2Br]·7MeCN (6·7MeCN) is a promising molecular catalyst for electrochemical proton reduction, with a mass transport coefficient, catalytic charge transfer resistance and current magnitude at the catalytic turnover region that are comparable to those of the reference electrocatalyst (Co(ii)Cl2).

Electrochemical Proton Reduction over Nickel Foam for Z-Stereoselective Semihydrogenation/deuteration of Functionalized Alkynes.

Selective reduction strategies based on abundant‐metal catalysts are very important in the production of chemicals. In this paper, a method for the electrochemical semihydrogenation and semideuteration of alkynes to form Z‐alkenes was developed, using a simple nickel foam as catalyst and H3O+ or D3O+ as sources of hydrogen or deuterium. Good yields and excellent stereoselectivities (Z/E up to 20 : 1) were obtained under very mild reaction conditions. The reaction proceeded with terminal and nonterminal alkynes, and also with alkynes containing easily reducible functional groups, such as carbonyl groups, as well as aryl chlorides, bromides, and even iodides. The nickel‐foam electrocatalyst could be recycled up to 14 times without any change in its catalytic properties.

Cover Feature: Electrochemical Proton Reduction over Nickel Foam for Z‐Stereoselective Semihydrogenation/deuteration of Functionalized Alkynes (ChemSusChem 1/2022)

The Cover Feature shows an alternative approach to the use of noble metals and flammable hydrogen gas for the semireduction of alkynes. Water electrolysis on the surface of commercially available nickel foam can deliver adsorbed hydrogen species that are chemo- and stereoselectively transferred to alkynes under ambient conditions. More information can be found in the Research Article by A. Valiente et al.

Enhancing Water Splitting Activity of Photocathode Using MoS2 Flakes Deposited on Copper Oxide Nanowire

In this work, we report the synthesis of copper oxide (CuO) nanowires on indium tin oxide (ITO) coated with a few–layers of MoS2 flakes by the thermal annealing and metal–organic chemical vapour deposition (MOCVD) methods at a temperature of 200 °C. As a result, the CuO/MoS2 photocathode gave the maximum photocurrent density (PCD) of −9.8 mA/cm2 (η = 0.75%) at –1.2 V (vs. RHE), which is higher than that of the pristine CuO nanowires photocathode (−6.08 mA/cm2, η = 0.44%). The high PCD of CuO/MoS2 photocathode is attributed to the high stability and more active sites of MoS2 flakes, lowering the electrochemical proton reduction over potential, as well as the built–in potential of CuO–MoS2 heterojunction, and minimizing the dark current of the photoelectrochemical (PEC) cell. Based on these findings, we advance the fabrication of the hybrid PEC device (MoS2 and traditional photocatalytic materials) for enhancing efficient hydrogen evolution reaction.

Effect of Lewis Basic Amine Site on Proton Reduction Activity of NNN‐Co Pincer Complex

Electrochemical proton reduction is a promising energy storage method because H2 molecule has a simple structure with a relatively low potential energy. Current interest in hydrogen catalysts has increased research efforts on synthetic analogs of hydrogenase active sites. In this study, we demonstrated the electrochemical H2 evolution reactivity of [NNNR‐Co(CH3CN)3]2+ (R  CH2 (1b), NCH3 (2b)) complexes and examined a proton‐relay process in the H2 evolution reaction (HER). Upon one‐electron reduction, the Co(II) ion center in a high‐spin state dissociated a CH3CN ligand, while opening a reaction site. Cyclic voltammograms of the Co complexes indicated quasi‐reversible Co(II/I) redox behaviors, and both complexes 1b and 2b showed catalytic H2 evolution activity. Interestingly, 2b, assisted by a proton‐relaying NCH3 group, exhibited more efficient catalytic activity than 1b.

