title: Soft, Wet-Chemical Synthesis of Metastable Superparamagnetic Hexagonal Close-Packed Nickel Nanoparticles in Different Ionic Liquids.
abstract: 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 (EB) of the electron from the Ni 2p3/2orbital of the very small 2(±1) nm diameter Ni particles by about 0.3 eV to EB=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 m2kg-1and of a Ni (10±4 nm) sample from [LMIm][NTf2] it is 0.99 A m2kg-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 H2in 1 h at 100 °C for 1-hexene and a turnover frequency (TOF) up to 1330 molhexane(molNi)-1h-1or in 60 h at 100 °C for benzene hydrogenation and TOF=23 molcyclohexane(molNi)-1h-1.
authors.affiliation: Institut für Anorganische Chemie und Strukturchemie, Heine-Universität Düsseldorf, 40204, Düsseldorf, Germany.
authors.forename: Susann
authors.initials: S
authors.lastname: Wegner
authors.affiliation: Institut für Anorganische Chemie und Strukturchemie, Heine-Universität Düsseldorf, 40204, Düsseldorf, Germany.
authors.forename: Christina
authors.initials: C
authors.lastname: Rutz
authors.affiliation: Institut für Anorganische Chemie und Strukturchemie, Heine-Universität Düsseldorf, 40204, Düsseldorf, Germany.
authors.forename: Kai
authors.initials: K
authors.lastname: Schütte
authors.affiliation: Gemeinschaftslabor für Elektronenmikroskopie RWTH-Aachen, Ernst Ruska-Centrum für Mikroskopie und Spektroskopie mit Elektronen, 52425, Jülich, Germany.
authors.forename: Juri
authors.initials: J
authors.lastname: Barthel
authors.affiliation: Physical Chemistry Department, University of Cologne, Luxemburger Str. 116, 50939, Cologne, Germany.
authors.forename: Alexey
authors.initials: A
authors.lastname: Bushmelev
authors.affiliation: Physical Chemistry Department, University of Cologne, Luxemburger Str. 116, 50939, Cologne, Germany.
authors.forename: Annette
authors.initials: A
authors.lastname: Schmidt
authors.affiliation: Lehrstuhl für Anorganische und Metallorganische Chemie, TU München, Lichtenbergstr. 4, 85748, Garching, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Dilchert
authors.affiliation: Lehrstuhl für Anorganische und Metallorganische Chemie, TU München, Lichtenbergstr. 4, 85748, Garching, Germany.
authors.forename: Roland A
authors.initials: RA
authors.lastname: Fischer
authors.affiliation: Institut für Anorganische Chemie und Strukturchemie, Heine-Universität Düsseldorf, 40204, Düsseldorf, Germany.
authors.forename: Christoph
authors.identifier: http://orcid.org/0000-0002-6288-9605
authors.initials: C
authors.lastname: Janiak
keywords: hexagonal close-packing; ionic liquids; nickel amidinate; nickel nanoparticles; soft wet-chemical synthesis