The unexpected reactivity of the o-carborane supported bis-silylene [(LSi:)C]2B10H101 {L= PhC(tBuN)2} towards carbon monoxide and 2,6-dimethylphenyl isocyanide is reported. While the reaction of 1 with CO leads selectively to the novel head-to-head coupling and C–O cleavage product 2 from two molecules 1 and four molecules CO, the reaction of 1 with 2,6-dimethylphenyl isocyanide affords solely the 1 : 2 molar head-to-tail coupling product 3 with a Si[double bond, length as m-dash]C bond.TU Berlin, Open-Access-Mittel - 201

Drieß, Matthias

Ruzicka, Ales

Szilvási, Tibor

Xiong, Yun

Yao, Shenglai

Chemical Communications

English

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This journal is©The Royal Society of Chemistry 2020 Chem. Commun., 2020, 56, 747--750 | 747Cite this:Chem. Commun., 2020,56, 747Homocoupling of CO and isocyanide mediated bya C,C0-bis(silylenyl)-substituted ortho-carborane†Yun Xiong, a Shenglai Yao, a Tibor Szilva´si,b Ales Ruzicka c andMatthias Driess *aThe unexpected reactivity of the o-carborane supported bis-silylene[(LSi:)C]2B10H10 1 {L= PhC(tBuN)2} towards carbon monoxide and2,6-dimethylphenyl isocyanide is reported.While the reaction of 1withCO leads selectively to the novel head-to-head coupling and C–Ocleavage product 2 from two molecules 1 and four molecules CO,the reaction of 1 with 2,6-dimethylphenyl isocyanide affords solely the1 :2 molar head-to-tail coupling product 3 with a SiQC bond.The activation of important small molecules such as CO, H2and N2 was earlier considered to be a domain of transitionmetals. However, the last decade witnessed increasing achieve-ments in the chemistry of main-group element (MG) com-pounds that mediate the cleavage of strong bonds in smallmolecules.1–7 Featuring a lone pair of electrons and a vacantp-orbital at the silicon center, silylenes are particularly reactivespecies which belong to the rare examples of low-valent MGcompounds being suitable for the activation of unreactivebonds and thus represent versatile building blocks for thesynthesis of value-added organosilanes. Since the isolation ofthe first isolable N-heterocyclic silylene (NHSi) by West et al.in 1994,8 the chemistry of stable silylenes has flourishedsubstantially. Several cyclic9–13 and acyclic14–18 silylenes havebeen successfully synthesised and their reactivity towardsnumerous small molecules such as O2, N2O, S8, P4, NH3, CO2,and even H2 has been developed.5,15–17,19 Nevertheless, the verystrong triple bonds of CO cannot be activated easily; one veryrecent example represents the homocoupling of CO moleculesto OCCO moieties mediated by an acyclic monosilylenereported by Aldridge and co-workers.20In order to learn whether two silylene (NHSi) moieties couldbe cooperative in cleaving unreactive bonds, we synthesisedseveral chelating bis-NHSis.21–24 To our delight, preliminaryinvestigations revealed that the xanthene and ferrocenesupported bis-NHSi A23a and B21b can even cleave the strongCRO bond (1072 kJ mol1) and mediates the deoxygenativecoupling of two CO molecules under mild reaction condi-tions (room temperature, 1 atm) to yield the ketene species Cand D, respectively (Scheme 1).23b,c Moreover, the reaction ofbis-silylene A with an equimolar amount of the isoelectronic2,6-dimethylphenyl isocyanide resulted in the dearomatisedsilene derivative E.23b Moreover, the reaction of A with twomolar equivalents of 2,6-dimethylphenyl isocyanide affordedF and G.Scheme 1 Bis-silylenes A, B and 1 as well as the products C–G resultingfrom the reaction of A and B with CO and 2,6-dimethylphenyl isocyanide,respectively.a Technische Universita¨t Berlin, Institute of