Octa-Imine POSS: Crystal Structure and Thermal Properties

Four novel octa-imine-functionalized polyhedral oligomeric silsesquioxane (POSS) derivatives, designated POSS-6 through POSS-9, have been reported by Mateusz Janeta and Sławomir Szafert in the Journal of Molecular Structure in 2025. These compounds were obtained by modular Schiff base condensation of octa(3-aminopropyl)silsesquioxane hydrochloride with the appropriate aromatic aldehyde in isolated yields ranging from 84 to 92 percent without chromatographic purification, and their structures were fully established by single-crystal X-ray diffraction. The identity of the peripheral substituent (hydroxyphenyl, bromophenyl, thiophene, or dibenzofuran) was found to govern both the non-covalent interaction topology in the solid state and the thermal behavior, with T_5% decomposition onset temperatures spanning from 305 to 326 °C across the four derivatives.

Synthesis of Octa-Imine POSS Derivatives by Schiff Base Condensation

All four compounds were obtained by modular Schiff base condensation of octa(3-aminopropyl)silsesquioxane hydrochloride with the appropriate aromatic aldehyde in methanol at room temperature in the presence of triethylamine as the base, which afforded the fully octa-substituted products in high isolated yields of 84% to 92% without any chromatographic purification. The integrity of the T₈ siloxane cage under these conditions was confirmed by ²⁹Si NMR spectroscopy, which revealed a single symmetric resonance near −66.7 ppm for all four derivatives, and by high-resolution mass spectrometry, which showed only molecular ions consistent with the intact T₈ framework.

Figure 1. Molecular structures of POSS-6, POSS-7, POSS-8, and POSS-9 determined by single-crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Color code: Si (gold), O (red), C (gray), N (blue), S (yellow), Br (brown).

Single-Crystal X-Ray Diffraction Analysis and Crystal Packing of POSS-6 through POSS-9

Single-crystal X-ray diffraction analysis constitutes the central structural contribution of this study. POSS-6, POSS-7, and POSS-8 crystallize in the triclinic centrosymmetric space group P-1, with half of the molecule in the asymmetric unit and the complete molecule generated by an inversion center. In each case the T₈ silsesquioxane cage adopts a square-prismatic geometry that deviates from idealized D_4h symmetry, with Si–O–Si bridging angles ranging from approximately 137° to 156° depending on the compound, which reflects the steric and electronic influence of the peripheral substituent on the siloxane framework. The Si–O bond lengths fall consistently between 1.615 and 1.637 Å, and the O–Si–O angles approach the ideal tetrahedral value of 109.5°, indicating local geometric regularity within each siloxane unit despite the overall distortion of the cage.

Figure 2. View of the three-dimensional network in the crystal structure of POSS-6. Yellow dotted lines indicate O···Si contacts; purple dotted lines indicate intramolecular O–H···N hydrogen bonds. The inset highlights the local geometry of these interactions. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity.

Non-Covalent Interactions and Host-Guest Behavior in the Imine-POSS Solid State

The supramolecular organization of POSS-6 in the solid state is governed by strong intermolecular O–H···N hydrogen bonds between the phenolic hydroxyl group and the imine nitrogen atoms of adjacent molecules, with H···N distances ranging from 1.82 to 1.88 Å and O–H···N angles of approximately 147° to 148°. These primary interactions are supplemented by O···Si contacts at 3.610 Å, approximately 2.2% shorter than the sum of the van der Waals radii, and by weaker C–H···O and C–H···C interactions, which together organize the molecules into an extended layered three-dimensional architecture.

Figure 3. View of the three-dimensional network in the crystal structure of POSS-7. Purple dotted lines indicate the non-covalent Br···H, Br···C, and C–H···N contacts that stabilize the crystal lattice. Hydrogen atoms not involved in these interactions are omitted for clarity.

In POSS-7, the eight 3-bromophenyl substituents introduce a rich network of halogen-based non-covalent interactions. Directional C–H···N hydrogen bonds and multiple short Br···H and Br···C contacts, several of which fall 1% to 5% below the sum of the van der Waals radii, together stabilize the crystal lattice and govern the packing of the brominated molecules. The Si–O–Si angles in POSS-7 range from 146.15° to 151.25° and the C–Br bond lengths fall between 1.866 and 1.907 Å, which confirms the expected structural configuration of the appended 3-bromophenyl imine groups.

