Silicon-Mediated Selective Homo- and Heterocoupling of Carbon

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Silicon-Mediated Selective Homo- and Hetero-Coupling of Carbon Monoxide Yuwen Wang, Arseni Kostenko, Terrance J. Hadlington, Marcel-Philip Luecke, Shenglai Yao, and Matthias Driess J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11899 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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Journal of the American Chemical Society

Silicon-Mediated Selective Homo- and Hetero-Coupling of Carbon Monoxide Yuwen Wang, Arseni Kostenko, Terrance J. Hadlington, Marcel-Philip Luecke, Shenglai Yao and Matthias Driess* Metalorganics and Inorganic Materials, Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 135, Sekr. C2, 10623 Berlin, Germany ABSTRACT: While the transformation of carbon monoxide to multicarbon compounds (fuels and organic bulk chemicals) via reductive scission of the enormously strong CO bond is dominated by transition-metals, splitting and deoxygenative reductive coupling of CO under nonmatrix conditions using silicon, the second most abundant nonmetal of the Earth’s crust, is extremely scarce and mechanistically not well understood. Herein we report the selective deoxygenative homocoupling of carbon monoxide by divalent silicon utilizing the (LSi:)2Xant 1a (Xant = 9,9-dimethyl-xanthene-4,5-diyl; L = PhC(NtBu)2) and (LSi:)2Fc 1b (Fc = 1,1’-ferrocenyl) as four-electron reduction reagents under mild reaction conditions (RT, 1 atm), affording the corresponding disilylketenes, Xant(LSi)2(µ-O)(µ-CCO) 2a and Fc(LSi)2(µ-O)(µ-CCO) 2b, respectively. However, the dibenzofuran analogue of 1b, compound 1c, was unreactive towards CO due to the longer distance between the two SiII atoms, which demonstrated the crucial role of the Si∙∙∙Si distance on cooperative CO binding and activation. This is confirmed by Density Functional Theory (DFT) calculations and further theoretical investigations on CO homocoupling with 1a and 1b revealed that the initial step of CO binding and scission involved CO acting as a Lewis acid (four-electron acceptor), in sharp contrast to CO activation mediated by transition-metals where CO serves as a Lewis base (two-electron donor). This mechanism was strongly reinforced by the reaction of 1a with isocyanide XylNC (Xyl = 2, 6-Me2C6H3), isoelectronic with CO. Treatment of 1a with one or two molecules of Xyl-NC furnished the unique (silyl)(imido)silene 3a and the C=C coupled bis(Xyl-NC) product 5, respectively, via the isolable doubly-bridged Xant(LSi)2(µXylNC)2 intermediate 4. Moreover, compound 3a reacts with one molar equiv of CO to give the disilylketenimine Xant(LSi)2(µO)(µ-CCNR) 6, representing, for the first time, a selective heterocoupling product of CO with isoelectronic isocyanide (CNR).

INTRODUCTION With the growing concerns about the shortage of fossil fuels,1 the importance of carbon monoxide serving as a versatile C1 building block to produce multicarbon compounds (fuels, solvents, organic bulk chemicals) cannot be overestimated. Since the C≡O bond (1072 kJ mol-1) is one of the strongest bonds in chemistry,2 scission of this triple bond is particularly challenging. Utilizing transition-metal catalysts enables the well-known Fischer-Tropsch synthesis, involving the reductive scission of the CO bond in the presence of dihydrogen (socalled ‘syn gas’) to form C−C bonds at elevated temperature.3 This process is used to produce tonnes of liquid hydrocarbons and oxygenates in the chemical industry each year.4 In the absence of H2, a reductive coupling of CO can only be achieved using very strong metal reducing agents (e.g., alkali metals, lanthanoids, actinoids or metal hydrides), affording ketenes (C=C=O),5 the dianionic oxocarbon homologues CnOn2- (n = 26)6 and ethenediolate derivatives.7 In stark contrast to the established domain of metal-mediated CO activation, nonmetal-based reductive activation of CO largely lags behind and faces greater challenges in view of limited redox capability. Examples of metal-free CO activation and coupling utilizing low-valent8 (Figure 1a, A8f) or multiply bonded9 (Figure 1a, B9b) main-group

Figure 1. a) Selected examples of main-group element compounds for CO activation. b) CO activation by a lithium disilenide. c) Employing bis(NHSis) for CO homo-and heterocoupling.

