Transformation of Germanium Dioxide to Microporous Germanate 4-Connected Nets

Extensive research efforts worldwide continue to aim for new compositions and framework topologies of zeolites and related microporous crystalline materials due to their current widespread applications in catalysis and separation technologies1-3 and the advantages they present as platforms for the assembly of spatially organized molecular recognition systems.4,5 Although, 4-connected nets of zeolitic silicates and aluminosilicates represent a vast, well-established, and useful class of materials,6 the analogous germanates remain conspicuously absent and largely unexplored. We have employed a synthetic route similar to that employed for zeolites in the conversion of GeO2 to previously unknown, microporous germanate 4-connected nets. Here, two crystalline materials of GeO2 composition will be reported, namely, [GeO2]10‚ (DMA)(H2O), ASU-7, (DMA ) dimethylamine), and [GeO2]10‚ DABCO)(H2O), ASU-9, (DABCO ) 1,4-diazabicyclo[2,2,2]octane), with the first possessing a remarkable new zeolite-type net containing 1-D channels and the second adopting an octadecasil net with large cages. At the outset of this study, germanate framework structures have been limited to those constructed from combinations of tetrahedral GeO4, trigonal bipyramidal GeO5, and/or octahedral GeO6 centers7-10 with no 4-connected microporous structures based solely on the GeO4 tetrahedron reported yet. GeO2 (150 mg, 1.434 mmol) was dissolved in a 40% aqueous DMA (1.26 mL, 10.04 mmol) followed by the addition of pyridine (3.20 mL, 39.78 mmol) and 48-51% aqueous HF (0.02 mL, 0.572 mmol). The solution (pH ) 10.6) was heated in a Teflon-lined vessel to 165 °C for 4 days and then cooled to room temperature to give 90 mg (56% yield based on GeO2) of ASU-7 as rodlike crystals (shown in Figure 1 for a selected sample). Octahedrally shaped crystals of ASU-9 (Figure 1) were obtained by a similar procedure. Here, GeO2 (50 mg, 0.478 mmol) and DABCO (250 mg, 2.232 mmol) were dissolved in water (0.45 mL) followed by the addition of pyridine (1.60 mL, 19.89 mmol) and 48-51% aqueous HF (0.02 mL, 0.572 mmol) and then the solution (pH ) 8.8) was heated to 160 °C for 2 days to give 6 mg product (10% unoptimized yield based on GeO2). Elemental microanalysis11 and single-crystal X-ray studies12 on the two crystals confirmed that these conditions are ideal for transforming the original condensed structure of GeO2 into 4-connected microporous nets having cubes and tetrahedra as building blocks (shown below). In both structures, the cubes are bridged by tetrahedral units that connect each of the eight corners, resulting in larger building units with distinct stereochemistry having the cubes either rotated by 45° or identically oriented with respect to other cubes along the same direction. The first arrangement belonging to ASU-7 yields a 4-connected net containing extended 1-D channels running along the crystallographic c-axis, where a single water molecule occupies the center of each cube and the DMA guest molecules reside in the channels (Figure 2). This is a new type of microporous net having the vertex symbols, 6‚6‚62‚62‚128‚128 (vertex 1) and 4‚6‚4‚6‚4‚6 (vertex 2, on cube).13a To examine the stability of this network in the absence of the DMA guests, a sample (39.9 mg) of ASU-7 was heated in a thermal gravimetric apparatus to reveal a weight loss of 5.2% at 420-540 °C corresponding to the removal of DMA and water (5.7% calculated per formula unit). Decomposition of DMA