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
μB, 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.