Imine-POSS Crystal Structures and 3D Supramolecular Networks

Imine-POSS nanobuilding blocks for supramolecular hybrid materials, synthesized and structurally characterized by Janeta et al. at the University of Wrocław and published in Dalton Transactions in 2016, constitute the first complete series of fully octa-imino-functionalized polyhedral oligomeric silsesquioxane (POSS) compounds, with five single-crystal X-ray diffraction structures reported for the first time. The five compounds bearing phenyl, 4-bromophenyl, 4-nitrophenyl, 2-hydroxyphenyl, and 2-naphthyl groups form three-dimensional supramolecular networks governed by halogen bonds, CH⋅⋅⋅N contacts, and intramolecular hydrogen bonds. The 4-nitrophenyl derivative assembles a porous framework whose guest-accessible channels occupy approximately 20% of the unit cell volume.

Synthesis of Octa-Imino-POSS from Bench-Stable OAS-POSS Precursor Salts

The synthesis of the target compounds rests on the post-synthetic modification of air-stable octa(3-aminopropyl)silsesquioxane salts, prepared by one-step hydrolytic condensation of (3-aminopropyl)triethoxysilane with hydrochloric or trifluoromethanesulfonic acid, yielding [OAS-POSS-NH₃]Cl and [OAS-POSS-NH₃]CF₃SO₃, respectively. Free octa(3-aminopropyl)silsesquioxane is thermally unstable in solution and requires storage at low temperature under anhydrous conditions; the protonated salts circumvent this limitation entirely and serve as convenient, bench-stable precursors for imine formation. Condensation of the hydrochloride salt with five structurally diverse aryl aldehydes in the presence of triethylamine at room temperature delivered compounds 1–5 in high isolated yields after flash chromatography.

Scheme 1. Solvent-dependent self-assembly of compound 2 (4-bromophenyl imine-POSS): spherical nanoparticles (~50 nm) form in CHCl₃, whereas rod-like morphologies grow from MeOH, driven by the directional Br⋅⋅⋅Br halogen bonding network identified by single-crystal X-ray diffraction.

The aryl substituents were deliberately chosen to cover a wide range of electronic and steric environments: phenyl (1), 4-bromophenyl (2), 4-nitrophenyl (3), 2-hydroxyphenyl (4), and 2-naphthyl (5). All condensations were conducted at room temperature, as elevated temperatures promote partial decomposition of the siloxane cage. In solution, each compound exhibits idealized O_h symmetry, confirmed by single sets of symmetry-equivalent resonances in the ¹H, ¹³C, and ²⁹Si NMR spectra. The characteristic imine CH=N signal appears at approximately δ = 8.31 ppm in ¹H NMR, while the ²⁹Si NMR resonance at δ = −66.5 ppm confirms the exclusive presence of alkyl-substituted T8 silicon nuclei. High-resolution mass spectrometry confirmed the identity and purity of all five compounds.

A noteworthy side reaction was identified when the triflate salt was used in place of the hydrochloride: the CF₃SO₃⁻ anion promotes T8 → T10 cage rearrangement, consistent with the previously described CF₃SO₃⁻/H₂O-mediated reorganization mechanism. The T10 product was identified by ²⁹Si NMR, which showed a second resonance at δ = −68.7 ppm characteristic of D_5h cage geometry alongside the T8 signal at δ = −66.5 ppm. This observation establishes that the choice of counterion in the OAS-POSS precursor salt critically determines whether Si₈O₁₂ cage integrity is preserved throughout the condensation.

Five Single-Crystal X-Ray Diffraction Structures: Core Geometry and Cage Distortion

The most structurally significant contribution of this study is the determination of five single-crystal X-ray diffraction structures, deposited with the Cambridge Crystallographic Data Centre as CCDC 1456557–1456561. These represent the first reported solid-state structures of fully octa-imino-functionalized cage-like silsesquioxanes. All five compounds crystallize in the triclinic system and the centrosymmetric P−1 space group. The asymmetric unit in each case comprises four silicon atoms and four organoimine side chains covalently bonded to the Si₈O₁₂ core. The geometry of the siloxane cage is consistent with established T8-type ranges: Si–O bond lengths of 1.55–1.65 Å, O–Si–O angles of 107–112°, and Si–O–Si angles of 145–152°.

