Monofunctionalized POSS Self-Assembly and Pd Catalysis

Three studies, one published in Chemical Science in 2017 and two published in Chemistry – A European Journal and Journal of Catalysis in 2015, together define what a single reactive vertex on a silsesquioxane cage makes possible. The first two concern monofunctionalized POSS self-assembly, in which a platinum(II) terpyridine chromophore and a metal-free porphyrin, each carried by one cage, give solvent-switchable nanorings, nanoplates, vesicles and ordered sheets. The third turns the same architecture toward catalysis, where a cage-supported palladium center couples an aryl bromide with phenylboronic acid at a turnover number of 2.5 × 10⁴. Read together, the three papers show that one reactive arm is enough to program both morphology and catalytic behavior.

A Single Reactive Vertex on a Silsesquioxane Cage

A cubic silsesquioxane of the type R′R₇Si₈O₁₂ carries seven inert substituents and one reactive vertex, and that asymmetry is what converts an otherwise passive nanoscale filler into a molecular building block. The seven spectator groups set solubility, hydrophobicity and thermal behavior, while the eighth arm defines the single position at which further chemistry takes place. Because that site is unique, the products are discrete molecules rather than statistical mixtures, so their aggregation behavior and their reactivity can be interpreted without averaging over an unknown distribution of substitution patterns. The three studies discussed here exploit that property in different directions. Two of them use the rigid, strongly hydrophobic Si₈O₁₂ core as the driving force for solvent-controlled supramolecular assembly, and the third uses it as a soluble, well defined support for a palladium center.

Platinum(II) Terpyridine Complexes and Solvent Switchable Assembly

Yam and coworkers prepared a series of eight alkynylplatinum(II) terpyridine complexes, each bearing one POSS cage, through a reaction catalyzed by copper(I) between an alkyne carrying the cage and a chloroplatinum(II) terpyridine precursor. The first member of the series connects a heptaisobutyl cage to the platinum chromophore through an imine linkage (Scheme 1), and its behavior exposed the weakness of that choice. A red solution of this imine complex in 80% water in tetrahydrofuran reverted to yellow over several days, and the accompanying decay of the low energy absorption band was attributed to hydrolysis of the imine and loss of the hydrophobic cage. The remaining seven complexes therefore carry a more robust amide linkage. One of them is a direct amide analogue of the imine complex, and the others were modified systematically at the alkynyl ligand, at the terpyridine pincer ligand, at the cage substituents and through charged alkynyl tails in order to isolate the contribution of each interaction to the assembly process.

Scheme 1. Synthesis of the alkynylplatinum(II) terpyridine complex in which a heptaisobutyl POSS cage is attached to the alkynyl ligand through an imine linkage

The spectroscopic signature of assembly is unambiguous in this system. In tetrahydrofuran, the imine complex shows intraligand transitions at approximately 300 to 350 nm and a metal to ligand charge transfer (MLCT) band in the 400 to 500 nm region. Raising the water content depletes the MLCT band at 442 nm and drives the growth of a new shoulder near 560 nm, assigned to a metal metal to ligand charge transfer (MMLCT) transition, with well defined isosbestic points. A critical water content of about 62% is required to trigger this process. The emission tracks the same event, as the band at about 626 nm of predominantly triplet MLCT character loses intensity while a near infrared emission at about 765 nm, assigned to a triplet MMLCT state, grows in. Among the amide complexes bearing a heptaisobutyl cage, the phenylacetylide complex aggregates at roughly 56% water, its analogue with an extended π conjugated alkynyl ligand at 50% and its analogue lacking the phenyl spacer on the terpyridine at 54%, whereas the phenylacetylide complex with a heptaphenyl cage requires 58%. The authors ascribe the later onset of the heptaphenyl derivative to the lower hydrophobicity of its cage and the markedly earlier onset of the extended alkynyl derivative to stronger π–π stacking. Fitting the data for the heptaphenyl derivative to a nucleation elongation model gave ΔG⁰ of about −69 ± 6.1 kJ mol⁻¹, appreciably more negative than the values of −40 and −47 kJ mol⁻¹ reported for a simple organic system and an alkynylgold(I) system.

