Manipulation of organoactinide coordinative unsaturation. Synthesis, structures, and reactivity of thorium hydrocarbyls and hydrides with chelating bis(tetramethylcyclopentadienyl) ancillary ligands

Article abstract of DOI:10.1021/om00098a021

This contribution describes thorium hydrocarbyl and hydride chemistry based upon the chelating (CH₃)₂Si[(CH₃)₄C₅] ₂^2- ligand (Me₂SiCp″₂^2-). Precursor Me₂Si(Cp″H)₂ can be prepared by reaction of SiCl₄ with 2 equiv of Li(CH₃)₄C₅, followed by methylation (CH₃Li) and methanolic workup. Subsequent reaction with n-C₄H₉Li in 1,2-dimethoxyethane yields Me₂Si(Cp″Li)₂·2DME. The dilithium salt undergoes reaction with ThCl₄ to yield Me₂SiCp″₂ThCl₂·2LiCl·2pME, which, in turn, can be alkylated with lithium reagents to produce crystalline, thermally stable Me₂SiCp″₂ThR₂ complexes where R = CH₂Si(CH₃)₃, CH₂C(CH₃)₃, C₆H₅, n-C₄H₉, and CH₂C₆H₅. The Me₂SiCp″₂Th[CH₂Si(CH₃) ₃]₂ complex crystallizes in the monoclinic space group P2₁/m - C_2h² with two molecules in a unit cell of dimensions (20 ± 1°C) a = 11.960 (5) A?, b = 11.270 (5) A?, c = 12.395 (6) A?, and β = 99.32 (4)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.076 for 2586 independent reflections having I > 3σ(I). The molecular structure consists of monomeric Me₂SiCp′₂Th[CH₂Si(CH₃) ₃]₂ units with η⁵-Cp″ coordination and ∠ (ring centroid)-Th-(ring centroid) = 118.4°. There is considerable dispersion in the Th-C(ring) distances. The Th[CH₂Si(CH₃)₃]₂ ligation is highly distorted, with Th-C-Si angles of 123.7 (14)° and 149.5 (12)°, accompanied by corresponding Th-C distances of 2.54 (2) and 2.48 (2) A?, respectively. The thorium coordination sphere is more open than that in Cp′₂Th[CH₂Si(CH₃)₃] ₂. Hydrogenolysis of the dialkyl yields the dimeric hydride Me₂SiCp″Th(μ-H)₄ThCp″₂SiMe ₂, which crystallizes in the monoclinic space group P2₁/n with four molecules in a unit cell of dimensions (20 ± 1°C) a = 10.965 (2) A?, b = 19.843 (5) A?, c = 18.759 (3) A?, and β = 90.39 (2)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.075 for 3298 independent reflections having I > 3σ(I). The dimer exhibits approximate C₂ symmetry with (ring centroid)-Th-(ring centroid) angles of 118.1° and 117.7°. The Th-Th distance is 3.632 (2) A? which suggests, in combination with infrared spectral data, a Th(μ-H)₄Th structure. The above hydride catalyzes the hydrogenation of 1-hexene at a rate ca. 10³ faster than (Cp′₂ThH₂)₂. For trans-2-hexene, hydrogenation catalyzed by the ring-bridged hydride is ca. 30 times more rapid than by (Cp₂′ThH₂)₂.