The Influence of the Second and Outer Coordination Spheres on Rh(diphosphine)2 CO2 Hydrogenation Catalysts

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• 作者：J. Timothy Bays ; Nilusha Priyadarshani ; Matthew S. Jeletic ; Elliot B. Hulley ; Deanna L. Miller ; John C. Linehan ; Wendy J. Shaw
• 刊名：ACS Catalysis
• 出版年：2014
• 出版时间：October 3, 2014
• 年：2014
• 卷：4
• 期：10
• 页码：3663-3670
• 全文大小：385K
• ISSN：2155-5435

文摘

A series of [Rh(PCH₂X^RCH₂P)₂]⁺ complexes was prepared to investigate second and outer coordination sphere effects on CO₂ hydrogenation catalysis, where X is CH₂ (dppp) or X鈥揜 is N鈥揅H₃, N鈥揅H₂COOH (glycine), N鈥揅H₂COOCH₃ (Gly-OMe), or N鈥揅H₂C(O)N鈥揅H(CH₃)COOCH₃ (GlyAla-OMe). All of these complexes were active for CO₂ reduction to formate, with the N鈥揅H₃ derivative offering an 8-fold enhancement over the dppp derivative, which is consistent with increased electron density around the metal. Despite the increase in rate with the addition of the pendant nitrogen, the addition of electron withdrawing amino acids and dipeptides to the amine resulted in complexes with reductions in rate of 1 to 2 orders of magnitude, most consistent with a change in pK_a of the pendant amine, resulting in lower activity. Collectively, the data suggest multiple contributions of the pendant amine in this catalytic system.

Keywords:

CO₂ hydrogenation; amino acid catalysts; outer coordination sphere; homogeneous catalysis; in situ NMR