Electron and atomic force microscopy converted those thresholds into visible objects. At 30% water, the complex bearing the extended π conjugated alkynyl ligand forms nanorings with internal and external diameters of approximately 180 nm and 300 nm. Raising the water content to 60% turns the solution red and transforms the rings into platelike structures several micrometers long and about 12 nm thick (Fig. 1). The phenylacetylide analogue, whose alkynyl ligand offers a smaller π surface, produces only short fibers of helical appearance, a few hundred nanometers in length, at 30% water and no well defined morphology at higher water content. The comparison is instructive because the two complexes differ in a single structural element. According to the authors, the extended conjugated system strengthens the π–π stacking that, together with Pt···Pt interactions, favors one dimensional stacking of the alkynyl units and therefore ordered growth into plates.

Fig. 1. TEM images of the superstructures formed by the complex bearing the extended π conjugated alkynyl ligand (2 × 10⁻⁴ M) in (a) 30% and (b) 60% water in THF.¹

The two charged derivatives behave in a way that a simple hydrophobic model would not predict. The complex carrying a zwitterionic sulfobetaine tail is already aggregated in pure tetrahydrofuran, showing a low energy absorption band at about 505 nm and a structureless emission near 740 nm. Adding water first weakens the aggregate, turning the solution from red to yellow at 50% water, and then restores it, returning the solution to red at 70% water. The sequence corresponds to aggregation, partial deaggregation and renewed aggregation as hydrophilic and hydrophobic driving forces exchange dominance, and electron microscopy follows it through short fibrils at 10% water, spherical aggregates at 30% and circular aggregates at 50% to platelike species at 70% water. The complex carrying an anionic sulfonate tail is overall neutral, which reduces electrostatic repulsion between the molecules, and it is extraordinarily sensitive to water. Addition of only 0.5% water turns its yellow solution pink and gives rise to a low energy MMLCT band, and rodlike aggregates are observed at low water content. The authors therefore regard this class of complexes as candidates for sensing, which places them alongside other fluorescent silsesquioxane sensors.

Porphyrin Hybrids without a Metal Center

Liu, Zhang and co-workers reached comparable morphological control without any metal center. Their hybrid, a metal-free tetrakisphenyl porphyrin bearing a POSS cage and abbreviated H₂TPP-POSS, was assembled by copper-catalyzed azide-alkyne cycloaddition between an azide-functionalized cage and an alkyne-terminated porphyrin (Scheme 2). Concentration alone switches the outcome in chloroform, where solutions below 10⁻⁴ M give nanovesicles and solutions above 10⁻⁴ M give diffuse microrods. Solvent polarity provides a second, independent control, and across a range of solvents the same molecule delivers crescent-shaped micelles, spherical micelles, doughnut-shaped vesicles and ordered square sheets (Scheme 3). Since the porphyrin supplies π–π stacking, hydrogen bonding and a flat aromatic surface while the cage supplies a rigid hydrophobic volume, the hybrid behaves as an amphiphile in which both blocks are molecular rather than polymeric, a design also exploited in light-responsive amphiphilic POSS molecules.

Scheme 2. Synthesis of the metal-free porphyrin–silsesquioxane hybrid H₂TPP-POSS: copper-catalysed azide–alkyne cycloaddition between the azidopropyl heptaisobutyl cage and an alkyne-terminated zinc tetraphenylporphyrin, followed by demetalation with HCl and neutralisation with Et₃N.²

Scheme 3. Solvent-switchable self-assembly of H₂TPP-POSS. In methanol the hybrid forms hollow vesicles built from a bilayer of porphyrin and cage units; in chloroform the same molecule packs into lamellar ribbons. The process is reversible on exchanging the solvent.²

