An Isolable Bis(Silanone–Borane) Adduct

Abstract

The reaction of bis(silylenyl)‐substituted ferrocene 1 with two molar equivalents of BPh₃ yields the corresponding bis(silylene–borane) Lewis adduct 2. The latter is capable to activate CO₂ to furnish the borane‐stabilized bis(silanone) 3 through mono‐oxygenation of the dative Si^II→B silicon centers under release of CO. Removal of BPh₃ from 3 with PMe₃ affords the corresponding 1,3,2,4‐cyclodisiloxane and the Me₃P−BPh₃ adduct. All isolated new compounds were characterized and their molecular structures were determined by single‐crystal X‐ray diffraction analyses.

Si=O→B in a bis(silanone): Reaction of ferrocene‐bridged bis(silylene–borane) 2, prepared from the bis(silylene) and BPh₃, with CO₂ quantitatively generates a Lewis acid‐stabilized bis(silanone) 3 with two Si=O→B moieties. Removal off BPh₃ with PMe₃ yields 1,3,2,4‐cyclodisiloxane 4 through intramolecular Si=O head‐to‐tail dimerization. In contrast, reaction of 2 with elemental sulfur furnishes the borane‐free bis(silathione) 5.

The activation of small molecules using non‐ and semi‐metal‐based compounds is an attractive field in main‐group chemistry which led to the discovery of new activation modes and types of reactions.1 In this context, the concept of frustrated Lewis pairs (FLPs) for cooperative activation of inert bonds employing Lewis acids and bases, firstly reported by Stephan, Erker and co‐workers, is a landmark discovery.2 Since then, the rapid expansion of FLP chemistry has paved the way to different inter‐ and intramolecular systems in which the majority is based on sterically encumbered phosphorus‐ and nitrogen‐centered Lewis bases and organoboranes as Lewis acids.3 Although divalent carbon species such as N‐heterocyclic carbenes (NHCs) have also been successfully probed in FLP chemistry for the activation of CO₂, H₂ and N₂O, the use of analogous Lewis pairs‐containing silylenes is less known.4, 5 The silicon(II) atom in silylenes exhibits an ambiphilic character due to its vacant 3 p orbital (LUMO) and the 3 s‐centered lone pair (HOMO). Owing to their interesting property and reactivity, stable N‐heterocyclic silylenes (NHSis), the heavier analogues of NHCs, have been utilized successfully for the metal‐free activation of small molecules6 and as powerful steering ligands in homogeneous catalysis.7 After the first isolation of an N‐heterocyclic silylene in 1994 by Denk and West, the formation of a silylene–borane adduct was reported two years later, which, however, slowly rearranges to a silylborane through Si^II insertion into the B−C bond of B(C₆F₅)₃.8 Since then, an increasing number of compounds containing a dative Si^II→B^III bond with four‐ and five‐ coordinate Si^II centers have been isolated and structurally characterized.9

Due to a large polarization of the Si=O bond and the remarkably weak Si−O π bond (58.5 kJ mol⁻¹) compared to the Si−O σ‐bond strength (119.7 kJ mol⁻¹), compounds with a Si=O bond are intrinsically susceptible to auto‐oligomerization to the corresponding polysiloxanes.10 Thus, introduction of an electron donor at the Si atom or/and an acceptor at the O atom are needed to disfavor head‐to‐tail oligomerization of the polar Si=O bond.11 This led to the first Lewis acid‐base supported silanone complex, the silaformamide–borane A (Scheme 1), which was reported by us in 2007, starting from a silylene and H₂O⋅B(C₆F₅)₃.12 Roesky et al. described in 2011 the isolation of the acid anhydride B generated from the reaction of a chlorosilylene with H₂O⋅B(C₆F₅)₃ in the presence of NHC.13 Similarly, Roesky et al. reported also the silaformyl chloride complex C, resulting from an NHC‐stabilized silylene and H₂O⋅B(C₆F₅)₃.14 In 2019, the isolation of the first donor–acceptor‐supported silaaldehyde D was accomplished by the Inoue group.15 Remarkably, Kippings dream of isolable genuine silanones was realized in 2014 with the isolation of the first metallosilanone by Filippou16 and 2017 by the groups of Inoue and Rieger.17 Very recently, a silicon analogue of a ketone with an unperturbed Si=O bond was synthesized by Iwamoto and co‐workers.18

Scheme 1.

Selected Lewis acid/base‐supported Si=O compounds.

