The hierarchy of ab initio and DFT methods for describing an intramolecular non-covalent SiN contact in the silicon compounds using electron diffraction geometries

In the series of silatranes XSi(OCH2CH2)3N, 1 (X = Me, 1a; H, 1b; F, 1c) with the known gas electron diffraction (GED) structures, the problematic geometry of 1-methylsilatrane 1a has been revised. In particular, the new value of the SiN distance (dSiN) in 1a turned out to be ∼0.06 Å longer than the generally accepted one. This dSiN resolves the long-standing contradiction between the data of the structural and spectral experiments regarding the sensitivity of 1 to the medium effect. We also performed the ab initio and DFT study of the combined series of silatranes 1a-c, silylalkylamines H3Si(CH2)3NMe2 (2a) and F3SiCH2NMe2 (2b), silylhydrazines F3SiN(Me)NMe2 (2c) and F3SiN(SiMe3)NMe2 (2d), and silyloxyamines ClH2SiONMe2 (2e,f), (F3C)F2SiONMe2 (2g,h) and F3SiONMe2 (2i), in which the GED dSiN values are in a wide range of 2-3 Å. None of the involved quantum chemical methods has succeeded in reproducing all the experimental gas-phase dSiN values in 1a-c, 2a-i with an acceptable accuracy (0.01-0.03 Å). The problems of the used methods, primarily CCSD with the Pople basis sets, are caused by four molecules with the geminal SiNN and SiON fragments (2d,f-i) and dSiN < 2.3 Å. A reasonable hierarchy of computationally accessible theory levels for studying the physicochemical manifestation of the non-covalent intramolecular SiN interactions can be constructed only at dSiN > 2.3 Å: MP2 < PBE0 ∼ B3PW91 ∼ SCS-MP2 < CCSD < CCSD(T).

Hydrosilyation of CO2 using a silatrane hydride: structural characterization of a silyl formate compound

The silatrane hydride compound, [N(CH2CH2O)3]SiH, reacts with CO2 in the presence of the [tris(2-pyridylthio)methyl]zinc hydride complex, [Tptm]ZnH, to afford the silyl formate and methoxide derivatives, [N(CH2CH2O)3]SiO2CH and [N(CH2CH2O)3]SiOCH3. The molecular structure of [N(CH2CH2O)3]SiO2CH has been determined by X-ray diffraction, thereby demonstrating that the formate ligand adopts a distal conformation in which the uncoordinated oxygen atom resides with a trans-like disposition relative to silicon. Density functional theory calculations indicate that the atrane motif of [N(CH2CH2O)3]SiO2CH is flexible, such that the energy of the molecule changes relatively little as the Si···N distance varies over the range 2.0–3.0 Å.

Dipole-Bound Anions of Intramolecular Complexes

Dipole-bound molecular anions are often envisioned as unperturbed neutral, polar molecules with single excess electrons. We report the observation of intramolecular structural distortions within silatrane molecules due to the formation of their dipole-bound anions. The combination of Rydberg electron transfer-anion photoelectron spectroscopy (RET-PES) and ab initio computational methodologies (CCSD and MP2) was used to study 1-hydro- (HS) and 1-fluoro- (FS) silatranes and their dipole bound anions, HS^- and FS^-. The vertical detachment energies (VDEs) of HS^- and FS^- were measured to be 48 and 93 meV, respectively. Ab initio calculations accurately reproduced these VDE values as well as their photoelectron spectral profiles. This work revealed significant shortening (by ∼0.1 Å) of dative Si ← N bond lengths when HS and FS formed dipole-bound anions, HS^- and FS^-. Detailed computational (Franck-Condon) analyses explained the absence of vibrational features in the photoelectron spectra of HS^- and FS^-.

Radical cations of phenyl silatrane

Electrochemical oxidation of phenylsilatrane (1) in CH₃CN/0.1 M Bu₄NPF₆ has been studied by voltammetry, UV-Vis and EPR-coupled spectroelectrochemistry supported by DFT calculations. One-electron withdrawal from the HOMO of 1, formed with a predominant contribution of the atrane N atom to the 3c-4e system, results in a short lived radical cation, in which the atrane nitrogen atom is almost planar and carries most of the spin density showing strong coupling with the protons of the axially directed C-H bonds of the three adjacent α-methylene groups (g = 2.0037, a^αH_ax = 37.93 G, a^αH_lat = 0.23 G and a^βH = 1.8 G). EPR spectroscopy and DFT calculations attest that the unpaired electron in the radical cation does not reside at the Si atom.

Assignment of photoelectron spectra of intramolecular silicon complexes: 1-vinyl- and 1-phenylsilatranes

The problematic photoelectron spectra of 1-vinyl- and 1-phenylsilatranes were theoretically studied and the correlation between the VIE and thed_SiNwas proposed to be used for the identification of orbitals in the Si←N coordination bond in XSi[OCH₂CH₂]₃N.

Electrochemical Oxidation and Radical Cations of Structurally Non-rigid Hypervalent Silatranes: Theoretical and Experimental Studies

Using 18 silatranes XSi(OCH₂ CH₂ )₃ N (1) as examples, the potentials of electrochemical oxidation E⁰ of the hypervalent compounds of Si were calculated for the first time at the ab initio and DFT levels. The experimental peak potentials E_p (acetonitrile) show an excellent agreement (MAE=0.03) with the MP2//B3PW91 calculated E⁰ (C-PCM). Radical cations of 1 reveal a stretch isomerism of the N→Si dative bond. Localization of the spin density (SD) on the substituent X and the short (s) coordination contact Si⋅⋅⋅N (d_SiN <2.13 Å) along with the high five-coordinate character of Si are typical for the first isomer 1^+.(s) , whereas the second one, 1^+.(l) , has a longer (l) Si⋅⋅⋅N distance (d_SiN >3.0 Å), the four-coordinate Si and the SD localized on the silatrane nitrogen atom N_s . The vertical model of adiabatic ionization (1→1^+.(s) or 1→1^+.(l) ) was developed. It allows, in accordance with an original experimental test (electrooxidation of 1 in the presence of ferrocene), a reliable prediction of the most probable pathways of the silatrane oxidation. The reliable relationships of E⁰ (1) with the strength characteristics of the dative contact N→Si were revealed.

Computational Study of Ways by Which exo-Silatranes Might Be Prepared

exo-Silatranes involve cage structures where the nitrogen lone pair points away from the cage rather than into it. This distinguishes them from the well-established endo-silatranes. exo-Silatranes have not been observed experimentally, consistent with a significant benefit to silicon-nitrogen interactions inside the cages as suggested for endo-silatranes. Identifying examples of exo-silatranes would prove useful in understanding Si-N interactions, as they would represent the "no interaction" extreme of the spectrum. We have found four means by which exo-silatranes might be synthesized: (1) employing smaller cages; (2) employing constrained rings to stiffen the cage backbones; (3) employing steric interactions to enhance preference for the less crowded exo-geometry around nitrogen; (4) modifying the Lewis acidity and basicity of the silicon and nitrogen so significantly as to remove their desire to interact. The preference for exo geometries is established using the parameter Δ, representing the distance between the nitrogen atom and the least-squares plane containing the adjacent carbon atoms. In some cases, Δ values for exo-silatranes are greater than 0.3 Å. In others, they are near zero, indicating a nearly planar nitrogen atom. There are no obvious structural markers besides Δ that distinguish between exo- and endo-silatranes.