An unprecedented gas-phase condensation reaction mediated by a bissilylated chloronium ion intermediate

Abstract

Gas-phase Cl₂Si(OH)⁺ and Si₂OCl_4−n(OH)_n+1⁺ (n = 0–4) ions are shown to undergo a novel condensation reaction with SiCl₄ that provides a potentially useful pathway for a variety of networks containing the –(Si–O–Si)– backbone. Theoretical calculations reveal that these reactions proceed via a bissilylated chloronium ion intermediate.

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Silicon alkoxides, Si(OR)₄, and related alkoxysilanes have been extensively explored as the molecular precursors of a large variety of silicon-containing glasses, gels, ceramics and nanomaterials via sol–gel chemistry.^1,2 For these substrates, sol–gel processes are well known to be initiated by the hydrolysis of the alkoxy groups followed by successive condensation reactions leading to the formation of ≡Si–O–Si≡ networks. While the kinetics and thermodynamics of the hydrolysis processes have been characterized,^3,4 a full understanding of the initial processes is essential for the purpose of designing materials with well-defined properties.⁵ This has provided the impetus for (1) theoretical studies addressing the mechanisms of the initial hydrolysis–condensation processes,^6–10 and (2) attempts to recreate some of these reactions in the gas-phase by mass spectrometric techniques in order to probe the intrinsic reactivity pathways of alkoxysilanes.^11–20

Because chemical modifications of the alkoxide precursors allow for the incorporation of organic functions and thus alter the functionality and nature of the end material,^21–24 there is considerable interest in unravelling new approaches to hydrolysis–condensation reactions. In this communication, we present an unusual set of gas-phase condensation reactions between Cl_3−nSi(OH)_n⁺ (n = 1–3) ions and SiCl₄ that provide a useful approach for a variety of ≡Si–O–Si≡ networks.

Cl_3−nSi(OH)_n⁺ (n = 1–3) ions were generated in an in-house built Fourier transform ion cyclotron resonance (FT-ICR) spectrometer by the progressive hydrolysis of gas-phase SiCl₃⁺ ions as described in earlier studies.^25,26 The initial hydrolysed ions, the Cl₂Si(OH)⁺ isotopologues, were selectively isolated in the ICR cell using multiple ejection techniques²⁵ and allowed to react with SiCl₄ at typical pressures of (3.5 ± 0.5) × 10⁻⁸ mbar. Under these conditions, reaction (1) was observed to proceed readily.

Cl₂Si(OH)⁺ + SiCl₄ → ⁺Cl₂SiOSiCl₃ + HCl (1)

The Cl₃SiOSiCl₂⁺ ions formed in (1) were then found to undergo rapid hydrolysis with background water (used to generate Cl₂Si(OH)⁺ and related species) to yield a series of Si₂OCl_5−n(OH)_n⁺ (n = 1–5) ions in an analogous fashion to that described in our previous work.²⁵

Si₂OCl_5−n(OH)_n⁺ + H₂O → Si₂OCl_4−n(OH)_n+1⁺ + HCl (n = 0–4) (2)

The rate constant for reaction (1) was estimated from the successive half-lives taking into account the subsequent reaction (2). Correction for the ion gauge readings to obtain the absolute pressure of SiCl₄ leads to a rate constant of (2.3 ± 0.7) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at a cell temperature of 333 ± 5 K. This value is of the order of 10% of the calculated ion-neutral collision rate constant.

The fact that no equivalent reaction to (1) has been previously observed in the neutral chemistry of SiCl₄ and Si₂Cl₆^27–31 prompted us to investigate more thoroughly the mechanism of reaction (1). This is of considerable interest because of the important role that chlorosiloxane species play in the functionalization of silica particles.^32–34 We therefore decided to explore the mechanistic details for this reaction through computational chemistry. Theoretical calculations were carried out with the Gaussian 03 suite of programs³⁵ to address the possible pathways, to characterize the intermediates and the transition states, and to map out the energy profile of the reaction. Initial calculations for the structure and energy of reagents, products, intermediates and transition states were carried out at the B3LYP/6-311+G(d,p) level of theory with vibrational frequencies corrected by a factor of 0.9679 to estimate zero-point energies (ZPE).³⁶ For comparison purposes, calculations were also performed at the MP2/6-311+G(d,p) level with vibrational frequencies corrected by a factor of 0.9523,³⁷ and with the M05-2X functional with the 6-311+G(d,p) basis set. These different methods were useful for assessing the robustness of the calculated energies and structures in view of possible isomeric structures.³⁸ Transition states were identified by a single imaginary frequency, and intrinsic reaction coordinate (IRC) calculations were carried out to characterize the connection between reaction intermediates and transition states.

The results of these calculations indicate that reaction (1) proceeds by the initial formation of a chloronium ion intermediate followed by an exchange of the bridging chlorine with an OH group as outlined in Scheme 1. The calculated energy profile for this multi-step reaction (Fig. 1) reveals that this pathway is very favourable with the two transition states lying well below the energy of the reagents. Furthermore, reaction (1) is predicted to be exothermic and thus consistent with the ease of the observed ion/molecule reaction. Fig. 2 illustrates the calculated structures of the intermediates and the transition states of the energy profile shown in Fig. 1. Although the calculated values with the B3LYP method are considerably different from those obtained with the MP2 method and the M05-2X functional (see Table 1), the general features of the energy profile are the same.

