Remarkable Enhancement of Catalytic Activity of Cu-Complexes in the Electrochemical Hydrogen Evolution Reaction by Using Triply Fused Porphyrin

A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β-β, meso-meso, β'-β' triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non-fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 10⁷  s^-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth-abundant 3d transition metals.

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.

A bio-inspired mononuclear manganese catalyst for high-rate electrochemical hydrogen production

[FeFe]-hydrogenase (H₂ase) catalyzes hydrogen evolution reactions (HERs), with an excellent performance that rivals that of platinum, the active site of which is built with crucial structural features required for efficient H-H bond formation. Herein, we report a mononuclear manganese complex (1) that contains a square pyramid coordination sphere and an intramolecular aniline as the proton relay, consistent with the crucial features of the active site in H₂ase. Benefitting from these features, complex 1 electrocatalyzes the HER with a turnover frequency (TOF) exceeding 10 000 s^-1 at -1.45 V (versus the ferrocenium/ferrocene couple) using anilinium tetrafluoroborate as a proton source. This work provides the first Mn-based functional model of H₂ase, serving as a new paradigm for a high performance, low cost, environmentally benign hydrogen production electrocatalyst.

A dinuclear porphyrin-macrocycle as efficient catalyst for the hydrogen evolution reaction

We report an unprecedented dinuclear catalyst for the electrochemical hydrogen evolution reaction (HER). A macrocyclic porphyrin complex comprising two nickel centres connected via redox mediating linker molecules gives rise to efficient catalysis, significantly outperforming a mononuclear reference catalyst.

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.

Electrocatalytic proton reduction by an air-stable nickel(ii)-thiolato PNN pincer complex

We report an air-stable nickel(ii)-thiolato PNN pincer complex [(PNN)NiII(SC6H4Me)]+[MeC6H4SO3]- (PNN = 2-((di-tert-butylphosphinomethyl-6-diethylaminomethyl)pyridine)) which is capable of reducing protons at an overpotential of 0.54 V at low acid concentrations. The proton reduction can be catalysed using weak or strong acids such as acetic acid and trifluoroacetic acid respectively. In contrast, the chloro and nitrate derivatives of the nickel pincer complex behave as poorer catalysts. A mechanism accounting for the role of the ligand in proton reduction is also briefly outlined.

Mono- and bimetallic manganese-carbonyl complexes and clusters bearing imidazol(in)ium-2-dithiocarboxylate ligands

Five complexes with the generic formula fac-[MnBr(CO)₃(S₂C·NHC)] were obtained by reacting [MnBr(CO)₅] with a set of representative imidazol(in)ium-2-dithiocarboxylate zwitterions. These ligands are the adducts of N-heterocyclic carbenes (NHCs) and carbon disulfide. The mononuclear Mn(i) derivatives were coupled with Na[Mn(CO)₅] to afford bimetallic [Mn₂(CO)₆(S₂C·NHC)] clusters. Yet, the most convenient strategy to access these dinuclear Mn(0) products implied a direct carbonyl substitution from the [Mn₂(CO)₁₀] dimer. The molecular structures of three monometallic and four bimetallic compounds were elucidated by single crystal X-ray diffraction analysis. In the monometallic complexes, the NHC·CS₂ ligands exhibited a bidentate κ²-S,S' coordination mode with an S-C-S bite angle of about 116°. In the dinuclear clusters, the CS₂^- unit acted as a chelate toward one manganese center and as a pseudoallylic ligand toward the other one. The S-C-S bite angle was reduced to ca. 104°. Thus, the zwitterions displayed a remarkable flexibility, which also permitted a staggered arrangement of the carbonyl groups in the bimetallic systems. Examination of the [small nu, Greek, macron]_CO absorption bands on IR spectroscopy helped identify the presence of fac-Mn(CO)₃ or Mn₂(CO)₆ motifs, while the ¹³C NMR chemical shift of the CS₂^- moiety was a reliable indicator for monitoring its hapticity. Whereas the dinuclear clusters were air- and moisture-stable crystalline solids, mononuclear halido derivatives displayed only a limited stability under aerobic conditions. Both types of compounds underwent rather unselective, extensive fragmentations in the gas phase, in sharp contrast with the analogous rhenium derivatives that led to clean sequential decarbonylation processes upon collision-induced dissociation.

Dinuclear Nickel Complexes of Thiolate-Functionalized Carbene Ligands and Their Electrochemical Properties

Four dimeric nickel(II) complexes [Ni₂Cl₂(BnC₂S)₂] [1], [Ni₂Cl₂(BnC₃S)₂] [2], [Ni₂(PyC₂S)₂]Br₂ [3]Br₂, and [Ni₂(PyC₃S)₂]Br₂ [4]Br₂ of four different thiolate-functionalized N-heterocyclic carbene (NHC) ligands were synthesized, and their structures have been determined by single-crystal X-ray crystallography. The four ligands differ by the alkyl chain length between the thiolate group and the benzimidazole nitrogen (two −C₂– or three −C₃– carbon atoms) and the second functionality at the NHC being a benzyl (Bn) or a pyridylmethyl (Py) group. The nickel(II) ions are coordinated to the NHC carbon atom and the pendent thiolate group, which bridges to the second nickel(II) ion creating the dinuclear structure. Additionally, in compounds [1] and [2], the fourth coordination position of the square-planar Ni(II) centers is occupied by the halide ions, whereas in [3]²⁺ and [4]²⁺, the additional pendant pyridylmethyl groups complete the coordination spheres of the nickel ions. The electrochemical properties of the four complexes were studied using cyclic voltammetry and controlled-potential coulometry methods. The thiolate-functionalized carbene complexes [1] and [2] appear to be poor electrocatalysts for the hydrogen evolution reaction; the complexes [3]Br₂ and [4]Br₂, bearing an extra pyridylmethyl group, show higher catalytic activity in proton reduction, indicating that the pyridine group plays an important role in the catalytic cycle.

Electrochemical proton reduction catalysed by selenolato-manganese carbonyl complexes

Four manganese selenolato carbonyl complexes have been synthesized and used as electrocatalyst for proton reduction.