Bioinspired Molecular Electrocatalysts for H2 Production: Chemical Strategies

Beyond S and Se: Electrocatalytic Hydrogen Production by Tellurolate-Bridged Co(III)-Mn(I) Heterodinuclear Complexes

In the pursuit of efficient electrocatalysts for the hydrogen evolution reaction (HER), a series of manganese and cobalt heterodinuclear complexes have been synthesized and characterized that have a stark resemblance with the [NiFe]-hydrogenase active site structure. Irradiation of [Mn2(CO)10] in the presence of 1.5 eq of [NaEPh] [E = S, Se, Te] followed by reaction with [Cp*CoCl]2 led to the formation of half-sandwiched trichalcogenate-bridged heterodinuclear complexes [{Mn(CO)3}(μ-EPh)3(CoCp*)] [E = S (C1); Se (C2) and Te (C3)]. The reaction of these heterodinuclear trichalcogenate-bridged complexes with [LiBH4·THF] yielded the corresponding dichalcogenate hydride-bridged heterobimetallic complexes [(CO)3Mn(μ-EPh)2(μ-H)(CoCp*)] [E = S (C5); Se (C6) and Te (C7)], which closely imitate the Ni-R intermediate of [NiFe]-hydrogenase. The resultant complexes (C5-C7) displayed impressive H2 production in DMF in the presence of HBF4, whereas the Te-based complex (C7) showcased the highest TON (184 h-1) with an impressive Faradaic efficiency of >98%. The DFT investigations revealed a unique role of bridging chalcogens in catalysis, wherein, depending on the identity of the chalcogen (S, Se, or Te), protonation could occur via two distinct routes. This study represents a rare example of the full trio of S/Se/Te-based heterodinuclear complexes whose electrocatalytic HER activity has been probed under analogous conditions.

Metal-Ligand Cooperation with Thiols as Transient Cooperative Ligands: Acceleration and Inhibition Effects in (De)Hydrogenation Reactions

ConspectusOver the past two decades, we have developed a series of pincer-type transition metal complexes capable of activating strong covalent bonds through a mode of reactivity known as metal-ligand cooperation (MLC). In such systems, an incoming substrate molecule simultaneously interacts with both the metal center and ligand backbone, with one part of the molecule reacting at the metal center and another part at the ligand. The majority of these complexes feature pincer ligands with a pyridine core, and undergo MLC through reversible dearomatization/aromatization of this pyridine moiety. This MLC platform has enabled us to perform a variety of catalytic dehydrogenation, hydrogenation, and related reactions, with high efficiency and selectivity under relatively mild conditions.In a typical catalytic complex that operates through MLC, the cooperative ligand remains coordinated to the metal center throughout the entire catalytic process, and this complex is the only catalytic species involved in the reaction. As part of our ongoing efforts to develop new catalytic systems featuring MLC, we have recently introduced the concept of transient cooperative ligand (TCL), i.e., a ligand that is capable of MLC when coordinated to a metal center, but the coordination of which is reversible rather than permanent. We have thus far employed thiol(ate)s as TCLs, in conjunction with an acridanide-based ruthenium(II)-pincer catalyst, and this has resulted in remarkable acceleration and inhibition effects in various hydrogenation and dehydrogenation reactions. A cooperative thiol(ate) ligand can be installed in situ by the simple addition of an appropriate thiol in an amount equivalent to the catalyst, and this has been repeatedly shown to enable efficient bond activation by MLC without the need for other additives, such as base. The use of an ancillary thiol ligand that is not fixed to the pincer backbone allows the catalytic system to benefit from a high degree of tunability, easily implemented by varying the added thiol. Importantly, thiols are coordinatively labile enough under typical catalytic conditions to leave a meaningful portion of the catalyst in its original unsaturated form, thereby allowing it to carry out its own characteristic catalytic activity. This generates two coexisting catalyst populations─one that contains a thiol(ate) ligand and another that does not─and this may lead to different catalytic outcomes, namely, enhancement of the original catalytic activity, inhibition of this activity, or the occurrence of diverging reactivities within the same catalytic reaction mixture. These thiol effects have enabled us to achieve a series of unique transformations, such as thiol-accelerated base-free aqueous methanol reforming, controlled stereodivergent semihydrogenation of alkynes using thiol as a reversible catalyst inhibitor, and hydrogenative perdeuteration of C═C bonds without using D2, enabled by a combination of thiol-induced acceleration and inhibition. We have also successfully realized the unprecedented formation of thioesters through dehydrogenative coupling of alcohols and thiols, as well as the hydrogenation of organosulfur compounds, wherein the cooperative thiol serves as a reactant or product. In this Account, we present an overview of the TCL concept and its various applications using thiols.

