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 Si12O3012− 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 K12Si12O30·2α-CD·nD2O (n ≈ 30–40), which crystallizes in the orthorhombic C2221 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 29Si MAS NMR measurement of 3Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and <SiOSi> bond angle for 3Q Si species in DnR cages (n = 3, 4, 6).

Hexagonal Prismatic Siloxanes Functionalized with Organosilyl Groups as Building Blocks of Nanoporous Materials

“…The D6R siloxane has a unique structure with a D 6h symmetry, which differs from the recently reported cage silsesquioxanes R 12 Si 12 O 18 (R=styryl) with a D 2d symmetry [8h] . Hydrolysis and condensation of tetraalkoxysilane in the presence of KOH and α‐cyclodextrin (αCD) lead to the selective formation of D6R‐type potassium silicate sandwiched by two αCDs (K 12 Si 12 O 30 ⋅ 2αCD ⋅ 36H 2 O, D6R‐αCD ) [8f,g] . D6R siloxanes exhibit larger sizes and cavities compared to D4R siloxanes.…”

Alkoxy- and Silanol-Functionalized Cage-Type Oligosiloxanes as Molecular Building Blocks to Construct Nanoporous Materials

“…For example, in the presence of tetramethylammonium (TMA) cations, cubic silicate (Si 8 O 20 8− ) with a D4R structure is almost quantitatively formed (Figure 2a), while double-three-ring (D3R) and D5R silicates become dominant in the presence of tetraethylammonium (TEA) cations and tetrabutylammonium (TBA) cations, respectively [16,22]. In addition, D6R silicate anion (Si 12 O 30 12− ) can be obtained as an inclusion complex with α-cyclodextrin [23,24]. Furthermore, cage-type organosiloxane oligomers can be selectively formed by hydrolysis and polycondensation of a methylene-bridged bis-trialkoxysilane [(EtO) 3 Si-CH 2 -Si(OEt) 3 ] in the presence of TMAOH [19].…”

Direct cross-linking of silyl-functionalized cage siloxanes via nonhydrolytic siloxane bond formation for preparing nanoporous materials

“…1 Bottom-up assembly of well-defined siloxane oligomers as building blocks is one of the promising methods for precisely controlling the framework structures at the molecular level. 2 Cage-type siloxanes with double- n -ring (D n R, n = 3–6) structures 3 are especially useful as building blocks because of their simple synthesis, framework rigidity, high symmetry, and multiple reactive sites that are available for cross-linking and chemical modification. 2 d , e Various functional groups and reactions have been employed for the intermolecular linking of cage siloxanes via Si–O–Si, 4 Si–C, 5 a , b C–C, 5 c – h and Si–O–C 6 bond formations.…”