Cyclic Silylcarbodiimides as Precursors for Porous Si/C/N Materials: Formation, Structures, and Stabilities

Abstract

Exchange reactions of bis(trimethylsilyl)carbodiimide (BTSC) with diorganodichlorosilanes were investigated with the aim to synthesize substitution products of the types R 2 SiCl(NCNSiMe 3 ), R 2 Si(NCNSiMe 3 ) 2 , and [R 2 Si(NCN)] n (R = Me, Ph, PhCH 2 , and Vin), which can be used as precursors for Si/C/N materials with tuned pore structures. Extensive 29 Si NMR spectroscopic investigations indicated that complex product mixtures containing several of the mentioned compounds are formed, independent of the starting materials and reaction conditions. The relative stabilities of the cyclic silylcarbodiimides of ring sizes 4n with n = 2-6 were calculated with DFT methods and compared, also considering the respective isomeric cyanamides with rings containing 2n atoms. According to these quantum chemical studies, the cyclic tetramer [Ph 2 Si(NCN)] 4 was predicted to be the most stable among the phenyl-substituted rings. It was experimentally synthesized via pyridine-catalyzed reactions of Ph 2 SiCl 2 with BTSC, isolated as [Ph 2 Si(NCN)] 4 3 (dioxane) 2 and structurally analyzed. In the course of attempts to selectively prepare oligosilylcarbodiimides from cyclosilazanes, high-quality crystals of hexaphenylcyclotrisilazane [Ph 2 Si-NH] 3 were obtained and characterized by X-ray single crystal structure analysis.

Sol–gel derived Si/C/O/N‐materials: molecular model compounds, xerogels and porous ceramics

“…The 29 Si NMR spectrum of cyclo-[Ph 2 Si-O-SiPh 2 (NCN)] 2 (δ = −42.9 ppm in CDCl 3 ) is very similar to that of octaphenylcyclotetrasiloxane (δ = −43.0 ppm in dioxane) and the fully carbodiimide bridged system [Ph 2 Si(NCN)] 4 (δ = −44.4 ppm in dioxane) [13] . Considering a [Si-O-Si(NCN)] 2 12-membered cyclus in an isodesmic reaction; [35] this reaction can be written like in Scheme 2, where the tetramer carbodiimide and the cyclotetrasiloxane act as counterparts.…”

“…[8,9,13,15] Without pyridine, a more complex structure is formed, as can be seen from the 29 Si CP/MAS NMR, with differently bonded NCN units. This is most likely the reason that the generally weak 13 C signal for the carbodiimide group is not visible in Fig. 5b (top).…”

“…Additional signals attributed to pyridine were observed in the samples with catalyst at about 145 ppm. [13,15] The FT-IR and Raman spectra of the as-synthesized Si/C/O/N xerogels shown in Fig. 6a and 6b clearly indicate the presence of N C N units with the characteristic valence vibrations at 2168 and 1533 cm −1 , as well as the corresponding deformation vibration at 800 cm −1 .…”

“…via condensation with BTSC and via base-supported HCl-elimination with cyanamide. [13] (Scheme 1, top). The latter route proved also feasible to create mixed siloxane/silylcarbodiimide-functionalized heterocycles with alternating Si-O-Si and Si-N C N-Si sequences, the use of chlorosiloxanes being a prerequisite (Scheme 1, bottom).…”

“…Owing to the pseudo-chalcogenic character of the carbodiimide group, it adopts the role of the oxygen atom, whereas the trimethylsilyl group can be considered as an analog of the protons in a hydrolytic process. [13,16,17] Several materials in the systems Si/C, [18] B/C/N, [19,20] Si/C/N, [21,22] Si/B/C/N, [23 -25] and Si/Al/C/N [26] possessing high thermal resistance and chemical stability, have been synthesized using this particular kind of sol-gel route. Several further reports on related work on BTSC-derived element carbodimides and their use as precursors for hybrid and ceramic materials have been published.…”