Boron-mediated activation of carbon dioxide

Accepted Manuscript Title: Boron-Mediated Activation of Carbon Dioxide Author: S´ebastien Bontemps PII: DOI: Reference:

S0010-8545(15)00207-6 http://dx.doi.org/doi:10.1016/j.ccr.2015.06.003 CCR 112089

To appear in:

Coordination Chemistry Reviews

Received date: Revised date: Accepted date:

26-3-2015 1-6-2015 4-6-2015

Please cite this article as: S. Bontemps, Boron-Mediated Activation of Carbon Dioxide, Coordination Chemistry Reviews (2015), http://dx.doi.org/10.1016/j.ccr.2015.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights:  Stoichiometric activations of CO2 by i) hydroborate and ii) Lewis Pairs featuring a borane Lewis acid

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 Catalytic reductions of CO2 by i) hydroborane, ii) diborane or silylborane and iii) hydrosilane activated by B(C6F5)3

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Edited June 4

Boron-Mediated Activation of Carbon Dioxide

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Sébastien Bontemps

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CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France. Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France Keywords: Carbon Dioxide, Hydroborane, Lewis Pair, Hydride, Hydroborate

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Contents

1. Introduction............................................................................................................................................. 3

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2. Activation of CO2 by a Lewis pair ......................................................................................................... 3 2.1 Phosphine / borane activation .............................................................................................................. 4 2.2 Amine / borane activation..................................................................................................................... 5

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3. Formation of formatoborate compounds from CO2 ............................................................................... 6 3.1 Hydride transfer from hydroborate compounds ................................................................................... 6

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3.2 Hydride transfer mediated by a borane................................................................................................ 7 4. Catalytic reduction of CO2 by hydroborane ........................................................................................... 9

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4.1 Metal hydride catalysts......................................................................................................................... 9 4.2 Lewis pair catalysts ............................................................................................................................ 10

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4.3 Lewis base catalysts............................................................................................................................ 11 4.4 Hydroborate catalysts......................................................................................................................... 11 4.5 Catalyst performances ........................................................................................................................ 11 4.6 Reductive functionalization of CO2 .................................................................................................... 12 5. Catalytic reduction of CO2 by diborane or silylborane ........................................................................ 12 6. Catalytic reduction of CO2 by borane-activated hydrosilanes.............................................................. 13 7. Concluding remarks.............................................................................................................................. 14 8. Acknowledgements............................................................................................................................... 14 9. References............................................................................................................................................. 14 ABSTRACT: Carbon dioxide is a very abundant molecule that is used as building block by nature. It would be a sustainable resource for our carbon-based societies if we could emulate nature in transforming CO2 under mild conditions. Despite the high thermodynamic stability of CO2, the last decade witnessed increasing interests in the homogeneous reduction of CO2 with dihydrogen, hydroborane or hydrosilane as reducing agents. With the use of the last two reductants very mild conditions could be used (T < 100°C, PCO2 < 5 atm), mechanistic insights were gained and a variety of compounds were described including more complex molecules resulting from the reductive functionalization of CO2 with amines. The versatile properties of boron-containing molecules are key to activate CO2 in various ways, and not only via hydroboration of CO2. At the stoichiometric level, Lewis pairs

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Edited June 4 featuring a strong Lewis acidic borane moiety are shown to form CO2 adducts, whereas formatoborate compounds can be obtained by the reaction of CO2 with hydroborates or with metal hydride complex mediated by boranes. Catalytic reductions of CO2 can be achieved i) with hydroboranes when the hydride transfer is catalyzed by a metal hydride complex, an ambiphilic compound, a strong Lewis base or a hydroborate, ii) with diborane or silylborane to promote the abstraction of one oxygen atom to afford CO and iii) with hydrosilane activated by strong Lewis acidic perfluorinated borane compound.

of hydrosilanes and hydroboranes to reduce CO2 is not economically interesting when simple molecules are synthesized. However, very mild conditions were used (T < 100°C, PCO2 < 3 atm) that allowed one to gain mechanistic insights and to synthesize a variety of compounds including more complex molecules resulting from the reductive functionalization of CO2 with amines. In addition, the versatile properties of boron-containing molecules allow one to activate CO2 in various ways, and not only via hydroboration of CO2. The present review focuses on these different modes of activation (scheme 1). At the stoichiometric level, a) Lewis pairs containing a strong Lewis acidic borane moiety form CO2 adducts, whereas b) formatoborate compounds can be obtained from the reaction of CO2 with hydroborates or with metal hydride complexes mediated by boranes. Catalytic reductions of CO2 are achieved c) with hydroboranes when the hydride transfer is catalyzed by a metal hydride bond, an ambiphilic compound, a strong Lewis base (L.B.) or a hydroborate, d) with diborane or silylborane to promote the abstraction of one oxygen atom to afford CO and e) with hydrosilane activated by strong Lewis acidic perfluorinated borane.

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1. Introduction

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Carbon dioxide is a very attractive source of carbon because it is very abundant and rather non-toxic when compared to the other C1 sources employed by the chemical industry. Its use has thus attracted a lot of interest in many fields of chemistry from the most fundamental researches to the most applied one [1-16]. However it is also a very thermodynamically stable molecule and only in few cases the balance between the cost of the required energy input (physical or chemical energy) and the created value of the product is economically favorable [1719]. Many interconnected factors – going beyond chemistry – have an impact on this balance. At the fundamental level, a more efficient and controlled activation of CO2 would allow one to i) decrease the level of energy input and / or ii) increase the molecular complexity of the product and as a consequence the added-value of the compounds synthesized from CO2. The last decade witnessed intense interests in the homogeneous reduction of CO2 with dihydrogen [20-23], hydrosilane [24] and hydroborane as reducing agents. While CO2 hydrogenation appears as the ideal reaction in term of atom economy, a sustainable source of “carbon free” dihydrogen and milder reaction conditions remain to be described. The use

L.B.

BR 3 L.B. / BR3

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C O O

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Scheme 1: Stoichiometric and catalytic activation of CO2 by boron-containing molecule

a)

H-BR 3

O C O

c)

Cat

HBR2

Hydroboration products

A

d)

H A

BR3

C O O

Stoichiometric transformations

b)

e) Cat / B(C 6F5 )3 HSiR 3

Cat R 2B-E

CO + R 2BOE

Hydrosilylation products

Catalytic transformations

E = BR 2 or SiR3 If an ambiphilic compound is used - the Lewis base and the borane are connected - a cyclic zwitterion is obtained. Scheme 2: Activation of CO2 by a Lewis base and a borane

2. Activation of CO2 by a Lewis pair No CO2-borane adduct has been reported so far while only strong Lewis bases form stable adduct with CO2. However, when combining borane with a Lewis base – even L.B. that does not activate CO2 by itself – CO2 adducts 1-14, 16-21, were characterized. The Lewis base – a phosphine or an amine – coordinates to the electrophilic central carbon, while the borane coordinates to the nucleophilic oxygen atom of CO2, formally generating a zwitterionic molecule featuring phosphonium or ammonium and borate moieties (scheme 2).

L.B.

BR 3 L.B. / BR 3 O

O

L.B. + BR2 CO2

L.B. = PR3 or NR 3

L.B.

