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(N i Pr) 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, [BF 4 ] − , hexafluorophosphate, [PF 6 ] − , and bis(trifluoromethylsulfonyl)imide, [NTf 2 ] − ) 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 [BF 4 ] − to ≈10 nm for [NTf 2 ] − (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 Ni 0 , 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][NTf 2 ] is 2.08 A m 2 kg −1 and of a Ni (10±4 nm) sample from [LMIm][NTf 2 ] it is 0.99 A m 2 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 H 2 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 .