Electronic Properties of Octameric Silsesquioxanes

HOMO-LUMO Gap and Insulating Character of the Silsesquioxane Core

Quantum-mechanical calculations performed for octahydrooctasilsesquioxane showed that the highest occupied molecular orbital (HOMO) of this compound is composed of atomic orbitals associated with the lone pairs of the oxygen atoms, whereas the lowest unoccupied molecular orbital (LUMO) is spherical and is located at the center of the silsesquioxane core, as shown in the figure below. The calculations further indicated that the energy gap between the HOMO and the LUMO amounts to approximately 6 to 7 eV. This value exceeds the 3 eV threshold for conductivity, which confirms that the silsesquioxane core behaves as an insulator.

Figure 1. Molecular orbitals of octahydrooctasilsesquioxane: (a) HOMO and (b) LUMO.

Electron-Withdrawing Behavior Compared to the Trifluoromethyl Group

In view of the low electronegativity of silicon (1.90, compared with 2.55 for carbon on the Pauling scale), the POSS core might be expected to act as an electron-donating group. Experimental studies have shown, however, that the silsesquioxane core behaves as an electron-withdrawing group. Feher and Budzichowski demonstrated that the 4-(chloromethyl)phenyl group attached to a silsesquioxane is resistant to hydrolysis and does not undergo substitution. On the basis of the chemical shifts observed in the ¹³C NMR spectra of octameric silsesquioxanes, they established that the electron-accepting properties of silsesquioxane are comparable to those of the trifluoromethyl group (–CF₃), consistent with the silsesquioxane cage polarizing an approaching electrophile.

Fluoride Anion Encapsulation within the Octasilsesquioxane Cage

Further evidence for the electrophilic character of the silsesquioxane core is provided by the ability to trap a fluoride anion within the cage. Such systems can be obtained by using tetra-n-butylammonium fluoride during the condensation of triethoxysilane, or by reacting tetramethylammonium fluoride with an octameric silsesquioxane (Scheme 1). These species can be isolated only when the organic side groups are weakly electron-accepting, as is the case for phenyl, vinyl, and fluorinated alkyl groups, an approach conceptually related to fluoride-selective silsesquioxane sensors. What the encapsulated anion does not do is bond to the cage. X-ray diffraction together with ¹⁹F and ²⁹Si NMR spectroscopy indicates only weak electrostatic interactions between the fluoride and the silicon atoms that surround it. In the crystal structure of octaphenylsilsesquioxane with an entrapped fluoride (Figure 2) the Si···F distance is 2.65 Å, far longer than the 1.71 Å of a covalent Si–F bond, and the separations between silicon atoms at opposite corners of the cage, 5.31 and 5.38 Å, are only marginally shorter than in the empty cage. The spectroscopic signatures point the same way: the ¹⁹F resonance at −26.4 ppm lies in the range expected for ionic fluoride salts, and the ²⁹Si signal of the cage at −80.6 ppm is displaced by a mere 0.9 ppm from that of fluoride-free octaphenylsilsesquioxane. Calculations on (HSiO_1.5)₈ place the endohedral F⁻/(HSiO_1.5)₈ host–guest arrangement 60–80 kcal mol⁻¹ below the exohedral alternative, whereas for group 1 and group 2 cations the preference is reversed. The cavity attracts the anion and repels the cation, which is exactly what an electrophilic core should do.

Scheme 1. Preparation of octameric silsesquioxanes with a fluoride anion encapsulated inside the POSS core: (a) condensation of a trialkoxysilane with n-Bu₄NF, and (b) reaction of an octameric silsesquioxane with Me₄NF in THF.

Figure 2. Solid-state X-ray structure of an octasilsesquioxane with a fluoride anion trapped inside the POSS core (hydrogen atoms omitted).

Full CitationBassindale, A. R.; Pourny, M.; Taylor, P. G.; Hursthouse, M. B.; Light, M. E. "Fluoride-Ion Encapsulation within a Silsesquioxane Cage." Angewandte Chemie International Edition, 2003, 42, 3488-3490.
DOI: 10.1002/anie.200351249
Full text: Angewandte Chemie International Edition → Wiley

References

(1) Pauling, L. The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativity of Atoms. J. Am. Chem. Soc. 1932, 54, 3570–3582.

(2) Feher, F. J.; Budzichowski, T. A. Syntheses of Highly-Functionalized Polyhedral Oligosilsesquioxanes. Journal of Organometallic Chemistry 1989, 379, 33–40.

(3) Bassindale, A. R.; Pourny, M.; Taylor, P. G.; Hursthouse, M. B.; Light, M. E. Fluoride-Ion Encapsulation within a Silsesquioxane Cage. Angew. Chem. Int. Ed. 2003, 42, 3488–3490.

(4) Bassindale, A. R.; Parker, D. J.; Pourny, M.; Taylor, P. G.; Horton, P. N.; Hursthouse, M. B. Fluoride Ion Entrapment in Octasilsesquioxane Cages as Models for Ion Entrapment in Zeolites. Further Examples, X-Ray Crystal Structure Studies, and Investigations into How and Why They May Be Formed. Organometallics 2004, 23, 4400–4405.

(5) Anderson, S. E.; Bodzin, D. J.; Haddad, T. S.; Boatz, J. A.; Mabry, J. M.; Mitchell, C.; Bowers, M. T. Structural Investigation of Encapsulated Fluoride in Polyhedral Oligomeric Silsesquioxane Cages Using Ion Mobility Mass Spectrometry and Molecular Mechanics. Chem. Mater. 2008, 20, 4299–4309.

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Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry. (2022). Electronic Properties of Octameric Silsesquioxanes. https://doi.org/10.59350/kt6p9-exj58