A recurring structural feature is the “overstretching” of the Si₈O₁₂ core relative to ideal cubic symmetry: the steric demands of the n-propyl-aryl-imine side chains exert an outward repulsive force that elongates the cage along one crystallographic axis, rendering it more cuboid than cubic in the solid state. This distortion is absent in solution, where all silicon atoms remain equivalent by ²⁹Si NMR. Compound 2 displays the most pronounced deformation, with non-bonding Si⋅⋅⋅Si distances spanning 5.387(2)–5.433(2) Å and estimated cage dimensions of approximately 3.074 × 3.133 × 3.148 Å.

Fig. 1. Single-crystal X-ray diffraction view of the supramolecular packing of compound 2 (4-bromophenyl imine-POSS) along the b axis. Blue dotted lines: Br⋅⋅⋅Br type I halogen bond contacts at 3.497 and 3.540 Å (5.5% below the van der Waals sum). Violet dotted lines: Si⋅⋅⋅Br short contacts at 3.867 Å. The n-propyl spacer and these directional interactions enforce a cylindrical C_2v geometry unique among functionalized POSS species. Reproduced from Janeta et al., Dalton Trans. 2016, 45, 12312 (The Royal Society of Chemistry).

Three-Dimensional Supramolecular Networks: Halogen Bonds, CH⋅⋅⋅N Contacts, and Porous Channels

The three-dimensional supramolecular networks formed by compounds 1–5 differ markedly in topology. In compound 1, the Si₈O₁₂ clusters assemble into a 3D grid through CH_Ar⋅⋅⋅N and CH_Ar⋅⋅⋅C interactions. Compound 2 uniquely adopts a cylindrical C_2v packing enforced by directional Br⋅⋅⋅Br and Si⋅⋅⋅Br contacts, an arrangement not previously observed for functionalized polyhedral oligomeric silsesquioxane materials. Compound 3 (4-nitrophenyl) exhibits a hierarchical two-level self-assembly: primary grid modules formed by N⋅⋅⋅O and O⋅⋅⋅C_Ar contacts organize further through C–H⋅⋅⋅O and CH⋅⋅⋅HC interactions into a layered 3D structure with gated void channels.

Fig. 2. Single-crystal X-ray diffraction view of the 3D interpenetrated supramolecular network in compound 3 (4-nitrophenyl imine-POSS) along the b axis. Two independent framework strands (red and green) are interdigitated. Chloroform guest molecules occupy the gated void channels, constituting approximately 20% of the unit cell volume (PLATON). Reproduced from Janeta et al., Dalton Trans. 2016, 45, 12312 (The Royal Society of Chemistry).

PLATON calculations indicate that the free void space within the crystal of compound 3 constitutes approximately 20% of the unit cell volume and accommodates four chloroform molecules per formula unit, confirmed independently by thermogravimetric analysis of isolated single crystals. After thermal removal of solvent, new diffraction peaks appear in the powder X-ray diffraction (PXRD) pattern at 2θ = 7.47°, 11.54°, and 22.86°, indicating formation of an additional crystalline phase. The occurrence of guest-accessible channels within an octa-functionalized POSS assembly demonstrates that imine-POSS nanobuilding blocks can be used to engineer porous hybrid frameworks with defined void architectures.

Salicylidenimine Tautomerism and Reduction to Secondary Amine-POSS

Among the most chemically nuanced findings is the tautomeric behavior of compounds 4 and 5, which bear 2-hydroxyphenyl and 2-naphthyl substituents, respectively. Both belong to the salicylidenimine class, in which the position of the transferable proton in the intramolecular O–H⋅⋅⋅N hydrogen bond defines whether the enol-imine or keto-enamine tautomer predominates. Compound 4 crystallizes in the enol-imine form: the C–O bond length of 1.351 Å is consistent with a phenolic C–O single bond (reference: 1.360 Å), and the C–N distance of 1.280(5) Å closely matches the reference value for C=N (1.279 Å). Compound 5 instead adopts a zwitterionic form in the solid state, with C–O distances of 1.245–1.326 Å and C–N distances of 1.316–1.439 Å intermediate between enol-like and enamine-like values, which significantly depresses its melting point (78 °C) relative to compound 4 (124 °C).

