Formation, Structure and Reactivity of a Beryllium(0) Complex with Mg^δ+-Be^δ- Bond Polarization

Attempts to create a novel Mg-Be bond by reaction of [(^DIPePBDI*)MgNa]₂ with Be[N(SiMe₃)₂]₂ failed; ^DIPePBDI*=HC[(tBu)C=N(DIPeP)]₂, DIPeP=2,6-Et₂C-phenyl. Even at elevated temperatures, no conversion was observed. This is likely caused by strong steric shielding of the Be center. A similar reaction with the more open Cp*BeCl gave in quantitative yield (^DIPePBDI*)MgBeCp* (1). The crystal structure shows a Mg-Be bond of 2.469(4) Å. Homolytic cleavage of the Mg-Be bond requires ΔH=69.6 kcal mol^-1 (cf. CpBe-BeCp 69.0 kcal mol^-1 and (^DIPPBDI)Mg-Mg(^DIPPBDI) 55.8 kcal mol^-1). Natural-Population-Analysis (NPA) shows fragment charges: (^DIPePBDI*)Mg +0.27/BeCp* -0.27. The very low NPA charge on Be (+0.62) compared to Mg (+1.21) and the strongly upfield ⁹Be NMR signal at -23.7 ppm are in line with considerable electron density on Be and the formal oxidation state assignment of Mg^II-Be⁰. Despite this Mg^δ+-Be^δ- polarity, 1 is extremely thermally stable and unreactive towards H₂, CO, N₂, cyclohexene and carbodiimide. It reacted with benzophenone, azobenzene, phenyl acetylene, CO₂ and CS₂. Reaction with 1-adamantyl azide led to reductive coupling and formation of an N₆-chain. The azide reagent also inserted in the Cp*-Be bond. The inertness of 1 is likely due to bulky ligands protecting the Mg-Be unit.