ALKYLIDENE-TRANSFER PROCESSES IN THE REACTIONS OF CP2TA(=CH2)(CH3) WITH SILANES

Cp2Ta(═CH2)(Me) (1; Cp ≡ η5-C5H5) reacts with relatively unhindered silanes such as Me2SiH2 and Me3SiH in tetrahydrofuran (THF) or benzene to produce equal amounts of Cp2Ta(H2C═CH2)(Me) and the bis(silyl) complexes Cp2Ta(SiR3)2(H) (SiR3 = SiMe2H, SiMe3) with concurrent liberation of methane. The reaction mechanism involves rate-limiting disproportionation of 1 to Cp2Ta(H2C═CH2)(Me) and the unsaturated intermediate Cp2Ta(Me), which subsequently reacts with silanes to yield the bis(silyl) complexes. In contrast, 1 reacts with the hindered silane H2Si(t-Bu)2 in THF to yield the silyl-substituted alkylidene hydride complex Cp2Ta(H)(═C(H)(SiH(t-Bu)2)) (2) and varying amounts of the bridging alkylidene complex {CpTa(μ2-η1:η5-C5H4)}2(μ-C(H)(SiH(t-Bu)2)) (3). In this instance, the reaction proceeds by a novel chain mechanism involving alkylidene transfer between tantalum centers in which the key chain-carrying species is the unsaturated intermediate Cp2Ta(Me). Reversible hydrogen transfer from tantalum to the alkylidene carbon in 2 is facile, and the resultant unsaturated alkyl can be readily trapped by donor ligands to produce Cp2Ta(L)(CH2SiH(t-Bu)2) (4a,b; L = CO, PMe3). The reaction of 1 with H2Si(t-Bu)2 in benzene also produces the phenyl methylene complex Cp2Ta(═CH2)(Ph) (5) in addition to 2, as a result of competing benzene C‒H activation during the reaction. Finally, the order of relative rates for ligand to alkylidene migratory insertion in complexes of the type Cp2Ta(═CHR)(X) is observed to be X = H > SiH(t-Bu)2 ≫ Ph > Me. Compounds 3 and 4a have been structurally characterized by single-crystal X-ray diffraction methods (3, triclinic, P1, Z = 2, a = 8.001 (1) Å, b = 10.389 (1) Å, c = 13.747 (2) Å, α = 68.80 (1)°, β = 78.49 (1)°, γ = 76.97 (1)°, V = 1029.1 Å3; 4a, monoclinic, C2/c, Z = 8, a = 30.922 (4) Å, b = 8.094 (2) Å, c = 20.517 (3) Å, β = 96.15 (1)°, V = 5105.2 Å3). © 1990, American Chemical Society. All rights reserved.