Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent

Elizabeth A. McLoughlin, Kate M. Waldie, Srinivasan Ramakrishnan, Robert M. Waymouth

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

Protonation of the Co(I) phenylazopyridine (azpy) complex [CpCo(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co(I) complex [CpCo(azpyH)]+ 3 with no change in oxidation state at Co. The N-H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N-H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N-H bond of 3 exhibits a pKa of 12.1, a hydricity of ΔG°H- = 89 kcal/mol, and a bond dissociation free energy (BDFE) of ΔG°H• = 68 kcal/mol in CD3CN. Hydride transfer from 3 to the trityl cation (ΔG°H- = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di-tert-butyl-4-(4′-nitrophenyl)phenoxyl radical (tBu2NPArO·, ΔG°H• = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.

Original languageEnglish (US)
Pages (from-to)13233-13241
Number of pages9
JournalJournal of the American Chemical Society
Volume140
Issue number41
DOIs
StatePublished - Oct 17 2018
Externally publishedYes

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Protonation
Cobalt
Hydrides
Protons
Hydrogen
Ligands
Atoms
Thermodynamics
Mental Competency
Free energy
Cations
Nitrogen
Oxidation
Positive ions

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

McLoughlin, Elizabeth A. ; Waldie, Kate M. ; Ramakrishnan, Srinivasan ; Waymouth, Robert M. / Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent. In: Journal of the American Chemical Society. 2018 ; Vol. 140, No. 41. pp. 13233-13241.
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abstract = "Protonation of the Co(I) phenylazopyridine (azpy) complex [CpCo(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co(I) complex [CpCo(azpyH)]+ 3 with no change in oxidation state at Co. The N-H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N-H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N-H bond of 3 exhibits a pKa of 12.1, a hydricity of ΔG°H- = 89 kcal/mol, and a bond dissociation free energy (BDFE) of ΔG°H• = 68 kcal/mol in CD3CN. Hydride transfer from 3 to the trityl cation (ΔG°H- = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di-tert-butyl-4-(4′-nitrophenyl)phenoxyl radical (tBu2NPArO·, ΔG°H• = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.",
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Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent. / McLoughlin, Elizabeth A.; Waldie, Kate M.; Ramakrishnan, Srinivasan; Waymouth, Robert M.

In: Journal of the American Chemical Society, Vol. 140, No. 41, 17.10.2018, p. 13233-13241.

Research output: Contribution to journalArticle

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N2 - Protonation of the Co(I) phenylazopyridine (azpy) complex [CpCo(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co(I) complex [CpCo(azpyH)]+ 3 with no change in oxidation state at Co. The N-H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N-H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N-H bond of 3 exhibits a pKa of 12.1, a hydricity of ΔG°H- = 89 kcal/mol, and a bond dissociation free energy (BDFE) of ΔG°H• = 68 kcal/mol in CD3CN. Hydride transfer from 3 to the trityl cation (ΔG°H- = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di-tert-butyl-4-(4′-nitrophenyl)phenoxyl radical (tBu2NPArO·, ΔG°H• = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.

AB - Protonation of the Co(I) phenylazopyridine (azpy) complex [CpCo(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co(I) complex [CpCo(azpyH)]+ 3 with no change in oxidation state at Co. The N-H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N-H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N-H bond of 3 exhibits a pKa of 12.1, a hydricity of ΔG°H- = 89 kcal/mol, and a bond dissociation free energy (BDFE) of ΔG°H• = 68 kcal/mol in CD3CN. Hydride transfer from 3 to the trityl cation (ΔG°H- = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di-tert-butyl-4-(4′-nitrophenyl)phenoxyl radical (tBu2NPArO·, ΔG°H• = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.

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