Silicon-29 NMR Structural Characterization of Two Novel Germanosilicate Cages in a Tetramethylammonium Germanosilicate Solution

We have identified two novel germanium-substituted silicate cages in an aqueous tetramethylammonium germanosilicate solution by means of high-field (11.7 T) silicon-29 NMR spectroscopy, with ²⁹Si isotopic enrichment. Our results indicate that these anions exist in the form of a double four-membered ring (cubic octameric cage) and a double three-membered ring (prismatic hexameric cage), in which a single silicon site is replaced by germanium. Introduction of chemical shift nonequivalence by means of heteroatom incorporation leads to complex spin-spin splitting patterns, which may, however, be readily interpreted by means of two-dimensional homonuclear correlated spectroscopy (2D-COSY) or by spectral simulation. The ²J couplings are 7.5 Hz in the double four-membered ring and 4.3 Hz in the double three-membered ring. A ⁴J coupling of ~ 1.0 Hz is also deduced for the double three-membered ring. These results represent the first solution NMR observation of heteroatom incorporation into silicate cages and provide a novel means for investigating the structures and stabilities of such species.