Soft, Wet-Chemical Synthesis of Metastable Superparamagnetic Hexagonal Close-Packed Nickel Nanoparticles in Different Ionic Liquids

The microwave-induced decomposition of bis{N,N′-diisopropylacetamidinate}nickel(II) [Ni{MeC(NiPr)₂}₂] or bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)₂] in imidazolium-, pyridinium-, or thiophenium-based ionic liquids (ILs) with different anions (tetrafluoroborate, [BF₄]⁻, hexafluorophosphate, [PF₆]⁻, and bis(trifluoromethylsulfonyl)imide, [NTf₂]⁻) yields small, uniform nickel nanoparticles (Ni NPs), which are stable in the absence of capping ligands (surfactants) for more than eight weeks. The soft, wet-chemical synthesis yields the metastable Ni hexagonal close-packed (hcp) and not the stable Ni face-centered cubic (fcc) phase. The size of the nickel nanoparticles increases with the molecular volume of the used anions from about 5 nm for [BF₄]⁻ to ≈10 nm for [NTf₂]⁻ (with 1-alkyl-3-methyl-imidazolium cations). The n-butyl-pyridinium, [BPy]⁺, cation ILs reproducibly yield very small nickel nanoparticles of 2(±1) nm average diameter. The Ni NPs were characterized by high-resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction. An X-ray photoelectron spectroscopic (XPS) analysis shows an increase of the binding energy (E_B) of the electron from the Ni 2p_3/2 orbital of the very small 2(±1) nm diameter Ni particles by about 0.3 eV to E_B=853.2 eV compared with bulk Ni⁰, which is traced to the small cluster size. The Ni nanoparticles show superparamagnetic behavior from 150 K up to room temperature. The saturation magnetization of a Ni (2±1 nm) sample from [BPy][NTf₂] is 2.08 A m² kg⁻¹ and of a Ni (10±4 nm) sample from [LMIm][NTf₂] it is 0.99 A m² kg⁻¹, ([LMIm]=1-lauryl-3-methyl- imidazolium). The Ni NPs were active catalysts in IL dispersions for 1-hexene or benzene hydrogenation. Over 90 % conversion was reached under 5 bar H₂ in 1 h at 100 °C for 1-hexene and a turnover frequency (TOF) up to 1330 mol_hexane (mol_Ni)⁻¹ h⁻¹ or in 60 h at 100 °C for benzene hydrogenation and TOF=23 mol_cyclohexane (mol_Ni)⁻¹ h⁻¹.