Scheme 1 General view of the intermediates involved in the mechanism of reaction (1).

Fig. 1 Calculated energy profile (including zero point energies) for reaction (1) at the MP2/6-311+G(d,p) level of theory.

Fig. 2 Optimized geometries calculated at the MP2/6-311+G(d,p) level of theory for the intermediates and transition states outlined in Scheme 1 and Fig. 1. Distances are in Å and angles in degrees.

Table 1 Calculated values for (E + ZPE) in kcal mol⁻¹ for the stationary points and transition states of Fig. 1 using different computational chemistry models. The values are referenced to the energy of the reactants

	B3LYP	MP2	M05-2X
RC	−18.3	−25.6	−25.4
TS1	−6.9	−14.6	−12.9
Int	−35.3	−47.8	−44.6
TS2	−12.1	−23.5	−19.9
PC	−26.3	−35.4	−34.1
Products	−9.0	−13.6	−10.5

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The mechanism outlined for reaction (1) is particularly significant because it is found to proceed through a bissilylated halonium ion intermediate. These species have been recognized in recent years as important intermediates for carbon–halogen activation and as precursors of highly reactive Lewis acids.^39–43 Similar types of species had been predicted to be stable intermediates in some early theoretical modeling of the reaction between Si⁺ and SiX₄ (X = F, Cl).⁴⁴

Several other aspects of reactions similar to (1) deserve further consideration. For example, the progressively hydrolyzed ions, Si₂OCl_4−n(OH)_n+1⁺ (n = 0–4), can undergo further condensation reactions with SiCl₄ in an analogous fashion to reaction (1). These secondary reactions were not extensively investigated because they are considerably slower than (1) and the ionic products fall in a mass range above the useful range of our spectrometer.²⁵ Nevertheless, they do provide a convenient pathway for the formation of larger ≡Si–O–(SiCl₂–O–Si(OH)Cl)_n–O–Si≡ networks and variations thereof as shown below.

Different ligands can also be introduced in the networks by making use of the ease with which ions such as Cl₃SiOSiCl₂⁺ and Si₂OCl₄(OH)⁺ can undergo gas-phase solvolysis type reactions by analogy to our previous work.²⁵ For example, reaction (3) is readily observed in experiments carried out in the presence of methanol at ∼1.5 × 10⁻⁸ mbar,

Si₂OCl₄(OH)⁺ + MeOH → Si₂OCl₃(OH)(OMe)⁺ + HCl (3)

These reactions are predicted to be considerably exothermic by our theoretical calculations. It should be noticed that further condensation reactions of the ionic product of reaction (3) with SiCl₄ pave the way for a variety of possible ≡Si–O–(SiCl₂–O–Si(OH)R)_n–O–Si≡ motifs containing different organic groups R. On the other hand, a similar approach to these reactions using SiF₄ and the corresponding fluorinated ions is predicted to be unfavourable. In fact, the reaction of gaseous F₂Si(OH)⁺ with SiF₄, analogous to reaction (1), is estimated to be endothermic by 7.7 (B3LYP), 5.1 (MP2) and 6.9 (M05-2X) kcal mol⁻¹ and has not been observed in the earlier study of Speranza and coworkers.⁴⁵

In summary, we have shown that a new type of gas-phase reaction represents a novel approach for building up a variety of modified ≡Si–O–Si≡ networks. This type of reaction can be further pursued with ions obtained from siloxanol, siloxanediols⁴⁶ or oligomer diols containing the siloxane structure.⁴⁷ Furthermore, the mechanism of these reactions illustrates the important role that bissilylated chloronium ions can play in promoting condensation type reactions.

Experimental

The experimental techniques and the FT-ICR spectrometer have been previously described.²⁵ Cl₂Si(OH)⁺ ions were generated from the reaction of SiCl₃⁺ ions (formed inside the ICR cell by the electron ionization of SiCl₄) with H₂O maintained at a nominal pressure of 1.4 ± 10⁻⁸ mbar. The reaction and kinetics of Cl₂Si(OH)⁺ ions (isolated by the removal of all other ions) with SiCl₄ maintained at a nominal pressure of Cl₂Si(OH)⁺ of (3.5 ± 0.5) × 10⁻⁸ mbar were followed through several half-life periods up to 6 s. The nominal pressures measured by an ionization gauge were corrected to absolute pressures by a procedure similar to that previously used in this laboratory.⁴⁸ Nevertheless, we estimate that this procedure leads to an uncertainty of 30% in the actual values of the absolute pressures for the present case. The temperature of the cell heated by the filament used for electron ionization in these experiments has been previously measured with a thermocouple and is typically 333 ± 5 K.

Acknowledgements

The authors thank the Brazilian Research Council (CNPq) for continuous support of this work and Jair J. Menegon for technical assistance.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Cartesian coordinates and calculated energies for all the species in Fig. S1. See DOI: 10.1039/c5nj02250a

‡ Present address: Laboratoire de Chimie Physique, CNRS UMR 8000, Université Paris-Sud, 15, rue Jean Perrin, 91405 Orsay Cedex, France. E-mail: E-mail: thiago.firmino@u-psud.fr

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