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

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    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><sup>⧧</sup> = 12.2(1) kcal mol<sup>–1</sup> and Δ<i>S</i><sup>⧧</sup> = 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><sup>⧧</sup> = 15.0(2) kcal mol<sup>–1</sup>; and Δ<i>S</i><sup>⧧</sup> = −4.2(7) eu. DFT studies support the experimentally measured rotation activation energy (cf. calculated Δ<i>H</i><sup>⧧</sup> = 11.1 kcal mol<sup>–1</sup>) as well as the presence of a high-energy triplet intermediate (Δ<i>H</i> = 8.8 kcal mol<sup>–1</sup>)
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