Electrocatalytic hydrogen evolution by molecular Cu(II) catalysts

Two mononuclear copper complexes [Cu(QCl-tpy)(OAc)(Cl)] (1′), and [Cu(4Ql-tpy)(OAc)(OH2)]Cl (2′) (QCl-tpy corresponds to 2-choloro-3-(2,6-di(pyridin-2-yl)pyridine-4-yl) quinoline and 4Ql-tpy represents 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)quinoline) have been synthesized and characterized using various analytical techniques like HRMS, EPR, single-crystal XRD, CV etc. Both the complexes were employed for electrochemical proton reduction in 95:5 DMF/H2O (v/v) in the presence of AcOH as a proton source. Among the two complexes, 2′ exhibits higher catalytic activity for proton reduction and achieves a TOF value of 1682 s−1. The complexes also performed electrochemical water reduction in 0.1 M phosphate buffer at environmentally benign pH 7.0, with lower overpotential than that of a blank solution by 400 mV and 350 mV for complex (1′) and (2′) respectively. The acid-base equilibria study in Britton- Robinson buffer predicts that both the complexes show comparable pKa value around 4.4, which is for [CuII(L)(OAc)(H2O)]+ ⇆ [CuII(L)(OAc)(OH)] + H+, where L is QCl-tpy or 4Ql-tpy. The Nyquist plot shows that the Heyrovsky step is the rate-determining step in both the catalysts. As a result, both the complexes follow the same mechanistic cycle, wherein the deprotonated species [CuII(L)(OAc)(OH)] on PCET generates [Cu0(L)(OAc)(OH2)]-. The [Cu0(L)(OAc)(OH2)]- converts into the CuII-H, in the presence of H+ and further CuII-H evolves H2 in the presence of H2O to complete the cycle.

Mononuclear nickel(II) dithiolate complexes with chelating diphosphines: Insight into protonation and electrochemical proton reduction

Inspired by the metal active sites of [FeFe]- and [NiFe]‑hydrogenases, a series of mononuclear Ni(II) ethanedithiolate complexes [{(Ph2PCH2)2×}Ni(SCH2CH2S)] (X = NCH2C5H4N-p (2a), NCH2C6H5 (2b), NCH2CHMe2 (2c), and CH2 (2d)) with chelating diphosphines were readily synthesized through the room-temperature treatments of mononuclear Ni(II) dichlorides [{(Ph2PCH2)2×}NiCl2] (1a-1d) with ethanedithiol (HSCH2CH2SH) in the presence of triethylamine (Et3N) as acid-binding agent. All the as-prepared complexes 1a-1d and 2a-2d are fully characterized through elemental analysis, nuclear magnetic resonance (NMR) spectrum, and by X-ray crystallography for 1b, 2a-2d. To further explore proton-trapping behaviors of this type of mononuclear Ni(II) complexes for catalytic hydrogen (H2) evolution, the protonation and electrochemical proton reduction of 2a-2c with aminodiphosphines (labeled PCNCP = (Ph2PCH2)2NR) and reference analogue 2d with nitrogen-free diphosphine (dppp = (Ph2PCH2)2CH2) are studied and compared under trifluoroacetic acid (TFA) as a proton source. Interestingly, the treatments of 2a-2d with excess TFA resulted in the unexpected formation of dinuclear Ni(II)-Ni(II) dication complexes [{(Ph2PCH2)2×}2Ni2(μ-SCH2CH2S)](CF3CO2)2 (3a-3d) and mononuclear Ni(II) N-protonated complexes [{(Ph2PCH2)2N(H)R}Ni(SCH2CH2S)](CF3CO2) (4a-4c), which has been well supported by high-resolution electrospray ionization mass spectroscopy (HRESI-MS), NMR (31P, 1H) as well as fourier transform infrared spectroscopy (FT-IR) techniques, and especially by X-ray crystallography for 3d. Additionally, the electrochemical properties of 2a-2d are investigated in the absence and presence of strong acid (TFA) by using cyclic voltammetry (CV), showing that the complete protonation of 2a-2d gave rise to dinuclear Ni2S2 species 3a-3d for electrocatalytic proton reduction to H2.

