Structural Studies of Cesium, Lithium/Cesium, and Sodium/Cesium Bis(trimethylsilyl)amide (HMDS) Complexes

Reacting
cesium fluoride with an equimolar n-hexane solution
of lithium bis­(trimethylsilyl)­amide (LiHMDS) allows the isolation
of CsHMDS (1) in 80% yield (after sublimation). This
preparative route to 1 negates the need for pyrophoric
Cs metal or organocesium reagents in its synthesis. If a 2:1 LiHMDS:CsF
ratio is employed, the heterobimetallic polymer [LiCs­(HMDS)2]∞ 2 was isolated (57% yield). By
combining equimolar quantities of NaHMDS and CsHMDS in hexane/toluene
[toluene·NaCs­(HMDS)]∞ 3 was isolated
(62% yield). Attempts to prepare the corresponding potassium-cesium
amide failed and instead yielded the known monometallic polymer [toluene·Cs­(HMDS)]∞ 4. With the aim of expanding the structural
diversity of Cs­(HMDS) species, 1 was reacted with several
different Lewis basic donor molecules of varying denticity, namely,
(R,R)-N,N,N′,N′-tetramethylcyclohexane-1,2-diamine
[(R,R)-TMCDA] and N,N,N′,N′-tetramethylethylenediamine (TMEDA), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), tris­[2-(dimethylamino)­ethyl]­amine
(Me6-TREN) and tris­[2-(2-methoxyethoxy)­ethyl]­amine
(TMEEA). These reactions yielded dimeric [donor·NaCs­(HMDS)2]2 5–7 [where
donor is (R,R)-TMCDA, TMEDA and
PMDETA respectively], the tetranuclear “open”-dimer
[{Me6-TREN­·Cs­(HMDS)}2­{Cs­(HMDS)}2] 8 and the monomeric [TMEEA·Cs­(HMDS)] 9. Complexes 2, 3, and 5–9 were characterized by X-ray crystallography
and in solution by multinuclear NMR spectroscopy.