Organo-f-Element Thermochemistry. Thorium vs. Uranium and Ancillary Ligand Effects on Metal-Ligand Bond Disruption Enthalpies in Bis(pentamethylcyclopentadienyl)actinide Bis(hydrocarbyls) and Bis(pentamethyIcyclopentadienyl)alkoxyactinide Hydrides and Hydrocarbyls

Joseph W. Bruno, H. A. Stecher, Tobin J. Marks, Lester R. Morss, David C. Sonnenberger

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Abstract

Thorium- and uranium-ligand bond disruption enthalpies (D(M-R)) have been determined for a series of complexes of the type Cp’2M(R)X, where M = Th or U, Cp’ = η5-(CH3)5C5, R = alkyl, aryl, or hydride, and X = alkyl, chloride, or alkoxide. The thermodynamic data were obtained by anaerobic, batch-titration, solution calorimetry from the enthalpies of solution of the organo-f-element complexes in toluene and from the subsequent enthalpies of alcoholysis with tert-butyl alcohol or methanol. Average (of Cp’2U(R)-R and Cp’2U(OR’)-R) values for the nonstepwise alcoholysis of the Cp’2UR2 series (gas phase, solution phase, in kcal/mol, quantity in parentheses = 2σ) are as follows: 67.8 (3.5), 71.8 (3.3) (R = CH3), 61.8 (2.7), 58.3 (2.1) (R = benzyl), and 73.3 (3.3), 73.3 (3.1) (R = CH2Si(CH3)3). For the Cp/2U(Cl)R series, D(U-R) = 70.1 (1.7), 74.6 (1.6) (R = CH3), 66.1 (2.8), 63.0 (2.7) (R = benzyl), 86.7 (2.6), 85.5 (2.6) (R = phenyl), and 90.5 (2.3), 89.2 (2.2) for Cp’2Th(C6H5)Cl. Data for hydride/alkyl pairs include Cp’2U(R)-OR’ (R’ = Si[C(CH3)3](CH3)2) where D(U-R) = 76.4 (1.1), 82.4 (7) (R = H) and 72.4 (1.4), 76.0 (1.2) (R = CH3) as well as Cp’2Th(R)-OR’ (R’ = CH[C(CH3)3]2) where D(Th-R) = 87.8 (1.5), 93.3 (1.2) (R = H) and 81.5 (3.1), 81.2 (3.1) (R = n-C4H9). For Cp’2Th(H){O-2,6-[O(CH3)3]C6H3}, D(Th-H) = 84.9 (1.2), 92.1 (1.0). For corresponding Th/U pairs, D(Th-R)-D(U-R) ≈ 10 kcal/mol. Alkoxide ancillary ligands appear to strengthen the M-R bond by several kcal/mol and to depress D(M-H)gas-D(M-R)gas to ≲5 kcal/mol. Chloride coligands have a smaller effect on D(M-R). These results show that ancillary ligands should significantly affect the thermodynamics of important M-H/M-C transposition processes such as 0-H elimination.

Original languageEnglish
Pages (from-to)7275-7280
Number of pages6
JournalJournal of the American Chemical Society
Volume108
Issue number23
DOIs
Publication statusPublished - Jan 1 1986

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ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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