Palladium Catalysis on a Silsesquioxane Support

The third study turns the same synthetic logic toward catalysis. Ervithayasuporn and co-workers prepared pyridine-triazole ligands on azide-functionalized silsesquioxane cages and treated them with Pd(COD)Cl₂, where COD denotes 1,5-cyclooctadiene, to give the mononuclear T₈ complex and the multinuclear T₁₀ material, the latter carrying approximately 4.6 palladium centers per cage and an unusually high loading of 1.61 mmol Pd g⁻¹ (Scheme 4). In a 1:1 ethanol and water mixture the soluble mononuclear complex operates homogeneously with an initial turnover frequency of 870 h⁻¹, only modestly above the 690 h⁻¹ of the insoluble amorphous material. The heterogeneous catalyst compensates on every other axis. Coupling 4-bromoanisole with phenylboronic acid at a loading of 3.6 × 10⁻³ mol% palladium under aerobic conditions gave the product in 91% yield, corresponding to a turnover number of 2.5 × 10⁴, and the solid was recovered by centrifugation and reused for at least five cycles without loss of activity. Single-crystal X-ray diffraction on the mononuclear complex confirms the square planar palladium centre, chelated by the pyridyl–triazole unit and completed by two chlorides, with the silsesquioxane cage held at the end of a flexible propyl tether (Fig. 2).

Scheme 4. Synthesis of the palladium(II) complex on a monofunctionalized silsesquioxane cage: copper-catalysed cycloaddition between the azide-bearing heptaisobutyl cage and 2-ethynylpyridine, followed by ligand exchange with [Pd(COD)Cl₂]. The mononuclear T₈ complex is shown; the multinuclear T₁₀ catalyst is made the same way from the decameric cage.³

Fig. 2. Molecular structure of the palladium(II) complex in which the cubic silsesquioxane acts as the ligand. The palladium centre is square planar, bound to the two nitrogen atoms of the pyridyl–triazole chelate and to two chlorides.³

Conclusions

Taken together, these three papers make a consistent case. A silsesquioxane cage with one reactive vertex is not merely a bulky substituent but a design element whose hydrophobicity, rigidity and defined stoichiometry set the terms on which the attached functional unit behaves. In the platinum and porphyrin systems that translates into morphologies selectable by solvent composition and concentration, with the cage substituent shifting the aggregation threshold by about two percentage points of water. In the palladium system it translates into a support that keeps a single metal center soluble and accessible, or, when the cage is multiply functionalized, into a recoverable solid of exceptional metal density. The same principle recurs in POSS difluoroboron photocatalysts, and it suggests that the productive question is no longer whether a cage can be attached, but which single property of the cage the target application actually requires.

References

1. Au-Yeung, H.-L.; Tam, A. Y.-Y.; Leung, S. Y.-L.; Yam, V. W.-W. Supramolecular assembly of platinum-containing polyhedral oligomeric silsesquioxanes: an interplay of intermolecular interactions and a correlation between structural modifications and morphological transformations. Chem. Sci. 2017, 8, 2267–2276. https://doi.org/10.1039/C6SC04169H
2. Liu, F.; Zhang, Y.; Xu, L.; Zhang, W. Morphology-controlled self-assembly of an organic/inorganic hybrid porphyrin derivative containing polyhedral oligomeric silsesquioxane (POSS). Chem. Eur. J. 2015, 21, 5540–5547. https://doi.org/10.1002/chem.201405334
3. Ervithayasuporn, V.; Kwanplod, K.; Boonmak, J.; Youngme, S.; Sangtrirutnugul, P. Homogeneous and heterogeneous catalysts of organopalladium functionalized-polyhedral oligomeric silsesquioxanes for Suzuki–Miyaura reaction. J. Catal. 2015, 332, 62–69. https://doi.org/10.1016/j.jcat.2015.09.014

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Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry. (2017). Monofunctionalized POSS Self-Assembly and Pd Catalysis. https://doi.org/10.59350/1zhj1-qrw92