element compounds or frustrated Lewis pairs (FLPs)10 (Figure 1a, C10b) have been reported. However, a selective de-

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oxygenative homocoupling of CO to multicarbon compounds (e.g. CnO species) has not yet been achieved with a molecular nonmetal system. Elemental silicon and its subvalent compounds may hold a unique position among potentially suitable nonmetal containing systems, because it has a relatively high reduction ability in both elemental form and subvalent states (e.g, divalent silicon in silylenes) and is highly abundant with about 28% of the mass of the Earth’s crust. It has been shown that silicon atoms and small clusters react with CO in cryogenic matrices to yield carbonyl complexes which can photochemically rearrange to elusive cyclic four-membered Si2(µ-O)(µ-CSi) and Si2(µ-O)(µ-CCO) species.11 Previous efforts towards divalent silicon (‘silylene’)-mediated CO activation included the observation of transient silylene-CO adducts.12 More recently, the reactions of CO with 1, 4-disila (Dewar benzene) or cyclo-trisilenes led to carbonylation products.13,14 It has also been shown that a lithium disilenide allows scission of the CO bond and C−C coupling for which a ketenyl intermediate was proposed but could not be verified experimentally (Figure 1b).15 Despite all these advances, a silicon-based molecular system that can simultaneously accomplish reductive CO cleavage and deoxygenative C−C homocoupling to form isolable ketenes (C=C=O) is hitherto unknown. Although N-heterocyclic silylenes (NHSis), the silicon analogues of N-heterocyclic carbenes (NHCs), are capable of activation of numerous small molecules, yet, CO is notably absent from this list,16 due to the insufficient reduction ability of a single SiII atom in NHSis towards CO. This prompted us to investigate whether the inertness towards CO could be conquered if two SiII atoms are placed in close proximity in a molecular bis(NHSi) scaffold17 to enable cooperative CO bond scission. Here we report the observation of the facile deoxygenative reductive homocoupling of CO at room temperature and 1 atm with the bis(NHSi)xanthene 1a 17f and bis(NHSi)ferrocene 1b 17b to give the corresponding disilyl(µO)(µ-CCO) ketenes in high yields (Figures 1c and 2). Density Functional Theory (DFT) calculations unraveled a unique binding and reduction mechanism, involving the lone pairs of two SiII atoms which interact cooperatively with the π* orbitals of CO in the initial step of activation. Further, the unprecedented deoxygenative reductive heterocoupling of CO with an isocyanide had also been achieved (Figure 1c), with related isolable intermediates reinforcing the aforementioned DFT-derived reaction mechanism.

RESULTS AND DISCUSSION Exposure of Xant(LSi:)2 1a (Xant = 9,9-dimethyl-xanthene4,5-diyl, L = PhC(NtBu)2) to CO (1 atm) at room temperature for 12 h led to the gradual formation of Xant(LSi)2(µ-O)(µCCO) 2a which was isolated as colorless crystals in 83% yields (Figure 2). The 1H NMR spectrum of 2a displays two partially overlapping singlets at δ 1.33 ppm corresponding to the slightly inequivalent tBu groups. The 29Si NMR signal (δ -91.4 ppm) of 2a is markedly shifted upfield compared with that of the precursor 1a (δ 17.3 ppm), consistent with the change of SiII in

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1a to SiIV in 2a. Additionally, a strong infrared absorption band at ν = 2069 cm-1 assigned to the C=O stretching vibration mode was observed, revealing the presence of the C=C=O ketene moiety.

Figure 2. Synthesis of Xant(LSi)2(µ-O)(µ-CCO) 2a and Fc(LSi)2(µ-O)(µ-CCO) 2b (L = PhC(NtBu)2). *The SiII∙∙∙SiII distance in crystallographic structure. §The SiII∙∙∙SiII distance in optimized structure at B3LYP-D3 level with s6-31g* basis set for Fe and def2-SVP for all other atoms.

Figure 3. Molecular structures of 2a and 2b. Thermal ellipsoids in the structures are drawn at 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (o): for 2a, C1-C2 1.276(5), C2-O2 1.180(4), Si1-C1 1.871(3), Si2-C1 1.872(3), Si1-O1 1.711(2), Si2-O1 1.712(2), C1C2-O2 179.9(5), Si1-C1-Si2 90.98(14), Si1-O1-Si2 102.47(12), C1-Si1-O1 82.39(13), C1-Si2-O1 82.36(13); for 2b, C2-C3 1.282(6), C3-O2 1.194(6), Si1-C2 1.876(3), Si1a-C2 1.876(3), Si1-O1 1.7050(19), Si1a-O1 1.7050(19), C2-C3-O2 179.6(5), Si1C2-Si1a 88.03(18), Si1-O1-Si1a 99.70(15), C2-Si1-O1 83.02(12), C2-Si1a-O1 83.02(12).

