Title: Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

Abstract

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H ₂ by protonation of a formally Co(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the meso carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co(II) species is reduced to a monoanionic singlet Co(I) species. Subsequent reduction leads to a dianionic doublet, formally a Co(0) complex in which substantial mixing of Co and porphyrin orbitals indicates ligand redoxmore » noninnocence. The partial reduction of the ligand disrupts the aromaticity in the porphyrin ring. As a result of this ligand dearomatization, protonation of the dianionic species is significantly more thermodynamically favorable at the meso carbon than at the metal center, and the ET-PT mechanism leads to a dianionic phlorin species. According to the proposed mechanism, the carboxylate group of this dianionic phlorin species is reprotonated, the species is reduced again, and H ₂ is evolved from the protonated carboxylate and the protonated carbon. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms.« less

Authors:

Solis, Brian H. ^[1]; Maher, Andrew G. ^[2]; Honda, Tatsuhiko ^[2]; Powers, David C. ^[2]; Nocera, Daniel G. ^[2]; Hammes-Schiffer, Sharon ^[1]

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+ Show Author Affiliations

1. Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States
2. Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138-2902, United States

Publication Date:

2014-11-12

Research Org.:

Harvard Univ., Cambridge, MA (United States); Univ. of Illinois at Urbana-Champaign, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)

OSTI Identifier:

1165086

Alternate Identifier(s):

OSTI ID: 1454868

Grant/Contract Number:  

SC0009758; CHE-1305124

Resource Type:

Journal Article: Published Article

Journal Name:

ACS Catalysis

Additional Journal Information:

Journal Name: ACS Catalysis Journal Volume: 4 Journal Issue: 12; Journal ID: ISSN 2155-5435

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; dearomatization; electrocatalyst; hangman porphyrin; hydrogen evolution reaction; phlorin; proton relay; proton-coupled electron transfer