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