The [(DABCO)7·(LiCH2SiMe3)8] Octamer: More Aggregated than the Parent Starting Material [LiCH2SiMe3]6 but Also Higher in Reactivity

Herein, we report on the reaggregation of hexameric trimethylsilylmethyllithium [LiCH2SiMe3]6 with the donor base DABCO (1,4-diazabicyclo[2.2.2]­octane) to give the unprecedented octamer [(DABCO)7·(LiCH2SiMe3)8] (1). The structure consists of four dimers, forming Li2C2 four-membered rings, connected to two [(DABCO)3·{(LiCH2SiMe3)2}2] chain fractions, interconnected by a single DABCO molecule. Interestingly, two different conformers of (LiCH2SiMe3)2 dimers are present, caused by different steric demand. Higher steric strain in the center of the molecule causes an ecliptic arrangement of the Me3Si group along the Si–Cα bond, while at the periphery the more relaxed staggered conformation is enabled. The reactivity of trimethylsilylmethyllithium coordinated by DABCO was tested in the benchmark reaction with toluene. Although the aggregation of 1 is much higher than that of the parent [LiCH2SiMe3]6, the reactivity of the first is higher than that of the starting material, provided the octameric aggregation found in the solid state is maintained in nondonating solvents. While the hexamer would not react with toluene, the octamer gives benzyllithium, coordinated by DABCO. The reaction was monitored by 1H NMR spectroscopy. Revisiting that known structure with modern technology revealed that [(DABCO)·(LiCH2Ph)]∞ (2) crystallizes in the space group P21. 2 still is the only benzyllithium compound featuring the η3-coordination mode to the Cortho atom of the phenyl ring, presumably triggered by the singly donating DABCO molecule. More donor centers supersede this extra coordination to the carbanion.