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Modelling S_N2 nucleophilic substitution at silicon by structural correlation with X-ray crystallography.

Alan R. Bassindale, David J. Parker, Peter G. Taylor.

Department of Chemistry, Open University, Walton Hall, Milton Keynes,

The trajectory of bimolecular nucleophilic substitution at silicon is modelled using a series of closely related five-coordinate silicon complexes based on quinoline ligands. Each complex contains an internal 'nucleophile' which undergoes an intramolecular interaction with the silicon. By careful choice of 'leaving group' at the silicon, different extents of 'substitution' are observed. The extent of bond formation in solution and in the solid state are measured using multi-nuclear NMR spectroscopy and single crystal X-ray crystallography.

Modelling the trajectory of an S_N2 reaction pathway was first performed by Dunitz over 15 years ago.¹

The technique known as structural correlation involves the collection of X-ray crystallographic data for compounds containing the molecule of interest and then sequencing the structures on the basis of key crystal data (usually inter-atom distances).

The structures are arranged so that a gradual deformation is observed, each structure representing a frozen snapshot of the substitution at a particular point on the modelled reaction profile.

Pestunovich et al ² were the first to apply this methodology to substitution at silicon in their study of N-(amidomethyl)-chlorosilanes (1).

In response, a method for modelling the substitution in solution was developed by Bassindale³ using ¹³C and ²⁹Si NMR spectroscopy as a monitoring tool. The system first studied was that using substituted 2-pyridone ligands (2) with a variety of ring substituents and leaving groups.

By varying R and X in a series of 25 such complexes, the substitution was mapped. We have since extended this technique to a series of 2-quinolinone analogues (3).⁴

We present here the results of our X-ray crystallographic study of complexes (3a-d) and so for the first time compare solution and solid state descriptions of the complexes and the substitution they model.

Structure of fluoride complex (3a)

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Fluoride is the poorest leaving group of the series and so models the earliest point in the substitution with the longest nucleophile-silicon distance; the amido oxygen being the nucleophile and the halogen the leaving group.

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With an improved leaving group the oxygen is closer to the silicon by 0.12Å. The Si-Cl bond is markedly stretched from its normal length. The N-C₉-Si angle decreases slightly as ring closure begins to take place.

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With the nucleophile a further 0.09Å closer to the silicon, the Si-X bond is considerably stretched and weakened as the strengthened nucleophilic interaction causes it to depart further. The N-C₉-Si angle continues to decrease.

Structure of triflate complex (3d)

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The nucleophile is now 0.11Å closer to the silicon and the substitution is close to completion. The interaction of the triflate group with the silicon is best described as electrostatic. There are no further silicon-oxygen distances in the crystal corresponding to further bonding interaction.The axial bond angles are somewhat distorted from the 180° ideal due to the intramolecular nature of the substitution.

To describe the degree to the which the Si-X bond is "stretched" by the incoming nucleophile, a percentage Si-X extension term is introduced. We define 0% extension as that of a typical Si-X bond and 100% extension as the sum of the crystal ionic radii for Si and X.

The key solid-state data, along with the results of the NMR solution work are presented in the following table. ORTEP diagrams and further crystal data are summarised here.

The geometry at the silicon remains trigonal-bipyramidal for the majority of the 'reaction'. This is particularly suprising for complex (3a) where the nucleophile-silicon interaction is particularly weak and the silicon is essentially tetracoordinate.

The relationship between the extent of Si-O bond formation and the degree of Si-X bond stretching is shown below.

As expected, the degree of Si-X bond stretching increases with strengthening nucleophilic attack. In the case of the triflate structure (3d) the stretching is great enough that the Si-X distance is more characteristic of an inter-ion contact distance than a covalent interaction.

We can also compare the extent of Si-O bond formation between solid state and solution.

We have successfully modelled the nucleophilic S_N2 substitution at silicon both in solution and in the solid state for a series of complexes (3) based on 2-quinolinone ligands and shown how the geometry of the complexes is very similar between the solid state and solution phases.

From the crystallographic data we have demonstrated the concerted Si-nucleophile bond shortening and Si-leaving-group extension that is characteristic of this mechanism.

• The EPSRC for their funding of this project.
  
• The Open University for funding the study visit of David Parker to Berlin.
  

1. D. Britton, J.D. Dunitz; J. Amer. Chem. Soc.; 1981, 103, 2971-2979.

2. V.F. Sidorkin, V.V. Vladimirov, M.G. Voronkov, V.A. Pestunovich; J. Mol. Struct. (Theochem); 1991, 228, 1-9.

3. (a) A.R. Bassindale, M. Borbaruah; J. Chem. Soc. Chem. Commun.; 1991, 1499-1501. (b) A.R. Bassindale, M. Borbaruah; J. Chem. Soc. Chem. Commun.; 1991, 1501-1503.

4. A.R. Bassindale, D.J. Parker, P.G. Taylor; unpublished results.