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. 2020 Mar 27;59(14):5494-5499.
doi: 10.1002/anie.201914339. Epub 2020 Feb 11.

Synthesis and Reactivity of the First Isolated Hydrogen-Bridged Silanol-Silanolate Anions

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Free PMC article

Synthesis and Reactivity of the First Isolated Hydrogen-Bridged Silanol-Silanolate Anions

Robin F Weitkamp et al. Angew Chem Int Ed Engl. .
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Abstract

We report on the first examples of isolated silanol-silanolate anions, obtained by utilizing weakly coordinating phosphazenium counterions. The silanolate anions were synthesized from the recently published phosphazenium hydroxide hydrate salt with siloxanes. The silanol-silanolate anions are postulated intermediates in the hydroxide-mediated polymerization of aryl and alkyl siloxanes. The silanolate anions are strong nucleophiles because of the weakly coordinating character of the phosphazenium cation, which is perceptible in their activity in polysiloxane depolymerization.

Keywords: phosphazene; silanolates; silicone; siloxanes; weakly coordinating cations.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Scheme 1
Scheme 1
Industrial synthesis of silicones.4
Scheme 2
Scheme 2
Examples of eight‐membered siloxane rings.10, 14, 17
Scheme 3
Scheme 3
Equilibrium reaction of phosphazene 1, hydroxide salt 2, and silanol–silanolate salt 3.
Figure 1
Figure 1
Molecular structure of the silanolate anion in 3. The phosphazenium cation and minor occupied disordered atoms are not shown. Thermal ellipsoids set at 50 % probability. The hydrogen atoms of the methyl groups are omitted for clarity. Selected bond lengths [pm] and angles [°]: O1–O2 258.2(1), O1–O3 252.4(5), O1–Si1 159.9(1), O2–Si2 162.4(1), O3–Si3 161.8(3); Si1‐O1‐O2 118.0(1), Si1‐O1‐O3 124.6(1), O2‐O1‐O3 102.7(2).25
Scheme 4
Scheme 4
Reaction of cyclotrisiloxanes with in situ generated phosphazenium hydroxide.
Figure 2
Figure 2
Molecular structure of the [D3OH] anion in 4. The phosphazenium cation is not shown. Thermal ellipsoids set at 50 % probability. The donor hydrogen atom is disordered in a ratio of 1:1, bonded to O1 or O4, but only one is shown. The hydrogen atoms of the methyl groups are omitted for clarity. Selected bond lengths [pm] and angles [°]: O1–O4 242.8(2), O1–Si1 157.9(2), O4–Si3 158.9(1), O2–Si2 161.8(1), O2–Si1 165.8(1); O4‐O1‐Si1 124.4(1), O1‐O4‐Si3 113.2(1), O2‐Si2‐O3 112.7(1).25
Figure 3
Figure 3
Molecular structure of the [DPh2 3OH] anion in 5. The phosphazenium cation is not shown. Thermal ellipsoids set at 50 % probability. The donor hydrogen atom is disordered in a ratio of 1:1, bonded to O1 or O4, but only one is shown. The hydrogen atoms of the phenyl groups are omitted for clarity. Selected bond lengths [pm] and angles [°]: O1–O4 242.9(2), O1–Si1 158.4(1), O4–Si3 157.8(1), O2–Si1 164.9(1), O2–Si2 162.1(1); O1‐O4‐Si3 125.1(1), O4‐O1‐Si1 119.0(1), O2‐Si2‐O3 112.3(1).25
Scheme 5
Scheme 5
Depolymerization of trimethylsilyl‐end‐blocked polydimethylsiloxanes employing silanolate salt 4 (see also Tables S1–S3).24

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References

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