Chemistry: Metalorganic and InorganicMaterials, Sekr. C2, Strasse des 17. Juni 135, 10623 Berlin, Germany.E-mail: matthias.driess@tu-berlin.de; Fax: +49-30-314-29732b Department of Chemical & Biological Engineering University of Wisconsin-Madison,1415 Engineering Drive, Madison, WI 53706, USAcDepartment of General and Inorganic Chemistry, Faculty of Chemical Technology,University of Pardubice, Studentska 573, 532 10 Pardubice, Czech Republic† Electronic supplementary information (ESI) available: Experimental and computa-tional details. CCDC 1963423 and 1963424. For ESI and crystallographic data in CIFor other electronic format see DOI: 10.1039/c9cc08680cReceived 6th November 2019,Accepted 10th December 2019DOI: 10.1039/c9cc08680crsc.li/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 10 December 2019. Downloaded on 1/30/2020 9:44:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article OnlineView Journal  | View Issue748 | Chem. Commun., 2020, 56, 747--750 This journal is©The Royal Society of Chemistry 2020Compared to the bis-silylene A,23a the ortho-carboranesupported bis-NHSi 124 is more rigid, and we wondered itsreactivity towards CO and the isoelectronic 2,6-dimethylphenylisocyanide furnishes similar or diﬀerent coupling products.Herein, we report the unexpected novel coupling productsfrom CO and 2,6-dimethylphenyl isocyanide mediated by thebis(NHSi) 1.Exposure of the yellow solution of 1 in toluene to CO (1 atm)at 20 1C leads to a colour change from yellow to brown. Thereaction mixture was allowed to warm to room temperature togive a brown solution, from which the unprecedented couplingcompound 2 could be crystallised after one day at ambienttemperature in the form of colourless crystals in 62% yields(Scheme 2). In contrast to the reaction of A and B with CO,the corresponding disilaketene could even not be observedas intermediates, presumably owing to the rigidity and sterichindrance of the o-carborane backbone.In compound 2, the two diﬀerent sorts of 29Si nuclei giverise to diﬀerent 29Si NMR resonances. The resonance atd = 62.27 ppm can be assigned to the Si atoms of the CSiO2moiety, whereas that at d = 108.44 ppm corresponds to theC2SiO units; the latter value is close to the29Si NMR resonanceof C (d = 91.4 ppm).23b Interestingly, the CQC–O moieties in 2resonate at 22.34 and 173.78 ppm in the 13C NMR spectrum,which are reminiscent of those observed for the ketene moietyin C (19.7 and 165.5 ppm).A single crystal X-ray diﬀraction analysis revealed a poly-cyclic skeleton in 2 as depicted in Fig. 1. Compound 2 crystal-lises in the triclinic space group P%1. Its molecular structureconsists of an eight-membered Si2C4O2 ring and two four-membered SiOSiC rings. The penta-coordinate Si1 and Si3atoms in the eight-membered ring feature a distorted trigonalbipyramidal coordination geometry with the N1–Si1–C3 angleof 172.2(1) and the N6–Si3–C6 angle of 170.6(1)1 in the axialdirection, respectively. In comparison, the Si2 and Si4 atomsare also penta-coordinated (ON2C2), but adopt a strongerdistorted trigonal bipyramidal geometry with the C8–Si4–O3angle of 162.1(1) and N3–Si2–C3 of 161.3(1)1, respectively.The Si–O distances, ranging from 1.683(3) to 1.728(2) Å, arein the common range of Si–O bonds. The Si–C distances withthe carborane C atoms (Si2–C1 2.042(3) and Si4–C8 2.070(3) Å)are significantly longer than those of the C atoms in the eight-membered Si2C4O2 ring (1.876(3) to 1.905(3) Å). The C4–C2(1.537(3) Å) and C5–C7 (1.536(4) Å) interactions, bridging theeight-membered ring and the two carborane clusters, representC–C single bonds. However, the C–C distances within the eight-membered ring C3–C4 (1.324(4) Å) and C5–C6 (1.337(4) Å) aresignificantly shorter and indicative of CQC bonds. It is worthyof note that the C4–O4 (1.361(3) Å) and C5–O2 bonds (1.346(4) Å)aremuch shorter than a C–O bond in ethers, implying considerableelectron delocalisation in those C–C–O moieties.The reactivity of 1 towards the isoelectronic 2,6-dimethylphenylisocyanide has also been investigated (Scheme 3). 