Organo-f-Element Thermochemistry. Metal-Ligand Bond Disruption Enthalpies in (Pentamethylcyclopentadienyl)thorium Hydrocarbyls, Metallacycles, Hydrides, and Dialkylamides

Thorium-ligand bond disruption enthalpies (D) have been determined in the series Cp'2ThR2 (Cp’ = η5-(CH₃)5C5) for a range of R groups including R = alkyl, aryl, metallacycle, hydride, and dialkylamide. Enthalpy data were obtained in an anaerobic batch-titration isoperibol reaction calorimeter from heats of solution in toluene followed by heats of alcoholysis with tert-butyl alcohol. Calculated Th-hydrocarbyl bond disruption enthalpies fall in the range 60-90 kcal/mol, with C₆H₅ > CH₃ ≍ CH₂Si(CH₃)3 > CH₂CH₃ ≳ CH₂C₆H₅ ≍ n-C₄H₉ ≍ CH₂C(CH₃)3. Th-C bonds appear to be strengthened by ThOR coligands and possibly weakened by Th-Cl coligands. The strain energies in thoracyclobutanes Cp’ 2Th(CH₂)2C(CH₃)2 and Cp'2Th(CH₂)2Si(CH₃)2 are estimated to be ca. 16 and 8 kcal/mol, respectively. A Th-H bond is ca. 20 kcal/mol stronger than a Th-alkyl bond, while a Th-NR2 bond is ca. 18 kcal/mol stronger. These data afford a considerably expanded understanding of organoactinide reaction patterns involving C-H activation, /3-hydride elimination, and CO insertion as well as why they sometimes differ from those of middle and late first-row transition elements.