Synthesis and Characterizationof Dimethylbis(2-pyridyl)borateNickel(II) Complexes: Unimolecular Square-Planar to Square-PlanarRotation around Nickel(II)

摘要

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 ΔH^⧧ = 12.2(1) kcal mol^–1 and ΔS^⧧ = 0.8(5) eu were measured via NMR inversion recovery experiments. Complex >4 was also observed to isomerize via a relatively slow ring flip: ΔH^⧧ = 15.0(2) kcal mol^–1; and ΔS^⧧ = −4.2(7) eu. DFT studies support the experimentally measured rotation activation energy (cf. calculated ΔH^⧧ = 11.1 kcal mol^–1) as well as the presence of a high-energy triplet intermediate (ΔH = 8.8 kcal mol^–1).