title: Combined Spectroscopic and Electrochemical Detection of a Ni(I) ⋅⋅⋅H-N Bonding Interaction with Relevance to Electrocatalytic H2 Production.
abstract: The [Ni(P(R) 2 N(R') 2 )2 ](2+) family of complexes are exceptionally active catalysts for proton reduction to H2 . In this manuscript, we explore the first protonation step of the proposed catalytic cycle by using a catalytically inactive Ni(I) complex possessing a sterically demanding variation of the ligand. Due to the paramagnetic nature of the Ni(I) oxidation state, the protonated Ni(I) intermediate has been characterized through a combination of cyclic voltammetry, electron nuclear double resonance (ENDOR) spectroscopy, and hyperfine sublevel correlation (HYSCORE) spectroscopy. Both the electrochemical and spectroscopic studies indicate that the Ni(I) complex is protonated at a pendant amine that is endo to Ni, which suggests the presence of an intramolecular Ni(I) ⋅⋅⋅HN bonding interaction. Using density functional theory, the hydrogen bond was found to involve three doubly-occupied, localized molecular orbitals: the 3dxz , 3d z 2, and 3dyz orbitals of nickel. These studies provide the first direct experimental evidence for this critical catalytic intermediate, and implications for catalytic H2 production are discussed.
authors.affiliation: Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr (Germany).
authors.forename: Amélie
authors.initials: A
authors.lastname: Kochem
authors.affiliation: Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352 (USA).
authors.forename: Molly
authors.initials: M
authors.lastname: O'Hagan
authors.affiliation: Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352 (USA). eswiedner@pnnl.gov.
authors.forename: Eric S
authors.initials: ES
authors.lastname: Wiedner
authors.affiliation: Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr (Germany). maurice.van-gastel@cec.mpg.de.
authors.forename: Maurice
authors.initials: M
authors.lastname: van Gastel
keywords: HYSCORE; electrochemistry; hydrogen; nickel; protonation