Simple and high yield access to octafunctional azido, amine and urea group bearing cubic spherosilicates

Abstract

Spherosilicates and polyhedral oligomeric silsesquioxanes represent unique well-defined rigid building blocks for molecular and hybrid materials. Drawbacks in their synthesis are often low yields and the restricted presence of functional groups either based on incomplete transformation of all corners or the reactivity of the functional groups. Particularly amine-functionalization reveals some synthetic challenges. In this study we report the synthesis of a new class of octafunctionalized hydrogen bond forming spherosilicates via a facile route based on octabromo alkyl functionalized cubic spherosilicates. Four different alkyl chain lengths, namely C₄, C₅, C₆ and C₁₁, were realized starting from ω-alkenylbromides via hydrosilylation of Q₈M₈^H. Using sodium azide in a mixture of acetonitrile : DMF = 10 : 1, the octaazide was obtained quantitatively and could be rapidly transformed in an octaamine cube via catalytic hydrogenation over Pd/C in absolute ethanol. The following reaction to hydrogen bond forming spherosilicates was performed in situ by adding propyl isocyanate. All transformations proceed quantitatively at the eight corners of the cube, which was evidenced by NMR spectroscopy and ESI-MS measurements. The Q₈-target compound can be separated after each reaction step over simple chemical workup while no cage rearrangement was observed. The structures were confirmed using ¹H, ¹³C, ²⁹Si-NMR, FT-IR, elemental analysis and ESI-MS. The method opens a high yield route (overall isolated yield 83–88%) for structural building blocks in hybrid materials.