Angewandte Chemie International Edition

Abstract

The first soluble and stable Mg²⁺ dication stabilized only by weakly coordinating and chemically robust carborate anions [HexCB₁₁Cl₁₁]⁻ is described. Mg[HexCB₁₁Cl₁₁]₂ (1), prepared by reaction of Mg(ⁿBu)₂ with 2 equiv of [Ph₃C][HexCB₁₁Cl₁₁], consists, in the solid state, of a central Mg²⁺ surrounded by two [HexCB₁₁Cl₁₁]⁻ anions. In solution, experimental and classical molecular dynamics simulations (cMD) agree with cation/anion association being retained, reflecting the high electrophilicity of the Mg center. Yet, reflecting only weak anion/cation interactions, species 1 polymerizes 1-hexene and coordinates alkynes. However, 1 displays no reaction with HSiEt₃ at room temperature, consistent with a low hydridicity of the hard (HSAB) Mg²⁺ center. Contrasting with 1 (FIA = 264 kJ mol⁻¹; FIA: Fluoride Ion Affinity), salt Mg[(nBu)₃NB₁₂H₄Cl₇]₂ (2), incorporating the more basic ammoniododecaborate [(nBu)₃NB₁₂H₄Cl₇]⁻ anion, is significantly less Lewis acidic (FIA = 214.7 kJ mol⁻¹) and unreactive with alkenes and alkynes. Salt 1 effectively catalyzes alkene/alkyne hydrosilylation via an initial alkene/alkyne coordination/initiation, as suggested by experimental and computational data. It also efficiently catalyzes (with a catalyst loading down to 0.1 mol%) the hydrosilylation of CO₂ to CH₄ in the presence of HSiEt₃. Salt 1 smoothly promotes the catalytic transfer hydrogenation of 1,1-diphenylethylene, and it is also an active imine hydrogenation catalyst in the presence of H₂.

Graphical Abstract

Stable Mg²⁺ dication stabilized only by weakly coordinating carborates was first prepared and structurally characterized as a soluble salt in organic solvents. Combined experimental and theoretical data provided valuable insights on its behavior/reactivity in solution. All data agree with a strong Lewis acidic Mg²⁺ center, readily coordinating alkenes and alkynes and effectively catalyzing alkene/alkyne hydrosilylation and imine hydrogenation.