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′ = ή5-Me5C5), the rate law is first-order in organoactinide and first-order in H2, with kH2/kD2= 2.5 (4) and kTUF/ktoluene= 2.9 (4). for a series of complexes, hydrogenolysis rates span a range of ca. 105with Cp′2ThCH2C(CH3)2CH2≈ Cp′2U(CH2-t-Bu)(0-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> Cp22Th(η-Bu)2- Cp′2ThMe2> Cp′2Th(Me)(03SCF3) > Cp′2Th(η-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 H2complex 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry