Zinc Ammonio-dodecaborates: Synthesis, Lewis Acid Strength, and Reactivity

Two series of zinc salts, [EtZn]­[A] and Zn­[A]2, with weakly coordinating anions [A]− as counterions have been prepared, and their activities as catalysts for hydrosilylation reactions of 1-hexene, benzophenone, and acetophenone have been investigated. The counterions and per- and partially chlorinated 1-ammonio-closo-dodecaborate anions [Me3NB12Cl11]− [1]−, [Pr3NB12H5Cl6]− [2]−, [Bu3NB12H4Cl7]− [3]−, and [Hex3NB12H5Cl6]− [4]− were chosen as potential and more readily available alternatives to carborate anions such as [CHB11Cl11]− and [HexCB11Cl11]−. The basicity of anion [4]− was determined as being close to that of the triflimide anion [N­(SO2CF3)2]−, and the fluoride ion affinities (FIAs) of compounds [EtZn]­[2] and Zn­[2]2 are lower than those of the Lewis acids B­(C6F5)3 and Zn­[HexCB11Cl11]2. The higher anion basicity and the resulting lower Lewis acidity of the zinc centers result in low activity in 1-hexene hydrosilylation catalysis and only moderate activity in the hydrosilylation catalysis of benzophenone and acetophenone.