Tantalum−Benzylidene Complexes Supported by C₅Me₅ and Diazadiene Ligands: Synthesis, Kinetic Analysis of the Formation, and Reactive Studies

Reaction of half-metallocene dichloro complexes Cp*TaCl₂(η⁴-supine-Ar-DAD) (1a: Ar = p-MeO; 1b: Ar = p-Me; 1c: Ar = p-F; 1d: Ar = p-Cl) (Cp* = pentamethylcyclopentadienyl; Ar-DAD = 1,4-di(aryl)-1,4-diaza-1,3-butadiene) with 1 equiv of Mg(CH₂Ph)₂ afforded a series of dibenzyl complexes Cp*Ta(CH₂Ph)₂(η⁴-prone-Ar-DAD) (2a−d). Heating dibenzyl complexes 2a−d in toluene gave the corresponding benzylidene complexes Cp*Ta(CHPh)(η⁴-prone-Ar-DAD) (3a−d) via α-hydrogen elimination followed by the release of toluene. The crystal structure of Cp*Ta(CHPh)(η⁴-prone-p-MeOC₆H₄-DAD) (3a), determined by X-ray analysis, had a three-legged piano-stool geometry, and the TaCHPh moiety had a syn-configuration. In solution, complex 3a showed fluxional behavior through conversion between the syn- and anti-rotamers. Thermolysis of complexes 2a−d in toluene-d₈, producing the corresponding benzylidene complexes 3a−d, followed first-order decay: the magnitude of k_obs was in the order Me (2b) < OMe (2a) < F (2c) < Cl (2d) for the p-substituted aryl groups of p-XC₆H₄-DAD ligands. Neopentyl alcohol protonated 3a to give Cp*Ta(OCH₂CMe₃)(CH₂Ph)(η²-N,N′-prone-p-MeOC₆H₄-DAD) (4), and reaction of 3a with pyrrole afforded Cp*Ta(η¹-NC₄H₄)(CH₂Ph)(η⁴-supine-p-MeOC₆H₄-DAD) (5). Coupling reaction of 3a with carbon monoxide produced an η²-C,O-ketene complex 6. Treatment of 3a with ^tBuCN and carbodiimides such as ⁱPrNCNⁱPr and CyNCNCy yielded the corresponding imido complex Cp*Ta[NC(^tBu)CHPh](η²-N,N′-prone-p-MeOC₆H₄-DAD) (8) and azametallacyclobutanes Cp*Ta[NRC(NR)CHPh](η²-N,N′-supine-p-MeOC₆H₄-DAD) (11a: R = ⁱPr; 11b: R = Cy). The reaction of 3a with acetophenone in toluene afforded the oxo-bridged dimer 12 along with the 1-methyl-1,2-diphenylethene, whereas complex 3a reacted with methyl benzoate to give Cp*Ta(OMe)[OC(Ph)CHPh](η²-N,N′-prone-p-MeOC₆H₄-DAD) (14) via cleavage of the carbon−oxygen bond.