The molecular structure of 2a is unambiguously determined by a single-crystal X-ray diffraction analysis and shown in Figure 3. The central structural feature of 2a


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Figure 4. DFT-derived mechanisms of the reaction of 1a with two molecules of CO to form 2a at the B3LYP-D3/def2-SVP//B3LYPD3/def2-SVP level of theory.

is a four-membered planar Si2OC ring, where two SiIV atoms are five-coordinate with a Si∙∙∙Si distance of 2.6692(13) Å. This Si∙∙∙Si distance is significantly shorter than that of precursor 1a (4.316 Å) leading to the distinguished distortion of the planar xanthene ring. The C1−C2−O2 angle of 179.9(5)o is nearly liner with the C1−C2 and C2−O2 distances of 1.276 (5) Å and 1.180 (4) Å, respectively, definitely confirming the formation of the ketene (C=C=O) function. Employing Fc(LSi:)2 1b (Fc = 1, 1’-ferrocenediyl) does also split CO easily to form Fc(LSi)2(µ-O)(µ-CCO) 2b as confirmed by multinuclear NMR spectroscopy (Figure S8 to S10) and a single-crystal X-ray diffraction analysis (Figure 3). Notably, the related bis(NHSi)dibenzofuran 1c, being accessible in an analogous procedure to that of 1a, is unreactive towards CO due to the significantly longer SiII∙∙∙SiII distance of 6.045(1) Å compared with that in 1a [4.316(1) Å] (Figure 2). In fact, the decisive role of the SiII∙∙∙SiII distance on cooperative CO binding and reduction is confirmed by DFT calculations (see below). Reaction intermediates could not be detected by means of NMR spectroscopy even if the reaction progress is monitored at -60 °C. Isotopic labelling experiments were carried out to further investigate the reductive homocoupling of CO to ketene. The bis(NHSis) 1a and 1b were exposed to 13CO affording the respective 13C-labeled complexes 2a and 2b containing the 13C=13C=O moiety as anticipated. In each of the 13C NMR spectra of 2a and 2b, one doublet at higher field [δ 19.7 (2a), 21.8 (2b) ppm] and one at lower field [δ 165.5 (2a), 168.4 (2b) ppm] with the same coupling constant [1J13C-13C = 97.9 (2a), 1J13C13 = 99.1 (2b) Hz] are assigned to the 13C=13C=O functional C group. In both 29Si NMR spectra, the resonances at δ -91.4 (2a) and -84.8 (2b) ppm exhibit a doublet of doublets because of coupling with the two 13C atoms from ketene. Accordingly, the strong infrared band corresponding to the ketene stretching vibration mode is red shifted by ca. 65 cm-1 for 2a and 64 cm-1 for 2b, respectively. Numerous competing mechanisms for the formation of 2a were considered in an in-depth DFT analysis (Figures 4 and S47), which had alluded to the mechanism outlined in Figure 4

