Silke Wolf

Ionic Liquids: New Perspectives for Inorganic Synthesis?

Ionic liquids are credited with a number of unusual properties. These include a low vapor pressure, a wide liquid-phase range, weakly coordinating properties, and a high thermal/chemical stability. These properties are certainly of great interest for inorganic synthesis and the creation of novel inorganic compounds. On the other hand, the synthesis repertoire for preparing inorganic compounds has always been broad, ranging from syntheses in solutions and melts to solid-state reactions, and from crystal growth in the gas phase to high-pressure syntheses. What new aspects can ionic liquids then add to the synthesis of inorganic compounds? This Minireview uses some early examples to show that the use of ionic liquids indeed provides access to unusual inorganic compounds.

Cu2X(OH)3 (X = Cl−, NO3−): synthesis of nanoparticles and its application for room temperature deposition/printing of conductive copper thin-films

Uniform Cu2Cl(OH)3 and Cu2(NO3)(OH)3 nanoparticles are firstly prepared and characterized based on various analytical tools (i.e. DLS, SEM, XRD, FT-IR, UV-Vis, and DTA-TG). Accordingly, Cu2Cl(OH)3 and Cu2(NO3)(OH)3 exhibit mean diameters of 51 nm and 37 nm, respectively. Both nanomaterials show a bright green colour with the maxima of absorption at 505 nm (Cu2Cl(OH)3) and 503 nm (Cu2(NO3)(OH)3). Thermally Cu2Cl(OH)3 and Cu2(NO3)(OH)3 decompose below 600 °C and 240 °C, respectively, to form CuO. In addition, as-prepared Cu2Cl(OH)3 and Cu2(NO3)(OH)3 nanoparticles can be reduced to form copper metal at room temperature. While depositing/printing suspensions of the as-prepared nanoparticles in ethanol on silicon wafers, glass plates or paper, NaBH4-driven reduction leads to highly conductive copper thin-films. Namely these thin-films exhibit sheet resistances of 3–10 Ω□ and specific resistances of 1.3–4.3 × 10−3 Ω cm, which matches with the resistivity of bulk copper metal (1.7 × 10−3 Ω cm).

[(Pb6I8){Mn(CO)5}6](2-): an octahedral (M6X8)-like cluster with inverted bonding.

[BMIm]2[{PbMn(CO)5)}6I8] (BMIm: 1-butyl-3-methylimidazolium) is obtained by ionic liquid mediated reaction of PbI2 and Mn2(CO)10. Central is a cubelike (Pb6I8) unit containing a nonfilled Pb6 octahedron. Each Pb of this (Pb6I8) unit is terminated on its outside by Mn(CO)5, exhibiting Pb-Mn metal-to-metal bonding (280 pm). Structurally, the (Pb6I8) unit is similar to the well-known octahedral (M6Xn) cluster-type family (M = Zr, Nb, Ta, Mo, W; X = Cl, Br, I). In contrast to most similar cluster compounds, such as W6Br12 ([W6Br8]Br2/1Br4/2, according to Niggli notation) or the carbonyl cluster [Sn6{Cr(CO5)6}](2-), however, the nonfilled central Pb6 octahedron in [{PbMn(CO)5)}6I8](2-) does not exhibit any metal-to-metal bonding. Structure and bonding of the title compound are validated by single-crystal structure analysis, energy-dispersive X-ray analysis (EDX), infrared spectroscopy (FT-IR), and density functional theory (DFT) calculations. Based on the isolobal principle, electronegativity considerations, bond lengths, and DFT calculations including Mulliken population analysis and natural population analysis (NPA), in sum, the charge distribution of Pb is best reflected by an oxidation state of +1.

