Preparation, structural characterization, and reactions of tantalum-alkyne complexes TaCl3(R1CCR2)L2 (L2 = DME, bipy, and TMEDA; L = Py)

Tantalum-alkyne complexes with the general formula TaCl3(R(1)CdropCR(2))(dme) (1a, R-1 = R-2 = Et; 1b, R-1 = R-2 = n-C5H11; 1c, R-1 = Ph, R-2 = Me; 1d, R-1 = R-2 = 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(R(1)CdropCR(2))(py)(2) (2a, R-1 = R-2 = Et; 2b, R-1 = R-2 = n-C5H11; 2c, R-1 = Ph, R-2 = Me; 2d, R-1 = R-2 = Ph). Additionally, the reaction of 1a with the bidentate nitrogen ligands bipyridine and N,N,N',N'-tetramethyethylenediamine (= TMEDA) gave TaCl3(EtCdropCEt)(bipy) (3) and TaCl3(EtCdropCEt)(tmeda) (4), respectively. The eta(2)- 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 la, 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.