Structure of compounds E(SnMe3)4 (E = Si, Ge) as seen by high-resolution X-ray powder diffraction and solid-state NMR

The compounds tetrakis(trimethylstannyl)germane, Ge(SnMe3)(4) (1), and tetrakis(trimethylstannyl)silane, Si(SnMe3)(4) (2), have crystal structures with the quasispherical molecules in a closed-packed stacking. At room temperature both structures have the space group P (1) over bar (Z = 2) with a = 9.94457 (5), b = 14.52927 (8), c = 9.16021 (5) Angstrom, alpha = 90.53390 (30), beta = 111.73080 (30), gamma = 90.0049 (4)degrees, and V = 1229.414 (12) Angstrom(3) for (1) and a = 9.92009 (7), b = 14.51029 (11), c = 9.13585 (7) Angstrom, alpha = 90.4769 (4), beta = 111.6724 (4), gamma = 89.9877 (6)degrees, and V = 1222.037 (16) Angstrom(3) for (2). The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as shown by analyzing the distances between the atoms. Upon heating, both compounds undergo a first-order phase transition at temperatures T-c = 348 +/- 5 K, as characterized by a relative jump of the lattice parameter of similar to 16%. At 353 K, both structures have the space group P<(1over bar> (Z = 4), with a = 14.2037 (2) Angstrom, and = 2865.52 (7) Angstrom(3) for (1) and a = 14.1346 (2) Angstrom, and = 2823.90 (7) Angstrom(3) for (2). Rietveld refinements were performed for the low-temperature phases measured at T = 295 K [R-wp = 0.0844 for (1), R-wp = 0.0940 for (2)] and for the high-temperature phases measured at T = 353 K [R-wp = 0.0891 for (1), R-wp = 0.0542 for (2)]. The combination of high-resolution X-ray powder diffraction measurements and variable-temperature magic-angle-spinning C-13, Si-29 and Sn-119 NMR experiments demonstrates low crystallographic and molecular (C-1) symmetries for the low-temperature phases of (1) and (2) at temperatures T < 348 +/- 5 K and high crystallographic symmetry due to rotational disorder for the high-temperature phases at temperatures T > 348 +/- 5 K.