Mechanistic studies of hydrogen evolution in aqueous solution catalyzed by a tertpyridine-amine cobalt complex

Anna Lewandowska-Andralojc, Teera Baine, Xuan Zhao, James Muckerman, Etsuko Fujita, Dmitry Polyansky

Research output: Contribution to journalArticle

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Abstract

The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as CoI and CoIII-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)]n+ (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)]n+ where L is the pentadentate DPA-Bpy ligand or [Co(OH2)]n+ as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a CoII species results in weakening the Co-O bond. The further reduction to a CoI species leads to the loss of the aqua ligand and the formation of [CoI-VS)]+ (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [CoI4-L)(OH2)]+ species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the CoI species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [CoIII-H]2+. We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [CoI4-L)(OH2)]+ in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation.

Original languageEnglish
Pages (from-to)4310-4321
Number of pages12
JournalInorganic Chemistry
Volume54
Issue number9
DOIs
Publication statusPublished - May 4 2015

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Cobalt
Amines
Hydrogen
amines
cobalt
Protons
aqueous solutions
Ligands
hydrogen
ligands
protons
Coordination Complexes
Density functional theory
density functional theory
Reaction intermediates
Radiolysis
reaction intermediates
Photolysis
radiolysis
metals

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry

Cite this

Mechanistic studies of hydrogen evolution in aqueous solution catalyzed by a tertpyridine-amine cobalt complex. / Lewandowska-Andralojc, Anna; Baine, Teera; Zhao, Xuan; Muckerman, James; Fujita, Etsuko; Polyansky, Dmitry.

In: Inorganic Chemistry, Vol. 54, No. 9, 04.05.2015, p. 4310-4321.

Research output: Contribution to journalArticle

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abstract = "The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as CoI and CoIII-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)]n+ (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)]n+ where L is the pentadentate DPA-Bpy ligand or [Co(OH2)]n+ as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a CoII species results in weakening the Co-O bond. The further reduction to a CoI species leads to the loss of the aqua ligand and the formation of [CoI-VS)]+ (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [CoI(κ4-L)(OH2)]+ species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the CoI species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [CoIII-H]2+. We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [CoI(κ4-L)(OH2)]+ in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation.",
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