skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Nature of Hydrogen Interactions with Ni(II) Complexes Containing Cyclic Phosphine Ligands with Pendant Nitrogen Bases

Abstract

Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H2 have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P2PhN2Ph)2(CH3CN)](BF4)2, 3a, (where P2PhN2Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane) permits a limiting value of the H2 production rate to be determined. The turnover frequency of 350 s-1 establishes that the rate of H2 production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P2CyN2Bz)2](BF4)2, 3b, (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (Keq = 190 atm-1 at 25oC). The hydrogen addition product exists as three nearly iso-energetic isomers 4A-C, which have been identified by a combination of one and two dimensional 1H, 31P and 15N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggest a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to [HNi(P2CyN2Bz)2](BF4),more » 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity. This work was supported by the Office of Basic Energy Sciences of the US Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
909679
Report Number(s):
PNNL-SA-53453
Journal ID: ISSN 0027-8424; PNASA6; KC0302010; TRN: US200723%%131
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Proceedings of the National Academy of Sciences of the United States of America, 104(17):6951-6956; Journal Volume: 104; Journal Issue: 17
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; 36 MATERIALS SCIENCE; ELECTRON DENSITY; HYDROGEN; HYDROGEN ADDITIONS; LIMITING VALUES; NICKEL; NICKEL HYDRIDES; NITROGEN; PHOSPHINES; PRODUCTION; proton relays; molecular catalysts; electron density; hydrogen interactions

