Potential applications of ionic liquids depend on the properties of this class of liquid material. To a large extent the structure and properties of these Coulomb systems are determined by the intermolecular interactions among anions and cations. In particular the subtle balance between Coulomb forces, hydrogen bonds and dispersion forces is of great importance for the understanding of ionic liquids. The purpose of the present paper is to answer three questions: Do hydrogen bonds exist in these Coulomb fluids? To what extent do hydrogen bonds contribute to the overall interaction between anions and cations? And finally, are hydrogen bonds important for the physical properties of ionic liquids? All these questions are addressed by using a suitable combination of experimental and theoretical methods including newly synthesized imidazolium-based ionic liquids, far infrared spectroscopy, terahertz spectroscopy, DFT calculations, differential scanning calorimetry (DSC), viscometry and quartz-crystal-microbalance measurements. The key statement is that although ionic liquids consist solely of anions and cations and Coulomb forces are the dominating interaction, local and directional interaction such as hydrogen bonding has significant influence on the structure and properties of ionic liquids. This is demonstrated for the case of melting points, viscosities and enthalpies of vaporization. As a consequence, a variety of important properties can be tuned towards a larger working temperature range, finally expanding the range of potential applications.

This Perspective highlights recent developments in methane conversion into different hydrocarbons and C1-oxygenates. Our analysis identified possible directions for further research to bring the above approaches to a commercial level.

Photocatalytic CO2 reduction to produce valuable chemicals and fuels using solar energy provides an appealing route to alleviate global energy and environmental problems. Searching for photocatalysts with high activity and selectivity for CO2 conversion is the key to achieving this goal. Among the various proposed photocatalysts, metal‐free materials, such as graphene, nitrides, carbides, and conjugated organic polymers, have gained extensive research interest for photocatalytic CO2 reduction, due to their earth abundance, cost‐effectiveness, good electrical conductivity, and environmental friendliness. They exhibit prominent catalytic activity, impressive selectivity, and long durability for the conversion of CO2 to solar fuels. Herein, the recent progress on metal‐free photocatalysis of CO2 reduction is systematically reviewed. Opportunities and challenges on modification of nonmetallic catalysts to enhance CO2 transformation are presented. Theoretical calculations on possible reduction mechanisms and pathways as well as the potential in situ and operando techniques for mechanistic understanding are also summarized and discussed. Based on the aforementioned discussions, suitable future research directions and perspectives for the design and development of potential nonmetallic photocatalysts for efficient CO2 reduction are provided.

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For about ten years, the focus of a considerable number of scientific publications has been on ionic liquids (ILs), which are composed of ions and have melting points below 100 8C. [1] An essential reason for the interest in this substance class are some unusual and often very useful materials characteristics. There are for example large electrochemical windows, hardly measurable vapor pressures at ambient temperature, large fluid ranges, and outstanding solubility characteristics.[2] In contrast to conventional solvents, ILs have the advantage that properties can be varied by a large number of various combinations of possible cations and anions. To minimize Coulomb interactions and to reach low melting points, the usage of bulky 1,3-disubstituted imidazolium, pyridinium, or tertiary ammonium or phosphonium cations is very common. Extensive research activities are presently focused on the question of how specific physical chemical properties of ILs are correlated with electronic and steric effects (compositionstructure-property relations). [3] Metal-ion-containing ILs are a very interesting subclass of Ils: not only because of the above-mentioned properties, but also they may have interesting magnetic or catalytic properties.[4] Investigations on (BMIm) [FeCl 4 ] (BMIm = 1-butyl-3-methylimidazolium) were sensational: it was shown that droplets can be affected by magnets.[5] Therefore, ILs with paramagnetic transition metal cations have been discussed as suitable candidates for magnetic and magnetorheological fluids.[6] Apart from a few examples (mostly based on rareearth-metal ions), [7] investigations of transition-metal-based ILs have been limited to 3d elements (M) in monoanionic halogenido complexes of the type [MX 4 ] À (X = halide, usually X = Cl). [4][5][6] 8] In terms of potential applications, almost all of these substances have undesired properties, such as high viscosity, insolubility in water, or hydrolytic instability.Herein we present a new class of ionic liquids that contain doubly negatively charged tetraisothiocyanatocobaltate(II) anions and that have the appearance of blue ink.[9] Despite the fact that doubly charged anions are present, surprisingly some of these compounds have glass-transition temperatures that lie far below room temperature and have low viscosities. Furthermore, these ILs are distinguished by useful features, such as good stabilities towards water and oxygen and also good solubility in many solvents. The four compounds A x [Co(NCS) 4 ] with A = EMIm (1-ethyl-3-methylimidazolium) and x = 2 (1), A = BMIm (1-butyl-3-methylimidazolium) and x = 2 (2), A = EMDIm (3,3'-(ethane-1,2-diyl)bis(1-methylimidazolium) and x = 1 (3), and A = PPN (bis(triphenylphosphine)iminium) and x = 2 (4) were investigated in more detail. These ILs were characterized by single-crystal Xray diffraction, elemental analyses, IR and UV/Vis spectroscopy, temperature-dependent thermal analyses (differential scanning calorimetry), susceptibility measurements, and also temperature-dependent measurements of vis...