BR 2 O

O

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Edited June 4 non-fluorinated borane moieties. Interestingly, despite a less Lewis acidic borane, CO2 is activated and compound 14 isolated [33]. A similar structure was also found with an alane moiety in place of the borane [34]. In 2012, Maerten, Baceiredo et al. subjected a boryl(phosphine)carbene [35] to 1 atm of CO2 [36]. The outcome is completely different from the previous examples since the carbene is driving the reaction with its insertion into the C=O bond of CO2. The resulting compound 15 features a phosphacumulene ylide with the oxygen atom from CO2 trapped between the phosphorus and the boron atoms (scheme 3). Experimental and theoretical data for 1-14 are reported in Table 1. Compounds 1-9 and 11-13 form reversibly. Compound 1 loses CO2 at 80°C under vacuum and the other compounds are isolated at low temperature since the release of CO2 is observed upon warming. The less stable adducts (2, 59) release CO2 between – 20 or –14 °C. The energy of formation of adducts 2, 5, 6 and 8 have indeed been calculated between -2.9 and -4.7 kcal.mol-1. In contrast, compound 13 and 14 are stable under vacuum at 100 and 80 °C, respectively. The computed formation energy of 20.3 kcal.mol-1 for compound 13 is thus in good agreement with the experience. The solution and solid state characterizations are also reported but no clear general trend emerges from it. In the case of 13C, the data are comprised between 160.5 and 161.3 ppm, but for 12, 13 and 14 which exhibit comparatively deshielded signals at 170.9, 169.7 and 167.8 ppm, respectively, and for 10 which exhibit shielded signal at 156.3 ppm. The CO stretching frequencies are much weaker in the case of 12 and 13 (1617 and 1608 cm-1) than in the other compounds (1686 to 1719 cm-1). The expected change of geometry from linear to bent for the activated-CO2 is observed, but since the variations are minimal between the different compounds (between 123.9(3) ° and 129.5(4) °), this measure is hardly an indicator of the degree of activation. While the two C-O bond distances are identical in each diborane compounds 12 and 13, the other compounds exhibit markedly shorter C=O distances for the oxygen not involve in the coordination with the borane moiety. Finally, compound 10 features the shortest B-O bond (1.477(5) Å), probably due to the enhanced Lewis acidity and accessibility of the borane fragment.

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2.1 Phosphine / borane activation In 2009, Grimme, Stephan, Erker et al. first reported the activation of CO2 by a Frustrated Lewis Pair (FLP). The use of the Lewis pair PtBu3 / B(C6F5)3 gives rise to the adduct 1 (scheme 3) [25], and a recent theoretical analysis studied the motion involved in the formation of this compound, showing an influence of the relative volume of the Lewis acid and Lewis base [26]. An intramolecular FLP containing dimesitylphosphine (-PMes2) as the L.B. and a perfluorinated borane as the L.A. connected by an ethylene linker activates CO2 to afford compound 2. Following similar strategies, the CO2 adducts 3-14 were characterized (scheme 3). The same authors have explored other FLP systems and isolated compounds 3-9 [27]. The combination of triisopropylphosphine (PiPr3) with B(C6F5)3 does not afford the expected CO2 adduct. The fluoride in para-position of the C6F5 group was indeed substituted by the phosphine [28]. However, when using unsubstituted B(C6F4H)3 at the para position, the reaction with PiPr3 and PtBu3 in the presence of CO2 affords adducts 3 and 4, respectively. In compounds 5-9, PtBu3 was used as the Lewis base whereas B(C6F4H)2R with different R groups (5: hexyl, 6: Cy, 7: norbornyl, 8: Cl and 9: Ph) was used as the Lewis acid counterpart. The experimental data and theoretical calculations show that the different properties of the R groups have little impact on the CO2 activation energy profile by the Lewis pair, or on the stability of the resulting compounds. Equilibrium constants for the formation of 6 were determined by a microfluidic approach in order to circumvent the time scales and diffusion issues encountered when measuring gas-liquid interaction parameters [29]. In addition, abstraction of the chloride in compound 6 gave rise to a more robust structure due to the increased Lewis acidity of the borane moiety [30]. Stephan et al. have subsequently used diborane compounds as Lewis acid to afford compounds 10-13. With a diborane featuring an Obridge or an ortho-phenylene linker, the isolated CO2 adducts 10 [31] and 11 [32] exhibit a single B-O coordination. However, with a sp2 C-bridge linker, the CO2 adducts 12 and 13 were isolated in which each oxygen atom of the CO2 molecule interacts with a borane moiety. The same year, Slootweg, Lammertsma et al. used an intramolecular FLP system with a methylene linker between a phosphine and a

Scheme 3: Activation of CO2 by phosphine / borane Lewis pair

(tBu) 3 P

O

O

B(C 6 F5) 3 (Me 3 C6 H 2) 2P O

1

(tBu) 3 P

B(C 6F5 )2 R 3 P

O

2

(C 6 F5) 2 B O O

O 11

B(C 6F5 )2

R2 B O

O

B(C 6 F4H)3 (tBu) 3 P

O R = iPr, 3 tBu, 4

O

B(C 6 F4H)2 (R)

Cl

(tBu) 3P O R = hexyl: 5 , Cy: 6 , O norbornyl: 7, Cl: 8, Ph: 9

(iPr) 2 N BR 2 P (tBu) 2 P BPh2 O Mes O R = C6 F5 : 12 O 14 P(tBu)3 Cl: 13

B

N(iPr) 2 Mes

B 10

(iPr) 2 N CO2

Cl

O

Cl B Cl

P Mes C C O

O

B

N(iPr) 2 Mes

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Table 1: Analytical characterization of compounds 1-14

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Edited June 4 d(C=O) Å

d(C-O) Å

d(B-O) Å

d(P-C) Å

OCO °

CO cm-1

13C

11B

31P

1

T (°C) for CO2 E (kcal.mol1 loss )

JPC

1[25]

1.2081(1 1.2988(1 1.5474(15) 1.8931(12) 127.6(1) 5) 5)

1695

161.6

–2.7

46.1

93

80 under vacuum

2[25]

1.209(4) 1.284(4) 1.550(4)

1.900(3)

123.9(3)

1694

160.5

–2.4

0.6

89

-20

1700

161.3

–2.3

37.3

112

Upon warming

4[27]

1.203(2) 1.300(2) 1.556(2)

1.892(2)

127.9(2)

1699

162.2

–2.4

45.4

93

Upon warming

5[27]

1.208(5) 1.281(5) 1.579(5)

1.896(4)

129.5(4)

1693

160.9

41.3

92.3

–14

-3.5

6[27]

1.206(4) 1.289(4) 1.587(4)

1.893(3)

128.8(3)

1698

161.3

41.0

91.9

–16

-4.7

7[27]

1.210(3) 1.284(3) 1.592(3)

1.896(3)

128.8(3)

1686

8[27]

1.201(3) 1.297(3) 1.527(3)

1.888(2)

128.0(2)

1702

161.0 160.8

0.8

43.9

92.0

10[31]

1.176(5) 1.320(5) 1.477(5)

1.894(4)

127.0(4)

1719

156.3

18.0

48.4

94.6

11[32]

1.194(3) 1.296(3) 1.550(3)

1.886(3)

128.4(2)

21

45.5

12[32]

1.250(2) 1.257(2) 1.647(3)

1.896(2)

128.1(3)

1617

170.9

10.3

55.0

1.874(3)

126.3(2)

1608

169.7

10.3

1.2098(1 1.2938(1 1.5645(15) 1.8690(12) 125.85(11) 1690 5) 4)

167.8

5

1.672(3) 13[32]

1.245(3) 1.259(3) 1.577(4)

14[33]

