Achieving Reversible H₂/H⁺ Interconversion at Room Temperature with Enzyme-Inspired Molecular Complexes: A Mechanistic Study

Abstract

Inspired by the contribution of the protein scaffold to the efficiency with which enzymes function, we used outer coordination sphere features to develop a molecular electrocatalyst for the reversible production/oxidation of H2 at 25 °C: [Ni­(PCy 2NPhe 2)2]2+ (CyPhe; PR 2NR′ 2 = 1,5-diaza-3,7-diphosphacyclooctane, Cy = cyclohexyl, Phe = phenylalanine). Electrocatalytic reversibility is observed in aqueous, acidic methanol. The aromatic rings in the peripheral phenylalanine groups appear to be essential to achieving reversibility based on the observation that reversibility for arginine (CyArg) or glycine (CyGly) complexes is only achieved with elevated temperature (>50 °C) in 100% water. A complex with a hydroxyl group in the para-position of the aromatic ring, R′ = tyrosine (CyTyr), shows similar reversible behavior. NMR spectroscopy and molecular dynamics studies suggest that interactions between the aromatic groups as well as between the carboxylic acid groups limit conformational flexibility, contributing to reversibility. NMR spectroscopy studies also show extremely fast proton exchange along a pathway from the Ni–H through the pendant amine to the carboxyl group. Further, a complex containing a side chain similar to tyrosine but without the carboxyl group (CyTym; Tym = tyramine) does not display reversible catalysis and has limited proton exchange from the pendant amine, demonstrating an essential role for the carboxylic acid and the proton pathway in achieving catalytic reversibility. This minimal pathway mimics proton pathways found in hydrogenases. The influence of multiple factors on lowering barriers and optimizing relative energies to achieve reversibility for this synthetic catalyst is a clear indication of the intricate interplay between the first, second, and outer coordination spheres that begins to mimic the complexity observed in metalloenzymes.

Carbon‐Nanotube‐Supported Bio‐Inspired Nickel Catalyst and Its Integration in Hybrid Hydrogen/Air Fuel Cells

“…As already hypothesized, this might be caused by protonation of carboxylic acids (pK a in solution of 2.5). [21] We believe that protonation and electron transfer are tied to reversibility,a sp ostulated in two recent papers, [23,36] and the stark change at pH 2int he immobilized system is consistent with that interpretation.…”

“…Va lues of E Ni II =Ni 0 are very similar to those determined for the immobilized complex 1 under argon (compare Figure 2Band They follow the same Nernstian behavior and pH dependency (Figure S3 A). [21] We believe that protonation and electron transfer are tied to reversibility,a sp ostulated in two recent papers, [23,36] and the stark change at pH 2int he immobilized system is consistent with that interpretation. These low values show that immobilized complex 1 competes well with hydrogenases in terms of near-zero-overpotential requirement.…”

Frequency and potential dependence of reversible electrocatalytic hydrogen interconversion by [FeFe]-hydrogenases

“…Attached to an electrode, these enzymes catalyze rapid oxidation and reduction, with only a minimum overpotential being required to swap the current direction either side of the equilibrium potential (2)(3)(4)(5). Of particular interest are hydrogenases, which catalyze the oxidation and production of H 2 with activities that may rival platinum (6)(7)(8)(9)(10)(11) and are inspirational in the quest for future electrocatalysts as well as in artificial photosynthesis (12,13).…”

Bio-Inspired H2 Production Catalysts

“…catalyst immobilized on the 2-amino-6-napthoic acid modified single wall carbon nanotubes (CWNTs). Immobilization results in the activity on a broader range of pH and make it most performing hydrogen-oxidizing electrode till reported [44,45]. Recently Dey et al [46] have synthesized a [FeFe] H 2 -ase mimics having azadithiolato (ATD) bridge and a terminal alkyne.…”