Title: Utilizing Charge Effects and Minimizing Intramolecular Proton Rearrangement to Improve the Overpotential of a Thiosemicarbazonato Zinc HER Catalyst

Abstract

The zinc(II) complex of diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazonepyridine), ZnL¹ (1), was prepared and evaluated as a precatalyst for the hydrogen evolution reaction (HER) under homogeneous conditions in acetonitrile. Complex 1 is protonated on the noncoordinating nitrogen of the hydrazonepyridine moiety to yield the active catalyst Zn(HL¹)OAc (2) upon addition of acetic acid. Addition of methyl iodide to 1 yields the corresponding methylated derivative ZnL²I (3). In solution, partial dissociation of the coordinated iodide yields the cationic derivative 3'. Complexes 1–3 were characterized by ¹H NMR, FT-IR, and UV–visible spectroscopies. The solid-state structures of 2 and 3 were determined by single crystal X-ray diffraction. HER studies conducted in acetonitrile with acetic acid as the proton source yield a turnover frequency (TOF) of 7700 s^–1 for solutions of 1 at an overpotential of 1.27 V and a TOF of 6700 s^–1 for solutions of 3 at an overpotential of 0.56 V. For both complexes, the required potential for catalysis, E_cat/2, is larger than the thermodynamic reduction potential, E_1/2, indicative of a kinetic barrier attributed to intramolecular proton rearrangement. The effect is larger for solutions of 1 (+440 mV) than for solutions of 3 (+160 mV). Controlled potential coulometry studies were used to determine faradaic efficienciesmore » of 71 and 89% for solutions of 1 and 3, respectively. For both catalysts, extensive cycling of potential under catalytic conditions results in the deposition of a film on the glassy carbon electrode surface that is active as an HER catalyst. Analysis of the film of 3 by X-ray photoelectron spectroscopy indicates the complex remains intact upon deposition. A proposed ligand-centered HER mechanism with 1 as a precatalyst to 2 is supported computationally using density functional theory (DFT). All catalytic intermediates in the mechanism were structurally and energetically characterized with the DFT/B3LYP/6-311g(d,p) in solution phase using a polarizable continuum model (PCM). Finally, the thermodynamic feasibility of the mechanism is supported by calculation of equilibrium constants or reduction potentials for each proposed step.« less

Authors:

Cronin, Steve P.  ^[1];

• Search DOE PAGES for author "Cronin, Steve P."
• Search DOE PAGES for ORCID "0000-0001-6328-706X"

Mamun, Abdullah Al  ^[1];

• Search DOE PAGES for author "Mamun, Abdullah Al"
• Search DOE PAGES for ORCID "0000-0001-8941-8195"

Toda, Megan J.  ^[1];

• Search DOE PAGES for author "Toda, Megan J."
• Search DOE PAGES for ORCID "0000-0002-4913-0848"

Mashuta, Mark S. ^[1]; Losovyj, Yaroslav ^[2]; Kozlowski, Pawel M.  ^[1];

• Search DOE PAGES for author "Kozlowski, Pawel M."
• Search DOE PAGES for ORCID "0000-0002-4090-8078"

Buchanan, Robert M. ^[1]; Grapperhaus, Craig A.  ^[1]

• Search DOE PAGES for author "Grapperhaus, Craig A."
• Search DOE PAGES for ORCID "0000-0003-4889-2645"

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+ Show Author Affiliations

1. Univ. of Louisville, KY (United States)
2. Indiana Univ., Bloomington, IN (United States)

Publication Date:

Tue Sep 10 00:00:00 EDT 2019

Research Org.:

Univ. of Louisville, KY (United States)

Sponsoring Org.:

USDOE; National Science Foundation (NSF)

OSTI Identifier:

1800549

Grant/Contract Number:  

FG02-08CH11538; CHE-1665136; CHE-1800245; MRI-1126394

Resource Type:

Accepted Manuscript

Journal Name:

Inorganic Chemistry

Additional Journal Information:

Journal Volume: 58; Journal Issue: 19; Journal ID: ISSN 0020-1669

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; chemistry; redox reactions; reaction mechanisms; electrodes; catalysts; evolution reactions