Catalysis of Dioxygen Reduction by <i>Thermus thermophilus</i> Strain HB27 Laccase on Ketjen Black Electrodes

We present electrochemical analyses of the catalysis of dioxygen reduction by <i>Thermus thermophilus</i> strain HB27 laccase on ketjen black substrates. Our cathodes reliably produce 0.56 mA cm^–2 at 0.0 V vs Ag|AgCl reference at 30 °C in air-saturated buffer, under conditions of nonlimiting O₂ flux. We report the electrochemical activity of this laccase as a function of temperature, pH, time, and the efficiency of its conversion of dioxygen to water. We have measured the surface concentration of electrochemically active species, permitting the extraction of electron transfer rates at the enzyme-electrode interface: 1 s^–1 for this process at zero driving force at 30 °C and a limiting rate of 23 s^–1 at 240 mV overpotential at 50 °C

Modeling Dioxygen Reduction at Multicopper Oxidase Cathodes

We report a general kinetics model for catalytic dioxygen reduction on multicopper oxidase (MCO) cathodes. Our rate equation combines Butler–Volmer (BV) electrode kinetics and the Michaelis–Menten (MM) formalism for enzymatic catalysis, with the BV model accounting for interfacial electron transfer (ET) between the electrode surface and the MCO type 1 copper site. Extending the principles of MM kinetics to this system produced an analytical expression incorporating the effects of subsequent intramolecular ET and dioxygen binding to the trinuclear copper cluster into the cumulative model. We employed experimental electrochemical data on Thermus thermophilus laccase as benchmarks to validate our model, which we suggest will aid in the design of more efficient MCO cathodes. In addition, we demonstrate the model’s utility in determining estimates for both the electronic coupling and average distance between the laccase type-1 active site and the cathode substrate

Hydrogen Generation Catalyzed by Fluorinated Diglyoxime–Iron Complexes at Low Overpotentials

Fe^II complexes containing the fluorinated ligand 1,2-bis­(perfluorophenyl)­ethane-1,2-dionedioxime (dAr^FgH₂; H = dissociable proton) exhibit relatively positive Fe^II/I reduction potentials. The air-stable difluoroborated species [(dAr^FgBF₂)₂Fe­(py)₂] (<b>2</b>) electrocatalyzes H₂ generation at −0.9 V vs SCE with <i>i</i>_cat/<i>i</i>_p ≈ 4, corresponding to a turnover frequency (TOF) of ∼20 s^–1 [Faradaic yield (FY) = 82 ± 13%]. The corresponding <i>mono</i>fluoroborated, proton-bridged complex [(dAr^Fg₂H-BF₂)­Fe­(py)₂] (<b>3</b>) exhibits an improved TOF of ∼200 s^–1 (<i>i</i>_cat/<i>i</i>_p ≈ 8; FY = 68 ± 14%) at −0.8 V with an overpotential of 300 mV. Simulations of the electrocatalytic cyclic voltammograms of <b>2</b> suggest rate-limiting protonation of an Fe^“0” intermediate (<i>k</i>_RLS ≈ 200 M^–1 s^–1) that undergoes hydride protonation to form H₂. Complex <b>3</b> likely reacts via protonation of an Fe^I intermediate that subsequently forms H₂ via a bimetallic mechanism (<i>k</i>_RLS ≈ 2000 M^–1 s^–1). <b>3</b> catalyzes production at relatively positive potentials compared with other iron complexes

Fe₄ Cluster and a Buckled Macrocycle Complex from the Reduction of [(dmgBF₂)₂Fe(L)₂] (L = MeCN, ^<i>t</i>Bu^<i>i</i>NC)

We report the syntheses, X-ray structures, and reductive electrochemistry of the Fe^II complexes [(dmgBF₂)₂Fe­(MeCN)₂] (<b>1</b>; dmg = dimethylglyoxime, MeCN = acetonitrile) and [(dmgBF₂)­Fe­(^<i>t</i>Bu^<i>i</i>NC)₂] (<b>2</b>; ^<i>t</i>Bu^<i>i</i>NC = <i>tert</i>-butylisocyanide). The reaction of <b>1</b> with Na/Hg amalgam led to isolation and the X-ray structure of [(dmgBF₂)₂Fe­(glyIm)] (<b>3</b>; glyIm = glyimine), wherein the (dmgBF₂)₂ macrocyclic frame is bent to accommodate the binding of a bidentate apical ligand. We also report the X-ray structure of a rare mixed-valence Fe₄ cluster with supporting dmg-type ligands. In the structure of [(dmg₂BF₂)₃Fe₃(¹/₂dmg)₃Fe­(O)₆] (<b>4</b>), the (dmgBF₂)₂ macrocycle has been cleaved, eliminating BF₂ groups. Density functional theory calculations and electron paramagnetic resonance data are in accordance with a central Fe^III ion surrounded by three formally Fe^IIdmg₂BF₂ units

Copper(II) Binding to α-Synuclein, the Parkinson’s Protein

Copper(II) Binding to α-Synuclein, the Parkinson’s Protei

Rapid Water Reduction to H₂ Catalyzed by a Cobalt Bis(iminopyridine) Complex

A cobalt bis(iminopyridine) complex is a highly active electrocatalyst for water reduction, with an estimated apparent second order rate constant <i>k</i>_app ≤ 10⁷ M^–1s^–1 over a range of buffer/salt concentrations. Scan rate dependence data are consistent with freely diffusing electroactive species over pH 4–9 at room temperature for each of two catalytic reduction events, one of which is believed to be ligand based. Faradaic H₂ yields up to 87 ± 10% measured in constant potential electrolyses (−1.4 V vs SCE) confirm high reactivity and high fidelity in a catalyst supported by the noninnocent bis(iminopyridine) ligand. A mechanism involving initial reduction of Co²⁺ and subsequent protonation is proposed

