Actinide diene (2-Butene-1,4-diyl) complexes. Synthesis including a methyl-induced β-hydrogen elimination reaction, structures, structural dynamics, thermochemistry, and reactivity

The synthesis and properties of the actinide s-cis-diene/cis-2-butene-1,4-diyl complexes Cp′₂U(η⁴-C₄H₆) (2), Cp′₂Th(η⁴-C₄H₆) (3), and Cp′₂Th(η⁴-CH₂CMeCMeCH₂) (4) (Cp′ = η⁵-CH₃)₅C₅) 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. Cp′₂Th(η⁴-C₄H₆) crystallizes in the monoclinic space group P2₁/c - C_2h⁵ (no. 14) with four molecules in a unit cell of dimensions a = 9.236 (3) Å, b = 14.720 (4) Å, c = 17.437 (4) Å, and β = 105.48 (2)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.049 for 2450 reflections having 2θ_MoKᾱ <50.7° and I > 3σ(I). The molecular structure consists of an unexceptional "bent sandwich" Cp′₂Th fragment coordinated to an s-cis-η 4-butadiene ligand. The average Th-C distance to the terminal carbon atoms of the butadiene ligand, 2.57 (3, 3, 3, 2) Å, is only slightly less than that to the internal carbon atoms, 2.74 (3, 1, 1, 2) Å. The actinide butadiene complexes undergo inversion of the metallacyclopentene ring which is rapid on the NMR time scale at higher temperatures; ΔG^≠, kcal mol^-1 (T_c, 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 (t-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 σ 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.