Upon warming

-4.2

Upon warming

80: no loss under vacuum

r.t. with N2 purge 15

52.7

85.7

15

48.0

89.7

100: no loss 20.3 under vacuum

alone is calculated at -8.4 kcal.mol-1. Experimental and theoretical data for 16-21 are reported in Table 2. Contrary to the previous adducts, only compound 17 is reported to release CO2 at -20°C. The 13C of the CO2 carbon are markedly more shielded with resonances comprises between 146.1 and 159.4 ppm, the most shielded being compound 16. The recorded CO stretching frequencies exhibit more variation. The dicationic compound 20 exhibits a similar band at 1693 cm-1 as the phosphine adducts, while compounds 18 and 19 exhibit weaker C=O bonds with signal at 1646 and 1645 cm-1, respectively. In the case of compounds 16 and 17, the frequencies are indicative of much stronger C=O bonds with signals at 1742 and 1822 cm-1, respectively. In summary, compounds 1-21 all exhibit activation properties toward CO2 with a combination of a Lewis base and a borane. The nature of the Lewis pair either frustrated or in close contact does not seem to play an important role. It appears however that a certain degree of Lewis acidity and / or Lewis basicity is required to stabilize the CO2 adduct for its isolation. Most of the example presented here indeed feature strong Lewis base and / or strong Lewis acid. In case of ambiphilic compound, the linker can play an important stabilizing role to compensate for a less Lewis acidic borane moiety.

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2.2 Amine / borane activation The isolation of the compounds 16-21 (scheme 4) shows that amine can also be used as Lewis base to activate CO2 with boranes. Dureen and Stephan studied the reactivity of four membered-ring boron amidinate. This strained ring obtained from the reaction of HB(C6F5)2 with carbodiimide behaves as FLP toward various small molecules. The boron amidinate compound reacts with CO2 to afford compound 16, featuring a six-membered ring depicted in scheme 3 [37]. Stephan, Erker et al. used amine / borane Lewis pair to isolate compounds 1720 [38]. Compound 17 is obtained with PhCH2N(Me)2 and B(C6F5)3 in the presence of CO2. When using the secondary amine (Ph)(iPr)NH, the CO2 adduct 18 is isolated as an ion pair featuring a borate and an amine moieties. The countercation is the ammonium (Ph)(iPr)NH2. The generality of this reaction has been shown intramolecularly with the isolation of compounds 19 and 20. Cantat et al. used a strongly basic guanidine (TBD: 1,5,7-triazabicyclo[4.4.0]dec-5-ene) with 9-borabicyclo[3.3.1]nonane (9-BBN) to generate a FLP system that is able to trap CO2 to afford the adduct 21 [39]. In this case the FLP system features a less Lewis acidic borane moiety. The E of formation of 21 compare to the dimeric structure of the corresponding FLP (72, vide infra) and CO2

93.4

91.9

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1.585(3)

45.0

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2.1

–16

cr

9[27]

62.4

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3[27]

-2.9

Scheme 4: Activation of CO2 by amine / borane Lewis pair

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(tBu)H 2N NMe 2 B(C 6F5 )3 (Ph)(iPr) N B(C 6 F5)3 O O O O 17 18 N(iPr)(Ph)H 2 O (C 6F5 )3B N O N N N H tBu B N tBu O O 2 20 21 N B(C 6 F5)3 O O Table 2: Analytical characterization of compounds 16-21 Ph O iPr N B(C 6F5 )2 N H iPr 16

d(B-O) Å

d(N-C) Å

OCO °

16[37]

1.208(5)

1.292(5)

1.493(5)

1.402(7)

123.4(4)

17[38]

1.189(2)

1.280(2)

1.550(2)

1.545(2)

133.06(18)

18[38]

1.238(3) 1.243(3)

1.312(3) 1.305(3)

1.508(3) 1.516(3)

1.355(3) 1.357(3)

123.7(2) 123.3(2)

19[38]

1.245(5) 1.243(6)

1.320(6) 1.319(6)

1.499(7) 1.494(6)

1.362(6) 1.366(6)

123.3(4) 122.4(4)

20[38]

1.225(6)

1.323(5)

1.508(5)

1.379(6))

21[39]

1.222(2)

1.299(3)

1.537(3)

1.410(3)

M 124.6(4)

d

123.9(2)

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3. Formation of formatoborate compounds from CO2

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Two types of reaction affords formatoborate compound from CO2. The hydride transfer occurs either from a) a hydroborate compound with or without the implication of the countercation or b) a metal hydride complex with the assistance of a borane (scheme 5). Scheme 5: Formation of formatoborate from CO2

BR3

H

O

A

O

a)

b)

O C O

A R 3B H

O C O

BR 3

A H

3.1 Hydride transfer from hydroborate compounds The reactivity of CO2 toward various hydroborate compounds occurs via the formal insertion of CO2 into the B-H bond. The hydridic character of the hydroborate is the main factor when the counterion exhibit poor Lewis acidity. As early as 1958, Wartik and Pearson reported the reaction of NaBH4 with CO2 and hypothesized the insertion(s) of CO2 into B-H bond(s) [40]. In 1985, La Monica et al. have studied the reactivity of a

19

B(C 6F5) 3 O

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O

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d(C-O) Å

CO cm-1

13C

11B

1742

146.1

0.3

1822

150.1

1646

159.4

–4.0

1645

157.6

–4.2

1693

158.7

–4.7

152.4 (THF)

3.3 (THF)

153.1 (CD2Cl2)

3.9 (CD2Cl2)

an

d(C=O) Å

tBu N

[Cu][BH4] complex toward CO2 and reported the double and triple insertion of CO2 into B-H bonds affording complexes 22 and 23 (scheme 6) [41]. When working in wet THF, the formation of formatotrihydroxyborate complex 24 is observed [42]. In 2009, Ashley, O’Hare et al. used the [TMPH][H(B(C6F5)3)] ion pair – generated from the FLP activation of H2 – to observe the insertion of CO2 into the B-H bond at 100 °C, leading to the formatoborate complex [TMPH][HCO2B(C6F5)3] 25 [43]. Interestingly, when this FLP system is placed at 160 °C under H2, quantitative formation of CH3OB(C6F5)3 is observed. The generation of analogous ion pairs with protonated lutidine 26 or benzylmethylamine 27 have been subsequently reported by Mayer et al. [44] and Stephan, Erker et al. [38], respectively. In such systems, Grimme, Erker, Stephan et al. [27] and Piers et al. [45] showed that a second borane could coordinate the second oxygen atom of the formate as shown in compounds 28-30. Cantat et al. reported that the coordination of a substituted Meguanidine compound to 9-BBN, promotes the insertion of CO2 into the B-H bond, resulting in the isolation of compound 31, fully characterized by NMR [39]. Scheme 6: Formation of compounds 22-31

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Edited June 4 (phen)(PPh 3 )Cu BH n

O

H O O

O

4-n 22: n = 2 23 : n = 1

O

24 H P(tBu)3

B(C 6 F5)3 A

H

B(OH) 3

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H

moiety of the ligand backbone and the Lewis acidic B(C6F5)3 [48]. This structure is reminiscent to the structure characterized by Piers et al. from the reaction of a methylborate compound with a scandium countercation also bearing a nacnac ligand [49]. Scheme 8: Formation of zwitterionic compounds 32 and 33 [48] F F F Ar Ar F H N N F N CO2 2 Mg Mg O H B(C 6F 5) C N O O F H Ar 2 F O (C 6 F5) 3B F 33 F F