Discerning the oscillatory electrochemical response during electrodeposition of cobalt in the presence of but-2-yne-1,4-diol

We present a new observation of electrochemical oscillation during the reduction of Co2+ from sulfate solution in the presence of but-2-yne-1,4-diol (butynediol) as an additive. Cyclic voltammetry, hydrodynamic voltammetry at galvanostatic condition, and electrochemical impedance spectroscopic studies suggest that the electrochemical oscillation observed was a relaxation type and was the manifestation of adsorbed hydrogen formation by electrochemical reduction of protons on cobalt and their chemical removal by semi-hydrogenation of butynediol to butenediol during the initial stages of electrodeposition.

Metalloenzyme mimic: diironhexacarbonyl cluster coupled to redox-active 4-mercapto-1,8-naphthalic anhydride ligands

The non-innocent redox-active ligand, 4-mercapto-1,8-naphthalic anhydride (HS-NAH), has been used in the design and synthesis of a diironhexacarbonyl complex, [μ-(S-NAH)2Fe2(CO)6]. [μ-(S-NAH)2Fe2(CO)6] has been characterized by spectroscopic methods and cyclic voltammetry. Infrared spectrum of [μ-(S-NAH)2Fe2(CO)6] displays peaks corresponding to terminal metal CO groups (2081, 2044, 2006 cm−1) and peaks assigned to C=O of the naphthalic anhydrides (1780, 1740 cm−1). Electrochemical measurements of [μ-(S-NAH)2Fe2(CO)6] feature redox events that are metal-based (irreversible, Epc: − 1.13 V, − 1.60 V vs Fc/Fc+) and naphthalic anhydride centered (partially chemically reversible, Epc: − 1.99 V; Epa: − 1.90 V vs Fc/Fc+). Cyclic voltammetric analysis of [μ-(S-NAH)2Fe2(CO)6] in the presence of acetic acid show that the complex mimics the active site of [Fe–Fe]-hydrogenase by catalyzing the electrochemical reduction of protons to hydrogen with an overpotential of − 0.67 V. The bi-functional model, [μ-(S-NAH)2Fe2(CO)6], exhibits electronic coupling with synergistic metal–ligand interactions leading to transformation of protons to molecular hydrogen.

A flexible homoleptic pentadentate Cu(II) molecular catalyst for effective proton and water reduction

A copper (II) based molecular catalyst [Cu(tpen)](PF6)2 has been synthesized, and employed for the catalytic activity towards hydrogen evolution both in organic and aqueous media (where tpen = N,N,N′,N’-Tetrakis(2-pyridylmethyl)ethylenediamine). The acid-base equilibria of the complex in phosphate buffer suggest that the initial protonation occurs at the free N atom of the pyridine ring. The pKa value for the [Cu(tpenH)]3+ ⇆ [Cu(tpen)]2+ was found to be 4.6. The electrocatalytic activity of the [Cu(tpen)](PF6)2 towards proton reduction in organic media was determined in presence of acetic acid as an external proton source. Controlled potential electrolysis was performed for the [Cu(tpen)](PF6)2 complex in the presence of acetic acid with varying overpotential ranging from 555 mV to 955 mV vs. SCE for 2 h, and a Faradaic efficiency of 89 ± 5% was observed. Electrochemical proton reduction by the complex follows first-order dependency with respect to complex concentration, and it is second order with respect to the concentration of acetic acid. The ic/ip value for the [Cu(tpen)](PF6)2 complex in the acid independent region was found to be 133 at a scan rate of 100 mV s−1 with observed rate constant (kobs) of 3415 s−1. The [Cu(tpen)](PF6)2 complex also undergoes the electrochemical proton reduction in aqueous media to evolve hydrogen. The catalyst generates hydrogen from an aqueous medium at pH 7.0 in phosphate buffer. The plausible mechanisms for pH higher than the pKa value has been derived from the spectroscopic evidence and supported by DFT studies, wherein an EECC process is involved in the catalytic cycle (E = electrochemical; C = chemical).

