Structural and Crystallographic Information from 61Ni Solid-State NMR Spectroscopy: Diamagnetic Nickel Compounds
journal contributionposted on 03.08.2017, 18:20 by Peter Werhun, David L. Bryce
Despite the significance of nickel compounds, NMR spectroscopy of the active nickel isotope 61Ni remains a largely unexplored field. While nickel(0) compounds have been studied by 61Ni 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 61Ni relativistic computations, the first 61Ni solid-state NMR (SSNMR) spectra of diamagnetic compounds are reported here. Specifically, bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)2], tetrakis(triphenylphosphite)nickel(0) [Ni[P(OPh)3]4], and tetrakis(triphenylphosphine)nickel(0) [Ni(PPh3)4] were studied. 61Ni SSNMR spectra of Ni(cod)2 were used to determine its isotropic chemical shift (δiso = 965 ± 10 ppm), span (Ω = 1700 ± 50 ppm), skew (κ = −0.15 ± 0.05), quadrupolar coupling constant (CQ = 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)2 in C6D6 yielded a narrow 61Ni signal, and the temperature dependence of δiso(61Ni) was assessed (δiso being 936.5 ppm at 295 K). The solution is proposed as a secondary chemical shift reference for 61Ni NMR in lieu of the extremely toxic Ni(CO)4 primary reference. For Ni[P(OPh)3]4, 61Ni SSNMR was used to infer the presence of two distinct crystallographic sites and establish ranges for δiso in the solid state, as well as an upper bound for CQ (3.5 MHz for both sites). For Ni(PPh3)4, line shape fitting provided a δiso value of 515 ± 10 ppm, Ω of 50 ± 50 ppm, κ of 0.5 ± 0.5, CQ of 0.05 ± 0.01 MHz, and η of 0.0 ± 0.2. The study of Ni(PPh3)4, in particular, demonstrates the utility of 61Ni SSNMR given the lack of a previously reported crystal structure and transient nature of Ni(PPh3)4 in solution.