1H NMRmeasurements showed that compound 3 is formed as soleproduct independent of the molar ratio of the startingScheme 2 Reaction of bis-silylene 1 with CO to aﬀord 2 via 20 and 200.Fig. 1 Molecular structure of 2. Thermal ellipsoids are drawn at 50%probability level. Hydrogen atoms and Ph as well as t-Bu groups areomitted for clarity. Selected bond distances (Å) and angles (1): Si1–O1 1.690(1),Si2–O1 1.698(2), Si1–O2 1.728(2), Si3–O3 1.683(3), Si4–O3 1.705(2),Si3–O4 1.724(2), Si1–C3 1.876(3), Si2–C3 1.905(3), C3–C4 1.324(4), C4–C2,1.537(3), O4–C4 1.361(3), Si3–C6 1.878(3), Si4–C6 1.905(4), C5–C6 1.337(4),C5–O2 1.346(4), C5–C7 1.536(4), Si2–C1 2.042(3), Si4–C8 2.070(3),;Si1–O1–Si2 104.1(1), Si1–C3–Si2 89.9(1), Si3–O3–Si4 103.9(1), Si3–C6–Si489.7(1), C3–C4–O4 132.0(2), C6–C5–O2 132.2(3), C3–Si1–N1 172.2(1),C3–Si2–N3 161.3(1), N6–Si3–C6 170.6(1), C8–Si4–O3 162.1(1).Communication ChemCommOpen Access Article. Published on 10 December 2019. Downloaded on 1/30/2020 9:44:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2020 Chem. Commun., 2020, 56, 747--750 | 749materials chosen. Thus reaction of 1 with two molar equiva-lents of isocyanide at 20 1C in toluene aﬀords 3 in 78%isolated yields. This is in stark contrast to the reactions ofbis-NHSi A with the same isocyanide,23b which led to E, F, andG (Scheme 1) as products depending on the molar ratio of thestarting materials and the reaction conditions.The 29Si NMR spectrum of 3 shows two resonances atd = 32.3 ppm (SiQC) and 60.0 ppm (SiC3N2), assignable tothe four- and five-coordinate Si atoms, respectively. The 13C{1H}NMR spectrum exhibits a signal at d = 181.7 ppm for SiQC–Siand 190.4 ppm for R–NQC–Si (R = 2,6-dimethylphenyl),respectively. A single-crystal X-ray diffraction analysis revealedthat 3 crystallises in the monoclinic space group P21/n. Themolecular structure of 3 (Fig. 2) shows that the two moleculesisocyanide underwent NQC–N–C coupling and concomitantlySi–C bond formation. While the four-coordinate Si2 atomfeatures a distorted tetrahedral coordination environment, thefive-coordinate Si1 atom adopts a distorted trigonal bipyramidalcoordination geometry with the axial C2–Si1–C5 angle of 1651.Both of the four-membered SiC2N and the five-membered Si2C3rings are planar. Along the axial direction, the Si1–C2 (2.027(2) Å)and Si1–C5 (2.054(2) Å) distances are significantly longer thanthat of Si1–C3 (1.822(2) Å) in the equatorial direction. Moreover,the five-coordinate Si1 atom features also a longer Si1–C5distance (2.054(2) Å) than that of the four-coordinate Si2 atomin the Si2–C6 bond (1.905(2) Å). The shortest Si2–C3 distance(1.724(2) Å) among all Si–C distances in 3 lies even in the range ofSiQC bond distances of silaethylene.25 Similarly, the relativelyshort C2–N6 (1.281(2) Å) distance compared with that of C3–N5(1.474(2) Å) suggests CQN bond character. It is noteworthy thatcompound 3 represents an unique example of head-to-tailcoupling product of isocyanide compared to those usual head-to-head coupling products.26Because all attempts to detect an intermediate even at78 1C by means of multi nuclear NMR spectroscopy wereunsuccessful, Density Functional Theory (DFT) calculationswere performed to gain insight into the mechanism of