Kinetics and the potential well in electrochemical hydrogen evolution by [Co(4-tolyl-tpy)2]2+

The ligand 4-tolyl-tpy [4-tolyl-tpy = 2,6-di(pyridin-2-yl)-4-p-tolylpyridine] and the corresponding cobalt complexes like [Co(4-tolyl-tpy)2]Cl2 and [Co(4-tolyl-tpy)2]Br3 has been synthesized and characterized by different spectroscopic and electrochemical methods. The electrochemical studies were carried out in 95:5 (v/v) DMF/H2O using acetic acid (AcOH) as a proton source. The faradaic efficiency found to be f = (86 ± 4) % using potential range −1.4 V to −1.7 V vs. SCE for a period of 1 h. The ic/ip value found to be 57.9 for a scan rate of 100 mV s−1, and TOF was calculated to be 652 s−1. The electrocatalytic H2 production by [Co(4-tolyl-tpy)2]2+ complex was also employed in phosphate buffer pH 7, and it shows the Faradaic efficiency of (88 ± 4%) within the overpotential range from −1.2 V to −1.5 V vs. SCE. The TOF of the reaction was found to be 3900 mmol of H2 per mole of catalyst per hour. The compare of electrochemical proton reduction, as well as water reduction by [Co(4-tolyl-tpy)2]2+ with the basic scaffold [Co(tpy)2]2+, reveals that it shows higher catalytic activity and have lower overpotential of 150 mV for proton reduction and 100 mV for water reduction. Various kinetics and spectroscopic studies are utilized to predict the mechanism for proton reduction. The kinetic and mechanistic studies are depicted based on changes in absorbance with the addition of Co(Cp)2, followed by AcOH. The intermediates involved in the kinetic process was analysed through global fitting and SVD analysis. The formation of [CoI(L2)] → [CoIII(H)(L)(2L)] and [CoIII(L)(2L)S] → [CoII(L)2] are faster reactions in comparison to the formation of Co(III)-Hδ-⋅⋅⋅Hδ+ to evolve hydrogen (Where 2L is bidentate 4-toly-tpy and S = solvent). The pK value was calculated to be 16.5 for Co(III)–H species in DMF generated from CoI species i.e. CoI + H+ ⇆ Co(III)–H.

Neutral Lipophilic Palladium(II) Complexes and their Applications in Electrocatalytic Hydrogen Production and C‐C Coupling Reactions

Three neutral palladium(II) complexes [Pd(Ln)2] (n = 1,2,3) containing benzotriazole‐phenolate ligands HL1 = 2‐(2H‐benzotriazol‐2‐yl)‐6‐dodecyl‐4‐methylphenol; HL2 = 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐di‐tert‐pentylphenol; HL3 = 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐bis(1‐methyl‐1‐phenylethyl)phenol were synthesized and characterized by several spectroscopic methods (1H, 13C NMR, UV‐Visible), ESI mass spectrometry and single‐crystal X‐ray diffraction. The geometry around the palladium(II) centre for each complex is described as distorted square planar. The complexes were tested as potential catalysts for electrochemical proton reduction in DMF using trifluoroacetic acid (TFA). Gas analysis under electrocatalytic conditions at a boron‐doped diamond working electrode confirmed the hydrogen production. After performing similar gas analysis experiments using a working mercury pool electrode, no hydrogen was detected which supports the concept that the active species of the catalytic process are palladium particles formed on the surface of the working electrode. The palladium(II) benzotriazolyl phenolate complexes facilitate the formation of particles for electrocatalytic hydrogen production. Furthermore, the palladium(II) complexes PdL2 and PdL3 were able to catalyse microwave‐assisted Heck and Sonogashira C–C coupling reactions.