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 [Hex(3)NB(12)H(5)Cl(6)](-) [4](-) were chosen as potential and more readily available alternatives to carborate anions such as [CHB11Cl11](-) and [HexCB(11)Cl(11)](-). 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[HexCB(11)Cl(11)](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.