being the most likely case, in line with further experimental data (see below). In the proposed mechanism of initial CO binding, the HOMO and HOMO-1 of 1a, representing the lone pairs of the silylene moieties, can interact with the two π* orbitals of CO to form the intermediate B in an endergonic step of 4.3 kcal mol-1 (Figure 4). Formation of B comprises a unique situation of donoracceptor interaction in which CO acts as a Lewis acid. In the optimized structure of B, the C−O distance is 1.326 Å with Wiberg Bond Index (WBI) of 1.15 and natural charges of q(C) = -0.80 el. and q(O) = -0.81 el [in comparison to 1.130 Å, WBI = 2.26, q(C) = +0.50 el. and q(O)= -0.50 el. in ‘free’ CO] indicative of a C−O bond that is a result of the occupation of both π*(CO) orbitals by the two Si lone pairs (Table S17). Intermediate B can then react with one additional molecule of CO via TS(B-C) to afford the disiladicarbonyl Xant(LSi)2(µCO)2 intermediate C (at ΔG = 10.0 kcal mol-1). Unlike B in which the interaction of the bis(NHSi) moieties with CO was achieved via donation of the Si lone pairs to the empty π*(CO) orbitals, in C the Si atoms are in the formal oxidation state of IV and bear two common C=O carbonyl groups, with r(C−O) = 1.216, 1.214 Å and WBI(CO) = 1.81, 1.82 with q(C) = -0.15, 0.18 el., and q(O) = -0.56, -0.55 el., respectively (Table S17). In C, the carbon atoms of C=O groups, which are separated by 2.575 Å, exhibit a very weak attractive interaction with a C−C WBI of 0.06; the latter situation predisposes the carbonyl carbon atoms to result in the C=C bond of the ketene moiety in the final product 2a. The formation of the intermediates B and C comprises key reaction steps (Figure 5), and the reaction proceeds similarly for Fc(LSi)2 1b, with the analogues of B and C laying at 5.3 and 7.9 kcal mol-1 on the potential energy surface (PES), respectively. The overall reaction of 1b with two molecules of CO to form 2b is exergonic by 75.0 kcal mol-1. For the bis(NHSi) 1c with a benzofuran backbone, the reaction with two molecules of CO to form the corresponding ketene product is also predicted to be exergonic by 33.4 kcal mol-1; however, the formation of the analogous intermediates B (36.3 kcal mol1) and C (45.7 kcal mol-1) is predicted to be strongly endergonic,



that is, the desired transformation cannot proceed under similar reaction conditions as mentioned above. Apparently, the two NHSi moieties in 1c are too far apart to allow cooperative CO activation.

carbon and two oxygen atoms of the two CO groups is -1.55 el.. Thus the two CO units are reduced by 1.55 el. to furnish a C−C and two C=O bonds which represent an elusive ethenedione dianion moiety. Intermediate D rearranges to E in an exergonic step via TS(D-E). The latter step reflects the beginning of the formation of the ketene unit (C=C=O). Compound E rearranges to F via a low barrier of 4 kcal mol-1 [TS(E-F)]. In TS(E-F) the imaginary frequency corresponds to the stretching of the C−O bond (ν = -463.5 cm-1) that is being cleaved in the process. This is accompanied by the formation of the ketene unit and a new Si−O bond that originates from the cleaved C−O bond. The reaction is extremely exergonic (55.5 kcal mol-1), with intermediate F found in a shallow minimum on the PES rearranging to the final product 2a via a low barrier [TS(F-2a)] of only 1.9 kcal mol-1 (Figure 4, black path). In an alternative pathway, relevant for the reaction of 1a with isocyanide (see below), compound E rearranges to G. The barrier for this reaction TS(E-G) is 13.9 kcal mol-1 higher than TS(E-F) (the C−O cleavage) and thus the reaction is not expected to proceed via this pathway (Figure 4, red path). G can be expected to be in a thermal equilibrium with H via TS(G-H) that is 3.7 kcal mol-1 lower in energy than TS(G-2a) that ultimately leads to the final product 2a (Figure 4, red path). The reactivity between 1a and isocyanide was investigated in order to attain experimental data which may reinforce our proposed mechanism, given that CO and organic isocyanides are isoelectronic and isolobal species. An equimolar mixture of 1a with Xyl-NC (Xyl = 2, 6-Me2C6H3) led to the formation of a single deep red-pink species, 3a, after 24 h at ambient temperature (Scheme 1). Scheme 1. Syntheses of compounds 3a, 4 and 5.

Figure 5. Calculated geometries of the reactive intermediates B-H at the B3LYP-D3/def2-SVP level of theory.

The decisive role of the SiII∙∙∙SiII distance for cooperative CO binding and activation is confirmed from DFT calculations. Relaxed PES scans of approaching the two SiII atoms from an optimized geometry of the different free ‘spacer’ ligands to the distance in intermediate B, i.e. from 4.162 to 3.276 Å for xanthene, from 4.048 to 3.038 Å for ferrocene, and from 5.536 to 3.389 Å for dibenzofuran, revealed that achieving the required proximity requires 5.1, 4.9 and 31.8 kcal mol-1 respectively (Figure S48). The latter values are reflected in the relative stabilities of the intermediate B for each ‘spacer’ ligand. The first step of OC−CO homocoupling with 1a is accomplished when C is converted to D (Figure 5) via the TS(C-D) at ΔG‡ = 24.9 kcal mol-1. The high barrier (14.9 kcal mol-1) of this step is reflected in the relatively long reaction time of 12 h to reach completion. In D the distance between the carbonyl carbon atoms is 1.541 Å with WBI of 1.11, and r(C−O) of 1.230, 1.234 Å and WBI (CO) of 1.44, 1.45, respectively (Table S17). The sum of natural charges on the two

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L

L

Si

Si

L Si O

XylNC (1 equiv.)

