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

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 (δ_iso = 965 ± 10 ppm), span (Ω = 1700 ± 50 ppm), skew (κ = −0.15 ± 0.05), quadrupolar coupling constant (C_Q = 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 δ_iso(⁶¹Ni) was assessed (δ_iso 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 δ_iso in the solid state, as well as an upper bound for C_Q (3.5 MHz for both sites). For Ni­(PPh₃)₄, line shape fitting provided a δ_iso value of 515 ± 10 ppm, Ω of 50 ± 50 ppm, κ of 0.5 ± 0.5, C_Q 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.