Greetings from silicon valley: Alkali metal silanides (H(3)Si)(3)Si(-)M(+) were shown to be selectively accessible for the first time by the reaction of neopentasilane Si(SiH(3))(4) with tBuOM or iPr(2)NLi. The method allows the convenient derivatization of higher silicon hydrides and provides a simple access for unprecedented systematic studies on the chemical behavior of hydropolysilanes (see scheme).
The perhydropentasilanes (H(3)Si)(4)Si and Si(5)H(10) were chlorinated with SnCl(4) to give chlorohydropentasilanes without destruction of the Si-Si backbone. Tetrachloroneopentasilane (ClH(2)Si)(4)Si (2) was prepared in high yield from (H(3)Si)(4)Si and 3.5 equiv of SnCl(4), while Si(5)H(10) and an equimolar amount of SnCl(4) afforded a mixture of ∼60% of ClSi(5)H(9) (1) with polychlorinated cyclopentasilanes and unreacted starting material, which could not be separated by distillation. The selective monochlorination of Si(5)H(10) was achieved starting from MesSi(5)Cl(9) (3; Mes = 2,4,6-trimethylphenyl) or TBDMP-Si(5)Cl(9) (4; TBDMP = 4-tert-butyl-2,6-dimethylphenyl). 3 or 4 was successfully hydrogenated with LiAlH(4) to give MesSi(5)H(9) (6) or TBDMP-Si(5)H(9) (7), which finally gave 1 along with aryl-H and Si(5)H(10) after treatment with an excess of liquid anhydrous HCl. All compounds were characterized by standard spectroscopic techniques. For Si-H derivatives, the coupled (29)Si NMR spectra were analyzed in detail to obtain an unequivocal structural assignment. The molecular structures of 2-4 were further confirmed by X-ray crystallography.
The monofunctionalized cyclohexasilanes XSi 6 Me 11 [X = -OH (2); -NH 2 (3)] are easily accessible from XSi 6 Me 11 and H 2 O/Et 3 N or NH 3 , respectively. The crystal structure of 2 as determined by single crystal X-ray crystallography exhibits the cyclohexasilane ring in chair conformation with the OH group in an unusual equatorial position due to intermolecular hydrogen bonding. Full geometry optimization (B3LYP/6-31+G * ) of the gas-phase structures of 2 and 3 affords six minima on the potential energy surface for chair, twist and boat conformers with the heterosubstituents either in axial or equatorial positions all being very close in energy. According to time-dependent DFT B3LYP/TZVP calculations contributions of several conformers to the observed solution UV absorption spectra of dodecamethylcyclohexasilane (1), 2 and 3 need to be considered in order to achieve satisfactory agreement of calculated and experimental data.
The reactions of the chloropermethylcyclohexasilanes Si6Me12−n
Cl
n
(1; n = 1), 1,3-Cl2Si6Me10 and 1,4-Cl2Si6Me10 (2 and 3, respectively; n = 2), and 1,3,5-Cl3Si6Me9 (4; n = 3) with NH3 or NaNH2, respectively, afforded the corresponding amino derivatives Si6Me12−n
(NH2)
n
(5−8), which are surprisingly stable toward self-condensation in the pure state. In the presence of traces of NH4Cl, working as an acid catalyst, 5−8 slowly decompose by loss of NH3 to give polysilazanes still containing intact cyclohexasilanyl moieties. In the case of 1,4-diaminodecamethylcyclohexasilane (7) the intramolecular condensation product 9 is formed along with some polymeric material. The X-ray structure analysis of 9, which is the first structurally characterized 7-azahexasilanorbornane, exhibits a norbornane-like structure with the cyclohexasilane ring in a boat conformation. Aminoundecamethylcyclohexasilane (5) is easily deprotonated by n-BuLi to give the expected lithium amide LiHNSi6Me11. With NaNH2, however, the open-chain ring scission product 1,5-dihydrodecamethylpentasilane (12) was formed nearly exclusively.
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