Starting from an in situ generated silylene–borane adduct, Teng et al. reported in 2016 on the activation of THF leading to the isolation of a corresponding ring‐opening product.20 Recently, Braun and co‐workers used a silylene–borane Lewis adduct as a tool for trapping a single water molecule, affording a zwitterionic silanol stabilized by intramolecular hydrogen bonds.23 In 2017, our group reported the first intramolecular silylene–borane FLP which activates H₂, O₂, CO₂ and even dehydrogenates water yielding a borane‐stabilized silanone E with a dative Si=O→B bond.19 Herein, we present the synthesis of the bis(silylene–borane) adduct 2 with the ferrocene spacer and its mild oxidation with CO₂ yielding the first borane‐stabilized bis(silanone) adduct 3. Removal of BPh₃ from 3 by addition of PMe₃ leads to the corresponding 1,3,2,4‐cyclodisiloxane through intramolecular Si=O head‐to‐tail dimerization. Moreover, the reaction of 2 with elemental sulfur yields a bis(silathione) with two ‘borane‐free’ Si=S moieties.

The reaction of the ferrocene‐derived bis(silylene)24 1 with two molar equivalents of triphenylborane in toluene at room temperature leads to the formation of the bis(silylene‐borane) adduct 2 which was isolated in 74 % yields as a red crystalline solid (Scheme 2). The identity of 2 was proven by elemental analysis, single‐crystal X‐ray diffraction analysis and multinuclear NMR spectroscopy in the solid state and in solution. Crystals suitable for an X‐ray diffraction analysis were obtained in a concentrated toluene solution of 2 at −30 °C, the crystals are a mixture of the two rotational conformers (Figure 1; see also the Supporting Information).

Scheme 2.

Synthesis of the bis(silylene–borane) adduct 2 from 1 and its reactivity towards CO₂ to give 4 and 3, respectively.

Figure 1.

Molecular structure of 2 (only one of the two rotational conformers) with thermal ellipsoids drawn at the 50 % probability level. Hydrogen and solvent atoms are omitted for clarity. Selected bond lengths [Å]: Si1−B1 2.089(2), Si2−B2 2.077(2). Selected bond angles [°]: C2‐Si1‐B1 130.74(9), C9‐Si2‐B2 130.63(9).

Compound 2 crystallizes in the monoclinic space group P12₁/c1 in which both silicon centers adopt a distorted tetrahedral geometry (∑Si1=356.72°, ∑B1=319.80°) with Si−B distances of 2.089(2) and 2.077(2) Å, similar to those of related silicon(II)–boranes adducts (1.9624(5)–2.108(2) Å).9 Given the low solubility of 2 in deuterated benzene and THF, only a broad ²⁹Si NMR signal of low intensity was observed at δ=54.0 ppm which is low‐field shifted compared to 1 (δ=43.3 ppm). The solid‐state ²⁹Si NMR (VACP/MAS) spectrum of 2 shows a singlet at δ=48.6 ppm (1: δ=41.6 ppm). The isotropic ¹¹B chemical shift was observed in [D₈]THF solutions at δ=−7.8 ppm (Δν _1/2=356 Hz) which is, as expected, low‐field shifted due to its coordination to the Si^II center (BPh₃: δ(¹¹B)=55.2 ppm, C₆D₆).9

Compound 2 is inert towards H₂ and CO but reacts with CO₂ in C₆D₆ under ambient conditions (1 bar, 298 K), resulting in the simultaneous formation of a pale‐yellow solid and CO as confirmed by an additional ¹³C‐labeling experiment (See the Supporting Information, S11). Resolving the solid in [D₈]THF and recording its multinuclear NMR spectra revealed the formation of a new species with a strongly high‐field shifted ²⁹S NMR singlet resonance at δ=−44.7 ppm (2: δ=+54.1 ppm). An X‐ray diffraction analysis of single crystals revealed the formation of the borane‐stabilized bis(silanone) 3, was isolated in 94 % yields (Figure 2).

Figure 2.

Molecular structure of 3 with thermal ellipsoids drawn at the 50 % probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selected distances [Å]: Si1−O1 1.557(4), Si2−O2 1.537(4), O1−B1 1.545(7), O2−B2 1.541(7); selected bond angles [°]: B1‐O1‐Si1 157.59, B2‐O2‐Si2 145.96,C9‐Si2‐B2 130.63(9).

The silicon center in 3 adopts a distorted tetrahedral geometry with a short Si−O distance of 1.557(4) and 1.537(4) Å in accordance with related four‐coordinated Lewis acid stabilized silanones (1.531–1.579 Å) containing a Si=O double bond.12, 13, 14, 15, 19, 21, 22 The Si−O distance is only slightly elongated when compared with recently reported genuine silanones (1.518–1.537 Å).15, 16, 17, 18 Bis(silanone) 3 is remarkable stable in solution ([D₈]THF) and no changes in the ¹H NMR spectra were observed upon heating to 60 °C. Compound 3 represents a rare example of borane‐stabilized silanones. Aldridge and co‐workers achieved the isolation of a stabilized silaaldehyde through chloride–hydride substitution using K[HBEt₃].21 Addition of B(C₆F₅)₃ to a cyclic amino(bora‐ylide(silanone)) reported by Kato et al., increased the stability of the pre‐formed free silanone.22 In the presence of B(C₆F₅)₃, Roesky et al. accomplished the isolation of a donor–acceptor stabilized silaformyl chloride.14 However, isolation of a borane‐stabilized silanone starting from a silylene–borane system is not reported so far.

