Abstract
A number of mono- and dioligosilanylated silocanes were prepared. Compounds included silocanes with 1-methyl-1-tris(trimethylsilyl)silyl, 1,1-bis[tris(trimethylsilyl)silyl], and 1,1-bis[tris(trimethylsilyl)germyl] substitution pattern as well as two examples where the silocane silicon atom is part of a cyclosilane or oxacyclosilane ring. The mono-tris(trimethylsilyl)silylated compound could be converted to the respective silocanylbis(trimethylsilyl)silanides by reaction with KOtBu and in similar reactions the cyclosilanes were transformed to oligosilane-1,3-diides. However, the reaction of the 1,1-bis[tris(trimethylsilyl)silylated] silocane with two equivalents of KOtBu leads to the replacement of one tris(trimethylsilyl)silyl unit with a tert-butoxy substituent followed by silanide formation via KOtBu attack at one of the SiMe3 units of remaining tris(trimethylsilyl)silyl group. For none of the silylated silocanes, signs of hypercoordinative interaction between the nitrogen and silicon silocane atoms were detected either in the solid state. by single crystal XRD analysis, nor in solution by 29Si-NMR spectroscopy. This was further confirmed by cyclic voltammetry and a DFT study, which demonstrated that the N-Si distance in silocanes is not only dependent on the energy of a potential N-Si interaction, but also on steric factors and through-space interactions of the neighboring groups at Si and N, imposing the orientation of the pz(N) orbital relative to the N-Si-X axis.
Highlights
Received: 16 December 2020Compared to carbon, for heavier group 14 elements hypercoordination is a fairly frequently observed phenomenon
The (Me3 Si)3 Si group is relatively bulky and we have previously found that double geminal tris(trimethylsilyl)silylation can be a challenge, NMR spectroscopic analysis of the reaction mixture indicated clean formation of 8 in satisfying yields
For this reason we decided to turn to the family of silocanes, which can be regarded as silatranes with one of the bridging arms between nitrogen and silicon missing
Summary
For heavier group 14 elements hypercoordination is a fairly frequently observed phenomenon. The noncoordinating exo-form of silocanes is favored in most cases over the endo-form (Scheme 1) [3,4,14,15] The reasons for this can be either steric (f.i., no structurally characterized N-Ph silocanes [16,17,18,19,20] with pronounced N→Si interaction are known, whereas a fair number of exo N-Me silocanes [6,20,21,22,23,24] are known) or electronic (at least one additional electronegative substituent seems to be required on the silocane Si to exhibit enough electrophilicity for a stable N→Si interaction). Calculations have shown that the diminished transannular Si-N interaction of silocanes makes them more susceptible for hydrolysis of the Si-O bonds [25]
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