Immobilization of Carbonylnickel Complexes: A Solid-State NMR Study

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.

Hybrid aniline/silica xerogel cation adsorption and thermodynamics of interaction

Aniline groups chemically immobilized on silica through the sol-gel process were employed to extract divalent nickel and manganese from aqueous solutions at room temperature. The maximum adsorption capacity of the xerogel was studied from adsorption isotherms using a batch technique. The isotherms obtained were adjusted following the Langmuir equation. The xerogel adsorbent appears to have better affinity for nickel than manganese. From calorimetric titration, thermodynamic data on cation/nitrogen basic atom interaction in the solid/liquid interface were determined. The enthalpic values, -0.46 +/- 0.02 and -.029 +/- 0.02 kJ mol(-1) for nickel and manganese, respectively, are in agreement with the low availability of the basic nitrogen atom on the aniline group and also the possible steric hindrance of the phenyl group bonded to nitrogen. However, thermodynamics indicated the existence of favorable conditions for such cation-nitrogen interactions.