Organometallic molecule-inorganic surface coordination and catalytic chemistry. In Situ CPMAS NMR delineation of organoactinide adsorbate structure, dynamics, and reactivity

William C. Finch, Ralph D. Gillespie, David Hedden, Tobin J Marks

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Abstract

A 75.4-MHz 13C CPMAS NMR spectroscopic study of the surface structures and reaction chemistry of a series of organoactinides adsorbed on various inorganic supports is reported. On Lewis acid surfaces such as dehydroxylated Al23, MgCl2, and Si2-Al23, it is found that organothorium complexes of the type Cp'2ThR2 (Cp' = η5-(CH3)5C5; R = 13CH3, 13CH213CH3), Cp'ThR3 (R = 13CH2C6H5), and Cp3ThR (Cp = η5-C5H5; R = 13CH3), undergo heterolytic Th - C scission to transfer an alkyl anion to the surface forming Cp'2ThR, CpThR3-n, or Cp3Th adsorbate species with "cation-like" character. Probe studies with paramagnetic Cp'2U(13CH3)2 indicate that the majority of the transferred methyl groups of Cp'2U(13CH3)2/DA and Cp'2U(13CH3)2/MgCl2 are located ≳5 Å from the U(IV) ion. On less dehydroxylated or more basic supports such as Si2-MgO, Si2, and MgO, μ-oxo species of the type Cp'2Th(CH3)O- are formed, by Th - C protonolysis or by transfer of an alkyl group to the surface. For Cp'2U(13CH3)2/Si2, the majority of the resulting 13CH3-Si(surface) functionalities are ≳5 Å from the actinide center. In agreement with heterogeneous catalytic studies, the NMR data reveal that only a small percentage of Cp'2Th(13CH3)2/DA or Cp'Th(13CH2C6H5)3/DA surface sites undergo reaction with ethylene or H2 at 25 °C. In contrast, 50 ± 10% of Cp'2Th(13CH3)2/MgCl2 sites undergo reaction with ethylene; >90 ± 10% of ethylene insertion/polymerization occurs at Th - CH3 with k(propagation)/k(initiation) ≈ 12 in the initial stages. There is no evidence for methane evolution via C - H functionalization nor for significant rates of Th(CH2CH2)n13CH3- 13CH3Mg(surface) alkyl group permutation. At 25 °C, a large percentage of Cp'2Th(13CH3)2/MgCl2 Th - CH3 and Mg - CH3 functionalities undergo hydrogenolysis, with Th-CH3 being slightly more reactive. In competition experiments, Th - CH3 is far more reactive than Mg - CH3 in migratory CO insertion, and products are inferred to be, inter alia, η2-acyl complexes. Cp'2Th(13CH3)2/MgCl2 undergoes reaction with propylene to yield methane (derived from Th-CH3), a Th(η3-allyl) complex, and what appear to be propylene oligomers.

Original languageEnglish
Pages (from-to)6221-6232
Number of pages12
JournalJournal of the American Chemical Society
Volume112
Issue number17
Publication statusPublished - 1990

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ASJC Scopus subject areas

  • Chemistry(all)

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