Metal, Bond Energy, and Ancillary Ligand Effects on Actinide-Carbon α-Bond Hydrogenolysis. A Kinetic and Mechanistic Study

Article abstract of DOI:10.1021/ja00260a007

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 k_H2/k_D2 = 2.5 (4) and k_THF/k_toluene = 2.9 (4).For a series of complexes, hydrogenolysis rates span a range of ca. 1E5 with <*> ca.Cp'2U(CH2-t-Bu)(O-t-Bu) (too rapid to measure accurately) > Cp'2Th(CH2-t-Bu) = 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 ca.Cp'2ThMe2 > Cp'2Th(Me)(O3SCF3) > Cp'2Th(n-Bu) ca.Cp'2Th(Me) > Cp'2ZrMe2 = Cp'2Th(p-C6H4NMe2)(O-t-Bu) > Cp'2Th(Ph)(O-t-Bu) > Cp'2U(Me) > Cp'2Th(Me).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 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).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 drwan.