Cooperative Trimerization of Carbon Monoxide by Lithium and

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Cooperative Trimerization of Carbon Monoxide by Lithium and Samarium Boryls Baoli Wang, Gen Luo, Masayoshi Nishiura, Yi Luo, and Zhaomin Hou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10108 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Journal of the American Chemical Society

Cooperative Trimerization of Carbon Monoxide by Lithium and Samarium Boryls Baoli Wang,†,‡ Gen Luo,†,‡ Masayoshi Nishiura,† Yi Luo,*,§ and Zhaomin Hou*,†,§ †

Organometallic Chemistry Laboratory and RIKEN Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ABSTRACT: The conversion of carbon monoxide (CO) to hydrocarbons and oxygenates on industrial solid catalysts (the Fischer– Tropsch reaction) largely relies on the cooperation of heteromultimetallic active sites composed of main group (such as alkali) and transition metals, but the mechanistic details have not been fully understood at the molecular level. Here we report the cooperative trimerization of CO by molecular lithium and samarium boryl complexes. We have found that in the coexistence of a samarium boryl complex and a lithium boryl complex, the trimerization of CO selectively occurred to give a diborylallenetriolate skeleton “BC(O)C(O)C(O)B”, in sharp contrast with the reaction of CO with either the lithium or the samarium boryl compound alone. The 13 C-labeled experiments and computational studies have revealed that the CO trimerization reaction took place exclusively by coupling of a samarium boryl oxycarbene species, which was generated by insertion of one molecule of CO into the samarium– boryl bond, with a lithium ketenolate species formed by insertion of two molecules of CO into the lithium–boryl bond. These results offer unprecedented insight into CO oligomerization promoted by heteromultimetallic components, and may help better understand the industrial F–T process and guide designing new catalysts.

INTRODUCTION The reductive oligomerization of carbon monoxide (CO) in the presence of H2 on solid catalysts to give hydrocarbons and oxygenates was discovered more than 90 years ago by Fischer and Tropsch.1 Since then, this transformation has attracted tremendous attention in both industry and academia, because it offers an economically attractive process for the production of low molecular weight commodity chemicals and liquid fuels from coal or synthesis gas.2–7 The heterogeneously catalyzed Fischer–Tropsch (F–T) process is generally non-selective and usually produces a Schulz–Flory distribution of hydrocarbons and oxygenates. It has been reported that addition of a main group metal component such as an alkali metal compound to transition metal-based catalysts could significantly improve the activity and selectivity.2,7 However, the cooperative effects of the heteromultimetallic components have not been fully understood at the molecular level. To have a better understanding of the F–T reaction mechanism and thereby develop more efficient and selective catalysts, extensive studies on the activation and coupling of CO by molecular organometallic complexes have been carried out over the past decades.8–12 It has been found that the dimerization or oligomerization of CO can be achieved through reduction with various low-valent metal complexes13– 23 or through insertion of CO into a metal‒R bond (R = alkyl,24–28 hydrogen,29–33 silicon,34 or boron35–37). In most cases, the resulting CO coupling products were composed of an even number of CO units such as dimers,13–19,25,26,29,30,34–38 tetramers,15,23,27,28 and hexamers,33 although some trimerization products were also reported with the structure and composition largely depending on the metal complexes employed.20–22,31,32