(phen) 2Cu

BR(C 6F5) 2

H O

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O O BR(C 6F5) 2 25: A = H-TMP 26: A = H-lutidine 28: R = Cy: 27 : A = H-N(PhCH 2)(Me) 2 29 : R = norbornyl N N

B(C 6F5) 3

H

B

O O

N Me O

30

3.2 Hydride transfer mediated by a borane CO2 is known to insert into metal hydride bond to afford formate ligand when the metal is basic enough. Several examples have been reported in the literature showing that borane stabilizes the resulting formate and even promote the hydride transfer during the reaction. In 2012, Mitton and Turculet characterized the formatoborate complexes of Pd and Pt (34-37) via the activation of CO2 by the borane compounds B(C6F5)3 and BPh3 and the related M-H complexes (scheme 9) [50]. The reaction occurs even in the absence of borane but the resulting formate complexes are less stable. A related study was conducted by Miller, Labinger and Bercaw with hydride donor complexes (Ni, Rh and Re [51]). By increasing the amount of a moderately Lewis acidic compound, the equilibrium is shifted toward the formation of formatoborates 38-40, characterized by NMR [52]. Interestingly, the use of the stronger Lewis acid B(C6F5)3 gave rise to hydride transfer to the borane but no CO2 insertion into the resulting hydroborate was detected at room temperature. In these studies, the main role of boranes is thus to stabilize the formate product. Scheme 9: Formatoborate compounds 34-40 [50-52]

an

H TMP

31 O H In 2013, Piers, Maron, Eisenstein et al. described that the hydroborate H(B(C6F5)3) anion with the strong Lewis acidic scandium countercation Cp*2Sc+ readily activate CO2 (1 atm) at room temperature to form compound 32 (scheme 7) [46]. Different factors explains this enhanced reactivity compare to systems giving rise to 25 (12 h, 100°C, 1 atm of CO2) and 26 (few hours, r.t., 4 atm of CO2). Despite an electrophilic counterion, no contact with the hydride are observed due to steric protection of the cation and of its preferred interaction with two fluoride atoms of the perfluorinated moiety [47]. It was calculated that these features allow the Sc fragment to interact with CO2 in a first step, activating the later for the hydride transfer from the hydroborate fragment as depicted in scheme 7. Scheme 7: Formation of zwitterionic compounds 32 [46] F F (Cp*) 2 Sc

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d

M

B(C 6F5 )3

F

F

F

H

CO 2

B(C 6F5 )2

B(C 6F5 )3

PCy2

PCy2

O

(Cp*) 2 Sc

O

Me Si M

32

H

O C

Me Si M O

O

BR3

PCy2 H

PCy2

M = Pd, R = C6F5: 34 , R = Ph: 35 M = Pt, R = C 6F5 : 36, R = Ph: 37

O Sc

CO2

H B(C 6F5)3

H B(C 6 F5 )2

Using alkaline-earth Mg hydride complex associated with the same strong H(B(C6F5)3) anion, Hill et al. reported the activation of formally two molecules of CO2 at room temperature within 2h with the isolation and full characterization of compound 33 (scheme 8) [48]. The final complex can be described as the insertion of one molecule of CO2 into a Mg-H bond, whereas the second equivalent is trapped, as in a Lewis pair, between the nucleophilic methine

H

B

B

O tBu

O 38 : A = [(dmpe) 2Ni][PF6] 39 : A = [(dmpe) 2(H)Rh(ACN)][OTf]

O R 2P CO OC H Re OC CO 40 R 2P BBN

O

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Edited June 4 In summary, CO2 reacts with hydroborates via its insertion into the B-H bond (Table 3 for analytical characterization). The formation of the strong B-O bond is the main driving force of the reaction when the countercation exhibit poor Lewis acidity. However with stronger Lewis acid, the reactivity is enhanced by the ability of the cation to facilitate the hydride transfer in activating CO2 and in stabilizing the resulting formatoborate compound. On another hand, the hydride transfer from a hydride donor complex to CO2 is facilitated by the formation of a strong B-O bond in the resulting formatoborate compound, but also in one case by the borane activation of CO2.

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cr

ip t

In 2013, Berke et al. reported the reaction of CO2 with a Re-H complex in the presence of B(C6F5)3. A first intermediate retaining the Re-H bond was characterized by NMR. In this complex, CO2 is not only activated by the metal center but also by the borane (scheme 10). Within 4h at room temperature, CO2 inserts into the Re-H bond to afford the formatoborate complex 41 [53]. It is worth mentioning that such reaction does not occur in the absence of the borane. This strongly acidic borane is thus able to activate CO2 to facilitate the hydride transfer from the hydride donor complex. Scheme 10: Formatoborate compounds 41 [53] B(C 6F5 )3 (C 6F5 )3B O O H H O Br O CO2 Br Re (iPr) 3 P (iPr) 3P NO Re P(iPr) 3 (iPr) 3 P 2 NO 41

d(C=O) Å

d(C-O) Å

d(B-O) Å

an

Table 3: Analytical characterization of compounds 22-41 d(H-C) Å

d(A-O) Å

22[41]

M

23[41] 24[42] 1.236(3)

1.288(3)

1.546(3)

26[44]

1.221(5)

1.302(5)

1.53 +/-0.20

27[38]

1.213(5)

1.291(5)

1.535(3)

1.253(6)

1.255(7)

29[27]

1.249(3)

1.249(3)

31[39] 32[46] 33ab[48] 34[50] 35[50] 36[50] 37[50]

0.950

1.612(8) 1.566(4)

1.255(3)

1.268(3)

1.583(3) 1.586(3)

1.236(5)

1.272(5)

1.560(5)

1.229(5)

1.270(5)

1.203(7)

1.219(7)

1.246(2)

1.268(3)

Ac ce p

30[45]

2.960(4)

1.04(4)

1.540(6)

0.950

OCO °

13C

11B

1685

1.2

1699

3.6

1590 1662

2.774(3)

te

28[27]

1.567(7)

0.986

d

25[43]

CO cm-1

9.5 122.1(3)

169.8

122.29 +/-0.20

153.3

-2.4

8.24 8.31

2.727(5)

1621

123.9(4)

170.2

-2.2

8.29

>5

1631

121.5

173.4

5.1

8.04

1638

121.2

172.7

4.3

8.04

120.2(2)

174.5

122.7(3)

172.9

0.941

>5

0.950

3.254(3) 3.224(3)

0.960

2.088(3)

1.538(6)c

2.115(3)

127.7(4)

173.5

0.948

1.979(4)

130.2(6)

182.7

8.44

163.6

1.553(3)

1H

2.1164(15)

1622

8.33 -0.7

8.77

-2.32

8.31

173.6

8.0

9.06

168.5

2.7

8.83

1592 8.25

38[52]

171.7

39[52]

173.3

5.9

170.8

-3.9

40[52] 1620

41[53]

8.49 8.85 8.68 8.45

a: CO2 in between the boron and magnesium atoms; b: bridging CO2; c: d(C-C)

8 Page 8 of 17

Edited June 4

R = Cy: 45 Cyp, 46 Ar

us

M

42

te

N

iPr Cu H

[M]-H =

d

O PtBu2

iPr

N

iPr

iPr

PPh2 Me Si Pd H PPh2 52

Detailed calculations were carried out by Guan, Wang et al. on the reaction catalyzed by complex 42 [61]. Three catalytic cycles are proposed to account for the 6-electrons reduction of CO2 into the methoxyborane 43 via compounds 53 and 54 (scheme 15). In each of these three cycles, two elementary steps are involved: i) insertion of C=O bond into a Ni-H bond and ii) formal addition of the hydroborane across the formed Ni-O bond that regenerates the catalyst and releases the product. The three compounds featuring a C=O bond that is hydroborated are i) CO2, ii) formoxyborane 53 and iii) formaldehyde 54. The hydroboration of methoxyborane to form methane has not been reported. It has been calculated that the formal insertion of the C-O single bond of 43 into the Ni-H bond is highly unfavorable [61, 62]. Scheme 15: Proposed mechanism [61]