Si 1a

CNXyl

O

L

O

C

Si

Si

L

L 3a

3b

N

XylNC (1 equiv.) tBu N L

L

L Ph

N tBu

Si C O XylN

Xyl = 2,6-Me2C6H3

NXyl C

C Si NXyl

O

C

Si

L

L 4

Si

XylN

5

The 1H NMR of 3a revealed that the methyl groups of the isocyanide are now in different chemical environments, and that the former aromatic protons of this fragment are high-field shifted, indicating a break in the aromaticity of this ring. The related 29Si NMR spectrum contains two distinct singlets (δ 3.6 and -0.3 ppm), also indicative of different chemical environments for the two Si-centers in the formed product. Structural data elucidated that 3a is a


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Figure 6. Molecular structures of 3a, 4, 5 and 6. Thermal ellipsoids in the structures are drawn at 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (o): for 3a, Si1-C46 1.853(3), Si2-C46 1.760(3), Si1-C52 1.913(3), C46-N5 1.423(3), C47-N5 1.311(3), C47-C48 1.437(4), C48-C49 1.357(4), C49-C50 1.446(4), C50-C51 1.341(4), C51-C52 1.497(4), C47-C52 1.534(4), Si1-C46-Si2 142.67(15), C46-Si1-C52 90.00(11), C46-N5-C47 115.1(2), Si1-C52-C47 100.98(16), N5-C47-C52 118.6(2), Si2C46-N5 107.47(17); for 4, Si1-C46 1.971(4), Si1-C55 2.002(4), Si2-C46 1.955(4), Si2-C55 1.974(4), C46-N5 1.285(5), C55-N6 1.281(5), Si1-C55-Si2 96.75(18), Si1-C46-Si2 98.41(18), C46-Si1-C55 80.81(17), C46-Si2-C55 81.91(17); for 5, C47-C56 1.350(3), Si1-C56 1.949(2), Si1-N5 1.7673(15), Si2-C47 1.9706(19), Si2- N6 1.7754(16), C47-N5 1.459(2), C56-N6 1.444(2), Si1-C56-C47 88.71(13), Si2C47-C56 87.17(13), N5-C47-C56 104.69(16), N6-C56-C47 106.82(16), N5-Si1-C56 73.35(8), N6-Si2-C47 73.37(8); for 6, C46-C47 1.296(3), C47-N5 1.229(3), Si1-C46 1.879(2), Si2-C46 1.880(2), Si1-O2 1.7111(16), Si2-O2 1.7143(16), C46-C47-N5 171.2(3), C46- N5C48 133.1(3), Si1-C46-Si2 90.28(10), Si1-O2-Si2 102.14(8), C46-Si1-O2 82.55(9), C46-Si2-O2 82.44(9).

highly conjugated (silyl)(imido)silene derivative (Figure 6), which results from initial formation of the isocyanide adduct of 1a (i.e intermediate 3b) (Scheme 1), analogous to intermediate B (Figure 4), with subsequent de-aromatisation of the aryl substituent by a SiII atom present in 1a, explaining the NMR observations described above. Thus, 3a represents a ‘trapped’ analogue of intermediate B, which could reform 3b upon rearomatization of the xylyl ring.18 Results of DFT calculations suggest that the isolated de-aromatized compound 3a is 1.0 kcal mol-1 more stable than 3b, and that 3a and 3b can exist in a thermal equilibrium via a transition state of only ΔG‡ = 16.4 kcal mol-1 (Figures S46). This equilibrium therefore enables the formation of 3b, which can further react with an additional molar equiv. of isocyanide (Scheme 1). Interestingly, unlike the endergonic reaction of 1a with one molar equivalent of CO (ΔG = 4.3 kcal mol-1, Figure 4), the reaction of 1a with one molar equiv. of Xyl-NC to form 3a is exergonic (ΔG = 5.6 kcal mol1), explaining the possibility of its isolation (Figure S46). As aforementioned, 3a represents a (silyl)(imido)silene derivative, and as such contains one short Si−C double bond, one longer Si−C bond at its core [d(Si2−C46) = 1.759(3) Å; d(Si1−C46) = 1.855(3) Å] (Figure 6). Observation of the formerly aromatic xylyl ring in product 3a clearly indicates its dearomatization; the ring now deviates from planarity, contains a tetrahedral sp3 carbon center (i.e. C52) and only two C=C bonding interactions [d(C48−C49) = 1.357(4) Å; d(C50−C51) = 1.341(4) Å]. The intensely colored pink-red solutions of 3a have an absorbance in the visible region of the UV-vis spectrum at 514 nm.