Hydrogen Generation Catalyzed by Fluorinated Diglyoxime–Iron Complexes at Low Overpotentials

Fe^II complexes containing the fluorinated ligand 1,2-bis­(perfluorophenyl)­ethane-1,2-dionedioxime (dAr^FgH₂; H = dissociable proton) exhibit relatively positive Fe^II/I reduction potentials. The air-stable difluoroborated species [(dAr^FgBF₂)₂Fe­(py)₂] (<b>2</b>) electrocatalyzes H₂ generation at −0.9 V vs SCE with <i>i</i>_cat/<i>i</i>_p ≈ 4, corresponding to a turnover frequency (TOF) of ∼20 s^–1 [Faradaic yield (FY) = 82 ± 13%]. The corresponding <i>mono</i>fluoroborated, proton-bridged complex [(dAr^Fg₂H-BF₂)­Fe­(py)₂] (<b>3</b>) exhibits an improved TOF of ∼200 s^–1 (<i>i</i>_cat/<i>i</i>_p ≈ 8; FY = 68 ± 14%) at −0.8 V with an overpotential of 300 mV. Simulations of the electrocatalytic cyclic voltammograms of <b>2</b> suggest rate-limiting protonation of an Fe^“0” intermediate (<i>k</i>_RLS ≈ 200 M^–1 s^–1) that undergoes hydride protonation to form H₂. Complex <b>3</b> likely reacts via protonation of an Fe^I intermediate that subsequently forms H₂ via a bimetallic mechanism (<i>k</i>_RLS ≈ 2000 M^–1 s^–1). <b>3</b> catalyzes production at relatively positive potentials compared with other iron complexes

Hydrogen Evolution from Pt/Ru-Coated p‑Type WSe₂ Photocathodes

Crystalline p-type WSe₂ has been grown by a chemical vapor transport method. After deposition of noble metal catalysts, p-WSe₂ photocathodes exhibited thermodynamically based photoelectrode energy-conversion efficiencies of >7% for the hydrogen evolution reaction under mildly acidic conditions, and were stable under cathodic conditions for at least 2 h in acidic as well as in alkaline electrolytes. The open circuit potentials of the photoelectrodes in contact with the H⁺/H₂ redox couple were very close to the bulk recombination/diffusion limit predicted from the Shockley diode equation. Only crystals with a prevalence of surface step edges exhibited a shift in flat-band potential as the pH was varied. Spectral response data indicated effective minority-carrier diffusion lengths of ∼1 μm, which limited the attainable photocurrent densities in the samples to ∼15 mA cm^–2 under 100 mW cm^–2 of Air Mass 1.5G illumination

Groups 5 and 6 Terminal Hydrazido(2−) Complexes: N_β Substituent Effects on Ligand-to-Metal Charge-Transfer Energies and Oxidation States

Brightly colored terminal hydrazido(2−) (dme)­MCl₃(NNR₂) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)­WCl₄(NNR₂) complexes have been synthesized and characterized. Perturbing the electronic environment of the β (NR₂) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the N_β lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize N_β in order to reduce the overlap between the NbN_α π bond and the N_β lone pair. For W complexes, increasing the energy of N_β eventually leads to reduction from formally [W^VIN–NR₂] with a hydrazido(2−) ligand to [W^IVNNR₂] with a neutral 1,1-diazene ligand. The photophysical properties of these complexes highlight the potential redox noninnocence of hydrazido ligands, which could lead to ligand- and/or metal-based redox chemistry in early transition metal derivatives

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

Recent work has shown that Cp*Rh­(bpy) [Cp* = pentamethylcyclopentadienyl, bpy = 2,2′- bipyridine] undergoes <i>endo</i> protonation at the [Cp*] ligand in the presence of weak acid (Et₃NH⁺; p<i>K</i>_a = 18.8 in MeCN). Upon exposure to stronger acid (e.g., DMFH⁺; p<i>K</i>_a = 6.1), hydrogen is evolved with unity yield. Here, we study the mechanisms by which this catalyst evolves dihydrogen using density functional theory (M06) with polarizable continuum solvation. The calculations show that the complex can be protonated by weak acid first at the metal center with a barrier of 3.2 kcal/mol; this proton then migrates to the ring to form the detected intermediate, a rhodium­(I) compound bearing <i>endo η</i>⁴-Cp*H. Stronger acid is required to evolve hydrogen, which calculations show happens via a concerted mechanism. The acid approaches and protonates the metal, while the second proton simultaneously migrates from the ring with a barrier of ∼12 kcal/mol. Under strongly acidic conditions, we find that hydrogen evolution can proceed through a traditional metal–hydride species; protonation of the initial hydride to form an H–H bond occurs before migration of the hydride (in the form of a proton) to the [Cp*] ring (i.e., H–H bond formation is faster than hydride–proton tautomerization). This work demonstrates the role of acid strength in accessing different mechanisms of hydrogen evolution. Calculations also predict that modification of the bpy ligand by a variety of functional groups does not affect the preference for [Cp*] protonation, although the driving force for protonation changes. However, we predict that exchange of bpy for a bidentate phosphine ligand will stabilize a rhodium­(III) hydride, reversing the preference for bound [Cp*H] found in all computed bpy derivatives and offering an appealing alternative ligand platform for future experimental and computational mechanistic studies of H₂ evolution