Order–disorder phenomena determined by high-resolution powder diffraction: the structures of tetrakis(trimethylsilyl)methane C[Si(CH₃)₃]₄ and tetrakis(trimethylsilyl)silane Si[Si(CH₃)₃]₄

Abstract

The compounds tetrakis(trimethylsilyl)methane C[Si(CH(3))(3)](4) (TC) and tetrakis(trimethylsilyl)silane Si[Si(CH(3))(3)](4) (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. At room temperature both structures have space group Fm{\bar 3}m (Z = 4) with a = 13.5218 (1) Å, V = 2472.3 (1) Å(3) for TSi, and a = 12.8902 (2) Å, V = 2141.8 (1) Å(3) for TC. X-ray scattering data can be described by a molecule with approximately sixfold orientational disorder, ruling out a structure with free rotating molecules. Upon cooling, TSi exhibits a first-order phase transition at T(c) = 225 K, as is characterized by a jump of the lattice parameter of Deltaa = 0.182 Å and by an exothermal maximum in differential scanning calorimetry (DSC) with DeltaH = 11.7 kJ mol(-1) and DeltaS = 50.0 J mol(-1) K(-1). The structure of the low-temperature phase is refined against X-ray powder data measured at 200 K. It has space group P2(1)3 (Z = 4), a = 13.17158 (6) Å and V = 2285.15 (2) Å(3). The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as is shown by analyzing distances between atoms and by calculations of the lattice energy in dependence on the orientations of the molecules. TC has a phase transition at T(c1) = 268 K, with Deltaa(1) = 0.065 Å, DeltaH(1) = 3.63 kJ mol(-1) and DeltaS(1) = 13.0 J mol(-1) K(-1). A second first-order phase transition occurs at T(c2) = 225 K, characterized by Deltaa(2) = 0.073 Å, DeltaH(2) = 6.9 kJ mol(-1) and DeltaS(2) = 30.0 J mol(-1) K(-1). The phase transition at higher temperature has not been reported previously. New NMR experiments show a small anomaly in the temperature dependence of the peak positions in NMR to occur at T(c2). Rietveld refinements were performed for the low-temperature phase measured at T = 150 K [space group P2(1)3, lattice parameter a = 12.609 (3) Å], and for the intermediate phase measured at T = 260 K [space group Pa{\bar 3}, lattice parameter a = 12.7876 (1) Å]. The low-temperature phase of TC is formed isostructural to the low-temperature phase of TSi. In the intermediate phase the molecules exhibit a twofold orientational disorder.

Dynamic disorder in solid tetrakis(trimethylstannyl)methane, C(SnMe3)4, investigated by one- and two-dimensional variable-temperature119Sn and13C NMR spectroscopy

“…Following this argument, the 5-fold dynamic disorder for 1 would indicate a much higher degree of "spherical shape" for the C(SnMe 3 )4 than for the C(SiMe 3 )4 molecule. In line with this pic- ture is the finding that 1 apparently does not undergo a phase transition to an ordered low-temperature phase, at least not at temperatures T> 30 K, while C(SiMe3)4 exists in its ordered low-temperature phase at temperatures T < 225 K [6]. From this qualitative consideration we may conclude that C(SnMe 3 ) 4 , 1, is not likely to be a good, that is a very closely related, model compound to indirectly characterize the dynamic properties of the intermediate-temperature phase of C(SiMe3)4.…”

“…The intermediate-temperature phase of C(SiMe 3 )4 has the same space group symmetry (Pa) as 1, C(SnMe3)4, but displays only 2-fold disorder. It might be argued [6] that the degree n of n-fold disorder may be taken as a measure of how "spherical" the topology of a molecule is: the higher n, the better a spherical shape is approximated in this description of disorder. Following this argument, the 5-fold dynamic disorder for 1 would indicate a much higher degree of "spherical shape" for the C(SnMe 3 )4 than for the C(SiMe 3 )4 molecule.…”

“…The crystal structures, structural phase transitions, and molecular dynamic properties of the closely related compounds C(SiMe3)4, 2, and Si(SiMe 3 ) 4 , 3, have recently been reported [5,6]. At room temperature, 2 and 3 crystallize in space group Fm3m (high-temperature phase), at T < 225 K both compounds exist in their respective orientationally ordered low-temperature phases (isothe crude product has been recrystallized from chloroform and subsequently sublimed as reported in the literature.…”

Order–disorder phenomena in crystalline phases of compounds E(XMe3)4 where E = C, Si, Ge and X = Si, Sn

“…The intermediate-temperature phase of C(SiMe 3 ) 4 also crystallizes in space group Pa 3, though with only two fold orientational disorder. 2 Hence, our initial motivation for studying the properties of C(SnMe 3 ) 4 was viewing this compound as a slowed-down model of the solid-state dynamics of the intermediate-temperature phase of C(SiMe 3 ) 4 , which itself is not suitable for in-depth solidstate NMR investigations of this kind as all the molecular dynamics in this phase are too fast for this purpose. In this situation, the exchange rate constants for both large-and small-angle jumps could be equal (model (b)) or differ from each other (models (c) and (d)).…”

Structures of Tetrasilylmethane Derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the Gas Phase, and their Dynamic Structures in Solution

“…In contrast, the permethyl species, C(SiMe 3 ) 4 , has been the subject of numerous reports, using NMR spectroscopy [18][19][20][21][22], Xray diffraction [23][24][25], gas electron diffraction (GED) [26,27], quantum chemical calculations [28,29], and vibrational spectroscopy [29].…”