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*TaCl24-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)24-prone-Ar-DAD) (2ad). Heating dibenzyl complexes 2ad in toluene gave the corresponding benzylidene complexes Cp*Ta(CHPh)(η4-prone-Ar-DAD) (3ad) 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 2ad in toluene-d8, producing the corresponding benzylidene complexes 3ad, followed first-order decay: the magnitude of kobs 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.