Solvated Silylium Cations:  Structure Determination by NMR Spectroscopy and the NMR/Ab Initio/IGLO Method

Abstract

Sixty R3SiX/solvent (S) and R2HSiX/S systems with R = methyl, ethyl, butyl and S = methylene chloride, DMPU, DMSO, sulfolane, HMPA, acetonitrile, pyridine, N-methylimidazole, and triethylamine were investigated with the help of NMR spectroscopy for different concentration ratios of R3SiX/S and R2HSiX/S as well as different temperatures. With the help of measured δ29Si and δ13C chemical shifts as well as 1 J Si - C and 2 J Si - P coupling constants, typical NMR parameters for R3SiX and R2HSiX, R3Si(S)+, R2HSi(S)+, and R2HSi(S)2 + were established and discussed to distinguish between possible silylium cation−solvent complexes and equilibria between them. In addition, the NMR/ab initio/IGLO method (based on the continuum solvent model PISA and IGLO-PISA chemical shift calculations) was used to determine geometry, stability, and other properties of Me3Si(S) n + and Me2HSi(S) n + complexes in different solutions. NMR measurements and ab initio calculations clearly show that R3Si(S)+ and R2HSi(S)+ complexes with tetracoordinated Si are formed with solvents (S) more nucleophilic than methylene chloride while complexes with two S molecules and a pentacoordinated Si atom can only be found for R3 - n H n Si+ cations with n ≥ 1. This is a result of internal (hyperconjugative) stabilization of R3Si+ by alkyl groups and external stabilization by S coordination, as well as of steric factors involving R and S. Complex binding energies are in the range of 40−60 kcal/mol, which is significantly different from complex binding energies in the gas phase. In all cases investigated, (weakly) covalent bonds between Si and S are formed that exclude any silylium cation character for the solvated R3Si+ and R2HSi+ ions.

Enantioselective Ring Opening of Epoxides with Silicon Tetrachloride in the Presence of a Chiral Lewis Base: Mechanism Studies

“…The signal at À110 ppm was a triplet with a coupling constant of 9 Hz, consistent with those spectra observed by Cremer and Bassindale for HMPA complexes of trialkylsilyl cations. [46] The signal at À110 ppm was tentatively assigned as the bis-HMPA complex of a trichlorosilyl cation i, with its chemical shift falling squarely in the middle of the range for a pentacoordinate silicon species (À50 to À150 ppm).…”

Valence Bond Study of the SiH3−F Bond

“…11 Of course, behavior in solution and in the solid state with regard to R 3 Si + X -heterolysis is the result of competition between the R 3 Si-X bond energy and the other interactions, with solvent molecules in solution, or with other R 3 Si-X units or their components in the solid state. [36][37][38][39] Silyl compounds have high binding energies when attached to electronegative atoms such as oxygen, nitrogen, or fluorine. 12,38 The binding energy of an acetonitrile solvent molecule to Me 3 Si + in solution has been calculated to be ∼40 kcal/mol with covalent bonding characteristics for the Si-N bond.…”

“…37 The low-temperature crystal structure of SiH 3 F shows a significant Si‚‚‚F nonbonded interaction. 38 After taking account of all the condensed phase interactions, in the balance it may very well be that R 3 Si-X will prefer to remain molecular even though its internal charge distribution shows a large charge separation between the R 3 Si and X groups. 40,41 In the case of SiH 3 -F the large binding energy is not directly due to the ionic character of the Si-F bond but rather to a strong resonance interaction between the ionic and covalent bonding structures.…”

Complexation of tris(pentafluorophenyl)silanes with neutral Lewis bases

“…Silyltriflates are known to be highly electrophilic species serving as powerful silylating reagents [7] and as Lewis acids [8]. For example, trimethylsilyl triflate (TMSOTf) readily transfers Me 3 Si-group onto a variety of Lewis basic molecules such as pyridines, amides, and phosphineoxides (Ph 3 PO, HMPA) to afford tetracoordinate salts Me 3 Si-L + OTf À [5,9]. When we mixed TPFS-OTf with 1 equiv.…”

“…In general, M-O-P (M = metal) angles in HMPA complexes vary in range 142-165°, the largest magnitudes were observed in complexes with lantanides and actinides [18]. Concerning the literature examples of Si-HMPA derivatives, the Si atoms always adopt the 178.30(6) C(11)-Si(1)-C (21) 121.23 (9) 118.80 (9) 113.35 (9) 119.98 (7) 111.84 (8) 113.27 (8) 121.5 118.6 119.8 C(11)-Si(1)-C (31) 119.37 (9) 119.05 (9) 110.4(1) 120.87 (7) 112.26 (7) 113.39 (8) 119.2 119.0 121.1 C(21)-Si(1)-C (31) 119.14(9) 122.15 (9) 110.21 (9) 119.10 (7) 112.90 (7) 106. 24 [12][13][14][15].…”