Location of Pyridine Guest Molecules in an Electroneutral {^3_∞}[SiO_(4/2)] Host Framework: Single-Crystal Structures of the As-Synthesized and Calcined Forms of High-Silica Ferrierite

Abstract

Single crystals (up to 580 × 480 × 20 μm) of highly silicious ferrierite (Si-FER, 1), suitable for single-crystal X-ray investigations, are synthesized under organothermal conditions. The structures of the as-synthesized (1a) and the calcined (1b) Si-FER are determined at room temperature. Both structures are refined in the orthorhombic space group Pnnm (No. 58, standard setting) with a = 743.0(1) pm, b = 1409.2(2) pm, c = 1882.0(2) pm, V = 1970.5(4) × 10^6 pm^3, Z = 1, R = 0.041 (1a) and a = 741.8(1) pm, b = 1407.0(2) pm, c = 1871.3(2) pm, V = 1953.1(4) × 10^6 pm^3, Z = 1, R = 0.037 (1b). The structure solution when combined with chemical analysis and ^1H and ^(13)C MAS NMR gives a unit cell content of [Si_(36)O_(72)]{py_(4-x) ap_x} (x = 0−1, py = pyridine, ap = 1-amino-n-propane) and [Si_(36)O_(72)] for 1a and 1b, respectively. The structure of 1a shows only weak host−guest interactions between the {^3_∞}[SiO_(4/2)] framework and the occluded, orientationally disordered pyridine molecules by means of relatively long organic-to-framework distances, d(C_(py)···O) ≥ 354(2) pm. ^(29)Si MAS NMR spectra from the organic-containing Si-FER 1a and the organic-free form 1b are in good agreement with the crystallographic results in that they conform to the well-known linear relationship between the cosine expression of the T−O−T angles and the chemical shift of the respective tetrahedral sites (T-sites). A new modification of this relationship is presented here and offers an improved linear correlation between the X-ray and NMR data for 1a and 1b, as well as for other high-silica microporous materials. Application of this new correlation to denser SiO_2 compounds is discussed.