Two Pathways for Electrocatalytic Oxidation of Hydrogen by a Nickel Bis(diphosphine) Complex with Pendant Amines in the Second Coordination Sphere

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• 作者：Jenny Y. Yang ; Stuart E. Smith ; Tianbiao Liu ; William G. Dougherty ; Wesley A. Hoffert ; W. Scott Kassel ; M. Rakowski DuBois ; Daniel L. DuBois ; R. Morris Bullock
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：July 3, 2013
• 年：2013
• 卷：135
• 期：26
• 页码：9700-9712
• 全文大小：598K
• 年卷期：v.135,no.26(July 3, 2013)
• ISSN：1520-5126

文摘

A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(P^Cy₂N^t-Bu₂)₂](BF₄)₂ (P^Cy₂N^t-Bu₂ = 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 Ni⁰ complex [Ni(P^Cy₂N^t-Bu₂H)₂](BF₄)₂. Using the pK_a values and Ni^II/I and Ni^I/0 redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P^Cy₂N^t-Bu₂)₂]²⁺ was determined to be 鈭?.9 kcal mol^鈥?. 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^鈥? is observed at 23 掳C.