Application of Bis(amido)alkyl Magnesiates toward the Synthesis of Molecular Rubidium and Cesium Hydrido-magnesiates

Abstract

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(SiMe₃)(Dipp), and alkyl is nBu or CH₂SiMe₃, are reported. More stable than their nBu analogues, the reactivities of the CH₂SiMe₃ 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(SiMe₃)(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 μ₃-(AM)₂Mg environments, compared to μ₂-Mg₂ environments in inverse crowns.

Scheme 1. Cocomplexation Syntheses of Alkali Metal Bis(amido)alkyl Magnesiates 1–4 and Conversion of 3 and 4 into Bis(amido)hydrido-magnesiate Dimers 5 and 6 with Yields Shown where Pure Tangible Product Could be Obtained

Figure 1

Sections of the infinite supramolecular chains of [(AM)MgN′₂nBu]_∞ (AM = Rb, 1, top; Cs, 2, bottom) with thermal ellipsoids drawn at 50% probability and all hydrogen atoms omitted for clarity.

Figure 2

Sections of the infinite supramolecular chains of [(AM)MgN′₂CH₂SiMe₃]_∞ (AM = Rb, 3, top; Cs, 4, bottom) with thermal ellipsoids drawn at 50% probability and all hydrogen atoms omitted for clarity.

Figure 3

Molecular structures of hydrido-magnesiates [(AM)MgN′₂H]₂ (AM = Rb, 5, top; Cs, 6, bottom) with thermal ellipsoids drawn at 50% probability and all hydrogen atoms other than metal-bound hydrides omitted for clarity.

Figure 4

Crystallographically characterized alkali metal hydride complexes related to 5 and 6. N-bound Dipp groups have been simplified as phenyl groups for clarity.

Figure 5

Contrast of [(AM)(N)(Mg)(N)]₂ units encapsulating two hydride ligands where AM = Na and amide is aliphatic (left) and AM = Cs and amide is aromatic (right).

Figure 6

Space filling diagrams of complexes 6 (left) and E (right), with pink Cs atoms and green Mg atoms.