To remove the boranes from the bis(silanone–borane) complex 3, trimethylphosphane (PMe₃, 5 equiv) was added. This resulted in the clean formation of the corresponding Lewis pair Me₃P→BPh₃ (³¹P NMR: −15.3 ppm) and the 1,3,2,4‐cyclodisiloxane 4 (head‐to‐tail dimer of Si=O moieties). The latter is identical with the isolated product from the reaction of 1 with CO₂ in 76 % yields (Scheme 2). Single crystals of 4 suitable for X‐ray diffraction analysis were obtained from a concentrated solution in a 1:1 benzene/hexane mixture at room temperature (Figure 3). The formation of Me₃P→BPh₃ was additionally confirmed by a single‐crystal X‐ray analysis obtained in the reaction mixture of 3 and PMe₃ in THF solutions (see the Supporting Information).

Figure 3.

Molecular structure of 4 with thermal ellipsoids drawn at the 50 % probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths [Å]: Si1−O1 1.709(4), Si2−O2 1.681(4). Selected bond angles [°]: Si1‐O1‐Si2 93.5(2), Si1‐O2‐Si2 93.9(2), O2‐Si2‐O1 84.4(2).

As expected, the five‐coordinate silicon centers in 4 show a drastically high‐field shifted ²⁹Si NMR chemical shift at δ=−92.1 ppm (3: δ=−44.7 ppm). The Si−O distance of 1.709(4) and 1.681(4) Å are elongated compared to those observed for 3 (1.557(4), 1.537(4) Å) in accordance with the presence of Si−O single bonds.25 Reaction of 4 with an excess amount of BPh₃ in toluene at room temperature does not regenerate 3.

Interestingly, reaction of the bis(silylene–borane) 2 with 10 equivalents of PMe₃ led to the formation of a new species 2' in the course of borane‐deprotection of one Si^II moiety in 2 (Scheme 3, see the Supporting Information). This process is reversible because removal of the solvent and PMe₃ in vacuum and re‐dissolving of the residue in C₆D₆ furnishes compound 2 as shown by NMR spectroscopy.

Scheme 3.

Reversible reaction of 2 with PMe₃ forming the monoborane adduct 2'.

In contrast to the oxygenation of 2 with CO₂, treatment of 2 with elemental sulfur in toluene at room temperature leads to the selective formation of the ‘borane‐free’ bis(silathione) 5. Compound 5 is identical with the product from the reaction of bis(silylene) 1 with elemental sulfur in toluene at room temperature, which was isolated in 54 % yield (Scheme 4). Similar to the product of an intramolecular silylene–borane FLP with elemental sulfur reported by our group,19 no Si=S→B interaction was observed. The structure of 5 (Figure 4) features two Si=S bonds with a low‐field shifted singlet ²⁹Si NMR signal at δ=12.1 ppm. The Si=S distances of 1.9867(13) and 1.9858(13) Å are consistent with related silathiones with four‐coordinate silicon atoms [{PhC(NtBu)₂}Si(S)Cl] (2.079(6) Å) and as reported for a Si=S product from sulfuration of an intramolecular silylene–borane FLP with elemental sulfur (1.9795(10) Å).19, 26 Bis(silathione) 5 is stable in C₆D₆ solutions over a period of several weeks which can be explained by a less polarized Si=S bond (ΔEN=0.7) compared to the Si=O bond (ΔEN=1.7) based on their electronegativities (EN).

Scheme 4.

Reaction of 1 or 2 with elemental sulfur affording 5.

Figure 4.

Molecular structure of 5 with thermal ellipsoids drawn at the 50 % probability level. Hydrogen and solvent atoms are omitted for clarity. Selected bond lengths [Å]: Si1−S1 1.9867(13), Si1−S2 1.9858(13). Selected bond angles [°]: C1‐Si1‐S1 120.70(12).

In summary, the synthesis of bis(silylene‐borane) Lewis adduct 2 containing two Si^II–BPh₃ moieties in a single molecule was presented. Exposure of 2 to CO₂ yields the corresponding borane‐supported bis(silanone) complex 3 featuring two Si=O→B units. Removal of the borane with PMe₃ yields 1,3,2,4‐cyclodisiloxane 4 through intramolecular Si=O head‐to‐tail dimerization. In contrast, the reaction of 2 with elemental sulfur yields exclusively the borane‐free bis(silathione) 5 which shows no tendency to undergo dimerization.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

M.-P. Luecke, E. Pens, S. Yao, M. Driess, Chem. Eur. J. 2020, 26, 4500.