Citation Formats

Wilson, Aaron, Shoemaker, R. K., Miedaner, Alex, Muckerman, J. T., DuBois, Daniel L., and DuBois, Mary R. Nature of Hydrogen Interactions with Ni(II) Complexes Containing Cyclic Phosphine Ligands with Pendant Nitrogen Bases. United States: N. p., 2007. Web. doi:10.1073/pnas.0608928104.
Wilson, Aaron, Shoemaker, R. K., Miedaner, Alex, Muckerman, J. T., DuBois, Daniel L., & DuBois, Mary R. Nature of Hydrogen Interactions with Ni(II) Complexes Containing Cyclic Phosphine Ligands with Pendant Nitrogen Bases. United States. doi:10.1073/pnas.0608928104.
Wilson, Aaron, Shoemaker, R. K., Miedaner, Alex, Muckerman, J. T., DuBois, Daniel L., and DuBois, Mary R. Tue . "Nature of Hydrogen Interactions with Ni(II) Complexes Containing Cyclic Phosphine Ligands with Pendant Nitrogen Bases". United States. doi:10.1073/pnas.0608928104.
@article{osti_909679,
title = {Nature of Hydrogen Interactions with Ni(II) Complexes Containing Cyclic Phosphine Ligands with Pendant Nitrogen Bases},
author = {Wilson, Aaron and Shoemaker, R. K. and Miedaner, Alex and Muckerman, J. T. and DuBois, Daniel L. and DuBois, Mary R.},
abstractNote = {Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H2 have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P2PhN2Ph)2(CH3CN)](BF4)2, 3a, (where P2PhN2Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane) permits a limiting value of the H2 production rate to be determined. The turnover frequency of 350 s-1 establishes that the rate of H2 production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P2CyN2Bz)2](BF4)2, 3b, (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (Keq = 190 atm-1 at 25oC). The hydrogen addition product exists as three nearly iso-energetic isomers 4A-C, which have been identified by a combination of one and two dimensional 1H, 31P and 15N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggest a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to [HNi(P2CyN2Bz)2](BF4), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity. This work was supported by the Office of Basic Energy Sciences of the US Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.},
doi = {10.1073/pnas.0608928104},
journal = {Proceedings of the National Academy of Sciences of the United States of America, 104(17):6951-6956},
number = 17,
volume = 104,
place = {United States},
year = {Tue Apr 24 00:00:00 EDT 2007},
month = {Tue Apr 24 00:00:00 EDT 2007}
}
  • A series of new iron(II) complexes that contain cyclic diphosphine ligands with pendant amine bases, P2RN2R’, have been synthesized and characterized (where P2RN2R’ are substituted 1,5-diaza-3,7-diphosphacyclooctanes). These compounds include [Fe(P2PhN2Ph)(CH3CN)4](BF4)2 (1), cis-[Fe(CH3CN)2(P2PhN2Ph)2](BF4)2 (2a), cis-[Fe(CH3CN)2(P2CyN2Bz)2](BF4)2 (2b), cis-Fe(CH3CN)2(P2PhN2Bz)2](BF4)2 (2c), cis-Fe (P2PhN2Ph)2(Cl)2 (3), and trans-[HFe(CH3CN)(P2PhN2Ph)2](BF4), (4). The molecular structures of 1, 2b, and 4 have been confirmed by X-ray diffraction studies. For all complexes the cyclic diphosphine ligands contain one six-membered ring in a chair conformation and one six-membered ring in a boat conformation. For complex 4, the two rings that are in boat conformations result in N--H distances between the pendant aminemore » nitrogens and the hydride ligand of 2.6 to 2.7 Å. Protonation of the pendant bases in complex 4 have been found to form several products. A structural assignment for a dominant protonated isomer has been assigned on the basis of 1H, 31P and 15N spectroscopic techniques. This work was supported by Grant CHE-0240106 from the National Science Foundation. D. L. D. acknowledges the support of the Office of Basic Energy Sciences of the Department of Energy, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less
  • A series of new iron(II)-hydride complexes that contain diphosphine ligands with pendant amine bases of the formula cis-[HFeL(PNP)2]+, where PNP = Et2PCH2NMeCH2PEt2 and L = CH3CN (3), CO (4), and P(OEt)3 (5), have been synthesized and characterized. Protonations of the pendant bases in the PNP complexes have been characterized, and for selected complexes, pKa values have been determined. Unlike the previously studied hydride complexes [HNi(PNP)2]+ and trans-[HFe(CH3CN)(PNP)(dmpm)]+, the new hydride complexes reported here do not show rapid intramolecular exchange between the protonated base of the diphosphine and the hydride ligand. “This work was supported by the Office of Basic Energymore » Sciences of the Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.”« less
  • Reactions of complexes of the type RuCl[sub 2](PPh[sub 3])(P-N) with H[sub 2] are reported, where P-N represents the chelating ferrocene-based ligands ([eta]-C[sub 5]H[sub 5])Fe([eta]-C[sub 5]H[sub 3](CHMeNMe[sub 2])PR[sub 2]-1,2) with R = i-Pr (the isoPFA ligand) or Ph (the PPFA ligand). Depending on the solvent(s) used, and absence or presence of added base, RuCl[sub 2](PPh[sub 3])(isoPFA) (2b) generates the dinuclear [eta][sup 2]-H[sub 2] complexes L[sub 2]([eta][sup 2]-H[sub 2])Ru([mu]-Cl)[sub 2]([mu]-H)Ru(H)(PPh[sub 3])[sub 2] where L[sub 2] = isoPFA (complex 3) or L[sub 2] = (PPh[sub 3])[sub 2] (complex 4). n-Butanol solutions of RuCl[sub 2](PPh[sub 3])(PPFA) (2a) under H[sub 2] also yield 4, asmore » well as Ru(H)Cl[sub 2](PPh[sub 3])(PPFA[center dot]H)(BuOH), a zwitterionic species containing a protonated amine moiety stabilized by coordinated n-BuOH, which is likely intermediate in heterolytic cleavage of the H[sub 2]: the complex RuH(Cl)(PPh[sub 3])(isoPFA) (5), together with a hybrido carbonyl species, is isolated from methanol solutions of 2b. Complexes 2b, 3, 4, and 5 are characterized by X-ray crystallography; the mononuclear complexes 2b and 5 are orthorhombic with space groups F2dd and P2[sub 1]2[sub 1]2[sub 1], respectively. Variable-temperature [sup 1]H- and [sup 31]P-NMR data for 3 reveal fast exchange between the [eta][sup 2]-H[sub 2] and the [mu]-H at 20[degrees]C and a slower exchange of this system with the terminal hydride; corresponding data for 4 and its P(p-tolyl)[sub 3] analogue 7 show a faster exchange involving all the hydrogens. Activation parameters are determined for the exchange processes. 48 refs., 13 figs., 7 tabs.« less
  • The iron complexes CpFe(PPh2NBn2)Cl (1-Cl), CpFe(PPh2NPh2)Cl (2-Cl), CpFe(PPh2C5)Cl (3-Cl) (where PPh2NBn2 is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, PPh2NPh2 is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and PPh2C5 is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction studies. These chloride derivatives are readily converted to the corresponding hydride complexes CpFe(PPh2NBn2)H (1-H), CpFe(PPh2NPh2)H (2-H), CpFe(PPh2C5)H (3-H)] and H2 complexes [CpFe(PPh2NBn2)(H2)]BArF4, [1-H2]BArF4, (where BArF4 is B[(3,5-(CF3)2C6H3)4]-), [CpFe(PPh2NPh2)(H2)]BArF4, [2-H2]BArF4, and [CpFe(PPh2C5)(H2)]BArF4, [3-H2]BArF4 as well as [CpFe(PPh2NBn2)(CO)]BArF4, [1-CO]BArF4. Structural studies are reported for [1-H2]BArF4, 1-H, 2-H, and [1-CO]BArF4. The conformations adopted by the chelate rings of the PPh2NBn2 ligand in the different complexes are determined by attractive ormore » repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BArF4 is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Experimental and theoretical studies of H/D exchange by the complexes [1-H2]+, [2-H2]+, and [3-H2]+ indicate that the relatively rapid exchange observed for [1-H2]+ and [2-H2]+ compared to [3-H2]+ is consistent with intramolecular heterolytic cleavage of H2 mediated by the pendant amine. These mononuclear FeII dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe] hydrogenase required for H-H bond formation and cleavage. We thank the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences, for support of this research. S.C. (DFT computations) and M. J. O. (NMR experiments) were supported by the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under FWP 56073. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.« less