Ni H

HBCat + CO2

O

51

Ac ce p

PtBu2

F4

The formoxyborane A is observed in most of the hydroboration systems but only complexes 51 and 52 catalyze its selective formation (scheme 14) [59]. Complex 51 described by Shintani and Nozaki is notably generated from addition of HBpin to the corresponding Cu alkoxy complex. The Pd complex 52 reported by Hazari et al. exhibits very high catalytic performances (vide infra) [60]. Scheme 14: Catalysts 50 and 51 for the selective reduction of CO2 into formoxyborane [59, 60] [M]-H H Bpin HBpin + CO2 + pinBOBpin C O

4.1 Metal hydride catalysts Guan et al. first reported the reduction of CO2 by catecholborane (HBCat) catalyzed by the Ni-H complex 42 bearing a tridentated pincer-type ligand. Methoxycatecholborane (CatBOCH3) 43 and the bis(boryl)ether CatBOBCat 44 were characterized as final products (scheme 12). The system is very selective and only a small amount of formoxyborane 53 was detected during the catalysis [54, 55]. Scheme 12: CO2 reduction catalyzed by 42 [54, 55] O

B(C 6F5 )

an

H BR 2

Ar N H Ga tBu N Ar 50

cr

N F M H N F Ar M = Mg: 48 Ca: 49

H H BR 2 + R2BOBR 2 C O H C D

Cat

(iPr) 2O P(iPr) 2 P HN Ru N (iPr) 2P HN N BPh4 F4 47

ip t

CO2 is reduced by hydroborane. Up to three subsequent hydroboration reactions can take place to afford i) the formoxyborane compound A, ii) the bis (boryl)acetal compound B, and iii) the methoxyborane compound C along with the bis(boryl)ether R2BOBR2 D (scheme 11). As shown for stoichiometric reactions in the previous section, the formation of the strong B-O bond is the driving force of the reaction. However the initial elementary step is the hydride transfer and requires a catalyst which can be i) a metal hydride complex, ii) an ambiphilic compound, iii) a strong Lewis base or iv) a hydroborate compound. Mechanistic insights will be presented as well as reductive functionalization of CO2 with amine. Scheme 11: Successive hydroboration reactions at CO2 H H BR2 BR2 Cat Cat H C O O C O C O H BR2 O H BR 2 O A B BR2

B

O C

PR3 H H Ru 2 H2 H PR3

4. Catalytic reduction of CO2 by hydroborane

CatBOCH3 + CatBOBCat 43 44

Following this report, Bontemps, Sabo-Etienne et al. [56] and Stephan et al. [57] described similar processes with ruthenium catalysts 45 and 47 and HBpin (scheme 13). The ruthenium complex 47 is notably isolated from the reaction of a related hydride complex with CO2. Hill et al. [48] and Aldridge et al. [58] reported the same transformation using complexes 48-50, which are the first non-transition metal based catalysts of this reaction, although less efficient. Scheme 13: Catalysts for the reduction of CO2 into methoxyborane C [48, 56-58]

BCat [Ni] H BCat H C O C O O 53 + [Ni]-H O BCat O [Ni]-H H2C O H BCat C + (CatB) 2O H H 44 O + [Ni]-H 54 [Ni] BCat [Ni] H BCat [Ni]-H H 3C O H 3C O + [Ni]-H 43

O C O

53

54

[Ni]-H H

Bontemps, Sabo-Etienne et al. reported NMR analyses on

Page 9 of 17

9

Edited June 4 ruthenium systems 45 and 46 which support this mechanism. The bis(boryl)acetal compound 55, was indeed characterized which suggests the occurrence of a related [M]-OCH2O-BR2 intermediate. Moreover, the (formoxy)(boryl)acetal compound 57 containing two reduced CO2 units was shown to result from the reaction of formoxyborane compound 56 with formaldehyde [56, 63]. Using the related ruthenium catalyst 46 bearing tricyclopentylphosphine ligands, free formaldehyde was then observed. This represents the first direct evidence of the reduction of CO2 into formaldehyde, a very important C1 source. In-situ reaction of formaldehyde with methoxyborane 58 was observed to give rise to C2 and C3 compounds 59 and 60 [64]. Scheme 16: In-situ characterization of compounds 55-60 with catalytic systems 45 and 46 [56, 63, 64] O O pinB C Bpin H2 55 O O O O O O pinB C + pinB C C C H2 H H H 56 H 57

O O CH3 + C H H 58

OCH 3 B O O

65 H 3B.SMe2 + CO2

BCat H 3CO

B

O 66

B

OCH3

cr

ip t

Very recently, a follow up study was conducted by Bouhadir, Bourissou, Fontaine et al. which shed new light on the understanding of such system [67]. Formaldehyde adducts 67 and 68 were characterized during the hydroboration reaction (scheme 19). While the CH2O unit resulted from the catalytic reduction of CO2, labelling experiments proved that this unit was not reduced further. These compounds were then isolated and used as catalyst for the hydroboration of CO2. In contrast with related ambiphilic compounds, no induction period was observed with 67 and 68 and the catalysis proceeds at room temperature. Theoretical calculations were conducted to explain the enhanced activity. Three isoenergetic pathways were predicted with compounds 67 and 68 acting as ambiphilic species able to co-activate HBCat and CO2. For the first calculated pathway, an oxygen center from the catechol unit acts as a L.B. while a proton of the formaldehyde unit is the acidic site. For the second and third calculated pathways, the boron-formaldehyde bond breaks which liberates the borane acting as L.A. and the oxygen atom from formaldehyde acting as L.B. This report proves that CO2 hydroboration could be catalyzed by Lewis pair exhibiting not only rather modest Lewis acidity but also low Lewis basicity. Scheme 19: Hydroboration catalysts 67 and 68 [67] R2 P CH 2 R = Ph: 67 O iPr: 68 B Cat In 2014, Stephan et al. reported that the related non-metallic systems 69 and 70 are able to catalyze the CO2 hydroboration to the methoxyborane level [68]. The FLP activation of CO2 is envisioned to be the first step to promote the hydride transfer from the hydroborane (scheme 20). Scheme 20: FLP catalysts 69 and 70 [68] O PR 2 R 2P C N CO2 O B N B N H R = tBu: 69 N PR 2 H N(iPr) 2 : 70 PR 2

O

Ac ce p

te

d

M

an

O C CH 3 H2 + 59 O O O pinB C C CH3 H2 H 2 60 An alternative mechanism with the initial activation of the hydroborane instead of CO2 could be envisioned. Guan et al. and Bontemps, Sabo-Etienne et al. reported the isolation of dihydroborate complexes 61-64 [65],[56, 64] observed during hydroboration reactions (scheme 17). However, Guan’s results have shown that dihydroborate complexes are resting states and not active species [65]. Scheme 17: Dihydroborate complexes 61-64 O PtBu2 PR 3 H H H Ni BR2 Ru H H Bpin OC PR 3 O PtBu2 pinB