Electrochemical reduction and protonation of a biomimetic diiron azadithiolate hexacarbonyl complex: Mechanistic insights

The electrochemical reduction and protonation of [Fe2(adtH)(CO)6] (1, adtH = SCH2N(H)CH2S) and [Fe2(pdt)(CO)6] (2, pdt = SCH2CH2CH2S) in the presence of moderately strong acid in acetonitrile was investigated by cyclic voltammetry (CV), focusing on the catalysis of hydrogen evolution reaction (HER) by a {2e-,2H+} pathway. The turnover frequencies at zero overpotential (TOF0) of the N-protonated product 1(H)+ and 2 for the HER were estimated from simulations of the catalytic CV responses at low acid concentration using a simple ECEC mechanism (two electrochemical and chemical steps). This approach confirmed that 1(H)+ is clearly a better catalyst than 2, pointing to a possible role of the protonable and biologically relevant adtH ligand in the enhancement of the catalytic performances. Density functional theory (DFT) calculations further suggested that, owing to a strong structural rearrangement in the course of the catalytic cycle, the HER catalysis by 1(H)+ only involves the iron center adjacent to the amine group in adtH and not the two iron centers as in 2. Since terminal hydride species (FeFe-H) are known to more easily undergo protonolyse to H2 than their bridging hydride isomers (Fe-H-Fe), this may explain here the enhanced activity of 1(H)+ over 2 for the HER.

Synthetic styrene-based bioinspired model of the [FeFe]-hydrogenase active site for electrocatalytic hydrogen evolution

Integration of molecular catalysts inside polymeric scaffolds has gained substantial attention over the past decade, as it provides a path towards generating systems with enhanced stability as well as enzyme-like morphologies and properties. In the context of solar fuels research and chemical energy conversion, this approach has been found to improve both rates and energy efficiencies of a range of catalytic reactions. However, system performance still needs to be improved to reach technologically relevant currents and stability, parameters that are heavily influenced by the nature of the incorporated molecular catalyst. Here, we have focused on the integration of a biomimetic {Fe2(μ-adt)(CO)6} (-CH2NHCH2S-, azadithiolate or adt2-) based active site ("[2Fe2S]adt"), inspired by the catalytic cofactor of [FeFe] hydrogenases, within a synthetic polymeric scaffold using free radical polymerization. The resulting metallopolymers [2Fe2S]adtk[DMAEMA]l[PyBMA]m (DMAEMA = dimethylaminoethyl methacrylate as water soluble monomer; PyBMA = 4-(pyren-1-yl)-butyl methacrylate as hydrophobic anchor for heterogenization) were found to be active for electrochemical H2 production in neutral aqueous media. The pyrene content was varied to optimize durability and activity. Following immobilization on multiwalled carbon nanotubes (MWNT) the most active metallopolymer, containing ∼2.3 mol% of PyBMA, could reach a turnover number for hydrogen production (TONH2) of ∼0.4 ×105 over 20 hours of electrolysis at an overpotential of 0.49 V, two orders of magnitude higher than the isolated catalyst counterpart. The study provides a synthetic methodology for incorporating catalytic units featuring second coordination sphere functional groups, and highlights the benefit of the confinement within the polymer matrix for catalytic performance.

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so-called DuBois' catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni(P2 Cy N2 Arg )2 ]6+ on a mechanistic model and its kinetic treatment that may apply to all DuBois' catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)-hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton-coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H2 uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics.

Exploiting the NADP+/NADPH-like Hydride-Transfer Redox Cycle with Bis-Imidazolium-Embedded Heterohelicene for Electrocatalytic Hydrogen Evolution Reaction

