[Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases

Calvin J. Curtis, Alex Miedaner, Rebecca Ciancanelli, William W. Ellis, Bruce C. Noll, M. Rakowski DuBois, Daniel L DuBois

Research output: Contribution to journalArticle

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Abstract

The reaction of Et2PCH2N(Me)CH2PEt2 (PNP) with [Ni(CH3CN)6](BF4)2 results in the formation of [Ni(PNP)2](BF4)2, which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF4)2, in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be -5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF4)2 undergoes reversible deprotonation to form [HNi(PNP)2](BF4) in acetonitrile solutions (pKa = 10.6). A convenient synthetic route to the PF6- salt of this hydride involves the reaction of PNP with Ni(COD)2 to form Ni(PNP)2, followed by protonation with NH4PF6. A pKa of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)2](BF4)2, was used to calculate homolytic and heterolytic Ni-H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)2](PF6). Oxidation of [HNi(PNP)2](PF6) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)2]2+ cation to the N atom to form [Ni(PNP)(PNHP)]2+. Estimates of the pKa values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1-2 pKa units. Cyclic voltammetry and proton exchange studies of [HNi(depp)2](PF6) (where depp is Et2PCH2CH2CH2PEt2) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)2](BF4)2 and [Ni(PNP)(dmpm)](BF4)2 (where PNBuP is Et2PCH2N(Bu)CH2PEt2 and dmpm is Me2PCH2PMe2) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.

Original languageEnglish
Pages (from-to)216-227
Number of pages12
JournalInorganic Chemistry
Volume42
Issue number1
DOIs
Publication statusPublished - Jan 2003

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Hydrogenase
Protons
protons
Hydrides
hydrides
Ligands
ligands
Atoms
Free energy
Cyclic voltammetry
atoms
Hydrogen
Ion exchange
free energy
thermodynamic cycles
Oxidation
Deprotonation
oxidation
crack opening displacement
electrocatalysts

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Curtis, C. J., Miedaner, A., Ciancanelli, R., Ellis, W. W., Noll, B. C., DuBois, M. R., & DuBois, D. L. (2003). [Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases. Inorganic Chemistry, 42(1), 216-227. https://doi.org/10.1021/ic020610v

[Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases. / Curtis, Calvin J.; Miedaner, Alex; Ciancanelli, Rebecca; Ellis, William W.; Noll, Bruce C.; DuBois, M. Rakowski; DuBois, Daniel L.

In: Inorganic Chemistry, Vol. 42, No. 1, 01.2003, p. 216-227.

Research output: Contribution to journalArticle

Curtis, CJ, Miedaner, A, Ciancanelli, R, Ellis, WW, Noll, BC, DuBois, MR & DuBois, DL 2003, '[Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases', Inorganic Chemistry, vol. 42, no. 1, pp. 216-227. https://doi.org/10.1021/ic020610v
Curtis CJ, Miedaner A, Ciancanelli R, Ellis WW, Noll BC, DuBois MR et al. [Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases. Inorganic Chemistry. 2003 Jan;42(1):216-227. https://doi.org/10.1021/ic020610v
Curtis, Calvin J. ; Miedaner, Alex ; Ciancanelli, Rebecca ; Ellis, William W. ; Noll, Bruce C. ; DuBois, M. Rakowski ; DuBois, Daniel L. / [Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases. In: Inorganic Chemistry. 2003 ; Vol. 42, No. 1. pp. 216-227.
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abstract = "The reaction of Et2PCH2N(Me)CH2PEt2 (PNP) with [Ni(CH3CN)6](BF4)2 results in the formation of [Ni(PNP)2](BF4)2, which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF4)2, in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be -5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF4)2 undergoes reversible deprotonation to form [HNi(PNP)2](BF4) in acetonitrile solutions (pKa = 10.6). A convenient synthetic route to the PF6- salt of this hydride involves the reaction of PNP with Ni(COD)2 to form Ni(PNP)2, followed by protonation with NH4PF6. A pKa of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)2](BF4)2, was used to calculate homolytic and heterolytic Ni-H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)2](PF6). Oxidation of [HNi(PNP)2](PF6) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)2]2+ cation to the N atom to form [Ni(PNP)(PNHP)]2+. Estimates of the pKa values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1-2 pKa units. Cyclic voltammetry and proton exchange studies of [HNi(depp)2](PF6) (where depp is Et2PCH2CH2CH2PEt2) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)2](BF4)2 and [Ni(PNP)(dmpm)](BF4)2 (where PNBuP is Et2PCH2N(Bu)CH2PEt2 and dmpm is Me2PCH2PMe2) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.",
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AU - Curtis, Calvin J.

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N2 - The reaction of Et2PCH2N(Me)CH2PEt2 (PNP) with [Ni(CH3CN)6](BF4)2 results in the formation of [Ni(PNP)2](BF4)2, which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF4)2, in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be -5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF4)2 undergoes reversible deprotonation to form [HNi(PNP)2](BF4) in acetonitrile solutions (pKa = 10.6). A convenient synthetic route to the PF6- salt of this hydride involves the reaction of PNP with Ni(COD)2 to form Ni(PNP)2, followed by protonation with NH4PF6. A pKa of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)2](BF4)2, was used to calculate homolytic and heterolytic Ni-H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)2](PF6). Oxidation of [HNi(PNP)2](PF6) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)2]2+ cation to the N atom to form [Ni(PNP)(PNHP)]2+. Estimates of the pKa values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1-2 pKa units. Cyclic voltammetry and proton exchange studies of [HNi(depp)2](PF6) (where depp is Et2PCH2CH2CH2PEt2) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)2](BF4)2 and [Ni(PNP)(dmpm)](BF4)2 (where PNBuP is Et2PCH2N(Bu)CH2PEt2 and dmpm is Me2PCH2PMe2) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.

AB - The reaction of Et2PCH2N(Me)CH2PEt2 (PNP) with [Ni(CH3CN)6](BF4)2 results in the formation of [Ni(PNP)2](BF4)2, which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF4)2, in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be -5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF4)2 undergoes reversible deprotonation to form [HNi(PNP)2](BF4) in acetonitrile solutions (pKa = 10.6). A convenient synthetic route to the PF6- salt of this hydride involves the reaction of PNP with Ni(COD)2 to form Ni(PNP)2, followed by protonation with NH4PF6. A pKa of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)2](BF4)2, was used to calculate homolytic and heterolytic Ni-H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)2](PF6). Oxidation of [HNi(PNP)2](PF6) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)2]2+ cation to the N atom to form [Ni(PNP)(PNHP)]2+. Estimates of the pKa values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1-2 pKa units. Cyclic voltammetry and proton exchange studies of [HNi(depp)2](PF6) (where depp is Et2PCH2CH2CH2PEt2) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)2](BF4)2 and [Ni(PNP)(dmpm)](BF4)2 (where PNBuP is Et2PCH2N(Bu)CH2PEt2 and dmpm is Me2PCH2PMe2) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.

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