Most of the reactions reported to date have been mainly based on homometallic complexes, while the reaction of gaseous CO with molecular heterometallic complexes has remained much less explored.39,40 In particular, the reaction of CO with a combination of an alkali metal complex and a transition metal (either d-block or f-block) complex has not been reported previously. Transition metal boryl complexes have fascinated scientists in organometallic and synthetic chemistry over the past decades, because of their important roles in various chemical transformations.41–47 However, the CO reaction chemistry of metal boryl compounds has remained almost unexplored to date.35–37 In the course of our recent studies on the fundamental chemistry and synthetic applications of rareearth (group 3 and lanthanide) metal complexes,48,49 we found that scandium boryl complexes could show unique behaviours in the reaction with CO, which yielded the corresponding boryl oxycarbene species that are highly reactive towards various substrates including the supporting ligands.35,36 These results promoted us to explore the CO reaction chemistry of new rare-earth metal boryl complexes bearing simpler and less reactive ligands. Herein, we report the synthesis and CO insertion reactions of a novel dichlorosamarium boryl complex. We have found that in the coexistence of the samarium boryl complex and a lithium boryl complex, the trimerization of CO selectively took place to give a unique diborylallenetriolate skeleton “BC(O)C(O)C(O)B”, in sharp contrast with the reaction of CO with either the samarium boryl complex or the lithium boryl complex alone. Experimental and computational studies have revealed that the CO trimerization reaction was accomplished exclusively by coupling of a samarium boryl oxycarbene species generated by

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Scheme 1. Synthesis of a Samarium Boryl Complex (2) and its Reactions with Carbon Monoxide in the Presence versus Absence of a Lithium Boryl Compound (1)

insertion of one molecule of CO into the samarium–boryl bond with a lithium ketenolate species formed by insertion of two molecules of CO into the lithium–boryl bond. This work represents the first example of selective CO oligomerization cooperated by molecular heteromultimetallic components, and may help better understand the industrial F–T process.

RESULTS AND DISCUSSION Synthesis of a chlorosamarium boryl complex. The metathetical reaction of SmCl3 with one equiv. of a lithium boryl compound {Li[B(N(Ar)CH)2](THF)2} (Ar = 2,6(iPr)2C6H3) (1)46,47 in THF took place rapidly at –35 ˚C to –5 ˚C, affording the corresponding chlorosamarium boryl complex 2 in 67% yield as dark-blue crystals within 30 minutes (Scheme 1). The use of two equiv. of 1 in this reaction gave the similar result. Complex 2 was thermally unstable and decomposed to a mixture of unidentified products in THF in 2 hours at room temperature. Nevertheless, single crystals of 2 suitable for X-ray diffraction studies were obtained by recrystallization from mixed THF and hexane at –35 ˚C. It was revealed that complex 2 possesses a Sm/Li heterobimetallic structure, in which the Sm and Li atoms are bridged by two chloride ligands (Fig. 1, left). A boryl ligand and a terminal chloride ligand are bonded to the Sm atom, and both the Sm and Li atoms each bear two THF ligands. Therefore, complex 2 could be viewed as a LiCl(THF)2 occluded complex of a dichlorosamarium boryl complex Cl2Sm[B(N(Ar)CH)2](THF)2 combined by two bridging chloride ligands. Complex 2 represents the first example of a chloride-ligated rare earth metal boryl complex as well as a rare example of structurally characterized f-transition metal boryl complexes.35,36,44,45 Despite the paramagnetic property of the Sm(III) ion, complex 2 showed well resolved 1H and 13C NMR spectra in THF-d8 at –20˚C. The methyl groups in the isopropyl units of the boryl ligand showed two doublets at δH 1.10 and 1.38, and the methine protons gave one septet at δH 5.19 in the 1H NMR spectrum, suggesting that the four isopropyl groups in 2 are equivalent in solution on the NMR time scale. The 11B NMR spectrum of 2 showed a broad signal at δB 24.3, which is com-

