Cyclodextrin-driven formation of double six-ring (D6r) silicate cage: NMR spectroscopic characterization from solution to crystals

Abstract

Identification and isolation of secondary building units (SBUs) from synthesis media of zeolites still represent a challenging task for chemists. The cage structure anion Si 12 O 3012− known as the double six-ring (D6R) was synthesized from α-cyclodextrin (α-CD) mediated alkaline silicate solutions and conditions of its stability and reactivity in aqueous solution were studied by using nuclear magnetic resonance (NMR) spectroscopy. A single crystal X-ray diffraction (XRD) analysis revealed a novel polymorph of the hybrid complex K 12 Si 12 O 30 ·2α-CD·nD 2 O (n ≈ 30–40), which crystallizes in the orthorhombic C222 1 space group symmetry with a = 14.841(4) Å, b = 25.855(6) Å, and c = 41.91(1) Å. The supramolecular adduct of the silicate anion sandwiched by two α-CDs forms a perfect symmetry matching the H-bonding donor-acceptor system between the organic macrocycle and the D6R unit. The driving force of such a hybrid assembly has found to be strongly dependent on the nature of the cation present as large alkali counter ions (K + , Rb + and Cs + ), which stabilize the D6R structure acting as templates. Lastly, we provided the first 29 Si MAS NMR measurement of 3 Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and bond angle for 3 Q Si species in DnR cages (n = 3, 4, 6).