R2 BOCH 3 + R 2BOBR2 PPh2

us

pinB

HBR 2 + CO2

R 2 = Cat: 61 = C 8 H16: 62

R = Cy: 63 Cyp, 64

4.2 Lewis pair catalysts In the previous examples, the catalysts feature a metal-hydride bond that promotes the hydride transfer to CO2. Non-metallic species also promotes such activation step. In 2013, Maron, Fontaine et al. reported the use of the ambiphilic phosphineborane compound 65 as catalyst for the hydroboration of CO2 with HBCat, 9BBN, HBpin and also BH3 into methoxyborane or methoxyboroxine 66 at 70 °C (scheme 18) [66]. This result is the first report of a catalytic hydroboration of CO2 with nonmetallic catalyst and of the use of the simple BH3 as hydroborating agent. Scheme 18: CO2 hydroboration catalyzed by the ambiphilic compound 64 [66]

Cantat et al. reported that compound 71 is able to catalyze a similar reaction (scheme 21) [39]. In the hydroboration process, the ambiphilic compound 72 is characterized by NMR and is calculated to activate CO2 in a FLP-fashion. Moreover, in this structure, the activated-CO2 promotes the hydride transfer by interacting with the borane. Such double activation of CO2 and HBR2 hold similarity with the mechanism proposed for compounds 67 ad 68. Scheme 21: FLP catalysts 72 [39]

Page 10 of 17

10

Edited June 4

N 71

N

9-BBN N B

N H

N 2

HBR 2

supported by the observation of an induction period when using PPh3 and P(p-MeC6H4)3 as Lewis base. This feature suggests the formation of an active species at early stage of the catalysis which is dependent on the nature of the phosphine. Scheme 24: Proposed key intermediates 80 and 81 [39, 77]

N

CO2 N R2 B

72

N O

C

O

N

H BR2

In the ambiphilic compounds presented herein a common feature is the modest Lewis acidity of the borane allowing the activation of CO2, but also the release of the product formed. With FLP systems featuring stronger Lewis acidic alane moiety, CO2 could be activated and stoichiometric reduction with ammonia borane was observed. In 2010, Ménard and Stephan reported the isolation of the CO2 adducts 73 and 74 (scheme 22) [69, 70]. These compounds readily react with ammonia borane (H3NBH3) within 15 min. to afford methanol in a maximum 51 % yield [71, 72]. Calculations have been carried out to understand the important factors governing this reduction [70, 73-75]. In order to characterize the elementary steps, substituted ammonia borane (Me3NBH3) was employed to reduce the CO2 adduct 75. Along the reduction to the methoxy level, compound 76 was isolated and structurally characterized. NMR studies indicate that the alane moiety decoordinates and reacts with the ammonia borane that can then reduce CO2 and afford 76 and other compounds [76]. Scheme 22: Stoichiometric hydroboration of CO2 by ammonia borane AlX 3 AlX3 (C 6F5)3Al Al(C 6F5)3 O O O O

B

O

O

L.B. =

Me

P(tBu) 3

N

N Me 77

78

81

cr

us

Cat =

H H BH 4 H H B Mes B Mes N N N N Ph2 P PPh2

NaBH4 84

Cl 83

82

4.3 Lewis base catalysts It was recently shown by Cantat et al. and Stephan et al. that the Lewis bases 77-79 also catalyze the hydroboration of CO2 into methoxyborane (scheme 23) [39, 77, 78]. Scheme 23: Lewis basic catalysts 77-79 [39, 77, 78] L.B. R 2BOCH 3 + R2BOBR 2 HBR2 + CO2 N

H

an

Al(C 6F5 )3

Ac ce p

76

B O

4.4 Hydroborate catalysts Very recently, hydroborate compounds 82-84 have been shown to catalyze the reduction of CO2 by BH3 into boroxine 66 (scheme 25). Fontaine et al. [79] and Mezailles et al. [80] used the compound 82 and 83 featuring similar BH2 fragment as catalyst of the reaction. The group of Mizuta used the simple hydroborate NaBH4 84 to promote the hydride transfer to CO2 in a similar fashion as metal hydride bond [81]. One or two equivalents of CO2 reacts with the borate to form mono or bis-formatoborate complexes B(H)4-x(O2CH)x that can then react with the hydroborane. The observation that the formate NaO2CH was also an active catalyst for this reaction supports this mechanism. Scheme 25: CO2 reduction catalyzed by hydroborate 82-84 [79-81] Cat H 3B.SMe2 + CO2 66

te

Me 3N

O

80

d

H

(tBu) 3 P

H

Me 3NBH3 H2 B

N Me

M

P(o-Tol) 3 75

P(Mes) 3 73 : X = Cl 74: X = Br

N

ip t

N

4.5 Catalyst performances The reduction of CO2 by hydroborane described in this section have been performed with a large variety of catalysts whose performances are reported in the following Table 4. The activity of the palladium system 52 described by Hazari et al. is an order of magnitude greater than any other systems in generating selectively a formoxyborane A with high TON of 63500 and catalyst loading of 0.001% at room temperature. Concerning the reduction of CO2 to the methoxyborane or boroxine level, non-metal catalysts exhibit the best performances with TON as high as 6043 for the Lewis base 79 (formation of methoxyborane) and 2950 for 65 (formation of boroxine). It is also worth mentioning the rather mild conditions employed in all those systems with maximum CO2 pressure of 5 atm and maximum temperature of 80 °C.

Me N Me N P N N 79

In the report of Cantat on the use of guanidine base compounds, the most active catalyst is the substituted MeTBD compound 77 [39]. The calculated mechanism involves the activation of the hydroborane by the nitrogen base affording compound 80 characterized by NMR, which is then able to transfer the hydride to CO2 (scheme 24). Stephan et al. reported the hydroboration of CO2 with 9-BBN catalyzed by PtBu3 [77]. The mechanism involves the dual activation of CO2 by the Lewis pair giving the intermediate 81 which then promotes the hydride transfer (scheme 24). This mechanism is

Table 4: Catalyst performances 42a[54] 45a[56] 47a[57] 48a[48] 49a[48] 50a[58] 51b[59]

PCO2 (atm) 1 1 1 1 1 1 1

TON

TOF (h-1)

495 10 9 10 10 10 8.5

495 2 0.1 0.07 0.1 2.5 0.4

Catalyst loading (%) 0.5 10 1 10 10 10 10

Temp (°C) 25 70 50 60 60 60 35

Page 11 of 17

11

Edited June 4 1 1 1 2 2 5 5 1 1 5 1 1 1 1

63500 10 2950 84 114 240 240 537 648 556 6043 18 2646 249

529 20 973 144 228 22 22 3.6 31 17 33 108 661 21

0.001 10 0.1 1 1 10 10 0.1 0.1 0.02 0.01 4 0.1 1

base and the hydroborane or iii) the dual activation of both the CO2 and the hydroborane by ambiphilic compounds featuring Lewis acid and base moieties of modest strength. More complex molecules have been synthesized from the reduction of CO2 by using amine in the reductive systems. Formamide, imine and methylamine have been characterized.