For the reaction of 1a with CO, our predicted mechanism suggests that B reacts with a further equivalent of CO to first yield the doubly-bridged Xant(LSi)2(µ-CO)2 intermediate C, followed by a string of OC−CO coupled intermediates (Figure 4). Concordantly, an equimolar mixture of Xyl-NC and 3a, after 4 weeks, converts near quantitatively to the isocyanide coupled product 5 (Scheme 1) which is an analogue of intermediate H (Figure 4). Very small amounts of doubly-bridged compound Xant(LSi)2(µ-XylNC)2 4 could also be isolated, which correlates directly with the predicted intermediate C. Notably, in the formation of both 4 and 5, the xylyl group in 3a is indeed rearomatised, markedly clear in the 1H spectrum of 5 which contains no signals for alkenyl protons, and two clear singlets for the methyl groups of the xylyl moiety. Further, a single signal is observed in the 29Si NMR spectrum (δ -93.6 ppm), indicating an equal chemical environment at both Si centers. The molecular structure of 5 contains two C−C coupled XylNC units at its core, forming a 1,2-disila-1,2-diamido alkene, with a C=C double bond [d(C47−C56) = 1.350(3) Å], and C−N single bonds [d(N5−C47) = 1.459(3) Å; d(N6−C56) = 1.444(3) Å] which are considerably longer than C−N bonds found in isocyanides. The DFT-derived mechanism for the reaction between 3a and Xyl-NC proceeds similarly to related steps in the reaction of 1a with CO, following re-aromatization to 3b, but negating C−N bond cleavage i.e. steps E to G, rather than E to F (Figure S46). Unlike the reaction of 1a with two molecules of CO, the analogous TS(E-G), which avoids C−N bond cleavage, is 12.3 kcal mol-1 lower in energy than the corresponding TS(E-F), which would proceed via C−N



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Figure 7. DFT-derived mechanisms of the reaction of 1a with one molecule of Xyl-NC and one molecule of CO to form 6 at the B3LYPD3/def2-SVP//B3LYP-D3/def2-SVP level of theory.

bond cleavage (Figure S46). Thus, 5 does not react further to give a compound analogous to ketene 2a, likely due to steric pressure ensued by the xylyl group. In the case of reaction of 1a with CO, calculations predict that the analogue of 5, (i.e. H) is 5.1 kcal mol-1 less stable than intermediate G (Figure 4). This is reversed for the reaction with isocyanide (Figure S46), yielding a good explanation for the formation of 5 as opposed to an analogue of intermediate G. Nevertheless, taken as a whole, the observation and isolation of 4 and 5 strongly reinforces our predicted mechanism for the CO cleavage-coupling reaction involving 1a. With compound 3a in hand we investigated whether its exposure to CO (1.4 atm) at room temperature enables a reductive hetero-coupling of CO with isocyanide, a reaction entirely unprecedented in the literature. After three weeks, compound 6, the ketenimine analogue of 2a, is formed as the sole product through complete CO bond cleavage and C=C heterocoupling with the isocyanide moiety (Scheme 2). Scheme 2. Syntheses of compound 6.