Generation of clean energy in a viable manner demands efficient and sustainable catalysts. One prospective method of clean energy generation is the electrochemical hydrogen evolution reaction (HER). Over the years, various transition metal-based complexes/polymeric organic materials were utilized in HER. However, the use of a redox-active small organic molecule as a catalyst for HER has not been explored well. The requirements of a strongly acidic solution, very high overpotential, and stability under acidic conditions pose several challenges for applying organic electrocatalysts for HER. Considering these challenges, herein, we demonstrated an NADP+-like organic system (NADP+ = nicotinamide adenine dinucleotide phosphate), a bis-imidazolium-fused heterohelicene, which acts as a catalyst for HER with mild acid (acetic acid) as a proton source at moderate overpotential. The unique structural backbone of this dicationic heterohelicene allowed to exploit the NADP+/NADPH-type (NADPH = reduced nicotinamide adenine dinucleotide phosphate) hydride transfer-based redox cycle efficiently under the applied conditions, where the NADPH-like hydride intermediate transfers the hydride to the proton of the mild acid to generate H2. The Faradaic efficiency and turnover number for the present HER were achieved up to 85 ± 5% and 50 ± 3, respectively. In addition, the maximum turnover frequency, TOFmax, value of 410 s-1 was observed, which is around 400 times that obtained for the existing reported NADP+-like organic compounds used as catalysts for HER. Thorough mechanistic studies were conducted experimentally and computationally to establish a plausible catalytic cycle. This advancement could help in designing efficient organic electrocatalysts for HER from a mild proton source.

Hybrids of [FeFe]- and [NiFe]-H2ase Active Site Models

Complexes of the type (diphosphine)Ni(μ-SR)2Fe(CO)3 are investigated with azadithiolate (adt, HN(CH2S-)2) as the dithiolate. The resulting complexes are hybrid models for the active sites of the [NiFe]- and [FeFe]-hydrogenases. The key complex (dppv)Ni(μ-adt)Fe(CO)3 (3) was prepared from the complex Ni[(SCH2)2NCbz](dppv), which contains a Cbz-protected adt ligand (Cbz = C(O)OCH2Ph, dppv = cis-1,2-(Ph2P)2C2H2). This complex combines with Fe2(CO)9 to give (dppv)Ni[(μ-SCH2)2NCbz]Fe(CO)3, which is readily deprotected to give 3. Complex 3 undergoes protonation at both Fe and N to give successively [(dppv)Ni(μ-adt)FeH(CO)3]+ ([H3]+) and [(dppv)Ni(μ-adtH)FeH(CO)3]2+ ([H3H]2+). The redox properties and dynamics of these complexes resemble previously reported analogues with propanedithiolate. Solutions of [H3]+ readily degrade to [(dppv)Ni[(μ-SCH2)2NCH2]Fe(CO)3]+ ([4]+), which features a methylene group linking N and Fe. Complex [4]+ can be made in high yield by reaction of [H3]+ with CH2O, and this conversion was also demonstrated with 13CH2O. Complex [4]+ undergoes hydrogenolysis by photochemical reaction with H2 to give [(dppv)Ni[(μ-SCH2)2NMe]FeH(CO)3]+, the N-methylated analogue of [H3]+. Upon treatment ith Me3O+, [4]+ undergoes quaternization, giving [(dppv)Ni[(μ-SCH2)2N(Me)CH2]Fe(CO)3]2+. In contrast with the lability of [H3]+, the phosphine-substituted derivative [(dppv)Ni(μ-adt)FeH(CO)2(PPh3)]+ did not degrade. Most complexes were characterized by X-ray crystallography.

Mononuclear manganese complexes as hydrogen evolving catalysts

Molecular hydrogen (H₂) is one of the pillars of future non-fossil energy supply. In the quest for alternative, non-precious metal catalysts for hydrogen generation to replace platinum, biological systems such as the enzyme hydrogenase serve as a blueprint. By taking inspiration from the bio-system, mostly nickel- or iron-based catalysts were explored so far. Manganese is a known oxygen-reducing catalyst but has received much less attention for its ability to reduce protons in acidic media. Here, the synthesis, characterization, and reaction mechanisms of a series of four mono-nuclear Mn(I) complexes in terms of their catalytic performance are reported. The effect of the variation of equatorial and axial ligands in their first and second coordination spheres was assessed pertaining to their control of the turnover frequencies and overpotentials. All four complexes show reactivity and reduce protons in acidic media to release molecular hydrogen H₂. Quantum chemical studies were able to assign and interpret spectral characterizations from UV–Vis and electrochemistry and rationalize the reaction mechanism. Two feasible reaction mechanisms of electrochemical (E) and protonation (C) steps were compared. Quantum chemical studies can assign peaks in the cyclic voltammetry to structural changes of the complex during the reaction. The first one-electron reduction is essential to generate an open ligand-based site for protonation. The distorted octahedral Mn complexes possess an inverted second one-electron redox potential which is a pre-requisite for a swift and facile release of molecular hydrogen. This series on manganese catalysts extends the range of elements of the periodic table which are able to catalyze the hydrogen evolution reaction and will be explored further.