parable with those of the gadolinium and erbium boryl complexes {(Me3SiCH2)2Ln[B(N(Ar)CH)2](THF)2} (Ar = 2,6(iPr)2C6H3) (Ln = Gd, δB 29.2; Ln = Er, δB 28.7).35 Reactions of samarium and lithium boryl complexes with CO. When the samarium boryl complex 2 was exposed to CO (1 atm) at –108 ˚C (frozen THF) to –78 ˚C in THF, the insertion of CO into the Sm–B bond rapidly took place, affording the boryl oxycarbene complex 3 in 73% yield as green crystals in 30 minutes (Scheme 1). When the reaction was carried out at –108 ˚C to 25 ˚C, a mixture of unidentified products was obtained. The analogous reaction of 13CO with 2 at –108 ˚C to –78 ˚C gave the 13C-enriched analogue 3-13C1, which displayed a singlet at δC 180.2 in the 13C NMR spectrum. The 11B NMR spectrum of 3 showed a broad signal at δB –58.8, which is 83.1 ppm up-filed shifted compared to that of the boryl precursor 2 (δB 24.3). Single crystals of 3 suitable for Xray diffraction analysis were obtained after recrystallization from mixed THF and hexane at –35 ˚C. The overall molecular skeleton of 3 was well established (Fig. 1, middle), although the CO-originated O1 atom was slightly disordered. The bond length (mean) of the C1−O1 bond (1.28(3) Å) in 3 is comparable with that found in the previously reported scandium boryl oxycarbene complex (1.266(3) Å).36 When a 1:1 mixture of the lithium boryl complex 1 and the samarium boryl complex 2 was exposed to CO (1 atm) in THF at –108 ˚C to 25 ˚C for 5 minutes, a diborylallenetriolate product 4 resulting from CO trimerization was obtained as dark-red crystals in 62% yield after recrystallization from THF/hexane at –35 ˚C (Scheme 1). Similarly, the reaction of 13 CO with 1 and 2 afforded the 13C-enriched analogue 4-13C3. Single crystals of 4 suitable for X-ray diffraction studies were grown from a THF/hexane solution at –35 ˚C. It was revealed that complex 4 possesses a novel diborylallenetriolate skeleton “BC(O)C(O)C(O)B”, in which the two end-CO units are each bonded to a boryl group at the carbon atom (Fig. 1, right). The six atoms of the “(CO)3” unit are coplanar and adopt a “Yshaped” structure, which is in sharp contrast with the CO trimerization products reported previously in the literature, such as the samarium ketenecarboxylate,21,22 uranium


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Figure 1. Crystal structures of 2 (left), 3 (middle) and 4 (right) with thermal ellipsoids at the 50% level except for the 2,6-(iPr)2C6H3 groups in the boryl unit and the THF ligands. Hydrogen atoms have been omitted for clarity. Selected metric parameters are reported as distances (Å) and angles (°). 2 (left): Sm1‒B1, 2.730(7). 3 (middle): Sm1–C1, 2.419(6); Sm1–O1, 2.30(3); C1–O1, 1.28(3); C1–B1, 1.575(8); B1–C1–Sm1, 176.7(4); B1–C1–O1, 111.9(12). 4 (right): Sm1–O1, 2.271(2); Sm1–O2, 2.261(2); Sm1–O4, 2.388(3); Sm1–O5, 2.404(3); Li1–O3, 1.745(6); Li1–O6, 2.005(8); Li1–O7, 1.986(7); C1–C3, 1.424(4); C2–C3, 1.423(4); C1–O1, 1.308(4); C2–O2, 1.310(4); C3–O3, 1.325(4); C1–B1, 1.592(5); C2–B2, 1.586(4); O1–C1–C3, 124.4(3); C1–C3–C2, 120.2(3); O3–C3–C1, 119.6(3); O2–C2–C3, 124.0(3); O3–C3–C2, 120.1(3); C3–O3–Li, 176.8(3).