O

L

L

Si

Si

C

CNXyl

O

N

Si

Si

L

L

3a

3b 13

tBu N L

Ph

N tBu

CO

L

L

Si

Si

O O

13

Si L

Xyl = 2,6-Me2C6H3

6a

C C NXyl and

O O

C

Si L

6b

13

C NXyl

Again, the xylyl ring in 6 is re-aromatized, indicating that the likely first step in its formation is the generation of 3b in-situ, the process likely resulting in the long observed reaction times. Further, as with 5, a single signal can be observed in the 29Si NMR spectrum of 6 (δ -88.4 ppm), pertaining to the two fivecoordinate Si centers in this compound. The molecular structure of 6 indicates a single O-atom bridging these two Si centers, originating from added CO (Figure 6). The carbon from this fragment is now bound by Xyl-NC, forming a disilaketenimine, for which there are no crystallographically characterized examples in the literature. The isocyanide unit contains clear C=C and C=N bonding interactions [d(C46−C47) = 1.296(3) Å; d(C47−N5) = 1.229(3) Å], and C47−N5−C48 bond angle of 133.1(3)° indicative of a lone-pair of electrons at N. The mechanism of reaction of 1a with one molecule of XylNC and one molecule of CO (Figure 7) is similar to the mechanism presented for the reaction of 1a with two molecules of CO (Figure 4). The formation of intermediate E proceeds via the same steps with the only difference is that intermediate C converts directly to E and not via D (Figure 4). Similar to the reaction of CO homocoupling, the TS(E-6) that involves C−O cleavage in intermediate E is lower in energy than the transition state TS(E-G) that leads to intermediate G by 12.4 kcal mol-1. E converts directly to the final product and not via intermediate H. To our surprise, the reaction of 13CO with 3a revealed that both the α- and β-carbon atoms of the C=C=N moiety in 6 were equally 13C-labeled (Scheme 2, 6a and 6b). This phenomenon is clearly indicated by both NMR and infrared spectroscopy. The 29Si NMR spectrum of the isotopically labelled compound contains both a doublet and a singlet centered at δ -88.4 ppm, the doublet through coupling


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Figure 8. Calculated mechanism of formation of compound 6 at the B3LYP-D3/def2-SVP level of theory. The pathway of formation of the isotopically scrambled product (right, black path) and the pathway of formation of the product without isotopic scrambling (left, red path) are presented.

to 13C (1JCSi = 71.6 Hz) (Figure S26). The related 13C NMR spectrum shows labeled singlet peaks relating to both α-and βC atoms of the C=C=N moiety, indicating that no one labeled molecule contains both α- and β-13C centers, as no coupling is observed (Figure S25). Finally, the IR spectrum of the labelled compound indicates an isotopic shift for both the C=C and C=N bonds (Figure S34). In contrast, such scrambling can be excluded for the ketene formation of 2a, in line with the result of the reaction of Xant(LSi:)2 1a with a mixture of 13CO and C18O (Figure S30). This isotopic labeling experiment led to two strong IR absorptions at 2044 and 2008 cm-1 assigned to 13C=O and C=18O ketene moieties instead of four absorptions according to 13C=O, C=18O, 13C=18O and C=O ketene species (Figure S30, Table S1), suggesting that there is no carbon atoms exchange during the ketene formation process. We propose that the isotopic scrambling in the reaction of 3a with 13CO originates from the formation of two intermediate isomers that can undergo either 13C−O or C−N cleavage, i.e. intermediate E and E’, respectively (Figure 8). Intermediate E forms from 3b as described in Figure 7 and shown here for comparison, via intermediate C (Figure 8, left red path). In contrast, E’ forms via the isomerization of 3b to L that reacts with a molecule of 13CO to form E’ (Figure 8, right black path). The barriers for the formation of E and E’ are very similar, thus both of these intermediates can form. Upon formation, E and E’ are not expected to be in thermal equilibrium under the reaction conditions since the barriers for the reverse reactions are too high, i.e. 33.2 and 30.2 kcal mol-1, respectively. Thus, although E is 4.6. kcal mol-1 more stable than E’, their formation is kinetically controlled and they form and act independently. The preferred pathway for the reaction of E is the 13C−O cleavage that leads to a direct formation of the final product 6 in which no isotopic scrambling occurs (Figure 8, left red path). E’ can also rearrange to final product but in a stepwise manner (Figure 8, right black path). E’ can undergo the C−N bond cleavage via a barrier of only 5.8 kcal mol-1 TS(E’-I) that leads to the intermediate I. I contains Si=NR and Si=C=13C=O moieties with the labeled carbon 13C still attached to the oxygen. I undergoes an exergonic (ΔG = -3.6 kcal mol-1) [2+2]

