In this Account of the small portion of the recent research in ionic liquids (ILs) by the Rogers Group, we fast forward through the first evolution of IL research, where ILs were studied for their unique set of physical properties and the resulting potential for tunable "green solvents", to the second evolution of ILs, where the tunability of the cation and anion independently offers almost unlimited access to targeted combinations of physical and chemical properties. This approach is demonstrated here with the field of energetic ionic liquids (EILs), which utilizes this design flexibility to find safe synthetic routes to ILs with high energy content and targeted physical properties.

Ionic liquids are shown to be good solvents for elemental sulfur, selenium, phosphorus and tellurium, and can be designed to maximise the solubility of these elements. The presence of the [S(3)](*-) radical anion in diluted solutions of sulfur in some ionic liquids has been confirmed, and is the origin of their intense blue colour (cf. lapis lazuli).

A well-known polyoxometalate, [PV₂Mo₁₀O₄₀]⁵⁻, in both acidic (acidic POM, H₅[PV₂Mo₁₀O₄₀]) and ionic liquid-compatible form ([C₂mim]POM, [1-ethyl-3-methylimidazolium]₄H[PV₂Mo₁₀O₄₀]), has been studied as a catalyst for the dissolution and delignification of wood in the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([C(2) mim]OAc). Differences were observed with variables such as the form of POM, POM loading, and reaction conditions. Generally, the addition of POM leads to a faster dissolution, a lower lignin content in the recovered cellulose-rich materials (isolated pulp), and a lower isolated yield of lignin due to its oxidation. Acidic POM decreases the lignin content of the pulp without compromising the yield of the pulp. [C₂mim]POM in the IL facilitates greater delignification (lower lignin content in pulp) than the IL with acidic POM; however, the overall pulp yield is also lower indicating some degradation of the carbohydrates. The POM can be recovered with [C₂mim]OAc after evaporation of the reconstitution solvent (e.g., acetone/water) and can be reused, albeit with some loss of POM and loss of POM activity under the current conditions.

Based on the surprisingly high solubility of AlCl3 in ionic liquids of type [cation][(CF3SO2)2N], a new class of highly acidic, ionic systems has been developed. First tests in toluene carbonylation revealed high potential of these new acidic ionic liquids as catalyst for arene functionaliziation reactions.

A series of mercury(II) ionic liquids, [C(n)mim][HgX(3)], where [C(n)mim] = n-alkyl-3-methylimidazolium with n = 3, 4 and X = Cl, Br, have been synthesized following two different synthetic approaches, and structurally characterized by means of single-crystal X-ray structure analysis ([C(3)mim][HgCl(3)] (1), Cc (No. 9), Z = 4, a = 16.831(4) Å, b = 10.7496(15) Å, c = 7.4661(14) Å, β = 105.97(2)°, V = 1298.7(4) Å(3) at 298 K; [C(4)mim][HgCl(3)] (2), Cc (No. 9), Z = 4, a = 17.3178(28) Å, b = 10.7410(15) Å, c = 7.4706(14) Å, β = 105.590(13)°, V = 1338.5(4) Å(3) at 170 K; [C(3)mim][HgBr(3)] (3), P2(1)/c (No. 14), Z = 4, a = 10.2041(10) Å, b = 10.7332(13) Å, c = 14.5796(16) Å, β = 122.47(2)°, V = 1347.2(3) Å(3) at 170 K; [C(4)mim][HgBr(3)] (4), Cc (No. 9), Z = 4, a = 17.093(3) Å, b = 11.0498(14) Å, c = 7.8656(12) Å, β = 106.953(13)°, V = 1421.1(4) Å(3) at 170 K). Compounds 1, 2, and 4 are isostructural and are characterized by strongly elongated trigonal [HgX(5)] bipyramids, which are connected via common edges in chains. In contrast, 3 contains [Hg(2)Br(6)] units formed by two edge-sharing tetrahedra. With melting points of 69.3 °C (1), 93.9 °C (2), 39.5 °C (3), and 58.3 °C (4), all compounds qualify as ionic liquids. 1, 2, and 4 solidify upon fast cooling as glasses, whereas 3 crystallizes. Cyclic voltammetry shows two separate, quasi-reversible redox processes, which can be associated with the 2Hg(2+)/Hg(2)(2+) and Hg(2)(2+)/2Hg redox couples.