cyclopropanonediolate,20 and magnesium cyclopropanetriolate The two end-CO groups in the complexes.31 “BC(O)C(O)C(O)B” moiety chelate a Sm atom via the oxygen atoms, while the middle CO group is bonded to a Li atom via the oxygen atom in the opposite direction. In addition to the chelate ligand, the Sm atom bears two terminal chloride ligands and two THF ligands. The Li atom also bears two THF ligands. The bond distances of the two C–C bonds (C1–C3: 1.424(4) Å, C2–C3: 1.423(4) Å) in the “BC(O)C(O)C(O)B” skeleton are almost identical, which are in-between a typical C–C single bond and a C=C double bond.50 The C–O bond distances of the two terminal CO units (C1–O1: 1.308(4) Å, C2–O2: 1.310(4) Å) are slightly shorter than that of the central CO group (C3–O3: 1.325(4) Å), but all lie in-between those of a typical C–O single bond and a C=O double bond.50 The 1H NMR spectrum of 4 in THF-d8 gave a singlet at δH 6.26 assignable to the four vinyl protons of the two diazaborolyl rings, suggesting that complex 4 possesses a highly symmetric structure in solution. The 11B{H} NMR spectrum of 4 showed a broad singlet at δB 20.4, which is 3.9 ppm up-field shifted from that of complex 2 (δB 24.3). The isotope labelled 4-13C3 displayed one triplet at δC 171.1 (J = 61.3 Hz) and one doublet at 201.0 (J = 61.3 Hz) in the 13C NMR spectrum, which are assignable to the middle-carbon and the two end-carbon atoms in the [BC(O)C(O)C(O)B] skeleton, respectively. The selective formation of the allenetriolate product 4 in the coexistence of 1 and 2 is in sharp contrast with the reaction of CO with 2 in the absence of 1, which did not give a characterizable product under the same conditions (–108 ˚C to 25 ˚C). To gain information on the reaction mechanism, we then carried out a series of control experiments. No reaction was observed between the lithium boryl complex 1 and the 13 C-enriched samarium boryl oxycarbene complex 3-13C1 in the absence of CO. However, rapid reaction took place under a CO atmosphere at –108 ˚C to 25 ˚C, which gave the allenetriolate product 4-13C1 that possesses one 13C-labelled

CO unit (Scheme 2). Similarly, the reaction of 1 and 3 with 13 CO afforded 4-13C2, in which two of the three CO units were 13 C-labelled (Scheme 2). The partially 13C-labelled complex 413 C1 showed a singlet at δC 201.0, while 4-13C2 gave two doublets at δB 171.1 and 201.0, respectively, in the 13C NMR spectrum. These NMR data are in consistence with those of the fully 13C-labelled analogue 4-13C3. These results suggest that the samarium boryl oxycarbene complex 3 may serve as a key intermediate in the formation of the CO trimerization product 4. Scheme 2. Synthesis of Partially 13C-Labelled Complexes 4-13C1 and 4-13C2 Cl Cl

O [B]

[Sm]

13 C

3- 13C 1

[Li] + Cl

CO (1 atm)

13

C

[Sm]

THF, –108 °C to 25 °C, 5 min

[Li]—[B]

[B] O

Cl Cl

C

O

[Li]

C

O 4- 13C 1, 59%

[B]

Cl

1

Cl

O [Sm] C

[B]

3

[Li] + Cl

13CO (1 atm)

[B] Cl

O

C

O

C13

13

[Sm] THF, –108 °C to 25 °C, 5 min

Cl

C

4- 13C 2 , 52% [Li] = Li(THF) 2

[B] =

N

B

O

[Li]

[B]

N

[Sm] = Sm(THF)2

To get information on the role played by the lithium boryl compound 1 in the formation of 4, the reaction of 1 with CO (1 atm) in the absence of 2 was carried out, which yielded almost quantitatively a new product 5 in 30 minutes at –108 ˚C to –78 ˚C in THF (Scheme 3). The similar reaction of 1 with 13CO gave the 13C-enriched analogue 5-13C4. Complex 5 was thermally unstable at room temperature. However, single crystals of 5 suitable for X-ray diffraction studies were grown from THF at –35 ˚C. It was established that 5 is composed of a



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cyclic ethene carbonate skeleton and an ethenediolate-chelated lithium metallacycle (Fig. 2), which could be viewed as a cyclodimerization product of two lithium borylketenolate species accompanied by 1,2-migration of a boryl group. Scheme 3. Reaction of a Lithium Boryl Compound (1) with Carbon Monoxide [Li] *CO (1 atm) [Li]—[B] 1

THF –108 °C to –78 °C 30 min

O

O

[B]

