[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

doi:10.1021/ic020610v

[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

Pacific Northwest National Laboratory

Research output: Contribution to journal › Article › peer-review

191 Citations (Scopus)

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 (pK_a = 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 pK_a 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 pK_a 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 pK_a 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 language	English
Pages (from-to)	216-227
Number of pages	12
Journal	Inorganic Chemistry
Volume	42
Issue number	1
DOIs	https://doi.org/10.1021/ic020610v
Publication status	Published - Jan 1 2003

ASJC Scopus subject areas

Physical and Theoretical Chemistry
Inorganic Chemistry

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@article{2c114a00d63145bb938cb070682c21e7,

title = "[Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases",

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.",

author = "Curtis, {Calvin J.} and Alex Miedaner and Rebecca Ciancanelli and Ellis, {William W.} and Noll, {Bruce C.} and DuBois, {M. Rakowski} and DuBois, {Daniel L.}",

year = "2003",

month = jan,

day = "1",

doi = "10.1021/ic020610v",

language = "English",

volume = "42",

pages = "216--227",

journal = "Inorganic Chemistry",

issn = "0020-1669",

publisher = "American Chemical Society",

number = "1",

}

TY - JOUR

T1 - [Ni(Et2PCH2NMeCH2 PEt2)2]2+ as a functional model for hydrogenases

AU - Curtis, Calvin J.

AU - Miedaner, Alex

AU - Ciancanelli, Rebecca

AU - Ellis, William W.

AU - Noll, Bruce C.

AU - DuBois, M. Rakowski

AU - DuBois, Daniel L.

PY - 2003/1/1

Y1 - 2003/1/1

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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