Biphenolate Phosphine Complexes of Tantalum

The preparation and structural characterization of tantalum complexes supported by 2,2′-phenylphosphino-bis(4,6-di-tert-butylphenolate) ([OPO]2−) are described. The reaction of Li2[OPO] with TaCl5, regardless of the molar ratio employed, in diethyl ether at −35 °C led to high-yield isolation of yellow crystalline [OPO]2TaCl. Alkylation of [OPO]2TaCl with MeMgBr or EtMgCl in diethyl ether at −35 °C generated the corresponding alkyl complexes [OPO]2TaR (R = Me, Et). Thermolysis of [OPO]2TaEt in benzene led to quantitative formation of [OPO]2TaH, which could also be prepared by treatment of [OPO]2TaCl with LiHBEt3 in diethyl ether at −35 °C. Hydrolysis of [OPO]2TaCl or [OPO]2TaR (R = H, Me, Et) generated [OPO]2TaOH. The reaction of [OPO]2TaOH with Me3SiCl in diethyl ether at room temperature afforded quantitatively [OPO]2TaCl. The solution structures of these complexes were all characterized by multinuclear NMR spectroscopy. The solid-state structures of [OPO]2TaCl, [OPO]2TaH, and [OPO]2TaOH were determined by X-ray crystallography. The spectroscopic and crystallographic data are all indicative of the coordination of both phosphorus donors to tantalum in these 7-coordinate complexes. Interestingly, the structure of [OPO]2TaH is markedly different from those of [OPO]2TaX (X = Cl, OH, Me, Et) on the basis of NMR and X-ray studies. Density functional theory computations reveal that the hydride structure found by X-ray crystallography is lower in energy by about 7 kcal/mol than that analogous to the established X-ray structures of [OPO]2TaCl and [OPO]2TaOH.