Silicon-29 2D NMR Evidence of Four Novel Doubly Germanium Substituted Silicate Cages in a Tetramethylammonium Germanosilicate Solution

We have identified four novel, doubly germanium substituted silicate cages in an aqueous tetramethylammonium germanosilicate solution by means of highfield silicon-29 NMR spectroscopy, in conjunction with ²⁹Si isotopic enrichment. Three of the species are derivatives of the double four-membered silicate ring (cubic octameric cage) in which two silicon sites have been replaced by germanium, notably including the species in which two adjacent silicon sites have been replaced. Although all three anions may coexist in solution, their relative concentrations indicate that germanium incorporation is site-selective, favoring maximum separation between germanate tetrahedra. The fourth germanosilicate species detected is a doubly germanium substituted double three-membered ring (prismatic hexameric cage). In this case we find no evidence of structures in which two adjacent silicon sites are replaced, perhaps because such cages are too strained. Two-dimensional ²⁹Si NMR homonuclear correlated spectroscopy (2D COSY) is used to determine the connectivities within the cages, and the results are supported by means of computer simulations.