Elsevier

Journal of Molecular Structure

Volume 1028, 28 November 2012, Pages 68-72
Journal of Molecular Structure

Structural studies of octahydridooctasilsesquioxane – H8Si8O12

https://doi.org/10.1016/j.molstruc.2012.06.033Get rights and content

Abstract

There has been an increasing amount of interest in silsesquioxanes recently. The most important group of polyhedral oligomeric silsesqioxanes (POSS) is octasilsesquioxanes. The attention is particularly focused on octahydridooctasilsesquioxanes, also known as T8H. The aim of the presented work is a detailed study of the structure of T8H = H8Si8O12. Powder X-ray diffraction and infrared spectroscopy FTIR are the main measurement techniques on which the detailed structural studies are based. The results undoubtedly point to the conclusion that T8H molecule symmetry is lower (C3i) than expected (Th) and matches site symmetry in the trigonal crystal structure.

Highlights

► Synthesis of octahydridooctasilsesquioxane (T8H). ► X-ray powder diffraction measurements. ► Structure determination based on the full pattern fit and Monte-Carlo calculations. ► Observation of T8H molecule symmetry lowering from expected Th to C3i. ► Infrared Spectroscopy measurements with DFT calculations.

Introduction

Silsesquioxanes are nanometer-sized molecules suitable for the production of organic–inorganic nanocomposite materials. Polyhedral oligomeric silsesquioxanes (POSS) are compounds of the empirical formula (RSiO1,5)n in which three oxygen atoms and R= hydrogen, aryl, alkyl or other organic group are attached to the silicon atom. The structure of silsesquioxanes can be random, ladder, cage or partial cage type [1]. The cage type silsesquioxanes are often referred to as spherosiloxanes because their polyhedral structures are topologically equivalent to a sphere.

Silsesquioxanes are nowadays very important and fascinating materials due to their applicational properties. The widest variety of cage structures is found for the hydridosilsesquioxanes (HSiO1,5)n, n = 8, 10, 12, 14, 16 and 18. These unique structures are synthesized by hydrolysis and condensation of silanes of the XSiY3 type, where X – chemically stable substituent and Y – highly reactive substituent. Most commonly used are: trialkoxysilanes [HSi(OR)3] or trichlorosilanes (HSiCl3) in the presence of acid or base catalyst and organic solvents [2].

This paper focuses on octahydridooctasilsesquioxane – T8H – H8Si8O12, particularly because of its inorganic nature and unusual molecular structure [3] as compared to octamethylsilasesquioxane (T8Me – (CH3)8Si8O12 which was studied previously [4]. Octahydridooctasilsesquioxane was obtained for the first time by Müller et al. from HSiCl3 in very low yield of 0.2% in 1959 [5]. The improved T8H synthesis method was proposed by Frye and Collins in 1970. They attained 13% yields from HSi(OCH3)3 catalyzed by HCl [6]. The 17.5% yield was achieved by Agaskar from HSiCl3 in the presence of partially hydrated FeCl3 in 1990 [7]. There are also modifications of Agaskar method, one using a surfactant [8] and second with the increased dilution of the trichlorosilane [9] but they do not change the yield significantly.

First published extensive studies on the symmetry of the molecule as well as crystal structure of octahydridooctasilsesquioxane were undertaken by Törnroos [10] by means of single crystal neutron diffraction. The proposed crystal symmetry was trigonal with the molecule with non-crystallographic cubic symmetry. Resent computational studies on this system made by Shutte and Pretorius [11] point out that the molecular symmetry in the crystal lattice is reduced to S6 (C3i).

Octahydridosilsesqioxane studied in this paper was obtained by synthetic route proposed by Bolln et al. [8] from triethoxysilane in the presence of FeCl3 as catalyst, petroleum ether as solvent and sodium dodecylsulfate as surfactant. The molecular structures of pure samples were determined by XRD and FT-IR methods. The main goal is to provide the direct experimental evidence of C3i symmetry of the T8H molecule in the crystal lattice as it was theoretically predicted by Shutte and Pretorius [11].

Section snippets

Sample preparation

Octahydridooctasilsesquioxane was synthesized according to the procedure described in literature [8] using triethoxysilane- HSi(OC2H5)3 (Sigma Aldrich 95%) as the precursor, sodium dodecylsulfate- C11H23SO3Na (POCH SA) as the surfactant, FeCl3 as the catalyst (Sigma Aldrich, 97%) and petroleum ether (LACH-NER, s.r.o.) as solvent.

Above mentioned ingredients created two-phase reaction mixture. Polar phase contained solution of FeCl3 in HCl and sodium dodecylsulfate. Non-polar phase was created by

Results and discussion

Powder diffraction results are presented in the Fig. 1. For clarity of the picture the figure includes inset of the narrower range (30–90°) with the y-axis scaling changed to square root in order to emphasize the details. Solid black line represents calculated diffraction pattern and the gray line corresponds to experimental results.

As a starting point for structural calculation, the structure of HSQ determined by Törnroos [10] was selected. Simple Rietveld refinement based on those parameters

Summary and conclusions

Pure T8H polycrystalline powder specimen synthesized by the modified method was studied to determine structure. Two structural sensitive methods – X-ray Powder Diffraction (XRD) and Infrared Spectroscopy (IR) were used in order to determine crystal structure as well as the symmetry of the T8H molecule in the crystal lattice. Results of all measurements were compared with calculated theoretical models. Until now, T8H molecule has been assign with cubic and non-crystallographic symmetry Th. The

Acknowledgements

This work was supported by Polish Ministry of Science and Higher Education under the project ‘‘Silsesquioxanes as ceramic precursors” N 507 265940. The numerical calculations were carried out in the Academic Computer Centre CYFRONET – AGH (grant No. MEiN/SGI3700/AGH/039/2006).

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