Details

Autor(en) / Beteiligte

• Wilson, Aaron D
• Shoemaker, R.K
• Miedaner, A
• Muckerman, J.T
• DuBois, Daniel L
• DuBois, M. Rakowski

Titel

Nature of hydrogen interactions with Ni(II) complexes containing cyclic phosphine ligands with pendant nitrogen bases

Ist Teil von

• Proceedings of the National Academy of Sciences - PNAS, 2007-04, Vol.104 (17), p.6951-6956

Ort / Verlag

United States: National Academy of Sciences

Erscheinungsjahr

2007

Link zum Volltext

Quelle

MEDLINE

Beschreibungen/Notizen

• 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 25°C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A-4C, 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, suggests 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.