Octaphenyl POSS: Synthesis, Structure and Ortho Bromination

Octaphenyloctasilsesquioxane, Ph₈Si₈O₁₂, is the aromatic counterpart of octamethylsilsesquioxane and one of the most consequential building blocks in cage chemistry, because its eight phenyl rings are not inert decoration but eight addressable reaction sites. First prepared by K. Olsson in 1958 in 9% yield, the compound is now accessible in 95% yield by a two-step route. What makes it distinctive is that electrophilic substitution on those rings disobeys the regiochemical rules learned for isolated arenes: bromination proceeds in the ortho position, with 85% selectivity, and requires no Lewis acid catalyst at all.

Topics covered in this post: preparation of octaphenyl-POSS from Olsson's hydrolysis to the Kawakami two-step route via a cyclic tetrasiloxanetetraol; the single-crystal structure and its O_h symmetry; uncatalyzed ortho bromination, iron-mediated ortho/meta bromination and regioselective para iodination with ICl; type II halogen bonds and the Br₄ synthon in the solid state; nitration and reduction to octa(aminophenyl)-POSS; and palladium-catalyzed cross-coupling of the halogenated cages.

From 9% to 95%: Routes to Octaphenyloctasilsesquioxane

Octaphenyloctasilsesquioxane was obtained for the first time by K. Olsson in 1958, as the product of hydrolysis of trichlorophenylsilane (PhSiCl₃) in methanol with aqueous hydrochloric acid as catalyst, in a yield of 9%. Numerous modifications of that preparation have since been described, and in many of them the principal product is accompanied by cage silsesquioxanes containing ten and twelve silicon atoms, a competition examined in the entries on decameric and dodecameric cages. The underlying equilibrium chemistry, in which solvent rather than stoichiometry decides which framework predominates, is the subject of the entry on phenylsilsesquioxane equilibration from cage to ladder.

The highest reported yield, 95%, belongs to the method developed in the group of Y. Kawakami. The sequence is deliberately divided in two. Trichlorophenylsilane is first hydrolyzed in a biphasic benzene and water system, which arrests the process at the cyclic tetrasiloxanetetraol rather than allowing it to run on to an ill-defined mixture. That intermediate is then condensed with benzyltrimethylammonium hydroxide to close the cube. Separating hydrolysis from condensation, and isolating a well-defined four-silicon intermediate between them, is what converts a statistical condensation into a reliable preparation.

Figure 1. Preparation of octaphenyloctasilsesquioxane after Y. Kawakami. Hydrolysis of PhSiCl₃ in a benzene and water system affords the cyclic tetrasiloxanetetraol, which is condensed with benzyltrimethylammonium hydroxide (BzTMAH) to give the octameric cage in 95% yield.

Molecular Structure in the Solid State

The crystal structure of octaphenyloctasilsesquioxane was determined by the group of S. L. Wunder. The eight silicon atoms occupy the vertices of a slightly distorted cube bridged by twelve oxygen atoms, with a phenyl ring directed outward from each vertex, and the molecule possesses high, idealized O_h symmetry. The structure is the reference point against which every substituted derivative discussed below is compared, because the substitution pattern introduced on the rings leaves the Si₈O₁₂ core essentially untouched while altering the intermolecular chemistry completely.

Figure 2. Molecular structure of octaphenyloctasilsesquioxane in the solid state, rendered from the deposited crystallographic coordinates. Silicon is shown in gold and oxygen in red. The eight silicon atoms define a cube and each carries one phenyl substituent directed away from the core, giving the molecule idealized O_h symmetry. Structure originally reported by Chinnam, P. R.; Gau, M. R.; Schwab, J.; Zdilla, M. J.; Wunder, S. L. Acta Crystallogr. Sect. C 2014, 70, 971.

Electrophilic Substitution: the Cage Overrides the Textbook Rules

The reactivity of a monosubstituted arene in electrophilic substitution is governed by the nature of the substituent. An electron-donating group activates the ring and directs the electrophile to the ortho and para positions, whereas an electron-withdrawing group deactivates it and directs to meta. Halogens form the familiar exception, deactivating the ring while still directing ortho and para. Against that background the behaviour of phenylsilsesquioxanes is anomalous in two separate respects.

Bromination of (PhSiO_1.5)_n proceeds in the ortho position, with a selectivity of 85% for n = 8, 70% for n = 10 and 60% for n = 12. Equally striking is that the reaction requires no catalyst, whereas halogenation of ordinary benzene derivatives normally demands a Lewis acid such as an iron(III) halide. Both observations are explained by the electronic structure of the silsesquioxane core, which behaves as an electron-withdrawing group comparable in strength to a trifluoromethyl group, together with the steric congestion around the cage. The mechanistic basis has been established computationally and is treated separately in the entry on ortho self-bromination of octaphenyl-POSS, where the cage is shown to polarize the approaching Br₂ molecule and deliver Br^δ+ to the ortho carbon, performing in situ the function otherwise assigned to a Lewis acid catalyst.

