Biphenolate phosphine complexes of tantalum

The preparation and structural characterization of tantalum complexes supported by 2,2′-phenylphosphino-bis(4,6-di-terf-butylphenolate) ([OPO]^2-) are described. The reaction of Li₂[OPO] with TaCI₂, regardless of the molar ratio employed, in diethyl ether at -35 °C led to high-yield isolation of yellow crystalline [OPO] ₂TaCI. Alkylation of [OPO]₂TaCI with MeMgBr or EtMgCI 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]₂TaCI with LiHBEt₃ in diethyl ether at -35 °C. Hydrolysis of [OPO]₂TaCI or [OPO] ₂TaR (R = H, Me, Et) generated [OPO]₂TaOH. The reaction of [OPO]₂TaQH with Me₃SiCI in diethyl ether at room temperature afforded quantitatively [OPO]₂TaCI. The solution structures of these complexes were all characterized by multinuclear NMR spectroscopy. The solid-state structures of [OPO]₂TaCI, [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] ₂TaCI and [OPO]₂TaOH.