A kinetic/mechanistic study of actinide hydrocarbyl ligand hydrogenolysis (An-R + H2 → An-H + RH) is reported. For the complex Cp′2Th(CH2-t-Bu)(O-t-Bu) (Cp′ = η5-Me5C5), the rate law is first-order in organoactinide and first-order in H2, with kH2/kD2 = 2.5 (4) and kTHF/ktoluene = 2.9 (4). For a series of complexes, hydrogenolysis rates span a range of ca. 105 with Cp′2ThCH2C(CH3)2CH2 ≈ Cp′2U(CH2-t-Bu)(O-t-Bu) (too rapid to measure accurately) > Cp′2Th(CH2-t-Bu)[OCH(t-Bu)2] = Cp′2Th(CH2-t-Bu)(O-t-Bu) > Cp′2Th(CH2-t-Bu)(Cl) > Me2Si(Me4C5)2Th(n-Bu)2 > Cp′2Th(n-Bu)2 ≈ Cp′2ThMe2 > Cp′2Th(Me)(O3SCF3) > Cp′2Th(n-Bu)[OCH(t-Bu)2] ≈ Cp′2Th(Me)[OSiMe2(t-Bu)] > Cp′2ZrMe2 = Cp′2Th(p-C6H4NMe2)(O-t-Bu) > Cp′2Th(Ph)(O-t-Bu) > Cp′2U(Me)[OCH(t-Bu)2] > Cp′2Th(Me)[OCH(t-Bu)2]. In the majority of cases, the rate law is cleanly first-order in organoactinide over 3 or more half-lives. However, for Cp′2ThMe2 → (Cp′2ThH2)2, an intermediate is observed by NMR that is probably [Cp′2Th(Me)(μ-H)]2. For Cp′2Th(Me)(O3SCF3), a follow-up reaction, which consumes Cp′2Th(H)(O3SCF3), is detected. Variable-temperature kinetic studies yield ΔH≠ = 3.7 (2) kcal/mol and ΔS≠ = -50.8 (7) eu for Cp′2Th(CH2-t-Bu)(O-t-Bu) and ΔH≠ = 9 (2) kcal/mol and ΔS≠ = -45 (5) eu for Cp′2U(Me)[OCH(O-t-Bu)2]. The present results are in accord with a polar "heterolytic" four-center transition state involving significant H-H bond cleavage. There is no chemical shift or spin-lattice relaxation time NMR spectroscopic evidence for an H2 complex in preequilibrium. There are approximate correlations between the hydrogenolysis rate and An-R bond disruption enthalpy, ancillary ligand electron-donor capacity, and An-R migratory CO insertion rate. Possible parallels between the present results and the activities of supported organoactinide catalysts, as well as the mechanism of molecular weight control by hydrogen in Ziegler-Natta catalysis, can be drawn.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1987|
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