25 25 70 25 20 60 60 25 25 60 20 80 80 25

5. Catalytic reduction of CO2 by diborane or silylborane In 2005, Sadighi et al. reported the use of the diborane B2pin2 as reductant and the carbene copper boryl complex 84 to disclose the reduction of CO2 (1 atm, r.t.) into CO with the concomitant formation of pinBOBpin (scheme 28) [85]. Scheme 28: Catalytic reduction of CO2 by a diborane [85] (IPr)Cu(Bpin) CO2 + B2(pin) 2 CO + pinBOBpin 84

ip t

52b[60] 64a[64] 65c[66] 67c[67] 68c[67] 69c[68] 70c[68] 71a[39] 77a[39] 78a[77] 79a[78] 82c[79] 83c[80] 84c[81]

a: formation of methoxyborane C; b: formation of formoxyborane A; c: formation of boroxine 66

cr

4.6 Reductive functionalization of CO2 In 2012, Cantat introduced an amine in hydrosilane reduction systems of CO2 which gave rise to the isolation of formamide, CO2 being the formylating agent [82, 83]. This finding opened the door to reductive functionalization of CO2 with amine [84]. Hydroborane reduction of CO2 systems have been used more recently in presence of an amine with catalysts 45, 46, 51, and 79. Shintani and Nozaki reported that the reduction of CO2 with HBpin in the presence of various primary and secondary amines affords related formamide compounds with the copper catalyst 51 (scheme 26) [59]. Bontemps, SaboEtienne et al. have shown that placing a bulky primary amine in the ruthenium systems 45 and 46 with the same hydroborane gives rise to the very selective generation of the corresponding imine [64]. Scheme 26: Formation of formamide and imine [59, 64] HBpin + CO2 + HNR1R2 51 O H

C

R1 = H

R2

N

te

45 or 46

d

M

an

us

Lin, Marder et al. conducted a detailed theoretical investigation on this catalytic process that shed light into the role of boron within the catalytic cycle (scheme 29) [86]. The first step concerns the insertion of CO2 into the Cu-Bpin bond of complex 84. In this step the nucleophilicity of the Cu-boryl bond is a key parameter for the activation of CO2 giving rise to its insertion. Somewhat surprisingly the C=O bond is oriented so that O approach the metal center and C approach the boron atom. The empty orbital at boron does not play a major role and the reverse approach involving O-B and C-Cu approach was not located. The insertion occurs then with the formation of Cu-O and C-B bonds, in agreement with the preferential approach calculated. It has to be noted that this intermediate could not be detected experimentally, even with aldehyde as substrate instead of CO2 [87]. After an isomerization step, the boryl fragment then migrates to the oxygen linked to the copper, concomitantly extruding CO. In this step the empty orbital at boron is a favorable feature. The subsequent addition of a molecule of diborane regenerates the active catalyst and forms pinBOBpin. Scheme 29: Catalytic cycle for the CO2 reduction into CO with catalyst 84 [86] IPrCu Bpin pinBOBpin CO 2

NR1R2

H

C

H

Ac ce p

The proazaphosphatrane superbase 79 catalyzes the mono- or di-methylation of various amines [78]. The proposed mechanism involves the dual role of the hydroborane to promote the formation of the formoxyborane 82 and compound 83 via dehydrogenative B-N coupling. Both 82 and 83 are intermediates in the formation of the methylamine (scheme 27). Scheme 27: Methylation of amine and key intermediates [78] 79 9-BBN + CO2 + R 2NH R 2N CH 3 + O(BBN)2

H C O O 82

BBN

R2N

B2 (pin) 2

O

IPrCu OBpin NHC

C

O

Cu Bpin

-CO IPrCu O Bpin

BBN

O

83

IPrCu O O (pin) B

Although, this result is a milestone in the reduction of CO2 with boron-containing molecule under mild conditions, no other diborane reduction of CO2 has been reported since. However, other research groups used silylborane compound as reductant instead of bisborane for the same transformation. Using complex (IPr)Cu(OtBu) 85 as catalyst, Kleeberg et al. have shown that the silylborane compound generates the active species (IPr)Cu(Si(Me)2(Ph)) with the release of tBuOBpin (scheme 30) [88]. CO2 then inserts into the active Cu-SiR3 bond, CO is subsequently extruded with the

In summary, the reduction of CO2 with hydroborane gives rise up to three hydroboration steps to reach the methoxyborane level. Methane has never been obtained from these hydroboration reactions. For each reduction, the first elementary step is the hydride transfer that require a catalyst. Electron-rich metal hydride bonds promote such reaction by CO2 insertion and hydroborate were very recently shown to exhibit similar behavior. Other non-metallic species also promote the hydride transfer via i) the Lewis base activation of the hydroborane, ii) the FLP activation of CO2 by a Lewis

Page 12 of 17

12

Edited June 4 formation of a Cu-OSiR3 intermediate. The active species is regenerated by the addition of a second equivalent of silylborane with the release of (Me)2(Ph)SiOBpin complex (IPr)Cu(OSi(Me)2(Ph)). Lindhardt, Skrydstrup et al. reported the same reduction reaction with silylborane, but using CsF as catalyst [89]. They hypothesized that the silylborane compound also promotes the regeneration of the catalytic species. In these examples, CO is extruded from a Cu-O2C-E intermediate with E = BR2 or SiR3. Experimental and theoretical studies have shown the specificities of boron or silicon for the promotion of such step when compare to Sn or aryl [90, 91]. In 2008, Hou et al. reported the carboxylation of organoboronic esters with CO2 catalyzed by the complex 85 (scheme 30). A similar metathesis of the boronic ester with the complex allows to install the aryl moiety at the copper center prior CO2 insertion, with the concomitant formation of the alkoxyboronic ester as driving force of the reaction [92]. No CO expulsion is occurring, but after CO2 insertion a carboxylated arene is generated. Scheme 30: Catalytic CO2 reduction by a silylborane compound [88, 92] (IPr)Cu OtBu + EBR 2 (IPr)Cu E + tBuOBR 2

tBu

PhH 2 C O

CH2Ph Zr

O O Me

tBu H B(C 6 F5) 3 H-TMP 89

tBu tBu 86

tBu PCy2 H

F

B(C 6F5 )3

(Cp*) 2 Sc

cr

PCy2 M = Pd: 87, Pt: 88

F

ip t

Me Si M

F

F

90

F B(C 6 F5) 2

H

us

Matsuo and Kawaguchi reported such reaction as early as 2006 at room temperature and 1 atm of CO2 with maximum TOF and TON of 7.3 h-1 and 211, respectively (scheme 33). Various silanes were used as reductant. The catalyst system is composed of the zirconium complex 86 and 1.5 equivalent of B(C6F5)3. The latter plays a dual role in generating an active cationic Zr complex – in a prototypical olefin polymerization co-activation role – able to react with CO2 and in activating the silane [100, 101]. In 2012, Turculet et al. used complexes 87 and 88 to catalyze a similar reaction [50]. In both systems, the catalyst 86/B(C6F5)3 and 87-88 are proposed to catalyze the hydrosilylation of CO2 into the bis(silyl)acetal B’, whereas free B(C6F5)3 catalyze the reduction of B’ into methane by activating the hydrosilane (scheme 29). To support such mechanism, Matsuo and Kawaguchi reported that if only 0.5 equivalent of B(C6F5)3 compare to [Zr] is added, CO2 is transformed into the bis(silyl)acetal B’ and Turculet et al. reported that the bis(silyl)acetal B’ is also formed when the less Lewis acidic borane BPh3 is employed. Scheme 33: Tandem catalyzes 86/B(C6F5)3, 87 and 88 [50, 100, 101]

