The synthesis and properties of the actinide complexes (2), (3), and (4) are described. They can be readily prepared from Cp’2MCl2 (M = U, Th) and the appropriate (THF)2Mg(CH2CRCRCH2) reagent. Complex 2 but not 3 can also be prepared from the methane-eliminating reaction of Cp’2U(Me)Cl and 3-butenyl Grignard. The molecular structure of 3 has been determined by single-crystal X-ray diffraction. crystallizes in the monoclinic space group (no. 14) with four molecules in a unit cell of dimensions a = 9.236 (3) A, b = 14.720 (4) A, c = 17.437 (4) A, and = 105.48 (2)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.049 for 2450 reflections having < 50.7° and I > The molecular structure consists of an unexceptional “bent sandwich” Cp’2Th fragment coordinated to an ligand. The average Th-C distance to the terminal carbon atoms of the butadiene ligand, 2.57 (3, 3, 3, 2) is only slightly less than that to the internal carbon atoms, 2.74 (3, 1, 1, 2) A. The actinide butadiene complexes undergo inversion of the metallacyclopentene ring which is rapid on the NMR time scale at higher temperatures; AG*, kcal mol-1 (Tc, K) = 17.0 ± 0.3 (394, 2), 15.0 ± 0.3 (299, 3), and 10.5 ± 0.3 (208, 4). Thermochemical studies of the thorium-butadiene bond disruption enthalpy in 3 and 4 using anaerobic batch-titration (f-BuOH) calorimetry indicate that the thorium-butadiene interaction enjoys no special stabilization. D(Th-butadiene) is comparable to that for relatively “weak” thorium-to-carbon a bonds. No evidence for s-trans-butadiene coordination is found in any of the actinide butadiene complexes. While the actinide-butadiene linkage undergoes facile hydrogenolysis and protonolysis, the activation of C-H bonds on exogenous hydrocarbon molecules is not observed. The bond disruption enthalpy data indicate that such process are thermodynamically unfavorable.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry