Mechanistic study of photoinduced β-hydride elimination. The facile photochemical synthesis of low-valent thorium and uranium organometallics

Joseph W. Bruno, Dale G. Kalina, Eric A. Mintz, Tobin J Marks

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Abstract

This paper reports a mechanistic investigation of the photochemistry of the actinide hydrocarbyls Th(C5E5)3R, E = H, D, R = CH3, i-C3H7, n-C4H9; Th(CH3C5H4)3(n-C4H 9); Th(indenyl)3(n-C4H9); U(C5E5)3R, E = H, R = CH3, i-C3H7, n-C4H9, sec-C4H9, E = D, R = n-C4H9. For the thorium cyclopentadienyl compounds with R ≠ CH3, UV photolysis in aromatic solvents produces 1:1 mixtures of alkane:alkene.(RH:R - (H)) and the trivalent thorium complex Th(C5E5)3 in nearly stoichiometric yield. On the basis of products, product yields, quantum yields, studies with 2H labels, matrix photochemistry, added reagents, and the behavior of the R = CH3 compound, a photoinduced β-hydrogen elimination mechanism is proposed. This reaction yields (irreversibly) olefin and Th(C5E5)3H, the latter species then reacting with another molecule of Th(C5E5)3R to produce alkane and Th(C5E5)3. A minor pathway for RH formation involves abstraction of C5E5 or solvent hydrogen (deuterium) atoms. On the basis of spectroscopic, magnetic, and chemical studies, the thorium photoproduct is formulated as a Th(η5-C5E5)3 compound, possibly with metal-bridging cyclopentadienyl ligands in the solid state. In contrast to these results, photolysis of the indenyl compound produces only alkane, the added hydrogen atom being derived from the indenyl ligand. For the uranium compounds in aromatic solvents, photoinduced β-hydrogen elimination (and U(C5H5)3 formation) occurs but is not the major pathway. Rather, hydrogen atom abstraction from cyclopentadienyl rings (analogous to the thermolysis pathway) predominates, with lesser abstraction from the solvent molecules. Photolysis in THF greatly increases the yield of U(C5H5)3.

Original languageEnglish
Pages (from-to)1860-1869
Number of pages10
JournalJournal of the American Chemical Society
Volume104
Issue number7
Publication statusPublished - 1982

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ASJC Scopus subject areas

  • Chemistry(all)

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