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

Abstract

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 molˉ1 (cf. CpBe‐BeCp 69.0 kcal molˉ1 and (DIPPBDI)Mg‐Mg(DIPPBDI) 55.8 kcal molˉ1). 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 MgII‐Be0. Despite this Mgδ+‐Beδˉ 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.