Pb(II)-promoted amide cleavage: Mechanistic comparison to a Zn(II) analogue

Eric S. Elton, Tingting Zhang, Rajeev Prabhakar, Atta M. Arif, Lisa M. Berreau

Research output: Contribution to journalArticle

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Abstract

Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl) methyl)amine) chelate ligand, [(epppa)Pb(NO3)2] (4-NO 3) and [(epppa)Pb(ClO4)2] (4-ClO4), were prepared and characterized. In the solid state, 4-NO3 exhibits κ5-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO3 is a 1:1 electrolyte with a coordinated NO3-, whereas 4-ClO4 is a 1:2 electrolyte. Treatment of 4-ClO4 with 1 equiv Me4NOH·5H2O in CH3CN:CH 3OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO 4)2 (3-ClO4). 1H NMR kinetic studies of the amide methanolysis reactions of 4-ClO4 and 3-ClO4 as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO 4 and 3-ClO4 differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO4; second-order behavior for 3-ClO4), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO4, and the properties of an initial intermediate isolated from each reaction upon treatment with Me4NOH·5H2O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO4 and 3-ClO4, indicate that the Zn(II)-containing 3-ClO4 initially undergoes amide deprotonation upon treatment with Me4NOH·5H2O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO4 instead directly forms a neutral amide-containing, epppa-ligated Pb(II)-OH/Pb(II)-OCH3 equilibrium mixture upon treatment with Me4NOH·5H2O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO4 is nucleophilic attack of the Pb(II)-OCH3 moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)-OH/Pb(II)- OCH3 mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO4.

Original languageEnglish
Pages (from-to)11480-11492
Number of pages13
JournalInorganic Chemistry
Volume52
Issue number19
DOIs
StatePublished - Oct 7 2013

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Amides
amides
cleavage
analogs
Protonation
methyl alcohol
Methanol
perchlorate
Deprotonation
chelates
kinetics
Electrolytes
Kinetics
electrolytes
ligands
Ligands
isotope effect
attack
acetonitrile
availability

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry

Cite this

Pb(II)-promoted amide cleavage : Mechanistic comparison to a Zn(II) analogue. / Elton, Eric S.; Zhang, Tingting; Prabhakar, Rajeev; Arif, Atta M.; Berreau, Lisa M.

In: Inorganic Chemistry, Vol. 52, No. 19, 07.10.2013, p. 11480-11492.

Research output: Contribution to journalArticle

Elton, Eric S. ; Zhang, Tingting ; Prabhakar, Rajeev ; Arif, Atta M. ; Berreau, Lisa M. / Pb(II)-promoted amide cleavage : Mechanistic comparison to a Zn(II) analogue. In: Inorganic Chemistry. 2013 ; Vol. 52, No. 19. pp. 11480-11492.
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abstract = "Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl) methyl)amine) chelate ligand, [(epppa)Pb(NO3)2] (4-NO 3) and [(epppa)Pb(ClO4)2] (4-ClO4), were prepared and characterized. In the solid state, 4-NO3 exhibits κ5-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO3 is a 1:1 electrolyte with a coordinated NO3-, whereas 4-ClO4 is a 1:2 electrolyte. Treatment of 4-ClO4 with 1 equiv Me4NOH·5H2O in CH3CN:CH 3OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO 4)2 (3-ClO4). 1H NMR kinetic studies of the amide methanolysis reactions of 4-ClO4 and 3-ClO4 as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO 4 and 3-ClO4 differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO4; second-order behavior for 3-ClO4), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO4, and the properties of an initial intermediate isolated from each reaction upon treatment with Me4NOH·5H2O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO4 and 3-ClO4, indicate that the Zn(II)-containing 3-ClO4 initially undergoes amide deprotonation upon treatment with Me4NOH·5H2O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO4 instead directly forms a neutral amide-containing, epppa-ligated Pb(II)-OH/Pb(II)-OCH3 equilibrium mixture upon treatment with Me4NOH·5H2O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO4 is nucleophilic attack of the Pb(II)-OCH3 moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)-OH/Pb(II)- OCH3 mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO4.",
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T2 - Mechanistic comparison to a Zn(II) analogue

AU - Elton, Eric S.

AU - Zhang, Tingting

AU - Prabhakar, Rajeev

AU - Arif, Atta M.

AU - Berreau, Lisa M.

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N2 - Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl) methyl)amine) chelate ligand, [(epppa)Pb(NO3)2] (4-NO 3) and [(epppa)Pb(ClO4)2] (4-ClO4), were prepared and characterized. In the solid state, 4-NO3 exhibits κ5-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO3 is a 1:1 electrolyte with a coordinated NO3-, whereas 4-ClO4 is a 1:2 electrolyte. Treatment of 4-ClO4 with 1 equiv Me4NOH·5H2O in CH3CN:CH 3OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO 4)2 (3-ClO4). 1H NMR kinetic studies of the amide methanolysis reactions of 4-ClO4 and 3-ClO4 as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO 4 and 3-ClO4 differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO4; second-order behavior for 3-ClO4), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO4, and the properties of an initial intermediate isolated from each reaction upon treatment with Me4NOH·5H2O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO4 and 3-ClO4, indicate that the Zn(II)-containing 3-ClO4 initially undergoes amide deprotonation upon treatment with Me4NOH·5H2O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO4 instead directly forms a neutral amide-containing, epppa-ligated Pb(II)-OH/Pb(II)-OCH3 equilibrium mixture upon treatment with Me4NOH·5H2O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO4 is nucleophilic attack of the Pb(II)-OCH3 moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)-OH/Pb(II)- OCH3 mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO4.

AB - Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl) methyl)amine) chelate ligand, [(epppa)Pb(NO3)2] (4-NO 3) and [(epppa)Pb(ClO4)2] (4-ClO4), were prepared and characterized. In the solid state, 4-NO3 exhibits κ5-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO3 is a 1:1 electrolyte with a coordinated NO3-, whereas 4-ClO4 is a 1:2 electrolyte. Treatment of 4-ClO4 with 1 equiv Me4NOH·5H2O in CH3CN:CH 3OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO 4)2 (3-ClO4). 1H NMR kinetic studies of the amide methanolysis reactions of 4-ClO4 and 3-ClO4 as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO 4 and 3-ClO4 differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO4; second-order behavior for 3-ClO4), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO4, and the properties of an initial intermediate isolated from each reaction upon treatment with Me4NOH·5H2O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO4 and 3-ClO4, indicate that the Zn(II)-containing 3-ClO4 initially undergoes amide deprotonation upon treatment with Me4NOH·5H2O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO4 instead directly forms a neutral amide-containing, epppa-ligated Pb(II)-OH/Pb(II)-OCH3 equilibrium mixture upon treatment with Me4NOH·5H2O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO4 is nucleophilic attack of the Pb(II)-OCH3 moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)-OH/Pb(II)- OCH3 mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO4.

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