85

M

an

R 2 = pin, E = SiMe 2Ph O R2 = E = Ar O

6. Catalytic reduction of CO2 by boraneactivated hydrosilanes

Ac ce p

te

d

The general mechanism for the hydrosilylation of CO2, similar to the mechanism of hydroboration, involves successive hydrosilylation steps to afford formoxysilane A’, bissilylacetal B’, methoxysilane C’ and methane (scheme 31). Scheme 31: General scheme for the reduction of CO2 by hydrosilane H SiR 3 Cat 1 H SiR 3 Cat 2 H C O O C O C O H SiR 3 O H SiR3 O A' B' SiR 3 Cat 3 Cat 4 CH 3OSiR3 B' CH 4 C' H SiR3 H SiR3 + + R 3SiOSiR 3 R 3SiOSiR3 D'

CO2

86/B(C 6F5 )3, 87 , or 88 HSiR3

(R 3 SiO) 2CH 2 B'

B(C 6F5 )3 HSiR3

CH 4 + R 2 SiOSiR2 D'

In 2010, Piers et al. described that the FLP system 89 with 1 additional equivalent of B(C6F5)3 catalyzes this reaction with Et3SiH as reductant [45]. The proposed mechanism involves the first activation of CO2 by the hydroborate affording compound 25 described by O’Hare et al. [43], and then successive hydrosilylation steps catalyzed by B(C6F5)3 leading to methane (scheme 34). The subsequent report by the group of Piers substantiated such mechanism with the support of theoretical calculations by Maron and Eisenstein. With the use of the same hydroborate species but with a (Cp*)2Sc countercation 90, the first activation step is much easier, leading to compound 40 (cf section 3) [46]. Solution state studies proved that compound 40 is in equilibrium with (Cp*)2Sc(O2CH) and free B(C6F5)3, explaining that catalysis proceeds to completion without additional B(C6F5)3. However, when additional B(C6F5)3 is added, a more pronounced induction period is observed, attributed to the change in the equilibrium between 40 and free B(C6F5)3 + (Cp*)2Sc(O2CH). After this induction period a much faster rate is observed due to the generation of a more active catalyst, which is proposed to be the ion pair 91.

The reduction of CO2 with hydrosilanes has been recently reviewed [24]. This section will concentrate on the hydrosilylation reactions catalyzed by boranes. The strong Lewis acid B(C6F5)3 is a known co-catalyst in hydrosilylation reactions as shown by Piers et al. and Gevorgyan and Yamamoto et al. in the reduction of carbonyl or alcohol functions [93-95]. It was elegantly proved that the borane compound activates the silane [96-99]. This silane borane complex cannot hydrosilylate CO2 on its own due to its inability to promote the first step. However, in the presence of an additional catalyst, B(C6F5)3 is involved in tandem catalyzes for the hydrosilylation of CO2. Catalysts 86-90 promotes the hydrosilylation of CO2 with B(C6F5)3 into methane (scheme 32). Scheme 32: Complexes 86-90 involved in tandem catalytic hydrosilylation of CO2 with B(C6F5)3

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Edited June 4 CO2 include the reduction by i) hydroborane, ii) diborane or silylborane and iii) borane-activated silanes. In the reaction with hydroboranes, up to three hydroboration steps can occur successively. The first elementary step is the hydride transfer which require a catalyst: either a metal complex featuring a hydride, a hydroborane, an ambiphilic compound featuring Lewis acid and base moiety of modest strength or even a strong Lewis base. When using diborane or silylborane as reductant, CO2 is reduced to CO with a copper catalyst or CsF. The reductant promotes the abstraction of the oxygen atom and the concomitant regeneration of the active catalyst featuring a key Cu-B or Cu-Si bond for the insertion of a new molecule of CO2. Finally, the specific properties of B(C6F5)3 in activating silanes are employed to co-catalyze the reduction of CO2 with hydrosilanes. This review offers an overview on various key features allowing the transformation of CO2. The main point is the need to catalyze the hydride transfer to CO2, which is the first elementary step both at the stoichiometric and catalytic level. Hopefully, the knowledge gained in the last decade will allow one to use the versatile properties of boron-containing molecule in catalytic amount and employ H2 as the reductant under similar mild conditions. Such findings would be an important breakthrough in the field of the reduction of CO2 but will also rely on a sustainable “carbon free” source of H2. Finally, more complex molecules are obtained by the reductive functionalization of CO2. One can assume that such multicomponent strategies could largely expand the scope of accessible compounds from the reduction of CO2 with hydroboranes in the future.

Scheme 34: CO2 reduction into methane by tandem catalysis 89-91 and B(C6F5)3 [45, 46] 89

A

O

CO 2

B(C 6 F5 )3 O B(C 6 F5 )3 HSiR3

H

or 90

CH4 + R 2SiOSiR 2 D'

A = TMPH: 25 (Cp*)2 Sc: 40

SiR 3

(C 6F 5) 3B O B(C 6 F5 )3

O O H

ip t

SiR3 O O H (Cp*)2 Sc O H H

B(C 6 F5 )3

91

O R

Sc O O

N O

H O B(C 6 F5) 3

Br Re

tBu

(iPr) 3P tBu

R = CH2SiMe 3

92

tBu

us

an

8. Acknowledgements

P(iPr) 3 H2

d

N

NO 93

te

tBu

O

M

(C 6 F5) 3B N

cr

Complexes 92 and 93 catalyze the selective reduction of CO2 into bis(silyl)acetal B’. Piers et al. synthesized the bispyridyl organoscandium catalyst 92 by the insertion of two molecules of CO2 into two Sc-R bonds (scheme 35) [102]. This system disfavors the formation of methane by limiting the concentration of free B(C6F5)3 due to its interaction with the ligand backbone. In a similar manner, the release of free B(C6F5)3 is presumably disfavored in the Re complex 93 [53]. Scheme 35: CO2 reduction into bis(silyl)acetal B’ by tandem catalysis with 92 and 93 [53, 102]

Ac ce p

Garcia et al., instead of using B(C6F5)3 as silane activator used BEt3 in the hydrosilylation of CO2. The nickel complex 94 was employed as co-catalyst to promote the selective formation of the formoxysilane A’ (scheme 36) [103]. Scheme 36: Tandem catalysis 94 / B(Et)3 [103] Pr2 Pr2 Pi Pi H Ni Ni H Pi Pi Pr2 Pr2 H SiR 3 94 C O CO2 + HSiR 3 O A' BEt

Support from CNRS and the ANR (Programme blanc “IRONHYC” ANR-12) is acknowledged. Didier Bourissou and Matthew Conley are warmly acknowledged for helpful discussions and comments on this manuscript. Special thanks to Sylviane Sabo-Etienne for our on-going collaboration on CO2 transformations.

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3

In summary, the known ability of the strongly Lewis acidic B(C6F5)3 to activate silane can be used in tandem catalytic system for the reduction of CO2. Methane and bis(silyl)acetal compounds are the two main compounds generated and key mechanistic insights have been recently disclosed.

7. Concluding remarks Various modes of activation of CO2 with boron-containing molecule have been presented. At the stoichiometric level, Lewis pairs form stable CO2 adducts when a certain strength either in the Lewis basicity or acidity is present. Formatoborate compounds are readily formed either from the direct reaction with hydroborate or with hydride donor complexes mediated by boranes. Catalytic transformations of

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Edited June 4 Graphical Abstract: Stoichiometric and catalytic activations of CO2 by boron-containing molecule are presented in this review, with an emphasis on the role of boron. H BR3

C O O

L.B. / BR 3

A A

BR 3

C O O

H-BR 3

ip t

L.B.

O C O

Hydroboration products

HBR 2

Cat / B(C6 F5 )3 HSiR3 Cat R2B-E

E = BR 2 or SiR3

Ac ce p

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CO + R 2BOE

Hydrosilylation products

cr

Cat

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