Heavy Alkali Metal Tris(trimethylsilyl)silanides:  A Synthetic and Structural Study

Abstract

Treatment of lithium tris(trimethylsilyl)silanide, Li(THF)3Si(SiMe3)3, with heavier alkali metal tert-butoxides yielded the alkali metal silanides MSi(SiMe3)3 (M = K, Rb, or Cs) in a simple, high-yielding, one-step procedure. Separation of the two solid reaction products was achieved by addition of crown ether, which also determines the formation of contact or separated ion pairs in the solid state. Here we report the synthesis and structural characterization of the contact ion K(18-crown-6)Si(SiMe3)3, 2, in addition to the separated [K(12-crown-4)2][Si(SiMe3)3], 1, [Rb(15-crown-5)2][Si(SiMe3)3], 3, and [Cs(18-crown-6)2][Si(SiMe3)3], 5. [Rb(18-crown-6)2][Si(SiMe3)3][Rb(18-crown-6)Si(SiMe3)3]2, 4, is a rare example where both contact and separated ions are observed in the solid state. The investigation of synthetic routes toward the target compounds also examined the previously published metalation of Si(SiMe3)4 with potassium tert-butoxide. This route proved to be temperamental: depending on reaction conditions and solvent systems, either adducts between product and unreacted starting material, namely, [{K(THF)Si(SiMe3)3}{KO t Bu}3], 6, or the target compound KSi(SiMe3)3 was isolated. All compounds were characterized by X-ray crystallography and NMR spectroscopy.

The Multidentate Ligand (MeOMe2Si)3Si−: Unusual Coordination Modes in Alkali Metal Silanides

“…356 pm) are significantly longer than in the closely related dimeric and monomeric hypersilanides [{(Me 3 Si) 3 Si(m-K)} 2 ] (337-342 pm), [12] [(Me 3 Si) 3 SiK-(C 6 H 6 ) 3 ] (332-335 pm), [12] [(Me 3 Si) 3 SiK(tmeda) 2 ] (339 pm), [18] and [(Me 3 Si) 3 SiK( [18]crown-6)] (345 pm). [19] Similar structural behavior has been observed in the tridentate 3-sila-b-diketiminates, [Me 3 SiSi{C(R)= NSiMe 3 } 2 M], in which the metal ion is coordinated to the internal N-donors to give either monomeric (Li) or dimeric (Na) zwitterionic cyclohexane structures; the latter, has significant intramolecular Si···Na (318 pm) contacts. The potassium derivative forms a dimer consisting of a central {Si 2 K 2 } core.…”

Synthetic, Structural, and Theoretical Investigations of Alkali Metal Germanium Hydrides—Contact Molecules and Separated Ions

“…With [18]crown-6 as a well-known match for potassium and subsequent formation of a contact molecules, the combination of rubidium and [18]crown-6 has been known to afford both contact molecules and separated ions, as demonstrated with a rubidium silanide, for which two separated ions [Rb( [18] 3 ] have been identified in the asymmetric unit. [30] In accordance with this result, 1.5 equivalents of [18]crown-6 were utilized in the reaction mixture, but only the contact molecule 4 and uncoordinated crown were isolated. The formation of the separated ions 2, 3, and 5 is the result of the small diameter of the crown cavity relative to the alkali metal diameter, as demonstrated with the combination of either [12]crown-4 or [15]crown-5 and potassium, or [18]crown-6 and cesium.…”

“…Pyramidal geometry was also observed in alkali metal trimethylsilyl, A C H T U N G T R E N N U N G -silanides, and A C H T U N G T R E N N U N G -germanides, with angles in the range of 1008. [21,30] While we recognize the significant uncertainty in locating hydrogen positions by using X-ray diffraction methods, a fact also supported by the large range of Ge À H distances in 1-4, as well as the significant underestimation of Ge À H distances in 2 and 3, we are confident about the assignment of pyramidal geometry. The pyramidal geometry is also in agreement with Bents rules, indicating that the coordination of an electropositive metal increases the p character in the metal-ligand bonds, an assumption further supported by the tetrahedral geometry in germyl derivatives exhibiting an exclusively organic environment.…”

“…These observations agree with ion-association trends in crown ether containing silanides and germanides. [21,30] In accordance with the small size of the GeH 3 À ion relative to GeA C H T U N G T R E N N U N G (SiMe 3…”

Der mehrzähnige Ligand (MeOMe2Si)3Si−: ungewöhnliche Koordinationsweisen in Alkalimetallsilaniden

“…356 pm) deutlich größer als in den verwandten dimeren und monomeren Hypersilaniden [{(Me 3 Si) 3 Si(m-K)} 2 ] (337-342 pm), [12] [(Me 3 Si) 3 SiK(C 6 H 6 ) 3 ] (332-335 pm), [12] [(Me 3 Si) 3 SiK(tmeda) 2 ] (339 pm) [18] und [(Me 3 Si) 3 SiK-([18]Krone-6)] (345 pm). [19] Eine ähnliche Struktur wurde für 3-Sila-b-diketiminate Me 3 SiSi[C(R) = NSiMe 3 ] 2 M beschrieben, bei denen die internen N-Donoren an das Metallion koordinieren, sodass entweder monomere (Li) oder dimere (Na) zwitterionische Cyclohexanstrukturen resultieren (letztere mit beträchtlichen intramolekularen Si···Na-Wechselwirkungen (318 pm)). Das Kaliumderivat bildet dagegen ein aus einem zentralen {Si 2 K 2 }-Kern bestehendes Dimer.…”