Scheme 2. Enol-imine / keto-enamine tautomeric equilibrium in compound 5 (2-naphthyl imine-POSS). The yellow-green enol-imine form predominates in non-polar media; the orange keto-enamine tautomer is favored in polar solvents. A zwitterionic form is found in the solid state by single-crystal X-ray diffraction.

In solution, the tautomeric equilibrium of compound 5 is strongly solvent-dependent. In non-polar CDCl₃, singlet resonances at δ = 8.64 and 14.45 ppm are assigned to the CH=N and O–H protons of the enol-imine tautomer. In polar DMSO-d₆, a doublet of triplets at δ = 14.10 ppm (³J_NH,H = 10.1, ¹J_N,H = 5.0 Hz) and a doublet at δ = 7.96 ppm (³J_HH = 8.3 Hz) are assigned to the =CH–NH fragment of the keto-enamine form. UV-Vis absorption at approximately 400 nm confirms the orange keto-enamine form in polar media, while absorption at 420 nm indicates the yellow-green enol-imine form in non-polar solvents.

Starting from imine-POSS 1–5, secondary amine-POSS compounds 6–10 were prepared by selective reduction with sodium triacetoxyborohydride under mild anhydrous conditions at room temperature, with no detectable cage decomposition confirmed by high-resolution mass spectrometry. This approach avoids the limitations of earlier nucleophilic substitution routes that required elevated temperatures (110 °C, 24 h) with accompanying cage rearrangement, and it demonstrates that imine-POSS derivatives provide a chemically versatile platform for the preparation of otherwise difficult-to-access secondary amine-POSS compounds. Notably, the imine-POSS framework described in this study has subsequently served as the direct synthetic precursor to difluoroboron-functionalized POSS photocatalysts capable of the selective aerobic photooxidation of sulfides to sulfoxides. From the standpoint of thermal properties, imine-POSS derivatives are substantially more thermally stable than their secondary amine counterparts: T_5% values for compounds 1–5 span 270–306 °C, whereas the T_5% of the corresponding secondary amine analogue falls to approximately 167 °C, a reduction of more than 125 °C. The two-stage TG-DTA decomposition pattern characteristic of functionalized POSS was observed throughout, with final SiO₂ residue masses agreeing with theoretical values to within 0.2%, confirming a sample purity of at least 99.8%.

Taken together, this study establishes a robust synthetic protocol for fully octa-imino-functionalized polyhedral oligomeric silsesquioxane under mild conditions and resolves five previously unknown single-crystal X-ray diffraction structures that reveal a rich diversity of three-dimensional supramolecular architectures: halogen-bonded cylindrical assemblies, porous interpenetrated networks with guest-accessible channels, and hydrogen-bonded frameworks governed by enol-imine and keto-enamine tautomerism. The demonstrated control over supramolecular topology and self-assembled morphology through aryl substituent variation and solvent selection positions imine-POSS as versatile nanobuilding blocks for the design of functional hybrid organic-inorganic materials. This line of research, which extends to catalyst-free solid-state polymerization of octa-functional POSS monomers, continues to expand the structural and functional landscape of silsesquioxane-based hybrid materials from the Janeta and Szafert groups at the University of Wrocław.

Full Citation“Multifunctional imine-POSS as uncommon 3D nanobuilding blocks for supramolecular hybrid materials: Synthesis, structural characterization, and properties.” Dalton Transactions, 2016, 45, 12312–12325.
DOI: 10.1039/c6dt02134d
Full text: Dalton Transactions → Royal Society of Chemistry
CCDC deposition numbers: 1456557 (compound 1), 1456558 (compound 2), 1456559 (compound 3), 1456560 (compound 4), 1456561 (compound 5)