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 Al2O3, MgCl2, and SiO2-Al2O3, it is found that organothorium complexes of the type Cp′2ThR2 (Cp′ = ′5-(CH3)5C3; 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, Cp′ThR3-n or Cp3Th adsorbate species with gcation-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 SiO2-MgO, SiO2, 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/SiO2, the majority of the resulting 13CH3-Si(surface) functionalities are ≳5 Å from the actinidecenter. In agreement with heterogeneous catalytic studies, the NMR data reveal that only a small percentage of Cp′2Th(13CH3)2/DA or CpTh(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)-13CH3-13CH3Mg(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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry