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. 2020 May 11;11(23):5895-5901.
doi: 10.1039/d0sc01427c. eCollection 2020 Jun 21.

A highly unsaturated six-vertex amido-substituted silicon cluster

Jan Keuter  1 , Christian Schwermann  2 , Alexander Hepp  1 , Klaus Bergander  3 , Jörn Droste  4 , Michael Ryan Hansen  4 , Nikos L Doltsinis  2 , Christian Mück-Lichtenfeld  3 , Felicitas Lips  1
Affiliations

A highly unsaturated six-vertex amido-substituted silicon cluster

Jan Keuter et al. Chem Sci. .

Abstract

Thermal treatment of the bicyclo[1.1.0]tetrasilatetraamide [Si4{N(SiMe3)Dipp}4] 1 resulted in the formation of a highly unsaturated six-vertex silicon cluster [Si6{N(SiMe3)Dipp}4] 2 with only four amine-substituents and two ligand-free silicon atoms. In solution, a major and a minor conformer of this cluster are in equilibrium according to multinuclear NMR spectroscopy, lineshape analysis, DFT calculations and molecular dynamics simulations. The bonding situation in the highly unsaturated cluster features lone pair type character at the ligand-free silicon atoms and partial single and double bond character in the upper butterfly-shaped ring of 2. This allows to consider 2 as the silicon analogue of a butalene isomer.

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Figures

Scheme 1
Scheme 1. Six-vertex silicon clusters.
Scheme 2
Scheme 2. Other silicon clusters with hemispheroidal silicon atoms (highlighted in bold).
Scheme 3
Scheme 3. Possible C6H4 isomers.
Scheme 4
Scheme 4. Synthesis of the amido-substituted silicon cluster 2.
Fig. 1
Fig. 1. (Top) Molecular structure of 2 (top view, Dipp and SiMe3 groups represented as wires for clarity). (Bottom) Cluster core of 2 in three different views illustrating the close similarity to a twisted prismane and a distorted octahedron missing two edges.
Fig. 2
Fig. 2. Comparison of the configuration of the ligand-free silicon atoms in F, D and 2.
Fig. 3
Fig. 3. 29Si NMR spectrum of 2 in THF-d8 at 210 K. Δ represents the major component and O the minor component.
Fig. 4
Fig. 4. 29Si{1H} CP/MAS NMR spectra of 2 recorded at 9.4 T employing (a) 10.0 kHz and (b) 4.0 kHz MAS. The inset in (a) enlarges the chemical shift region of the SiMe3 groups. The corresponding fit is shown in grey in (b). The isotropic 29Si resonances are marked by triangles and spinning sidebands are marked with an asterisk.
Fig. 5
Fig. 5. Result of the QTAIM analysis and IBOs 298, 299 and 300.
Fig. 6
Fig. 6. Possible resonance structure of 2.
Scheme 5
Scheme 5. Major and minor conformer and enantiomer of 2 in solution. Values in brackets are solvent corrected relative free energies ΔG(298)solv.
Fig. 7
Fig. 7. Potential energy diagram (TPSS-D3/def2-SVP) of the minimum energy path for the stepwise transformation of 2-A to the minority conformer 2-C and the formation of the enantiomer of 2-A (2-D).
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