Distinctly diﬀerent reactivity of bis(silylenyl)-versus phosphanyl-silylenyl-substituted o -dicarborane towards O 2 , N 2 O and CO 2 †

In stark contrast to the reactivity of the bis-silylenyl dicarborane CB–Si 2 (1) [CB = ortho - C , C 0 -C 2 B 10 H 10 , Si = PhC( t BuN) 2 Si] towards O 2 , N 2 O, and CO 2 , yielding the same dioxygenation product CB–Si 2 O 2 (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 QQQ P[N( t Bu)CH 2 ] 2 } affords with O 2 the CB–Si( QQQ O)P( QQQ O) silanone-phosphine oxide (4), with N 2 O the CB–Si( QQQ O)P silanone-phosphine (5), and with CO 2 the CB– Si(O 2 C QQQ O)P silicon carbonate-phosphine (6) and CB–C( QQQ O)OSiOP ester (7), respectively.

For many decades, the activation of ubiquitous small molecules such as H 2 , N 2 , O 2 , CO, CO 2 , N 2 O, 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][2][3][4][5][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][8][9][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 Nheterocyclic silylenes (NHSis) that feature two adjacent divalent silicon atoms as highly reactive sites. [13][14][15] Such bis-NHSis not only can serve as steering ligands for coordination of maingroup elements in unusual low oxidation states and transition metals for catalysis, [13][14][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 SiÁ Á ÁSi distances between 2.6 and 4.5 Å are capable reacting with CO at ambient temperature, affording disilaketenes. [16][17][18] Featuring a SiÁ Á ÁSi 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 0 -dicarborandiyl-silylene supported Ge 2 and Ge 4 species which could not be realised with 1. 22 Herein, we wish to report the markedly different reactivity of 1 and 3 towards O 2 , N 2 O, and CO 2 , respectively.
Exposure of solutions of 1 in diethyl ether at À20 1C to O 2 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 N 2 O and CO 2 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 squarepyramidal coordination geometry with the carborane-C atom at the apical position. The Si1-O1 [1.695(1) Å] and Si1-O1 0 [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 29 Si nuclei exhibit a drastic upfield shift in the 29 Si{ 1 H} NMR spectrum (d = À98.6 ppm) when compared with the precursor 1 (d = 18.9 ppm). 19 The reactions of O 2 , N 2 O, and CO 2 with various stable silylenes to give SiQO-containing compounds or further oxygenated Si-O-containing species are well-documented. 5,9,[24][25][26][27][28][29][30] We wondered whether a transient or even isolable SiQO 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 À20 1C resulted in the formation of the CB-Si(QO)P(QO) silanone-phosphine oxide (4) (Scheme 2 and Fig. 2), in which both P and Si atoms are monooxygenated. Compound 4 was isolated at À20 1C as a colorless solid in 68% yields. It is sparingly soluble in Et 2 O, but well soluble in THF. Solutions of 4 in ethereal solvents are only stable below À20 1C; its decomposition affords a mixture of unidentified products. In the solid state, however, 4 is stable at room temperature and decomposes above 153 1C.
The 31 P{ 1 H} NMR spectrum of 4 (measured at À20 1C in d 8 -THF) exhibits a singlet at d = 26.4 ppm, which is significantly up-field shifted relative to that of the precursor 3 (d = 102.9 ppm). 22 A similar up-field shift is observed for the 29 Si{ 1 H} NMR resonance of 4 (d = À52.8 vs. 17.5 ppm for 3). 22 The IR spectrum of 4 shows a very strong stretching vibration mode at n = 1193 cm À1 for the SiQO bond, which is slightly larger than the value observed for the SiQO bond in a CQO-supported silanone (1153 cm À1 ). 27 A single-crystal X-ray diffraction analysis confirmed the presence of SiQO and PQO bonds in 4 with a O1-Si1-P1-O2 torsion angle of 36.81 (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 ketosupported silanone adduct [1.532(2) Å], 27 indicating a more pronounced SiQO 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 SiQO 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 N 2 O activation to furnish a SiQO-(and/or PQO)-containing product, we exposed solutions of 3 in Et 2 O to N 2 O gas at À20 1C. To our surprise, the oxygenation took place only at the silicon centre to form the CB-(SiQO)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 N 2 O. Akin to 4, compound 5 is labile in Et 2 O and THF solutions above À20 1C and decomposes to an unidentified mixture of products. However, 5 is indefinitely stable in the solid state at ambient temperature and decomposes above 176 1C.
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 SiQO 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 SiQO stretching frequency of 1187 cm À1 , which is very close to the value observed for 4 (n = 1193 cm À1 ). The fourcoordinate Si atom in 5 shows a resonance at d = À51.9 ppm in the 29 Si{ 1 H} NMR spectrum (measured at À30 1C in d 8 -THF), matching well with that for 4 (d = À52.8 ppm). In contrast, the marginally different chemical shift at d = 114.2 ppm in the 31 P{ 1 H} NMR spectrum of 5 (vs. d = 102.9 ppm of its precursor 3) 22 confirms the presence of the unchanged phosphine moiety.
As aforementioned, previous studies on CO 2 activation toward silylenes revealed mono-oxygen transfer and liberation of CO to form either SiQO 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 SiQO intermediate with CO 2 . Similar to the reactivity of 3 towards N 2 O, 3 reacts with CO 2 in diethyl ether at À20 1C only at the Si(II) site to yield the CB-(SiO 2 CQO)P silicon carbonate-phosphine (6) (Scheme 2). The latter product was isolated in 77% yields after workup at low temperature. The 31 P{ 1 H} NMR spectrum of 6 shows a signal at d = 115.5 ppm (measured at À10 1C in d 8 -THF), indicating the presence of the unchanged phosphine moiety, whereas the 29 Si{ 1 H} NMR spectrum shows a resonance at d = À93.1 ppm, reminiscent of the five-coordinate 29 Si nuclei attached to a ferrocene spacer (d = À92.1 ppm). 29 The IR spectrum of 6 exhibits a stretching vibration mode at n = 1813 cm À1 for the CQO group, comparable to that observed in a bis-NHC-supported silicon dicarbonate (1746 cm À1 ). 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. Although the mechanism of 6 from 3 with CO 2 is still unknown, we propose that the initial step of the reaction is the formation of 5 with a SiQO moiety under release of CO. To prove this, we conducted the reaction of 5, obtained from 3 and N 2 O, with CO 2 at À20 1C (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 29 Si{ 1 H} NMR spectrum shows a doublet at d = À110.4 ppm ( 2 J (Si,P) = 9.4 Hz) comparable to that of 6 (d = À93.1 ppm), and the 31 P nucleus resonates at d = 119.6 ppm in the 31 P{ 1 H} NMR spectrum reminiscent of that for 6 (d = 115.5 ppm).
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 O 2 , N 2 O, and CO 2 . While all reactions of 1 with O 2 , N 2 O, and CO 2 let to the same dioxygenation product CB-Si(m-O) 2 Si 2, the activation products with 3 turned out to furnish different products featuring SiQO, PQO, or CQO functionalities.

Conflicts of interest
There are no conflicts to declare.