Distinctly different reactivity of bis(silylenyl)-versus phosphanyl-silylenyl-substituted o-dicarborane towards O₂, N₂O and CO₂

Abstract

In stark contrast to the reactivity of the bis-silylenyl dicarborane CB–Si₂ (1) [CB = ortho-C,C′-C₂B₁₀H₁₀, Si = PhC(tBuN)₂Si] towards O₂, N₂O, and CO₂, yielding the same dioxygenation product CB–Si₂O₂ (2) with a four-membered 1,3,2,4-disiladioxetane ring, the activation of the latter small molecules with the phosphanyl-silylenyl-functionalised CB–SiP (3) {P=P[N(tBu)CH₂]₂} affords with O₂ the CB–Si(=O)P(=O) silanone-phosphine oxide (4), with N₂O the CB–Si(=O)P silanone-phosphine (5), and with CO₂ the CB–Si(O₂C=O)P silicon carbonate-phosphine (6) and CB–C(=O)OSiOP ester (7), respectively.

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For many decades, the activation of ubiquitous small molecules such as H₂, N₂, O₂, CO, CO₂, N₂O, etc. has been considered to be a domain of transition-metal chemistry and metalloenzymes, including their low molecular-weight functional models. However, recent advances in low-valent non- and semi-metal chemistry have provided a plethora of impressive evidence with transition-metal-like reactivity of main-group elements in small molecule activation.^1–6 This is in part due to the availability of reactive lone pair electrons and relatively low-lying empty acceptor orbitals of main-group elements in unusual low oxidation states, which can mimic the electronic situation of transition-metal centres and even mediate chemical bond cleavage of unactivated small molecules. Striking examples comprise the activation of dihydrogen by acyclic silylenes with a highly reactive silicon(II) centre^7–10 and the fixation of dinitrogen by a borylene as a carbene analogue.^11,12 In recent years, we have been interested in the chemistry of chelating N-heterocyclic silylenes (NHSis) that feature two adjacent divalent silicon atoms as highly reactive sites.^13–15 Such bis-NHSis not only can serve as steering ligands for coordination of main-group elements in unusual low oxidation states and transition metals for catalysis,^13–15 they can also mediate cooperative small molecule activation through sufficiently close proximity of the two silicon(II) centres.^16,17 For instance, the bis-NHSis with a Si⋯Si distances between 2.6 and 4.5 Å are capable reacting with CO at ambient temperature, affording disilaketenes.^16–18 Featuring a Si⋯Si distance of about 3.3 Å, the two Si(II) centres in the ortho-dicarborane-based bis-NHSi CB–SiSi (1)¹⁹ (Scheme 1) are in a predestinate position to coordinate and subsequently activate CO.¹⁷ Moreover, the bis-NHSi 1 has been employed to stabilise a single zero-valent Si atom,²⁰ monovalent boron,²¹ and serves in nickel-mediated catalytic transformations.¹⁹ More recently we succeeded in the synthesis of the isolobal phosphanyl-silylenyl-functionalised o-dicarborane CB–SiP (3) where one silylenyl group in 1 is replaced by a phosphanyl moiety.²² Compound 3 enabled the formation of new classes of C,C′-dicarborandiyl-silylene supported Ge₂ and Ge₄ species which could not be realised with 1.²² Herein, we wish to report the markedly different reactivity of 1 and 3 towards O₂, N₂O, and CO₂, respectively.

Scheme 1 Reactions of 1 with O₂, N₂O, and CO₂ to yield 2.

