Cavity-Directed Synthesis of Labile Silanol Oligomers within Self-Assembled Coordination Cages

This paper introduces a concept that is referred to as cavity-directed synthesis by showing the selective oligomerization of trialkoxysilanes, RSi(OMe)₃ (7), in self-assembled hollow compounds. Pd(II)-linked coordination hosts (cage, bowl, or tube) are found to strictly control the oligomerization of 7 (R = 2-naphthyl) in such a way that their optimal guests are produced in their cavities. Thus, within coordination tube 1, one molecule of 7 is accommodated and subsequently hydrolyzed to give silanetriol RSi(OH)₃ (4). Under ordinary aqueous conditions, this reactive compound undergoes rapid polycondensation (so-called sol−gel condensation) leading to Si−O networks. Within the cavity of 1, however, 4 remains very stable and the polycondensation is completely suppressed. On the other hand, coordination bowl 2 and cage 3 give its dimers RSi(OH)₂OSi(OH)₂R (5) and cyclic trimers [RSi(OH)O]₃ (6), respectively. X-ray crystallographic studies clearly show that the cavity size and the shape of 1, 2, and 3 nicely fit with those of 4, 5, and 6, respectively, demonstrating that the cavities strictly direct the oligomerizaion reaction of 7.