Preparation, structural characterization, and reactions of tantalum-alkyne complexes TaCl₃(R¹C≡CR²)L₂ (L₂ = DME, Bipy, and TMEDA; L = Py)

Tantalum-alkyne complexes with the general formula TaCl₃(R¹C≡CR²)(dme) (1a, R¹ = R² = Et; 1b, R¹ = R² = n-C₅H₁₁; 1c, R¹ = Ph, R² = Me; 1d, R¹ = R² = Ph; DME = 1,2-dimethoxyethane) were synthesized by treatment of the corresponding alkynes with a low-valent tantalum derived by reduction of tantalum pentachloride with zinc powder in a mixed solvent of toluene and DME. The DME ligand can be replaced by 2 equiv of pyridine to afford the corresponding dipyridine complexes TaCl₃(R¹C≡CR²)(py)₂ (2a, R¹ = R² = Et; 2b, R¹ = R² = n-C₅H₁₁; 2c, R¹ = Ph, R² = Me; 2d, R¹ = R² = Ph). Additionally, the reaction of 1a with the bidentate nitrogen ligands bipyridine and N,N,N′,N′-tetramethyethylenediamine (= TMEDA) gave TaCl₃(EtC≡CEt)(bipy) (3) and TaCl₃(EtC≡CEt)(tmedia) (4), respectively. The η²- and 4e-coordination mode of an alkyne with a large contribution of a metalacyclopropene canonical structure was revealed by spectroscopic methods (NMR and IR) and crystallographic analyses of 1a, 1c, 2a, and 4. The reactivies of the tantalum-3-hexyne complexes 1a, 2a, 3, and 4 toward 3-phenylpropanal were investigated. Only the pyridine complex 2a reacted with a stoichiometric amount of the aldehyde to afford the corresponding allylic alcohol in 77% yield upon hydrolysis. The reaction proceeded via an oxatantalacyclopentene species.