*C

*C

*C

O

O

*C [B]

[Li]

5, 93% (NMR yield), 52% (isolated yield), *C = 12C 5- 13C 4, 47% (isolated yield), *C = 13C

[Li] = Li(THF) 2

[B] =

N

B

N

[Sm] = Sm(THF) 2

The 1H NMR spectrum of 5 in THF-d8 at –30 ˚C showed two singlets at δH 5.87 and 5.81, assignable to the four vinyl protons in the two diazaborolyl rings. The 13C NMR spectrum of 5-13C4 in THF-d8 at –30 ˚C showed two sharp doublets at δC 166.3 (J = 89.6 Hz) and 156.7 (J = 97.2 Hz) assignable to the two carbonate carbon atoms, and two broad doublets at δC 115.3 ppm (J = 97.2 Hz) and 104.5 (J = 89.6 Hz), assignable to the two boryl-substituted carbon atoms. The 11B{H} NMR signals of 5 appeared at δB 25.9 and 24.3, which are comparable with that of complex 4 (δB 20.4).

Figure 2. Crystal structure of 5 with thermal ellipsoids at the 50% level except for the 2,6-(iPr)2C6H3 groups in the boryl unit and the THF ligands. Hydrogen atoms have been omitted for clarity. Selected metric parameters are reported as distances (Å) and angles (°): Li1–O2, 1.82(2); Li1–O5, 2.00(3); L2–O4, 1.81(2); L2–O7, 1.96(2); L2–O8, 2.04(2); C1–C2, 1.380(10); C2–O3, 1.400(9); C2–O2, 1.256(9); O3–C3, 1.351(8); C3–O1, 1.429(8); O1–C1, 1.433(8); C3–C4, 1.314(9); C4–O4, 1.368(8); C1–B1, 1.521(11); C4–B2, 1.568(10).

Complex 5 represents a rare example of a well-defined CO coupling product associated with a lithium boryl compound.37 No reaction was observed between 3 and 5 at –30˚C in THF. At room temperature, 5 gradually changed to an unidentified new compound, while 3 remained unchanged in 12 hours as monitored by 1H NMR. The formation of 4 was not observed.

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These results may rule out the possibility of involvement of 5 in the formation of 4. Computational studies. To clarify the mechanistic details of the formation of 4, we then performed density functional theory (DFT) calculations on the associated reaction processes (see Supporting Information for details). Because of the relatively large size of the molecules, {B[N(C6H5)CH]2} was used in place of the {B(N(2,6-(iPr)2C6H3)CH)2} ligand by replacing each isopropyl group with an H atom for the calculations. It was found that the reaction of the samarium boryl complex 2′ (a model of 2) with one molecule of CO could give an adduct 2′-CO, which may easily transform to a thermodynamically stable boryl oxycarbene species36 ISm via a B−C bond forming transition state TS1Sm (Fig. 3a). Complex ISm is equivalent to 3 obtained experimentally. Although the reaction of 3 with CO in flask did not give a structurally characterizable product, the DFT calculation shows that insertion of another molecule of CO into the oxycarbene species in ISm to give a more stable ketenolate species34 IISm is possible by overcoming an energy barrier of 15.0 kcal/mol via an intermediate ISm-CO and a C−C bond forming transition state TS2Sm. However, the higher energy barrier for the insertion of the second CO via TS2Sm (∆G‡ = 15.0 kcal/mol) than that for the first CO insertion via TS1Sm (∆G‡ = 5.8 kcal/mol) could account for the isolation of 3 at low temperatures. The reaction of the lithium boryl complex 1′ (a model of 1) with CO may take place similarly, first giving a mono-CO insertion product ILi and then the CO coupling product IILi (Fig. 3b). The energy barrier for the first CO insertion (5.3 kcal/mol) is similar to that in the case of the Sm complex (5.8 kcal/mol). However, the energy barrier for the second CO insertion (4.9 kcal/mol) in the case of the lithium complex, which is comparable with that for the first CO insertion, is much lower than that in the case of the Sm complex (15.0 kcal/mol). The intermolecular Li···O interactions between two molecules of the lithium ketenolate species IILi could form a dimer II-IILi, which then undergoes C−O bond formation between the two ketenolate units to give IV via TS4 and finally afford the more stable product 5′ through intramolecular C−O bond formation (cyclization) and 1,2immigration of a boryl group via TS5 (top path in Fig. 3c). Complex 5′ could be viewed as equivalent to 5 isolated experimentally. The reaction of the samarium oxycarbene complex ISm with the lithium ketenolate IILi may easily take place to give IILi-ISm through the intermolecular Li···Cl and Sm···O interactions (bottom path in Fig. 3c). This process is exothermic by 21.2 kcal/mol. Subsequently, the intramolecular attack of the oxycarbene species to the ketene unit in IILi-ISm would occur to yield a CO-trimerization product III via transition state TS3. The subsequent release of one molecule of lithium chloride from III with migration of the remaining Li(THF)2 moiety to the middle CO unit would give 4′, which is equivalent to 4 obtained experimentally. It is worth noting that the energy barrier for the formation of III or 4′ (2.7 kcal/mol, bottom path in Fig. 3c) is significantly lower than that for the formation of IISm (15.0 kcal/mol, Fig. 3a) and also lower than that for the dimerization of IILi to give 5′ (11.2 kcal/mol, top path in Fig. 3c). This could account for the