Figure 4. View of the three-dimensional network in the crystal structure of POSS-8. Orange dashed lines indicate S···Si chalcogen bonding contacts; blue dashed lines indicate intramolecular S···N interactions; yellow dashed lines indicate S···O contacts. Hydrogen atoms are omitted for clarity.

POSS-8, functionalized with thiophene-containing imine groups, is distinguished by a pronounced set of directional chalcogen bonding interactions that are critical to its supramolecular organization. Each thiophene moiety participates in S···Si contacts at 3.856 Å, approximately 5.8% shorter than the sum of the van der Waals radii, and in S···O interactions at 3.319 Å, approximately 2.1% below the corresponding threshold. Particularly significant are the intramolecular S···N contacts at 3.086 Å, approximately 15% shorter than the van der Waals limit, which reflect pronounced electronic communication between the sulfur atom of the thiophene ring and the adjacent imine nitrogen and contribute substantially to the conformational rigidity of the peripheral linker units. The Si–O–Si angles in POSS-8 span the broadest range of the series, from 137.78° to 156.26°, which reflects the greatest degree of cage distortion among the four compounds.

Figure 5. View of the three-dimensional packing in the crystal structure of POSS-9 (a) and surface representation of the quasi-one-dimensional solvent-accessible channels formed by the arrangement of the dibenzofuran peripheral groups (b). The chloroform-filled channels extend along the crystallographic a axis and account for 17.8% of the unit cell volume.

POSS-9, which bears the most sterically demanding dibenzofuran substituents, presents a qualitatively distinct solid-state organization. Crystallographic analysis reveals that chloroform molecules are encapsulated within the lattice, occupying quasi-one-dimensional channels that extend along the crystallographic a axis and are formed by the arrangement of the peripheral dibenzofuran groups around the POSS cage. The solvent-accessible void volume, determined by PLATON analysis, corresponds to 320.26 ų, which represents 17.8% of the unit cell volume. The encapsulated chloroform molecules are stabilized by van der Waals forces and C–H···Cl interactions. Removal of the crystallization solvent from the lattice results in the appearance of new diffraction peaks in the powder X-ray diffraction pattern, indicating a phase transformation driven by the collapse or reorganization of the host–guest framework upon desolvation.

Figure 6. Thermogravimetric analysis (TGA) traces of POSS-6 through POSS-9 recorded under oxidative conditions (O₂:N₂ = 40:60, heating rate 10 °C per minute). Decomposition onset temperatures (T_5%) and theoretical ceramic yields for complete conversion to SiO₂ (in parentheses) are indicated.

Thermogravimetric Analysis and Ceramic Yield of Octa-Imine Silsesquioxane Derivatives

Thermogravimetric analysis under both oxidative and inert atmospheres confirmed high thermal stability across the series, with decomposition onset temperatures (T_5%) ranging from 305 °C for POSS-6 and POSS-7 to 326 °C for POSS-9. Thermal degradation in air proceeds through two principal mass-loss stages, which correspond to decomposition of the organic periphery and to the subsequent oxidative conversion of the silsesquioxane framework to silicon dioxide. The ceramic yields at 800 °C closely matched the theoretical predictions for complete formation of SiO₂, which confirms the clean and predictable degradation pathway of these hybrid materials.

Structure-Property Relationships in POSS-Based Hybrid Organic-Inorganic Materials

The findings of this study provide a crystallographically validated structure–property map for four distinct imine-POSS systems and demonstrate that the choice of peripheral substituent precisely controls both the nature of the non-covalent interactions in the solid state and the resulting thermal behavior. These results extend a sustained research program by the authors, which has previously established imine-POSS compounds as scaffolds for porous frameworks with exceptional iodine vapor adsorption, as multinuclear zinc complexes that catalyze the cycloaddition of CO₂ under ambient conditions, and, most recently, as difluoroboron-functionalized POSS photocatalysts for the selective oxidation of sulfides to sulfoxides. The present work advances the rational design of thermally stable, structurally well-defined POSS-based hybrid materials for applications in catalysis, environmental remediation, sensing, and molecular electronics.

Full Citation

Full CitationMateusz Janeta, Sławomir Szafert. “Rationally Designed Octa-Imine Silsesquioxanes: Insights into Structural and Thermal Properties.” Journal of Molecular Structure, 2025.
DOI: 10.1016/j.molstruc.2025.143517
Full text: ScienceDirect, Journal of Molecular Structure
Crystallographic data: CCDC 1873806 (POSS-6), 1522027 (POSS-7), 2453809 (POSS-8), and 2453808 (POSS-9), available free of charge at www.ccdc.cam.ac.uk.