Tricyanomethane or Dicyanoketenimine—Silylation makes the Difference

Pseudohalides such as tricyanomethanide, [C(CN)₃]^-, are well known in chemistry, biochemistry and industrial chemistry. The protonated species HC(CN)₃, a classic hydrogen pseudohalide Brønsted acid, is a very strong acid with a pK_a value of -5. However, HC(CN)₃ is difficult to handle as it tends to decompose rapidly or, more precisely, to oligo- and polymerize. Therefore, silylated pseudohalide compounds with the [Me₃Si]⁺ as the "big organometallic proton" have become interesting, exhibiting similar chemical properties but better kinetic protection. Here, the stepwise silylation of the pseudohalide anion [C(CN)₃]^- is reported, forming the heavier homologue of HC(CN)₃, namely [Me₃Si][C(CN)₃], and in presence of two additional [Me₃Si]⁺ cations even the dicationic species [(Me₃Si-NC)₃C]²⁺ as stable [B(C₆F₅)₄]^- salt. Surprisingly, in contrast to the protonated species HC(CN)₃, in which the proton is bound to the central carbon atom of [C(CN)₃]^-, silylation of the [C(CN)₃]^- anion occurs at one of the three terminal nitrogen atoms, thus forming the long-sought dicyanoketenimine [Me₃Si-NC-C(CN)₂]. All further silylation steps take place exclusively on the terminal N atoms of the three CN groups and not on the central carbon atom, until the intriguing, highly symmetrical dication, [(Me₃Si-NC)₃C]²⁺, is finally generated. The experimental data are supported by quantum chemical calculations in terms of thermodynamics and chemical bonding.