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Journal of the American Chemical Society Gsol kcal/mol

a

Cl

20

[B]

Cl O

O

O

TS1Sm

2'-CO

Cl O C

5.8

4.4

2'

0

[Li] Cl

C

Cl

CO

[B]

0.0 Cl

–10

[Sm]

[B]

[Li]

C 10

Cl

Cl

Cl [Sm]

O

C

TS2Sm

O

Cl

2'

C

Cl

[Li]

–3.9

ISm-CO

CO

[Li] Cl

[Li]

[Sm]

Cl [Sm]

[B]

[B]

Cl

Cl

[Sm]

C

G = 15.0

–12.9

Cl

ISm –20

–30

[Li] = Li(THF)2

[B] =

N

[Sm] = Sm(THF)2 B

[B]

N

[B]

0

Cl

O

O

[B] C

[B] C

[Li]

5.3

C

O

O

ILi-CO –10.1

CO

TS2Li

–12.9 O [B] C

–30

[B]

20 0.0 2 IILi

0

O [Li] C C O O C [Li] O C [B]

[Li]

–8.0

ILi

Gsol kcal/mol

[Li]

C

[Li]

[B]

O

G = 4.9

C

[B]

[Li]

O [Li] C C O O O C C [Li] [B]

[Li]

[B] [B]

O C

[Li]

C

O

O [Li]

9.0 TS4

O C

O O C C [B]

O

C O

[Li]

C C

[B] [Li]

7.9 TS5

2.5 IV

–40

–21.2 [B]

IILi

O

O

C

+ Cl

–60

O [B]

–80

C

[B] Cl [Li]

[Sm]

O

C

[Li]

[Sm] Cl

Cl [Li] IILi-ISm

C [B]

Cl

C

O

O C

C O

O

–23.5 5´

[B]

G = 2.7

[B]

C O Cl

C

C

C

TS3 –18.5

IILi-ISm

O C

O

[B]

Cl

C [Li]

[Sm]

[B]

O

Cl O

TS3

Cl

Cl

[Li]

O C

–65.8

ISm

C O

C [B]

III

[B]

[B] [Li]

[Li]

Cl [Li]

[Li]

Cl [Sm]

O C

O C

O

[Li] C

O

–31.4

–2.2 II-II Li

0.0

[B]

C

IILi

IILi + ISm –20

O

–34.5

1.9

40

C

IISm

O

–10

c

C

[B]

[Li]

TS1Li

1'-CO

0.0 [B]

Cl

[Sm]

C

C O 1'

[Li] Cl

[Li]

