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 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.

Yam and co-workers assembled a series of eight alkynylplatinum(II) terpyridine complexes, each bearing one POSS cage, by the copper(I)-catalyzed reaction of a POSS-functionalized alkyne with a chloroplatinum(II) terpyridine precursor. Complex 1 connects the heptaisobutyl cage to the platinum chromophore through an imine linkage, and its behavior exposed the weakness of that choice. A red solution of 1 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. Complexes 2 to 8 therefore replaced the imine with an amide linker, and were further modified at the alkynyl ligand, at the pincer ligand and at the cage substituents in order to isolate the contribution of each interaction to the assembly process.

Scheme 1. Structures of the POSS-functionalized alkynylplatinum(II) terpyridine complexes

The spectroscopic signature of assembly is unambiguous in this system. In tetrahydrofuran, complex 1 shows intraligand transitions at approximately 300–350 nm and a metal-to-ligand charge transfer band centered at 442 nm. Raising the water content drives the growth of a new shoulder near 560 nm of metal-metal-to-ligand charge transfer origin, with well defined isosbestic points, and a critical water content of about 62% is required to trigger the process. The emission tracks the same event, shifting from a band at about 626 nm of triplet metal-to-ligand charge transfer character to a near-infrared feature at about 765 nm. Cage hydrophobicity governs the threshold. The heptaisobutyl-substituted complexes 3, 4 and 6 aggregate at roughly 56%, 50% and 54% water, whereas the heptaphenyl analogue 5 requires 58%. Fitting the data for 5 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 purely organic system and an alkynylgold(I) system.

Electron and atomic force microscopy converted those thresholds into visible objects. At 30% water, complex 4, which carries an 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 plate-like structures several micrometers long and about 12 nm thick. Complex 3, whose phenyl-substituted alkynyl ligand offers a smaller π surface, produces only short helical fibers 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 one structural element. Extending the conjugated system strengthens the π–π stacking that, together with Pt⋅⋅⋅Pt contacts, enforces one-dimensional stacking and therefore ordered growth.

Fig. 1. Obrazy TEM agregujące struktury kompleksu platyny(II) z niesymetrycznym silseskwioksanem 2 w: (a) 30% roztworze wody w THF, (b) w 60% roztworze wody w THF.1

The two charged derivatives behave in a way that a simple hydrophobic model would not predict. Complex 7, which carries 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 breaks 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. Complex 8, in which an anionic sulfonate confers overall charge neutrality and removes electrostatic repulsion, is extraordinarily sensitive. Addition of only 0.5% water turns the yellow solution pink and generates rod-like aggregates, which places this class alongside other fluorescent silsesquioxane sensors as candidates for optical detection of solvent composition.

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. 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. 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.

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 3 and the multinuclear T₁₀ material 6, the latter carrying approximately 4.6 palladium centers per cage and an unusually high loading of 1.61 mmol Pd g⁻¹. 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.

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 several percent 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.

Full Citations 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.” Chemical Science, 2017, 8, 2267–2276.
DOI: 10.1039/C6SC04169H
Full text: Chemical Science → Royal Society of Chemistry

Liu, F.; Zhang, Y.; Xu, L.; Zhang, W. “Morphology-Controlled Self-Assembly of an Organic/Inorganic Hybrid Porphyrin Derivative Containing Polyhedral Oligomeric Silsesquioxane (POSS).” Chemistry – A European Journal, 2015, 21, 5540–5547.
DOI: 10.1002/chem.201405334
Full text: Chemistry – A European Journal → Wiley

Ervithayasuporn, V.; Kwanplod, K.; Boonmak, J.; Youngme, S.; Sangtrirutnugul, P. “Homogeneous and Heterogeneous Catalysts of Organopalladium Functionalized-Polyhedral Oligomeric Silsesquioxanes for Suzuki-Miyaura Reaction.” Journal of Catalysis, 2015, 332, 62–69.
DOI: 10.1016/j.jcat.2015.09.014
Full text: Journal of Catalysis → ScienceDirect