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

Abstract

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.

Exploring the solid state and solution structural chemistry of the utility amide potassium hexamethyldisilazide (KHMDS)

“…These K-N bond distances are similar to those found for the solvent-free (KHMDS) 2 [2.770(3) and 2.803(3) Å]. 33 The distinct K-N bond lengths for K1 and K2 observed in 1 result in two slightly distinct N-K-N angles [N-K1-N 94.58 (4) and N-K2-N 95.13(4)°], whilst the K-N-K 34,36,39,41,49,56,68,69 3 crystallises as a dimeric (KHMDS) 2 unit, and it contains two molecules of THF, one coordinating to each K atom (Fig. 3a) ] unit represents a branch point towards four distinct directions in the polymeric two-dimensional sheet, two through its K atoms and two via two methyl groups from each HMDS ligand.…”

“…A total of eleven new solvates of KHMDS (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) have been prepared and structurally characterised, both in solution and in the solid state. All synthetic protocols were optimised for obtaining high-quality crystalline samples for single crystal X-ray diffraction studies.…”

“…3 Additionally, the absence of β-hydrogen atoms prevents β-hydride elimination, and their lipophilic character makes alkali metal derivatives of the HMDS group special candidates for many homogeneous reactions. Since lithium, sodium and potassium HMDS reagents are commercially available, they have become prominent transfer reagents for the synthesis of s-block metal, 4 transition metal, [5][6][7] lanthanide 8 and actinide 9,10 HMDS complexes via salt metathesis. In synthesis, they are used in a wide variety of chemical transformations ranging from drug synthesis to polymer pro-duction, such as in the formation of kinetic enolate anions, [11][12][13] alkylations, 14 arylations, 15 isomerisations, 16 polymerizations, 17 ring closing reactions and Wittig reactions, 18,19 and also, in the deprotonative metallation of acidic C-H bonds, like cyclopentadiene, indene and fluorene.…”

Synthesis of Indoles through Domino Reactions of 2‐Fluorotoluenes and Nitriles

“…OHara and coworkers have demonstrated that CsN(SiMe 3 ) 2 could be obtained by metathesis between LiN(SiMe 3 ) 2 and CsF. [21] We ascribe the success of this indole synthesis to the enhanced reactivity of CsN(SiMe 3 ) 2 over LiN(SiMe 3 ) 2 .T he conditions in entry 4w ere next used to probe the impact of substrate stoichiometry.I ncreasing the loading of LiN(SiMe 3 ) 2 from 1t o2equiv improved the AY of 3aa to 64 %( entry 7). Further increasing the loading to 3equiv,h owever, had ad etrimental impact on the AY (entry 8).…”

Synthesis of Indoles through Domino Reactions of 2‐Fluorotoluenes and Nitriles

“…Other combinations either gave low AYs of 3 aa (entries 5 and 6) or did not promote the reaction at all (see the Supporting Information for details). O'Hara and co‐workers have demonstrated that CsN(SiMe 3 ) 2 could be obtained by metathesis between LiN(SiMe 3 ) 2 and CsF . We ascribe the success of this indole synthesis to the enhanced reactivity of CsN(SiMe 3 ) 2 over LiN(SiMe 3 ) 2 .…”