Hydrophobic stannaboroxine-based cyclic oligomers

Herein, we report the synthesis of a systematic series of stannaboroxine-based compounds and demonstrate their use as precursors for hydrophobic thin layers. A formyl-substituted stannaboroxine Ar(Ph)Sn(O₃B₂(4-CH[double bond, length as m-dash]O-C₆H₄)₂) (1), where Ar is 2,6-(Me₂NCH₂)₂C₆H₃, was prepared as a robust difunctional building block and subsequently converted via Schiff coupling into imino-functionalized stannaboroxines bearing alkyl and trialkoxysilyl substituents (Ar(Ph)Sn(O₃B₂(4-CH[double bond, length as m-dash]N(C₁₈H₃₇)-C₆H₄)₂) (2) and Ar(Ph)Sn(O₃B₂(4-CH[double bond, length as m-dash]N(C₃H₇Si(OEt)₃-C₆H₄)₂)) (3). Extension of this strategy to reactions of 1 with various diamines (1,4-diaminobenzene and 1,2-diaminoethane) afforded cyclic stannaboroxine hexamer (4) and dimer (5). Compounds 2-5 were processed into thin coatings by spin-coating on PE, silicon wafer and glass substrates. Surface properties were systematically analyzed by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Variable Angle Spectroscopic Ellipsometry (VASE), UV-Vis spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The water contact angle (WCA) measurement revealed that the ethylenediamine-bridged cyclic dimer 5 provides smooth, transparent layers with WCA up to ∼126°, while formulation with SiO₂ nanoparticles further enhanced the WCA to ∼146°, due to higher roughness of the thin layers. These findings establish stannaboroxine-based oligomers as a versatile, fluorine-free platform for advanced hydrophobic coating materials.