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 [( DIPeP BDI*)MgNa] 2 with Be[N(SiMe 3 ) 2 ] 2 failed; DIPeP BDI*=HC[(tBu)C=N(DIPeP)] 2 , DIPeP=2,6-Et 2 C-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 ( DIPeP BDI*)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 ( DIPP BDI)Mg-Mg( DIPP BDI) 55.8 kcal mol -1 ). Natural-Population-Analysis (NPA) shows fragment charges: ( DIPeP BDI*)Mg +0.27/BeCp* -0.27. The very low NPA charge on Be (+0.62) compared to Mg (+1.21) and the strongly upfield 9 Be NMR signal at -23.7 ppm are in line with considerable electron density on Be and the formal oxidation state assignment of Mg II -Be 0 . Despite this Mg δ+ -Be δ- polarity, 1 is extremely thermally stable and unreactive towards H 2 , CO, N 2 , cyclohexene and carbodiimide. It reacted with benzophenone, azobenzene, phenyl acetylene, CO 2 and CS 2 . Reaction with 1-adamantyl azide led to reductive coupling and formation of an N 6 -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.