Synthesis and Characterization of Dimethylbis(2-pyridyl)borate Nickel(II) Complexes: Unimolecular Square-Planar to Square-Planar Rotation around Nickel(II)

The syntheses of novel dimethylbis­(2-pyridyl)­borate nickel­(II) complexes <b>4</b> and <b>6</b> are reported. These complexes were unambiguously characterized by X-ray analysis. In dichloromethane solvent, complex <b>4</b> undergoes a unique square-planar to square-planar rotation around the nickel­(II) center, for which activation parameters of Δ<i>H</i>^⧧ = 12.2(1) kcal mol^–1 and Δ<i>S</i>^⧧ = 0.8(5) eu were measured via NMR inversion recovery experiments. Complex <b>4</b> was also observed to isomerize via a relatively slow ring flip: Δ<i>H</i>^⧧ = 15.0(2) kcal mol^–1; and Δ<i>S</i>^⧧ = −4.2(7) eu. DFT studies support the experimentally measured rotation activation energy (cf. calculated Δ<i>H</i>^⧧ = 11.1 kcal mol^–1) as well as the presence of a high-energy triplet intermediate (Δ<i>H</i> = 8.8 kcal mol^–1)