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

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posted on 2013-11-11, 00:00 authored by Marion Schär, Dirk Saurenz, Frank Zimmer, Ina Schädlich, Gotthelf Wolmershäuser, Serhiy Demeshko, Franc Meyer, Helmut Sitzmann, Oliver M. Heigl, Frank H. Köhler
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 13 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.