Evidence That Imidazolium-Based Ionic Ligands Can be Metal(0)/Nanocluster Catalyst Poisons in at Least the Test Case of Iridium(0)-Catalyzed Acetone Hydrogenation

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• 作者：Lisa Starkey Ott ; Sarah Campbell ; Kenneth R. Seddon ; Richard G. Finke
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：November 26, 2007
• 年：2007
• 卷：46
• 期：24
• 页码：10335 - 10344
• 全文大小：318K
• 年卷期：v.46,no.24(November 26, 2007)
• ISSN：1520-510X

文摘

This study begins with the question of whether ionic liquids (ILs), such as 1-butyl-3-methylimidazoliumhexafluorophosphate, [bmim][PF₆], can be catalyst poisons for transition-metal catalysts rather than a preferredstabilizing media as typically assumed in the literature. The test case of acetone (propanone) hydrogenation ispicked for two reasons: (i) acetone hydrogenation is important for its applications in heat pumps, H₂ storageschemes, and fuel cells and for the commercial value of the resultant product, propan-2-ol, and (ii) two recent,independent studies have reported putative Ir(0)_n nanocluster-catalyzed hydrogenations of acetone beginning ineach case with the identical precursor, [{(COD)IrCl}₂] (where COD = 1,5-cyclooctadiene) (1). A close comparisonof the results of those two literature studies and their related, but different, experimental conditions (vide infra)suggests the hypothesis that the IL is actually a catalyst poison. Indeed, the investigations herein (i) find that 1.0equiv of added IL, [bmim][PF₆], completely inhibits the formation of Ir(0)_n nanoclusters under conditions 1 in Table1 in the main text (namely, 3.6 mM precatalyst 1, 22 C, and 2.76 bar H₂) and (ii) demonstrate that 0.1 and 1.0equivs of this same IL, [bmim][PF₆], poisons 74 and 90%, respectively, of the acetone hydrogenation activity ofpremade, previously catalytically active nanoclusters. The above results in turn compelled a reinvestigation of theclaim that Ir(0)_n nanoclusters are the catalyst in what was reported as a colloidal suspension prepared underconditions 2 in Table 1 of the main text (namely, 52 mM precursor 1, 92 equiv of IL, 75 C, and 4.05 bar of H₂).We further (iii) find that the colloidal suspension prepared under conditions 2 is a mixture of unreacted precursor,1, some nanoclusters, and isolable bulk metal, and we also (iv) find, somewhat surprisingly, in light of the IL-poisoning results found under conditions 1, that the Ir(0) catalyst prepared under conditions 2 is active, preciselyas reported, for acetone hydrogenation. This, in turn, further demanded that we go on to (v) investigate the natureof the true catalyst under conditions 2, the results of which we are able to interpret only by the hypothesis that bulkmetal is the dominant, true catalyst under conditions 2. Overall, the results provide strong evidence that ILs canbe potent inhibitors of metal(0)/nanocluster catalysis, rather than the often-assumed superior solvent for nanoclustercatalysis. The results also fortify our recent report that, under conditions where stoichiometrically high amounts ofcoordinating ligands are present (vs the amount of surface metal atoms), bulk-metal catalysts can actually besuperior to nanocluster catalysts of the same metal, a seemingly heretical finding prior to our recent experimentalevidence for this (Besson, C.; Finney, E. E.; Finke, R. G. J. Am. Chem. Soc. 2005, 127, 8179; Besson, C.; Finney,E. E.; Finke, R. G. Chem. Mater. 2005, 17, 4925).