theformation of 2 and 3. The DFT-derived mechanism leading to 2(Scheme 2 and Fig. S3 in ESI†) follows initially similar reactionsteps that have been uncovered previously for the reaction ofCO and bis-NHSi A to give C (Scheme 1).23b In contrast tothe former case, however, the disilaketene analogue of C(compound 20, Scheme 2) is a reactive intermediate whichundergoes migration of one of the carbons of the carboranefrom Si to the central carbon of the ketene moiety to aﬀordanother intermediate (compound 200) with a SiQC–CQOmoiety; head-to-tail dimerisation of the latter furnishes thefinal product 2. We think that the newly observed cleavage ofthe Ccarborane–Si bond is the consequence of the weaker andmore polar nature of the C–Si bond as indicated by the lowerWiberg Bond Index (WBI, 0.53 in 1) compared to the WBI of0.69 in both A and B. In addition, the proposed ketene inter-mediate 20 is energetically 32.5 kcal mol1 less stable than itsdimer 2 in part owing to the steric congestion of 20 causedby both bridged m-CQCQO and m-O. In case of the reactionof 1 with 2,6-dimethylphenyl isocyanide, the isocyanide moietyreacts rapidly with 1 to form 30 which is attacked by anadditional 2,6-dimethylphenyl isocyanide molecule to give 3(Fig. 3). The latter reaction step shows a very small energybarrier of 1.9 kcal mol1 that is consistent with the experimentalfact that the conversion proceeds at 20 1C. NBO analysis(Tables S5–S7, ESI†) reveals the zwitterionic nature of 30 (Fig. 3)that enables the reactivity of 30 towards an additional isocyanide.In conclusion, the o-carborane supported bis-NHSi 1 is morereactive towards CO and 2,6-dimethylphenyl isocyanide thanthe previously reported bis-NHSis A and B.23b Owing to thediﬀerent steric and electronic eﬀects of the o-carborane back-bone, the outcome of the reaction of 1 with CO aﬀords solely,via a cascade of cleavage and coupling reactions, the new bis-silylene-mediated polycyclic CO coupling product 2 as the finalproduct. The reaction of 1 with 2,6-dimethylphenyl isocyanidefurnishes the new SiQC containing coupling product 3,irrespective of the chosen molar ratio of 1 to isocyanide. Theunexpected head-to-head of CO and head-to-tail of isocyanidehomocoupling products represent new reactivity features of aScheme 3 Reaction of 1 with 2,6-dimethylphenylisocyanide to form 3.Fig. 2 Molecular structure of 3. Thermal ellipsoids are drawn at 50%probability level. Hydrogen atoms are omitted for clarity. Selected bonddistances (Å) and angles (1): Si1–C3 1.822(2), Si1–C2 2.027(2),Si1–C5 2.054(2), Si2–C3 1.724(2), Si2–C6 1.905(2), C2–N6 1.281(2),C2–N5 1.413(2), C3–N5 1.474(2); C3–Si1–C2 70.7(1), C3–Si1–C5 94.3(1),C2–Si1–C5 165.0(1), C3–Si2–C6 101.6(1), N5–C3–Si2 139.9(1), N5–C3–Si197.3(1), Si2–C3–Si1 22.3(1).ChemComm CommunicationOpen Access Article. Published on 10 December 2019. Downloaded on 1/30/2020 9:44:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Online750 | Chem. Commun., 2020, 56, 747--750 This journal is©The Royal Society of Chemistry 2020bis-NHSi towards small molecules. More investigations con-cerning the remarkable reactivity of bis-NHSi 1 to furnish newclasses of organosilicon compounds are currently in progress.We thank the Deutsche Forschungsgemeinschaft [DR 226/19-4]for financial support. Support of A. 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Homocoupling of CO and isocyanide mediated by a C,C′-bis(silylenyl)-substituted ortho-carborane

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Homocoupling of CO and isocyanide mediated by a C,C′-bis(silylenyl)-substituted ortho-carborane

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