cycloaddition reaction to form intermediate J with a four membered ring (Si−O−13C−N). In J the labeled carbon is attached to both the oxygen (originating from the 13CO) and the nitrogen (originating from the isocyanide) making J a key intermediate in the isotopic scrambling process. J rearranges to K via an exergonic (ΔG = -3.6 kcal mol-1) ring expansion which leads to a cleavage of the original 13C−O bond attaching the labeled carbon to the nitrogen. The exergonic step (ΔG = -7.6 kcal mol-1) of the Si−N bond cleavage forms intermediate F that is similar to the intermediate F presented in Figure 4. F consequently rearranges to the final product 6 in which the 13C of the labeled carbon monoxide is attached to the nitrogen. The isotopic scrambling in the products presented for the reaction of 3a with 13CO is not observed when 2a is reacted with a mixture of 13CO and C18O. As explained above, the isotopic scrambling in the reaction of 3a and one molecule of 13CO originates from the formation of intermediate J (Figure 8). However, calculations suggest that in the reaction of 2a with a mixture of 13CO and C18O the formation of J is not possible. This is because upon 13C−O cleavage of intermediate E to form F (Figure 9), the barrier for the formation of the final product 2a is only 1.9 kcal mol-1, and the reaction is highly exergonic (ΔG = -16.9 kcal mol-1) (Figure 9, black path). In contrast, in order to form J, that can lead to the isotopically scrambled product, F would first need to rearrange to K via a high barrier of 19.2 kcal mol-1 in an endergonic process of ΔG = 10.9 kcal mol-1 (Figure 9, red path). K would then need to rearrange to J that is further higher in energy than K by 14.1 kcal mol-1. Therefore, in reaction of 2a with a mixture of 13CO and C18O the isotopic scrambling does not occur.



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AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions Y.W., T.H. and A.K. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Figure 9. Calculated mechanism of formation of the final product 2a (black path) vs. the formation of intermediate J (red path) that can lead to isotopically scrambled product, at the B3LYPD3/def2-SVP level of theory.

CONCLUSIONS Employing the bis(NHSi)xanthene 1a and bis(NHSi)ferrocene 1b, we could accomplish the selective dexoygenative homocoupling of CO at room temperature under 1 atm to furnish Xant(LSi)2(µ-O)(µ-CCO) 2a and Fc(LSi)2(µO)(µ-CCO) 2b, respectively. Both products 2a and 2b resulting from four-electron reduction of CO were fully characterized. In stark contrast, the bis(NHSi)dibenzofuran 1c with a markedly longer Si∙∙∙Si distance shows no reactivity towards CO, illustrating the significant role of the Si∙∙∙Si distance on the cooperative CO activation. In line with results from DFT calculations, the mechanism represents a unique initial CO binding step where the lone pairs of the two SiII atoms act as a Lewis base towards CO, which acts as a Lewis acid. Moreover, the (NHSi)xanthene 1a reacts with one or two molecules isocyanide (Xyl-NC, isoelectronic with CO) to give the (silyl)(imido)silene derivative 3a and the bis-isocyanide coupled product 5, respectively, via the isolable intermediate Xant(LSi)2(µ-XylNC)2 4. The latter reaction sequence supports the proposed mechanism of CO homocoupling. Remarkably, even the reductive heterocoupling of CO with Xyl-NC to give the disilyl(µ-O)(µ-CCNXyl) ketenimine 6 could also be realized for the first time. Interestingly, when 3a was exposed to 13CO, both the α- and β-carbon atoms of the C=C=N moiety in 6 were 13C-labeled, indicating a scrambling process, while such scrambling process could be excluded for the disilylketene formation of 2a based on the reaction of 1a with a mixture of 13CO and C18O. Both processes were clearly confirmed by results of DFT calculations. The cooperative reduction ability of two NHSi moieties with SiII atoms in close proximity opens a new avenue to facile transformations of CO with other unsaturated organic molecules than isocyanides into functional multicarbon compounds in the absence of a metal.

ASSOCIATED CONTENT Supporting Information.

Experimental procedures, characterizations, crystallographic analyses, and computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

This work was financially supported by the Deutsche Forschungsgemeinschaft [DR 226/19-1 and Cluster of Excellence UniCat (EXC 314-2)]. Y.W. gratefully acknowledges financial support by the China Scholarship Council. A.K. is grateful to the Alexander von Humboldt Foundation for a postdoctoral fellowship. We thank Dr. Zhenbo Mo for helpful discussion, Dr. Xiaohui Deng and Rodrigo Beltrán-Suito for cyclic voltammetry (CV) measurements and Paula Nixdorf for the assistance in the XRD measurements.

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Silicon-Mediated Selective Homo- and Heterocoupling of Carbon

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