fac versus mer Coordination for a Tridentate Diethylene Glyclolate Ligand in Tantalum Complexes: A Combined Experimental and Theoretical Study

A series of new tantalum-digol (digol = diethylene glycolate) complexes TaCp*Cl₂(3-digol) (1), TaCp*Me₂(3-digol) (2), TaCp*ClMe(3-digol) (3), and TaCp*Me(OTf)(3-digol) (OTf = triflate) (4) have been synthesized. Treatment of 2 with 1 equiv of H₂O·B(C₆F₅)₃ enabled the synthesis of a tantalum-oxo borane-stabilized complex, TaCp*{O·B(C₆F₅)₃}(3-digol) (5). Insertion processes into the methyl groups of complexes 2, 3, and 4 were not observed upon reaction with CO or isocyanides. In contrast, complex 4 reacted slowly with 1 equiv of xylylisocyanide (xylyl = 2,6-dimethylphenyl) to yield the dicationic aminocarbene complex [TaCp*{C(Me)NH(xylyl)}(OH₂)(3-digol)][OTf]₂ (6). All products were characterized by NMR spectroscopy and elemental analysis. The single-crystal structures of 1, 5, and 6 were determined. The molecular structures of 1, 2, 3, and 6 were also determined by means of DFT-based methods.