Roman Imhof

Structural and vibrational properties of the octanuclear silasesquioxane C6H13(H7Si8O12)

The crystal structure of hexylheptahydrooctasilasesquioxane C6H13(H7Si8O12) has been determined by single-crystal X-ray diffraction and compared with the structures of H8Si8O12, [Co(CO)4(H7Si8O12)] and (CH3Si8O12. The monosubstituted Silasesquioxane molecules have a crystallographic C1symmetry. The local symmetry of their octanuclear Si cage, however, is close to C3v. They are closer to C3v than H8Si8O12 which has an effective molecular symmetry of Th in the crystal. The major distortions originate in intermolecular O ⋯ Si contacts. The larger the number of such contacts per molecule, the larger is the distortion of the local symmetry of the cage. The structure of C6H13(H7Si8O12) compares well with that of [Co(CO)4(H7Si8O12)], and it is clearly the least distorted of the two due to a reduced influence of the n-hexyl ligand on the cage geometry and due to a smaller number of intermolecular interactions. The IR spectrum of C6H13(H7Si8O12) has been interpreted by correlating it with the three molecules H8Si8O12, C6H13[Si(OSiMe3)3] and HSi(OSiMe3)3.

Synthesis and crystal structure of [Co(CO)4(H7Si8O12)]. A new type of monosubstituted octanuclear silasesquioxane with a silicon–cobalt bond

Reaction of octacarbonyldicobalt [Co2(CO)8] with octahydrosilasesquioxane H8Si8O12 in toluene leads to [Co(CO)4(H7Si8O12)], a new monosubstituted, octanuclear silasesquioxane with a silicon–cobalt bond. The product has been analysed by 1H, 13C, 29Si NMR and IR spectroscopies, mass spectrometry and microanalysis. The crystal structure of [Co(CO)4(H7Si8O12)] has been determined by single-crystal X-ray diffraction and is compared with the structures of H8Si8O12, [Co(CO)4(SiCl3)] and [Co(CO)4(SiF3)]. The H7Si8O12 fragment exhibits structural distortions consistently around, and originating from the Si atom to which the Co(CO)4 fragment binds. The remaining part of the cage essentially retains C3 symmetry, but is, nevertheless, in principle closer to the ideal Oh symmetry for this type of cage, than is crystalline H8Si8O12, for which a non-crystallographic molecular symmetry of Th is present. The comparison of the structures of the Co(CO)4 fragments shows that these are consistently distorted and deviate from ideal C3v symmetry. The distortions are essentially of the same kind in the three compounds and differ only in magnitude. The Si–Co distance in [Co(CO)4(H7Si8O12)] is 2.285 A. The experimental findings have been compared with extended-Huckel molecular orbital calculations. Interaction between the Co(dz2) and the Si(Pz) orbitals leads to the bond between the two fragments. Overlap population analysis indicates small but significant bonding interaction between Si and Ceq and antibonding interaction between the Co and the nearest O(Si). Fractional molecular orbital analysis indicates that the electronic structure of [Co(CO)4(H7Si8O12)] consists of three parts: orbitals belonging only to one of the two fragments and orbitals shared by both fragments. The highest-occupied molecular orbital (HOMO) of [Co(CO)4(H7Si8O12)] consists of oxygen lone pairs localised on H7Si8O12. The lowest-unoccupied molecular orbital (LUMO) is identical with the LUMO of Co(CO)4 and the first electronic transitions observed in the near UV are of H7Si8O12(oxygen lone pair) to Co(CO)4 fragment charge-transfer type.

In situ attenuated total reflection FTIR investigations of H2O, HSiCl3 and Co2(CO)8 on ZnSe in the range 600–4000 cm−1

Abstract A new attenuated total reflection (ATR)-IR set-up attached to an external port of a BOMEM DA3 FTIR instrument is used for in situ spectroscopy on ZnSe surfaces in the range 600–4000 cm −1 . ATR-IR studies on silylating reactions had been restricted to the 1400–4000 cm −1 range because of the strong absorption bands corresponding to ν as (Si-O-Si) and other limiting factors. Noteworthy spectra of growing very thin water films are observed. HSiCl 3 is used to form polysiloxane films bearing Si-H units at the surface. Si-H bending at 802 and 820 cm −1 , ν(SiCl 3 ) at 596 cm −1 , Si-O-Si modes at 1150 and 1067 cm −1 , and bands at 910 and 880 cm −1 are shown to be useful to investigate the reaction. The Si-H groups at the surface are reacted with Co 2 (CO) 8 to form (-O) 3 Si-Co(CO) 4 complexes; this results in a decrease of the δ(Si-H) mode at 820 cm −1 . The final spectrum compares well with that of [Co(CO) 4 (H 7 Si 8 O 12 )] in the range of the carbonyl vibrations.

Correlation of the vibrational structure of H8Si8O12 and H10Si10O15

A normal coordinate analysis has been performed for H10Si10O15 by applying the modified general valence force field determined for H8Si8O12. Using group theory allows a quantitative correlation of all infrared and Raman active fundamentals of the two molecules, for which the experimental spectra are presented.

