4 research outputs found
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Beyond Metal-Hydrides: Non-Transition-Metal and Metal-Free Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation
A new pathway for
homogeneous electrocatalytic H<sub>2</sub> evolution
and H<sub>2</sub> oxidation has been developed using a redox active
thiosemicarbazone and its zinc complex as seminal metal-free and transition-metal-free
examples. Diacetyl-bisÂ(<i>N</i>-4-methyl-3-thiosemicarbazone)
and zinc diacetyl-bisÂ(<i>N</i>-4-methyl-3-thiosemicarbazide)
display the highest reported TOFs of any homogeneous ligand-centered
H<sub>2</sub> evolution catalyst, 1320 and 1170 s<sup>–1</sup>, respectively, while the zinc complex also displays one of the highest
reported TOF values for H<sub>2</sub> oxidation, 72 s<sup>–1</sup>, of any homogeneous catalyst. Catalysis proceeds via ligand-centered
proton-transfer and electron-transfer events while avoiding traditional
metal-hydride intermediates. The unique mechanism is consistent with
electrochemical results and is further supported by density functional
theory. The results identify a new direction for the design of electrocatalysts
for H<sub>2</sub> evolution and H<sub>2</sub> oxidation that are not
reliant on metal-hydride intermediates
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Metal-Assisted Ligand-Centered Electrocatalytic Hydrogen Evolution upon Reduction of a Bis(thiosemicarbazonato)Cu(II) Complex
In this study, we
report the electrocatalytic behavior of the neutral, monomeric CuÂ(II)
complex of diacetyl-bisÂ(<i>N</i>-4-methyl-3-thiosemicarbazonato),
CuL<sup>1</sup>, for metal-assisted ligand-centered hydrogen evolution
in acetonitrile and dimethylformamide. CuL<sup>1</sup> displays a
maximum turnover frequency (TOF) of 10 000 s<sup>–1</sup> in acetonitrile and 5100 s<sup>–1</sup> in dimethylformamide
at an overpotential of 0.80 and 0.76 V, respectively. The rate law
is first-order in catalyst and second-order in proton concentration.
Gas analysis from controlled potential electrolysis confirms CuL<sup>1</sup> as an electrocatalyst to produce H<sub>2</sub> with a minimum
Faradaic efficiency of 81% and turnover numbers as high as 73 while
showing no sign of degradation over 23 h. The H<sub>2</sub> evolution
reaction (HER) was probed using deuterated acid, demonstrating a kinetic
isotope effect of 7.54. A proton inventory study suggests one proton
is involved in the rate-determining step. Catalytic intermediates
were identified using <sup>1</sup>H NMR, X-ray photoelectron, and
UV–visible spectroscopies. All catalytic intermediates in the
proposed mechanism were successfully optimized using density functional
theory calculations with the B3LYP functional and the 6-311gÂ(d,p)
basis set and support the proposed mechanism
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Translation of Ligand-Centered Hydrogen Evolution Reaction Activity and Mechanism of a Rhenium-Thiolate from Solution to Modified Electrodes: A Combined Experimental and Density Functional Theory Study
The homogeneous,
nonaqueous catalytic activity of the rhenium-thiolate complex ReL<sub>3</sub> (L = diphenylphosphinobenzenethiolate) for the hydrogen evolution
reaction (HER) has been transferred from nonaqueous homogeneous to
aqueous heterogeneous conditions by immobilization on a glassy carbon
electrode surface. A series of modified electrodes based on ReL<sub>3</sub> and its oxidized precursor [ReL<sub>3</sub>]Â[PF<sub>6</sub>] were fabricated by drop-cast methods, yielding catalytically active
species with HER overpotentials for a current density of 10 mA/cm<sup>2</sup>, ranging from 357 to 919 mV. The overpotential correlates
with film resistance as measured by electrochemical impedance spectroscopy
and film morphology as determined by scanning and transmission electron
microscopy. The lowest overpotential was for films based on the ionic
[ReL<sub>3</sub>]Â[PF<sub>6</sub>] precursor with the inclusion of
carbon black. Stability measurements indicate a 2 to 3 h conditioning
period in which the overpotential increases, after which no change
in activity is observed within 24 h or upon reimmersion in fresh aqueous,
acidic solution. Electronic spectroscopy results are consistent with
ReL<sub>3</sub> as the active species on the electrode surface; however,
the presence of an undetected quantity of catalytically active degradation
species cannot be excluded. The HER mechanism was evaluated by Tafel
slope analysis, which is consistent with a novel Volmer–Heyrovsky–Tafel-like
mechanism that parallels the proposed homogeneous HER pathway. Proposed
mechanisms involving traditional metal-hydride processes vs ligand-centered
reactivity were examined by density functional theory, including identification
and characterization of relevant transition states. The ligand-centered
path is energetically favored with protonation of cis-sulfur sites
culminating in homolytic S–H bond cleavage with H<sub>2</sub> evolution via H atom coupling
Metal–Ligand Cooperativity Promotes Reversible Capture of Dilute CO<sub>2</sub> as a Zn(II)-Methylcarbonate
In this study, a series of thiosemicarbazonato–hydrazinatopyridine
metal complexes were evaluated as CO2 capture agents. The
complexes incorporate a non-coordinating, basic hydrazinatopyridine
nitrogen in close proximity to a Lewis acidic metal ion allowing for
metal–ligand cooperativity. The coordination of various metal
ions with (diacetyl-2-(4-methyl-thiosemicarbazone)-3-(2-hydrazinopyridine)
(H2L1) yielded ML1 (M = Ni(II), Pd(II)),
ML1(CH3OH) (M = Cu(II), Zn(II)), and [ML1(PPh3)2]BF4 (M = Co(III))
complexes. The ML1(CH3OH) complexes reversibly
capture CO2 with equilibrium constants of 88 ± 9 and
6900 ± 180 for Cu(II) and Zn(II), respectively. Ligand effects
were evaluated with Zn(II) through variation of the 4-methyl-thiosemicarbazone
with 4-ethyl (H2L2), 4-phenethyl (H2L3), and 4-benzyl (H2L4) derivatives.
The equilibrium constant for CO2 capture increased to 11,700
± 300, 15,000 ± 400, and 35,000 ± 200 for ZnL2(MeOH), ZnL3(MeOH), and ZnL4(MeOH), respectively.
Quantification of ligand basicity and metal ion Lewis acidity shows
that changes in CO2 capture affinity are largely associated
with ligand basicity upon substitution of Cu(II) with Zn(II), while
variation of the thiosemicarbazone ligand enhances CO2 affinity
by tuning the metal ion Lewis acidity. Overall, the Zn(II) complexes
effectively capture CO2 from dilute sources with up to
90%, 86%, and 65% CO2 capture efficiency from 400, 1000,
and 2500 ppm CO2 streams