The selectivity is not immutable. Adding iron dust to the bromination gives a product bearing bromine atoms in both the ortho and the meta positions, which confirms that the uncatalyzed pathway and the classical catalyzed pathway are genuinely different in their regiochemical outcome rather than merely different in rate. Iodination follows yet another course: treatment with iodine monochloride proceeds with high regioselectivity to place iodine in the para position, and the octa(para-iodophenyl) product is obtained in 90 to 95% yield for cages containing eight, ten or twelve silicon atoms alike.

Figure 3. Reactivity of octaphenyloctasilsesquioxane under electrophilic substitution. Bromine alone gives the ortho-brominated cage without any catalyst; bromine in the presence of FeBr₃ gives a product substituted at both the ortho and the meta positions; iodine monochloride delivers iodine selectively to the para position.

Halogen Bonding in the Crystal Structures of Bromo- and Iodophenyl Cages

Once halogens are installed on the periphery, the crystal chemistry of these cages is dominated by halogen bonding. In octakis(para-iodophenyl)octasilsesquioxane the I···I separation is 3.80 Å. The two C–I···I angles differ sharply, 90.6° at one iodine and 163.5° at the other, and that asymmetry is the diagnostic signature of a type II halogen bond, in which the electrophilic σ-hole of one halogen meets the nucleophilic equatorial belt of its partner.

A structural consequence follows from the geometry of the cage. Because the iodine atoms sit at the para positions, they point directly outward from the molecule, so the intermolecular contacts they form hold neighbouring cages apart rather than allowing them to nest. Solvent molecules used for crystallization become trapped between the arms, and guest species occupy 39% of the unit cell volume. A molecule designed as a rigid building block therefore delivers substantial void space as a by-product of its own substitution pattern.

Figure 4. Fragment of the crystal structure of octakis(para-iodophenyl)octasilsesquioxane, rendered from the deposited crystallographic coordinates. The I···I contacts measure 3.80 Å, with C–I···I angles of 90.6° and 163.5° characteristic of a type II halogen bond. Iodine is shown in violet, silicon in gold and oxygen in red.

The ortho-substituted bromo analogue behaves differently, as the altered substitution position would lead one to expect. In the crystal lattice of octakis(ortho-bromophenyl)octasilsesquioxane the intramolecular Br···Br contacts measure 3.54 Å, and intermolecular halogen bonds of 3.758 Å link neighbouring molecules into a four-bromine Br₄ structural motif. Moving the halogen from para to ortho thus changes the supramolecular outcome from a porous, solvent-filled lattice to a discrete recurring synthon, without any change to the cage itself.

Figure 5. Fragment of the crystal structure of octakis(ortho-bromophenyl)octasilsesquioxane, rendered from the deposited crystallographic coordinates. Intramolecular Br···Br contacts of 3.544 Å and intermolecular contacts of 3.758 Å assemble the Br₄ motif shown isolated at right. Bromine is shown in dark red, silicon in gold and oxygen in red.

Nitration and the Route to Octa(aminophenyl)-POSS

Nitration, Friedel–Crafts acylation and sulfonation of octaphenyloctasilsesquioxane all proceed at the ortho, meta and para positions in 80% yield. Nitration with fuming nitric acid gives a mixture of the three regioisomers in a 10:65:25 ratio, so the meta product dominates here, in contrast to the pronounced ortho preference of the uncatalyzed bromination. The nitro groups are then reduced to amines with a mixture of formic acid and triethylamine over palladium on carbon.

The product of that reduction, octa(aminophenyl)silsesquioxane, is one of the most heavily used cages in the whole field, because eight aromatic amines on a rigid monodisperse scaffold constitute a nanoconstruction site for imines, amides, polyimides and coordination networks. Its aliphatic counterpart, octa(3-aminopropyl)silsesquioxane, underpins much of the imine-functionalized POSS chemistry described elsewhere on this blog, and the comparison between the two is instructive: the aromatic amine is less nucleophilic and more sterically encumbered, but it is held rigidly and conjugated to the ring, which matters for optical and electronic applications in a way that a flexible propyl tether does not permit.

Figure 6. Preparation of an amino-functionalized derivative of octaphenyloctasilsesquioxane. Nitration with fuming HNO₃ affords a mixture of ortho, meta and para isomers in a 10:65:25 ratio, and the nitro groups are subsequently reduced with formic acid and triethylamine (Et₃N) over Pd/C.

