Structural and Crystallographic Information from <sup>61</sup>Ni Solid-State NMR Spectroscopy: Diamagnetic Nickel Compounds

2017-08-03T18:20:11Z (GMT) by Peter Werhun David L. Bryce
Despite the significance of nickel compounds, NMR spectroscopy of the active nickel isotope <sup>61</sup>Ni remains a largely unexplored field. While nickel(0) compounds have been studied by <sup>61</sup>Ni NMR in solution, solid-state experiments have been limited to Knight shift studies of nickel metal and nickel intermetallics. In conjunction with an NMR study of their ligands and <sup>61</sup>Ni relativistic computations, the first <sup>61</sup>Ni solid-state NMR (SSNMR) spectra of diamagnetic compounds are reported here. Specifically, bis­(1,5-cyclooctadiene)­nickel(0) [Ni­(cod)<sub>2</sub>], tetrakis­(triphenylphosphite)­nickel(0) [Ni­[P­(OPh)<sub>3</sub>]<sub>4</sub>], and tetrakis­(triphenylphosphine)­nickel(0) [Ni­(PPh<sub>3</sub>)<sub>4</sub>] were studied. <sup>61</sup>Ni SSNMR spectra of Ni­(cod)<sub>2</sub> were used to determine its isotropic chemical shift (δ<sub>iso</sub> = 965 ± 10 ppm), span (Ω = 1700 ± 50 ppm), skew (κ = −0.15 ± 0.05), quadrupolar coupling constant (<i>C</i><sub>Q</sub> = 2.0 ± 0.3 MHz), quadrupolar asymmetry parameter (η = 0.5 ± 0.2), and the relative orientation of the chemical shift and electric field gradient tensors. A solution study of Ni­(cod)<sub>2</sub> in C<sub>6</sub>D<sub>6</sub> yielded a narrow <sup>61</sup>Ni signal, and the temperature dependence of δ<sub>iso</sub>(<sup>61</sup>Ni) was assessed (δ<sub>iso</sub> being 936.5 ppm at 295 K). The solution is proposed as a secondary chemical shift reference for <sup>61</sup>Ni NMR in lieu of the extremely toxic Ni­(CO)<sub>4</sub> primary reference. For Ni­[P­(OPh)<sub>3</sub>]<sub>4</sub>, <sup>61</sup>Ni SSNMR was used to infer the presence of two distinct crystallographic sites and establish ranges for δ<sub>iso</sub> in the solid state, as well as an upper bound for <i>C</i><sub>Q</sub> (3.5 MHz for both sites). For Ni­(PPh<sub>3</sub>)<sub>4</sub>, line shape fitting provided a δ<sub>iso</sub> value of 515 ± 10 ppm, Ω of 50 ± 50 ppm, κ of 0.5 ± 0.5, <i>C</i><sub>Q</sub> of 0.05 ± 0.01 MHz, and η of 0.0 ± 0.2. The study of Ni­(PPh<sub>3</sub>)<sub>4</sub>, in particular, demonstrates the utility of <sup>61</sup>Ni SSNMR given the lack of a previously reported crystal structure and transient nature of Ni­(PPh<sub>3</sub>)<sub>4</sub> in solution.