Application of bis(amido)alkyl magnesiates towards the synthesis of molecular rubidium and caesium hydrido-magnesiates

Rubidium and cesium are the least studied naturally occurring s-block metals in organometallic chemistry but are in plentiful supply from a sustainability viewpoint as highlighted in the periodic table of natural elements published by the European Chemical Society. This underdevelopment reflects the phenomenal success of organometallic compounds of lithium, sodium, and potassium, but interest in heavier congeners has started to grow. Here, the synthesis and structures of rubidium and cesium bis(amido)alkyl magnesiates [(AM)MgN′ ₂alkyl] _∞, where N′ is the simple heteroamide ^-N(SiMe ₃)(Dipp), and alkyl is nBu or CH ₂SiMe ₃, are reported. More stable than their nBu analogues, the reactivities of the CH ₂SiMe ₃ magnesiates toward 1,4-cyclohexadiene are revealed. Though both reactions produce target hydrido-magnesiates [(AM)MgN′ ₂H] ₂ in crystalline form amenable to X-ray diffraction study, the cesium compound could only be formed in a trace quantity. These studies showed that the bulk of the ^-N(SiMe ₃)(Dipp) ligand was sufficient to restrict both compounds to dimeric structures. Bearing some resemblance to inverse crown complexes, each structure has [(AM)(N)(Mg)(N)] ₂ ring cores but differ in having no AM-N bonds, instead Rb and Cs complete the rings by engaging in multihapto interactions with Dipp π-clouds. Moreover, their hydride ions occupy μ ₃-(AM) ₂Mg environments, compared to μ ₂-Mg ₂ environments in inverse crowns.