Exposure of solutions of 1 in diethyl ether at −20 °C to O₂ gas leads to immediate disappearance of the yellow color and formation of the 1,3,2,4-disiladioxetane 2 (Scheme 1). Interestingly, 2 is also formed as single product through exposure of 1 towards N₂O and CO₂ under the same reaction conditions. After workup, 2 has been isolated as a colorless solid; it crystallises in the monoclinic space group C2/c. The two Si centres are five-coordinated and bridged by two oxygen atoms (Fig. 1). The Si atoms adopt a distorted square-pyramidal coordination geometry with the carborane–C atom at the apical position. The Si1–O1 [1.695(1) Å] and Si1–O1′ [1.708(1) Å] distances in 2 are slightly longer than the Si–O length in octamethyl cyclotetrasiloxane (ca. 1.65 Å).²³ As expected, the five-coordinate ²⁹Si nuclei exhibit a drastic up-field shift in the ²⁹Si{¹H} NMR spectrum (δ = −98.6 ppm) when compared with the precursor 1 (δ = 18.9 ppm).¹⁹

Fig. 1 Molecular structure of 2. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms and one toluene molecule are omitted for clarity. Selected bond distances (Å) and angles (°): Si1–O1 1.695(1), Si1–O1′ 1.708(1), Si1–C1 1.961(2), Si1–N1 1.861(1), Si1–N2 1.884(1), Si1–Si1′ 2.403(1); O1–Si1–O1′ 83.4(1), Si1–O1–Si1′ 89.8(1), O1–Si1–C1 96.6(1), O1′–Si1–C1 96.1(1).

The reactions of O₂, N₂O, and CO₂ with various stable silylenes to give Si=O-containing compounds or further oxygenated Si–O-containing species are well-documented.^5,9,24–30 We wondered whether a transient or even isolable Si=O species could be detected if one silylenyl moiety in 1 is replaced by a isoelectronic phosphanyl group; thus we employed the phosphanyl-silylenyl-functionalised carborane CB–SiP (3). Indeed, under the same reaction conditions, exposure of 3 to dioxygen gas at −20 °C resulted in the formation of the CB–Si(=O)P(=O) silanone-phosphine oxide (4) (Scheme 2 and Fig. 2), in which both P and Si atoms are mono-oxygenated. Compound 4 was isolated at −20 °C as a colorless solid in 68% yields. It is sparingly soluble in Et₂O, but well soluble in THF. Solutions of 4 in ethereal solvents are only stable below −20 °C; its decomposition affords a mixture of unidentified products. In the solid state, however, 4 is stable at room temperature and decomposes above 153 °C.

Scheme 2 Reactions of CB-PSi 3 with N₂O, O₂, and CO₂ to give 4, 5, 6 and 7.

Fig. 2 Molecular structure of 4 (There are two molecules in the asymmetric unit, only one is depicted). Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°) for molecule 1: Si1–O1 1.524(3), Si1–C1 1.943(4), Si1–N1 1.820(3), Si1–N2 1.814(3), P1–O2 1.456(3), P1–C2 1.899(4), P1–N3 1.638(3), P1–N4 1.656(3); O1–Si1–C1 122.5(2), O2–P1–C2 109.2(2).

The ³¹P{¹H} NMR spectrum of 4 (measured at −20 °C in d₈-THF) exhibits a singlet at δ = 26.4 ppm, which is significantly up-field shifted relative to that of the precursor 3 (δ = 102.9 ppm).²² A similar up-field shift is observed for the ²⁹Si{¹H} NMR resonance of 4 (δ = −52.8 vs. 17.5 ppm for 3).²² The IR spectrum of 4 shows a very strong stretching vibration mode at ν = 1193 cm⁻¹ for the Si=O bond, which is slightly larger than the value observed for the Si=O bond in a C=O-supported silanone (1153 cm⁻¹).²⁷ A single-crystal X-ray diffraction analysis confirmed the presence of Si=O and P=O bonds in 4 with a O1–Si1–P1–O2 torsion angle of 36.8° (Fig. 2). In 4 both silicon and phosphorus atoms adopt a distorted tetrahedral coordination environment. In line with the IR spectroscopic data, the Si–O distance of 1.524(3) Å in 4 is slightly shorter than that in the aforementioned keto-supported silanone adduct [1.532(2) Å],²⁷ indicating a more pronounced Si=O bond character.