2∞[Co{1,4-C6H4(CN)2}2{NTf2}2][SnI{Co(CO)4}3]2 – a 2D coordination network with an intercalated carbonyl cluster

Dark red transparent crystals of [Co{1,4-C(6)H(4)(CN)(2)}(2){NTf(2)}(2)][SnI{Co(CO)(4)}(3)](2) are obtained by reacting SnI(4), Co(2)(CO)(8) and 1,4-C(6)H(4)(CN)(2) in the ionic liquid [EMIm][NTf(2)] (EMIm: 1-ethyl-3-methylimidazolium; NTf(2): bis(trifluoromethylsulfonyl)imide). According to X-ray structure analysis based on single crystals, the title compound crystallizes in a triclinic manner and contains the novel (2)(∞)[Co{1,4-C(6)H(4)(CN)(2)}(2){NTf(2)}(2)] coordination network. This infinite 2D network is composed of Co(2+) ions that are planarily interlinked by four 1,4-dicyanobenzene ligands. As a non-charged 2D network, Co(2+) is furthermore coordinated by two [NTf(2)](-) anions. The (2)(∞)[Co{1,4-C(6)H(4)(CN)(2)}(2){NTf(2)}(2)] layers are stacked on top of each other with SnI[Co(CO)(4)](3) molecules intercalated in distorted cubic gaps between the layers. The title compound is furthermore characterized by energy dispersive X-ray (EDX) analysis, thermogravimetry (TG), infrared spectroscopy (FT-IR) and optical spectroscopy (UV-Vis).

[{Fe(CO)3}4{SnI}6I4]2−: The First Bimetallic Adamantane‐Like Cluster

Show some metal: the first bimetallic adamantane-like cluster, [{Fe(CO)(3)}(4){SnI}(6)I(4)](2-), was prepared by an ionic-liquid-based synthesis. The valence states of iron and tin were verified based on bond-length considerations, FT-IR and (119)Sn Mössbauer spectroscopy, as well as with DFT calculations.

Ionic-liquid-assisted synthesis of the phosphorus interhalides [PBr4][IBr2] and [PBr4][I5Br7].

The phosphorus interhalides [PBr4][IBr2] (1) and [PBr4]2[I5Br7] (2) were prepared by reaction of PBr5 and the interhalogen IBr in the ionic liquid [MeBu3N][N(Tf)2] (N(Tf)2: bis(trifluoromethylsulfonyl)amide). [PBr4][IBr2] (1) consists of tetrahedral [PBr4]+ cations and linear [IBr2]- anions. [PBr4]2[I5Br7] (2) also contains [PBr4]+ cations as well as the iodine bromide anion [I5Br7]2-. The latter represents the yet largest known polyiodinebromide. Moreover, (2) shows remarkable halogen release (IBr and Br2) of 96.8 wt% below 300 °C. For the ternary system P-Br-I, (1) and (2) are the first compounds that were characterized by crystal structure analysis. Composition, bonding situation and properties were further validated by energy dispersive X-ray (EDX) analysis, thermogravimetry (TG) and Raman spectroscopy.

[BMIm][Fe(OTf)3], [BMIm][Mn(OTf)3], [BMIm][Li(OTf)2] – Three One-dimensional Infinite Coordination Polymers

By heating of FeCl2 and MnCl2 in the ionic liquid [BMIm][OTf] (BMIm: 1-butyl-3- methylimidazolium, OTf: trifluoromethanesulfonate), the compounds [BMIm][M(OTf)3] (M: Fe+II, Mn+II) have been obtained as colorless crystals. Similarly, [BMIm][Li(OTf)2] was synthesized by heating of LiCl in [BMIm][OTf]. While the crystal quality of the [BMIm][M(OTf)3] (M: Fe+II, Mn+II) products thus obtained is low, mild oxidation of Fe(CO)5 or Mn2(CO)10 with GeI4 applied as an alternative in the same ionic liquid allowed a slow growth of well-formed, needle-shaped crystals. According to X-ray structure analysis based on single crystals, [BMIm][M(OTf)3] (M: Fe+II, Mn+II) crystals are monoclinic, and [BMIm][Li(OTf)2] crystals are triclinic. All compounds form infinite 1∞[M(OTf)3] (M=Fe, Mn) and 1∞[Li(OTf)2] chains. The compounds have further been characterized by FT-IR spectroscopy, energy-dispersive X-ray analysis (EDX), differential thermal analysis (DTA), thermogravimetry (TG), and magnetic measurements. Graphical Abstract [BMIm][Fe(OTf)3], [BMIm][Mn(OTf)3], [BMIm][Li(OTf)2] – Three One-dimensional Infinite Coordination Polymers