Pseudohalogen Chemistry in Ionic Liquids with Non-innocent Cations and Anions

Abstract

Within the second funding period of the SPP 1708 "Material Synthesis near Room Temperature",which started in 2017, we were able to synthesize novel anionic species utilizing Ionic Liquids (ILs) both, as reaction media and reactant. ILs, bearing the decomposable and non-innocent methyl carbonate anion [CO₃ Me]^- , served as starting material and enabled facile access to pseudohalide salts by reaction with Me₃ Si-X (X=CN, N₃ , OCN, SCN). Starting with the synthesized Room temperature Ionic Liquid (RT-IL) [nBu₃ MeN][B(OMe)₃ (CN)], we were able to crystallize the double salt [nBu₃ MeN]₂ [B(OMe)₃ (CN)](CN). Furthermore, we studied the reaction of [WCC]SCN and [WCC]CN (WCC=weakly coordinating cation) with their corresponding protic acids HX (X=SCN, CN), which resulted in formation of [H(NCS)₂ ]^- and the temperature labile solvate anions [CN(HCN)_n ]^- (n=2, 3). In addition, the highly labile anionic HCN solvates were obtained from [PPN]X ([PPN]=μ-nitridobis(triphenylphosphonium), X=N₃ , OCN, SCN and OCP) and HCN. Crystals of [PPN][X(HCN)₃ ] (X=N₃ , OCN) and [PPN][SCN(HCN)₂ ] were obtained when the crystallization was carried out at low temperatures. Interestingly, reaction of [PPN]OCP with HCN was noticed, which led to the formation of [P(CN)₂ ]^- , crystallizing as HCN disolvate [PPN][P(CN⋅HCN)₂ ]. Furthermore, we were able to isolate the novel cyanido(halido) silicate dianions of the type [SiCl_0.78 (CN)_5.22 ]^2- and [SiF(CN)₅ ]^2- and the hexa-substituted [Si(CN)₆ ]^2- by temperature controlled halide/cyanide exchange reactions. By facile neutralization reactions with the non-innocent cation of [Et₃ HN]₂ [Si(CN)₆ ] with MOH (M=Li, K), Li₂ [Si(CN)₆ ] ⋅ 2 H₂ O and K₂ [Si(CN)₆ ] were obtained, which form three dimensional coordination polymers. From salt metathesis processes of M₂ [Si(CN)₆ ] with different imidazolium bromides, we were able to isolate new imidazolium salts and the ionic liquid [BMIm]₂ [Si(CN)₆ ]. When reacting [Mes(nBu)Im]₂ [Si(CN)₆ ] with an excess of the strong Lewis acid B(C₆ F₅ )₃ , the voluminous adduct anion {Si[CN⋅B(C₆ F₅ )₃ ]₆ }^2- was obtained.

Keywords: borates; hydrogen bonding; ionic liquids; pseudohalides; silicates.