title: Two pathways for electrocatalytic oxidation of hydrogen by a nickel bis(diphosphine) complex with pendant amines in the second coordination sphere.
abstract: A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(P(Cy)2N(t-Bu)2)2](BF4)2 (P(Cy)2N(t-Bu)2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni(II) complex gives three isomers of the doubly protonated Ni(0) complex [Ni(P(Cy)2N(t-Bu)2H)2](BF4)2. Using the pKa values and Ni(II/I) and Ni(I/0) redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P(Cy)2N(t-Bu)2)2](2+) was determined to be -7.9 kcal mol(-1). The catalytic rate observed in dry acetonitrile for the oxidation of H2 depends on base size, with larger bases (NEt3, t-BuNH2) resulting in much slower catalysis than n-BuNH2. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt3 and t-BuNH2. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H2 using n-BuNH2 as a base and with added water, a turnover frequency of 58 s(-1) is observed at 23 °C.
authors: Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, USA. j.yang@uci.edu
authors: Jenny Y
authors: JY
authors: Yang
authors: Stuart E
authors: SE
authors: Smith
authors: Tianbiao
authors: T
authors: Liu
authors: William G
authors: WG
authors: Dougherty
authors: Wesley A
authors: WA
authors: Hoffert
authors: W Scott
authors: WS
authors: Kassel
authors: M
authors: M
authors: Rakowski DuBois
authors: Daniel L
authors: DL
authors: DuBois
authors: R Morris
authors: RM
authors: Bullock
mesh_terms: D000588:Amines / Q000737:chemistry*; D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry*; D055664:Electrochemical Techniques; D006859:Hydrogen / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009532:Nickel / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D000588:Amines; D056831:Coordination Complexes; D009532:Nickel; D006859:Hydrogen