Carbon Monoxide Activation by Organoactinides. A Comparative Synthetic, Thermodynamic, Kinetic, and Mechanistic Investigation of Migratory CO Insertion into Actinide-Carbon and Actinide-Hydrogen Bonds To Yield η²-Acyls and η²-Formyls

This paper reports the synthesis, characterization, and carbon monoxide chemistry of a series of sterically hindered thorium alkyls and hydrides of the type Cp′₂Th(R)(X) (Cp′ = η⁵-C₅Me₅) where R = H, D, Me, n-BU, and CH₂-t-Bu and X = OCH-t-Bu₂, OC₆H₃-2,6-t-Bu₂, and O-t-Bu. In addition, improved syntheses of the known complexes [Cp′₂Th(µ-H)(H)]₂, Cp′₂Th(O-t-Bu)(Cl) and Cp′₂Th(CH₂-t-Bu)(Cl) are presented. The alkyl complexes undergo facile, irreversible carbonylation to yield η²-acyls that were characterized by a variety of methods. Infrared and ¹³C NMR spectra of these complexes demonstrate that strong metal-(acyl)oxygen bonding takes place, fostering a pronounced carbene-like character. Thus, these complexes are characterized by low C-O infrared stretching frequencies (v_CO = 1450–1480 cm⁻¹) and low-field ¹³C NMR chemical shifts (δ¹³C 355–370). The hydrides undergo a rapid, reversible, migratory CO insertion to yield formyls that have been characterized spectroscopically at low temperature. Infrared and ¹³C NMR spectra of these species are quite similar to the corresponding acyls, suggesting an analogous η² structure. Variable-temperature equilibrium data show that the insertion of CO into thorium-hydrogen bonds is exothermic by ca. 5 kcal/mol, and this value is compared to that for the analogous alkyls. The equilibrium was also found to exhibit a distinct equilibrium isotope effect upon deuterium substitution, K_H/K_D = 0.31 at −78 °C. The carbonylation of the complex Cp′₂Th(n-Bu)(OCH-t-Bu₂) was found to obey a second-order rate law where rate = kP_co [complex], Likewise, the insertion of CO into the Th-H bond of Cp′₂Th(H)(OCH-t-Bu₂) was found by NMR methods to be first order in metal hydride. Labeling and crossover experiments offer further support for an intramolecular pathway for CO insertion, resulting in the formation of monomeric formyls. The insertion exhibits a primary kinetic isotope effect; at −54 °C, (k_H/k_D)forward = 2.8 (4) (insertion) and (k_H/k_D)_reverse = 4.1 (5) (extrusion). Thus, the insertion is inferred to involve rate-determining migration of the hydride ligand. Approximate rate data for the series of alkyls synthesized show that the rate of hydride migration greatly exceeds that of alkyl migration. For the complexes reported herein, k(H) ≈ 5 × 10³k(CH₂-t-Bu) ≈ 7 × 10^4k(n-Bu) ≈ 10⁸k(Me). The latter three rates partially mirror Th-C bond disruption enthalpy trends. The rate of migratory insertion is impeded when the steric bulk of the alkoxide coligand is increased and accelerated when it is replaced by chloride.