As mentioned above, the reaction of silylenes with dioxygen usually affords disiladioxetanes or related Si–O single bond containing products. The formation of 4 as an isolable Si=O species is apparently due to the presence of the phosphanyl moiety, acting as an oxygen atom acceptor in close proximity to the silicon atom. In order to figure out whether 3 is capable of N₂O activation to furnish a Si=O-(and/or P=O)-containing product, we exposed solutions of 3 in Et₂O to N₂O gas at −20 °C. To our surprise, the oxygenation took place only at the silicon centre to form the CB-(Si=O)P silanone-phosphine (5), which could be isolated in 51% yields (Scheme 2). This is consistent with the fact, that such substituted phosphines are inert toward N₂O. Akin to 4, compound 5 is labile in Et₂O and THF solutions above −20 °C and decomposes to an unidentified mixture of products. However, 5 is indefinitely stable in the solid state at ambient temperature and decomposes above 176 °C.

The molecular structure of 5 established by a single-crystal X-ray diffraction analysis revealed a four-coordinate silicon atom with a distorted tetrahedral coordination geometry (Fig. 3), while the phosphorus atom remains unchanged with respect to the precursor 3. The Si=O distance of 1.524(1) Å in 5 is identical to that in 4. In line with that, the IR spectrum of 5 exhibits a Si=O stretching frequency of 1187 cm⁻¹, which is very close to the value observed for 4 (ν = 1193 cm⁻¹). The four-coordinate Si atom in 5 shows a resonance at δ = −51.9 ppm in the ²⁹Si{¹H} NMR spectrum (measured at −30 °C in d₈-THF), matching well with that for 4 (δ = −52.8 ppm). In contrast, the marginally different chemical shift at δ = 114.2 ppm in the ³¹P{¹H} NMR spectrum of 5 (vs. δ = 102.9 ppm of its precursor 3)²² confirms the presence of the unchanged phosphine moiety.

Fig. 3 Molecular structure of 5. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Si1–O1 1.524(1), Si1–C1 1.930(2), Si1–N1 1.827(2), Si1–N2 1.814(2), P1–C2 1.949(1), P1–N3 1.717(2), P1–N4 1.677(2); O1–Si1–C1 120.2(1).

As aforementioned, previous studies on CO₂ activation toward silylenes revealed mono-oxygen transfer and liberation of CO to form either Si=O^26,28–30 or dimeric disiladioxetane species.^29,31 in less cases, silicon carbonates^31–34 could be isolated which resulted from trapping reaction of Si=O intermediate with CO₂. Similar to the reactivity of 3 towards N₂O, 3 reacts with CO₂ in diethyl ether at −20 °C only at the Si(II) site to yield the CB-(SiO₂C=O)P silicon carbonate-phosphine (6) (Scheme 2). The latter product was isolated in 77% yields after workup at low temperature. The ³¹P{¹H} NMR spectrum of 6 shows a signal at δ = 115.5 ppm (measured at −10 °C in d₈-THF), indicating the presence of the unchanged phosphine moiety, whereas the ²⁹Si{¹H} NMR spectrum shows a resonance at δ = −93.1 ppm, reminiscent of the five-coordinate ²⁹Si nuclei attached to a ferrocene spacer (δ = −92.1 ppm).²⁹ The IR spectrum of 6 exhibits a stretching vibration mode at ν = 1813 cm⁻¹ for the C=O group, comparable to that observed in a bis-NHC-supported silicon dicarbonate (1746 cm⁻¹).³⁵ A single-crystal X-ray diffraction analysis confirmed the molecular structure of 6 (Fig. 4), which features a five-coordinate silicon atom in a strongly distorted square-pyramidal coordination geometry with the carborane–C atom at the apical position. The Si–O distances of 1.718(1) and 1.770(1) Å in 6 are significantly longer than the Si=O lengths in 4 and 5, respectively, but close to the Si–O bonds in 2 [1.695(1), 1.708(1) Å]. The C3–O1 distance of 1.194(2) Å is considerably shorter than those of C3–O2 (1.362(2) Å) and C3–O3 (1.340(2) Å), implying a C=O bond between C3 and O1.