The monophenylhydrosilasesquioxanes PhHn–1SinO1.5n where n= 8 or 10

The first monosubstituted decasilasesquioxane, PhH9Si10O15, and the analogous PhH7Si8O12 molecule have been prepared and characterized by IR and Raman spectroscopy and X-ray crystallography. Both have crystallographic C1 symmetry, but their cages exhibit an approximate Cs, and an effective C3, symmetry, respectively in the crystalline state. The IR and Raman spectra of the two molecules are very similar and reflect the close similarity observed for the spectra of H8Si8O12 and H10Si10O15. They have been treated as a superposition of the spectra of the siloxane cage Hn-1SinO1.5n, n= 8 or 10, the phenyl substituent and the connecting moiety O3Si–C(CH)2 and assigned on the basis of spectral correlation and normal coordinate analysis. The siloxane cage vibrations are best understood by correlation with those of the unsubstituted cages, indicating that distortions of the Si8O12 and Si10O15 cages caused by the substituent are small. A comparison of the Si–C stretching force constants indicates that Si–Cvinyl and Si–Cphenyl are of similar strength while the Si–Calkyl bond is weaker. The notion of ring-opening vibrations, introduced for (HSiO1.5)2n, n= 2, 3, 4, etc., is also applicable to PhH7Si8O12 and PhH9Si10O15. The phenyl substituent does not influence the frequency range of four- and five-membered ring-opening vibrations, however the number of such vibrations is increased.

In situ attenuated total reflection FTIR investigations of thin water films in the silanization of ZnSe and Si

Abstract In situ attenuated total reflection ATR-FTIR spectroscopy is used to investigate the reaction of trimethylchlorosilane (CH 3 ) 3 SiCl in organic solvents at room temperature on ZnSe and on Si surfaces in the presence and absence of pyridine. In the absence of the base, siloxanes are formed, but they can be removed in situ with the reaction solvent along with the surplus of trimethylchlorosilane and no chemisorption is detected. Instead a thin water film is formed on the surface which shows a well resolved ATR-IR water spectrum with absorptions ν as ( O  H ) at 3477 cm −1 , ν s ( O  H ) at 3421 cm −1 , 2δ(HOH) at 3186 cm −1 , δ (HOH) + ν L at 2182 cm −1 where ν L is a librational mode of water and δ(HOH) at 1601 cm −1 . These thin water films are astonishingly stable and easy to handle, and their growth can be monitored. Addition of pyridine to the reaction solvent allows the detection of trimethylsilanol with an absorption band at 3706 cm −1 . Concurrently pyridinium chloride is formed and partly precipitates on the substrate surface.

Mass Spectra of Octaspherosiloxanes

The mass spectra of 14 different octasilasesquioxanes R8Si8O12 are presented. The well-known uniformly substituted compounds (R8=H8, Me8 or Et8) were re-examined and consequently used for comparison. The initial characterization of all the compounds examined was achieved with 1H-NMR spectroscopy. The influence of the different substituents (hydrogen, methyl, ethyl, ethyl phenyl and styrene) under electron impact ionization and collision-induced dissociation conditions was examined. The compounds with hydrogen substituents demonstrated minor cage destruction, whereas the compounds with organic substituents showed multiple losses of the substituents but no destruction of the cage system at all. The stability of the cubic Si8O12 system seems to increase further with an increasing number and size of organic substituents. The additional information gained by collision experiments was modest. All mass spectra also contained doubly charged ions.

Vibrational Structure of Zeolite A

A harmonic force field of the highly symmetrical octahydridosilasesquioxane in terms of internal force constants has been determined, based on extensive IR and FT-Raman data and on a normal coordinate analysis of H 8 Si 8 O 12 and D 8 Si 8 O 12 , and a step by step procedure starting from O h -H 8 Si 8 O 12 to the frameworks O h -(-O) 8 Si 8 O 12 , D 4h -(-O) 8 Si 8 O 12 , D 4h -(≡SiO) 8 Si 8 O 12 , and finally to D 2d -(≡TO) 8 T 8 O 12 , T = Si or Al and Si/Al = 1, has turned out to be an excellent, enlightening approach for qualitatively and quantitatively describing the vibrational structure of the zeolite A framework. An improved force field is reported and a correlation of the experimental and the calculated IR and Raman spectra of zeolite A has allowed to assign group frequencies to all fundamental modes. All fundamentals between 1110—950 cm −1 belong to antisymmetric T—O—T stretching vibrations, while the symmetric T—O—T stretching modes are at 860-830 cm −1 , 740—680 cm −1 , and 610—570 cm −1 . The fundamentals between 490—100 cm −1 can be described as O—T—O bending vibrations – with the exception of a D4R and two sodalite cage breathing modes and the 8-ring pore opening – whereas the T—O—T bending vibrations are below 100 cm −1 .

Vibrations of Monosubstituted Octasilasesquioxanes

The vibrational spectra of monosubstituted spherosiloxanes of the type RH7Si8O12 can be understood as a superposition of the spectral features of H8Si8012 and of the substituent R. We show this for the FT-Raman spectra of RH7Si8O12 with R = −CH2−CH2−C6H5 and −CH = CH−C6H5. The force fields of H8Si8012 and of the organic substituent, ethylbenzene and styrene, respectively, were combined and normal coordinate analysis was applied. The spectra of phenethyl-H7Si8O12 and styryl-H7Si8O12 were correlated with the spectrum of H8Si8O12. The Si-X stretching frequencies in the spherosiloxanes are larger than in most other siloxane compounds. We have shown that this is due to the different X-Si-O-Si conformations, which are anti and syn, respectively, and can be explained by the dependence of the bond order on the X-Si-O-Si dihedral angle.

Dalton communications. Synthesis of the first organometallic monosubstituted octanuclear silasesquioxane

The hexachloroplatinic acid-catalysed hydrosilylation of vinylferrocene by octahydrosilasesquioxane H8Si8O12 to the first organometallic monosubstituted octasilasesquioxane [(η-C5H5)Fe(η-C5H4CH2CH2)]H7Si8O12 has been achieved.