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.

Microemulsions (MEs) are ideal for obtaining high-quality inorganic nanoparticles. As thermodynamically stable systems with a nanometer-sized droplet phase that serves as a nanoreactor, MEs have obvious advantages for the synthesis of nanoparticles. MEs also have disadvantages, such as their complexity as multicomponent systems, the low amount of obtainable nanoparticles, their limited thermal stability, the fact that hydrolyzable or oxidizable compounds are often excluded from synthesis, the partly elaborate separation of nanoparticles, as well as the removal of surface-adhered surfactants subsequent to synthesis. This Review presents some strategies to further expand the options of ME-based synthesis of inorganic nanoparticles. This comprises the crystallization of nanoparticles in "high-temperature MEs", the synthesis of hollow nanospheres, the use of hydrogen peroxide or liquid ammonia as the polar droplet phase, and the synthesis of base metals and nitrides in MEs.

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The bimetallic metal-organic framework Cu-Ru-BTC with the stoichiometric formula Cu2.75Ru0.25(BTC)2·xH2O, which is isoreticular to HKUST-1, was successfully prepared in a direct synthesis using mild reaction conditions. The partial substitution of Cu(2+) by Ru(3+) centers in the paddlewheel structure and the absence of other Ru-containing phases was proven using X-ray absorption spectroscopy.

The crown-ether coordination compounds ZnX 2 (18-crown-6), EuX 2 (18-crown-6) (X: Cl, Br, I), MnI 2 (18crown-6), Mn 3 Cl 6 (18-crown-6) 2 , Mn 3 I 6 (18-crown-6) 2 , and Mn 2 I 4 (18-crown-6) are obtained by ionic-liquid-based synthesis. Whereas MX 2 (18-crown-6) (M: Zn, Eu) show conventional structural motives, Mn 3 Cl 6 (18-crown-6) 2 , Mn 3 I 6 (18-crown-6) 2 , and Mn 2 I 4 (18-crown-6) exhibit unusual single MnX 4 tetrahedra coordinated to the crown-ether complex. Surprisingly, some compounds show outstanding photoluminescence. Thus, rare Zn 2+ -based luminescence is observed and unexpectedly efficient for ZnI 2 (18-crown-6) with a quantum yield of 54%. Unprecedented quantum yields are also observed for Mn 3 I 6 (18-crown-6) 2 , EuBr 2 (18-crown-6), and EuI 2 (18-crown-6) with values of 98, 72, and 82%, respectively, which can be rationalized based on the specific structural features. Most remarkable, however, is Mn 2 I 4 (18crown-6). Its specific structural features with finite sensitizer−activator couples result in an extremely strong emission with an outstanding quantum yield of 100%. Consistent with its structural features, moreover, anisotropic angle-dependent emission under polarized light and nonlinear optical (NLO) effects occur, including second-harmonic generation (SHG). The title compounds and their optical properties are characterized by single-crystal structure analysis, X-ray powder diffraction, chemical analysis, density functional theory (DFT) calculations, and advanced spectroscopic methods.