[Li]

10 CO

O C

Cl

[Sm] O

ISm [B]

Gsol kcal/mol

b

–18.9 Cl

Cl [Sm]

O

Cl

4' –66.0 1/2 [(THF) 2Li(µ-Cl)] 2

Figure 3. Computational analysis of reaction pathways. a, The reaction of CO with 2′ (a model of 2). b, The reaction of CO with 1′ (a model of 1). c, The reaction of IILi with ISm (bottom path) and the dimerization of IILi (top path). TS = transition state, ∆Gsol = relative Gibbs free energy in solution.

selective formation of 4 under the coexistence of 1 and 2 with CO. The overall energy barrier for the CO trimerization process is less than 6 kcal/mol and the whole reaction is significantly exothermic. These results clearly demonstrate that the selectivity of CO oligomerization can be significantly influenced and altered by cooperation of two different metal components.

CONCLUSION In summary, by the reaction of SmCl3 with a lithium boryl compound 1, we have synthesized a novel chlorosamarium boryl complex 2. On exposure to an atmosphere of CO at low

temperature, 2 afforded a boryl oxycarbene complex 3. In contrast, the reaction of the lithium boryl complex 1 with CO gave a bicyclic product 5, which is composed of a cyclic ethene carbonate skeleton and an ethenediolate-chelated lithium metallacycle formed through dimerization of two lithium ketenolate species. Remarkably, in the coexistence of 1 and 2, the selective CO-trimerization took place to give a novel Li/Sm heterobimetallic diborylallenetriolate product 4 that consists of a “Y-shaped” [C(O)C(O)C(O)] skeleton. The 13 C-labelled experiments and DFT calculations have revealed that the formation of the unique diborylallenetriolate moiety in 4 is achieved exclusively by coupling of a samarium boryl



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oxycarbene species like 3, which is generated by insertion of one molecule of CO into the samarium–boryl bond, with a lithium ketenolate species formed by insertion of two molecules of CO into the lithium–boryl bond. This Sm/Li heterometal-mediated cross-coupling reaction can compete against the CO oligomerization reactions involved by the homometallic active species. This work has clearly demonstrated that the cooperation of two different metal species can significantly alter the reaction course of CO coupling, and has thus shed unprecedented light on the synergistic effects of heteromultimetallic components on CO oligomerization at the molecular level. The results observed in this work may also suggest that the selectivity of CO coupling could possibly be fine-tuned by changing the combination of different metal components, thus offering helpful hints for designing new catalysts.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental details, spectroscopic and analytical data, and DFT calculation (PDF) X-ray crystallographic data for 2 (CCDC 1563094), 3 (CCDC 1563095), 4 (CCDC 1563097) and 5 (CCDC 1563096) (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected] (Z.H.); *[email protected] (Y.L.)

Author Contributions ‡

These authors contributed equally to this work.

ORCID Baoli Wang: 0000-0001-5057-2339 Gen Luo: 0000-0002-5297-6756 Masayoshi Nishiura: 0000-0003-2748-9814 Yi Luo: 0000-0001-6390-8639 Zhaomin Hou: 0000-0003-2841-5120

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We appreciate financial support through a Grant-in-Aid for Scientific Research (C) (17K05824), a Grant-in-Aid for Scientific Research (S) (26220802) and a Grant-in-Aid for Scientific Research on Innovative Areas (17H06451) from JSPS and grants from the National Natural Science Foundation of China (21429201, 21674014). We gratefully appreciate accesses to RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of Dalian University of Technology for computational resources. We thank Mrs. Akiko Karube for micro elemental analyses.

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Table of Contents artwork hetero-bimetallic cooperation

B O C

Sm

C

Li

Sm/Li

O Sm

Sm

B Sm

Sm–B + Li–B

O

O

C

Li

O

C

B

O C

O

C O Li

C

B

C C

B

O Li

B

trimerization of CO


8

Cooperative Trimerization of Carbon Monoxide by Lithium and

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