Paramagnetic 18-Valence-Electron Alkylcyclopentadienylnickel(II) Bromide Dimers

The paramagnetic behavior of 18-electron cyclopentadienylnickel­(II) complexes of the [CpNi­(μ-Br)]₂ type with two unpaired electrons per metal ion has been investigated by ¹H and ¹³C NMR spectroscopy of [Cp‴Ni­(μ-Br)]₂ (1; Cp‴ = 1,3,4-tBu₃C₅H₂), [³CpNi­(μ-Br)]₂ (2; ³Cp = 1,3,4-iPr₃C₅H₂), and [⁴CpNi­(μ-Br)]₂ (3; ⁴Cp = 2,3,4,5-iPr₄C₅H). The tri-tert-butylcyclopentadienyl derivative 1 crystallizes in the triclinic space group P1̅ and has been investigated by X-ray crystallography. Solid-state magnetic susceptibility measurements of 3 revealed an effective magnetic moment at room temperature of 4.04 μ_B, confirming the presence of two d⁸ nickel­(II) ions. While antiferromagnetic coupling via the bromo bridges is weak (J = −2.4 cm^–1), zero-field splitting is substantial (D = +48.2 cm^–1). NMR spectra of complexes 1–3 show signals with half-widths up to 3600 Hz within a spectral window exceeding 500 ppm (¹H) or 2200 ppm (¹³C). An analysis of the spectra gave insight into the spin delocalization, the equilibrium orientation of the iPr substituents, and the presence of different conformers of compound 3. Paramagnetic behavior has also been observed for the Cp* derivative [Cp*Ni­(μ-Br)]₂ (Cp* = C₅Me₅) by ¹H NMR spectroscopy. The presence of two unpaired electrons is discussed in terms of the weak ligand field originating from the combined interaction of poorly π accepting alkylcyclopentadienyl and π donating bromo ligands with the nickel­(II) center.