Fig. 4 Molecular structure of 6. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Si1–O2 1.718(1), Si1–O3 1.770(1), Si1–C1 1.939(2), Si1–N3 1.811(2), Si1–N4 1.874(2), P1–C2 1.932(2), P1–N1 1.684(2), P1–N2 1.708(2), C3–O1 1.194(2), C3–O2 1.362(2), C3–O3 1.340(2), C1–C2 1.738(3); O2–Si1–C1 117.5(1), O3–Si1–C1 102.1(1), N3–Si1–C1 109.8(1), N4–Si1–C1 103.2(1), O2–Si1–O3 75.0(1), O1–C3–O2 126.9(2), O1–C3–O3 129.3(2), O2–C3–O3 103.7(2).

Although the mechanism of 6 from 3 with CO₂ is still unknown, we propose that the initial step of the reaction is the formation of 5 with a Si=O moiety under release of CO. To prove this, we conducted the reaction of 5, obtained from 3 and N₂O, with CO₂ at −20 °C (Scheme 2). Indeed, a clean formation of 6 confirmed this by multinuclear NMR spectroscopy measurements.

While 6 is stable at room temperature in the solid state, it isomerises quantitatively at room temperature in ethereal solvents to give 7 in the course of 24 hours (Scheme 2). Its molecular structure has been established by a single-crystal X-ray diffraction analysis. As depicted in Fig. 5, the silicon atom in 7 is five-coordinate and in a distorted trigonal-bipyramidal coordination environment with the N1 and O2 atoms in the axial positions. Thus, the Si1–O2 distance of 1.773(2) Å is longer than that of Si1–O3 (1.624(2) Å). The driving force of the isomerization of 6 to 7 is attributed to the oxygen affinity of phosphorus that induces CB C–P bond scission and CB C–CO bond formation. In addition, the steric congestion in 6 between the phosphine group and the CB cage is released after migration of the phosphine moiety from the carborane–C2 to the O3 atom. As a result, the C1–C2 distance of 1.652(3) Å in 7 is significantly shorter than that in 6 (1.738(3) Å). In accordance with the molecular structure of 7, its ²⁹Si{¹H} NMR spectrum shows a doublet at δ = −110.4 ppm (²J_(Si,P) = 9.4 Hz) comparable to that of 6 (δ = −93.1 ppm), and the ³¹P nucleus resonates at δ = 119.6 ppm in the ³¹P{¹H} NMR spectrum reminiscent of that for 6 (δ = 115.5 ppm).

Fig. 5 Molecular structure of 7. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Si1–O2 1.773(2), Si1–O3 1.624(2), Si1–C1 1.937(3), O3–P1 1.666(2), O2–C3 1.314(3), O1–C3 1.200(3), C2–C3 1.515(4), C1–C2 1.652(3); O3–Si1–O2 94.2(1), O3–Si1–N2 120.2(1), N2–Si1–C1 118.3(1), O3–Si1–C1 120.6(1), N1–Si1–O2 167.8(1), Si1–O3–P1 134.7(1), O2–C3–O1 125.5(2), O1–C3–C2 122.4(2), O2–C3–C2 112.1(2).

In summary, we have described the markedly different reactivity of the bis-NHSi dicarborane 1 and its isolobal phosphanyl-NHSi-functionalised dicarborane analogue 3 towards O₂, N₂O, and CO₂. While all reactions of 1 with O₂, N₂O, and CO₂ let to the same dioxygenation product CB–Si(μ-O)₂Si2, the activation products with 3 turned out to furnish different products featuring Si=O, P=O, or C=O functionalities.

Financial support by the Deutsche Forschungsgemeinschaft (Germany's Excellence Strategy–EXC 2008–390540038–UniSysCat and DR 226/21-1) and the Czech Science Foundation (A. R., project no. 21-02964S) is gratefully acknowledged.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods, crystallographic details. CCDC 2075598 (compound 2), 2075596 (compound 4), 2075600 (compound 5), 2075599 (compound 6), and 2075597 (compound 7). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1cc01939b

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