Cross-Coupling and Outlook

The bromo- and iodophenylsilsesquioxanes participate in palladium-catalyzed coupling reactions, among them the Suzuki–Miyaura, Heck and Sonogashira processes, which makes it possible to construct polyhedral silsesquioxanes carrying elaborate side arms built from aromatic rings. Taken together with the halogenation chemistry above, this defines the position octaphenyl-POSS occupies. The cage is prepared in near-quantitative yield from a commodity chlorosilane, it is halogenated without a catalyst and with a regiochemistry it imposes on itself, and the resulting aryl halide is then handed to the standard toolkit of aromatic cross-coupling. Each of the eight arms carries the modification, so the product is octafunctional by construction rather than by statistics.

What remains comparatively unexploited is the solid-state dimension. The contrast between the solvent-filled lattice of the para-iodo cage and the compact Br₄ synthon of the ortho-bromo cage shows that halogen position alone switches the packing motif, which suggests that halogenated phenylsilsesquioxanes could be treated as deliberate crystal engineering tectons rather than merely as coupling substrates. Given how reliably the Si₈O₁₂ core enforces a known geometry, that would be an unusually well-defined starting point.

References

(1) Olsson, K. Improved Preparation of Octakis(Alkylsilsesquioxanes). Ark. Kemi 1958, 13, 367–378.

(2) Tateyama, S.; Kakihana, Y.; Kawakami, Y. Cage Octaphenylsilsesquioxane from Cyclic Tetrasiloxanetetraol and Its Sodium Salt. J. Organomet. Chem. 2010, 695, 898–902. https://doi.org/10.1016/j.jorganchem.2010.01.013

(3) Chinnam, P. R.; Gau, M. R.; Schwab, J.; Zdilla, M. J.; Wunder, S. L. The Polyoctahedral Silsesquioxane (POSS) 1,3,5,7,9,11,13,15-Octaphenylpentacyclo[9.5.1.1^3,9.1^5,15.1^7,13]octasiloxane (Octaphenyl-POSS). Acta Crystallogr. Sect. C Struct. Chem. 2014, 70, 971–974. https://doi.org/10.1107/S2053229614019834

(4) Roll, M. F.; Mathur, P.; Takahashi, K.; Kampf, J. W.; Laine, R. M. [PhSiO_1.5]₈ Promotes Self-Bromination to Produce [o-BrPhSiO_1.5]₈. J. Mater. Chem. 2011, 21, 11167–11176. https://doi.org/10.1039/C1JM11536G

(5) Bahrami, M.; Hashemi, H.; Ma, X.; Kieffer, J.; Laine, R. M. Why Do the [PhSiO_1.5]_8,10,12 Cages Self-Brominate Primarily in the Ortho Position? Phys. Chem. Chem. Phys. 2014, 16, 25760–25764. https://doi.org/10.1039/C4CP03997A

(6) Roll, M. F.; Asuncion, M. Z.; Kampf, J.; Laine, R. M. para-Octaiodophenylsilsesquioxane, [p-IC₆H₄SiO_1.5]₈, a Nearly Perfect Nano-Building Block. ACS Nano 2008, 2, 320–326. https://doi.org/10.1021/nn700196d

(7) Roll, M. F.; Kampf, J. W.; Laine, R. M. Halogen Bonding Motifs in Polyhedral Phenylsilsesquioxanes: Effects of Systematic Variations in Geometry or Substitution. Cryst. Growth Des. 2011, 11, 4360–4367. https://doi.org/10.1021/cg200312g

(8) Brick, C. M.; Tamaki, R.; Kim, S.-G.; Asuncion, M. Z.; Roll, M.; Nemoto, T.; Ouchi, Y.; Chujo, Y.; Laine, R. M. Spherical, Polyfunctional Molecules Using Poly(Bromophenylsilsesquioxane)s as Nanoconstruction Sites. Macromolecules 2005, 38, 4655–4660. https://doi.org/10.1021/ma047963k

(9) Asuncion, M. Z.; Roll, M. F.; Laine, R. M. Octaalkynylsilsesquioxanes, Nano Sea Urchin Molecular Building Blocks for 3-D-Nanostructures. Macromolecules 2008, 41, 8047–8052. https://doi.org/10.1021/ma801480p

(10) Tamaki, R.; Tanaka, Y.; Asuncion, M. Z.; Choi, J.; Laine, R. M. Octa(Aminophenyl)Silsesquioxane as a Nanoconstruction Site. J. Am. Chem. Soc. 2001, 123, 12416–12417. https://doi.org/10.1021/ja011781m

Full Citation Tateyama, S.; Kakihana, Y.; Kawakami, Y. "Cage Octaphenylsilsesquioxane from Cyclic Tetrasiloxanetetraol and Its Sodium Salt." Journal of Organometallic Chemistry, 2010, 695, 898–902.
DOI: 10.1016/j.jorganchem.2010.01.013
Full text: Journal of Organometallic Chemistry → ScienceDirect

---

Cite this post

This post has its own persistent identifier. Please use the DOI below when citing it.

Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry. (2026). Octaphenyl-POSS: Synthesis, Structure and Electrophilic Substitution. https://doi.org/